The effect of light during embryonic development on laterality and exploration in Western Rainbowfish

Several factors affect the development of lateralization such as hormones and light exposure during early development. Laterality also often correlates with other behavioral traits. To examine whether there is a common mechanism underlying the development of laterality and other behaviors, we manipulated laterality by exposing embryos of the Western rainbowfish (Melatotaenia australis) to light or continuous darkness during early development and determined whether a shift in laterality was associated with a change in behavior in a novel environment test at two different ages. We found that exposing eggs to darkness led to offspring that displayed significantly less lateralized behavior in the mirror test two weeks after hatching than offspring from eggs exposed to light. Interestingly, the effects of rearing condition were lost by 3 months of age. These data suggest that exposure to light can influence laterality very early in development, but such bias can be overwritten by developmental processes post-hatch. Moreover, our manipulation of laterality apparently had no influence on exploration suggesting independent causal mechanisms. The experimental manipulation of light exposure during development could be a useful tool for enhancing individuals with a specific laterality and behavioral traits to aid future research into the causes and consequences of laterality.

Dynamics of maternal androgens and its metabolites during early embryonic development: embryonic modification of a maternal effect

In birds, exposure to maternal (yolk) testosterone affects a diversity of offspring post-hatching traits, which eventually affect offspring competitiveness. However, maternal testosterone is heavily metabolized at very early embryonic developmental stages to hydrophilic metabolites that are often assumed to be much less biologically potent. Either the rapid metabolism could either keep the maternal testosterone from reaching the embryos, opening the possibility for a mother-offspring conflict or the metabolites may facilitate the uptake of the lipophilic testosterone from the yolk into the embryonic circulation after which they are either converted back to the testosterone or functioning directly as metabolites. To test these possibilities, we injected isotope-labeled testosterone (T-[D5]) into the yolk of freshly laid Rock pigeon (Columba livia) eggs and determined the concentration and distribution of T-[D5] and its labeled metabolites within different egg fractions by liquid chromatography combined with tandem mass spectrometry at day 2, 5 and 10 of incubation. Although under a supraphysiological dosage injection, yolk testosterone decreased within 2 days and was metabolized into androstenedione, conjugated testosterone, etiocholanolone and other components that were unidentifiable due to methodological limitation. We show for the first time that testosterone, androstenedione and conjugated testosterone, but not etiocholanolone, reached the embryo including its brain. Their high concentrations in the yolk and extraembryonic membranes suggest that conversion takes place here. We also found no sex-specific metabolism, explaining why maternal testosterone does not affect sexual differentiation. Our findings showed that maternal testosterone is quickly converted by the embryo, with several but not all metabolites reaching the embryo providing evidence for both hypotheses.

Egg-mediated maternal effects in a cooperatively breeding cichlid fish

Mothers can influence offspring phenotype through egg-mediated maternal effects, which can be influenced by cues mothers obtain from their environment during offspring production. Developing embryos use these components but have mechanisms to alter maternal signals. Here we aimed to understand the role of mothers and embryos in how maternal effects might shape offspring social phenotype. In the cooperatively breeding fish Neolamprologus pulcher different social phenotypes develop in large and small social groups differing in predation risk and social complexity. We manipulated the maternal social environment of N. pulcher females during egg laying by allocating them either to a small or a large social group. We compared egg mass and clutch size and the concentration of corticosteroid metabolites between social environments, and between fertilized and unfertilized eggs to investigate how embryos deal with maternal signalling. Mothers in small groups produced larger clutches but neither laid smaller eggs nor bestowed eggs differently with corticosteroids. Fertilized eggs scored lower on a principal component representing three corticosteroid metabolites, namely 11-deoxycortisol, cortisone, and 11-deoxycorticosterone. We did not detect egg-mediated maternal effects induced by the maternal social environment. We discuss that divergent social phenotypes induced by different group sizes may be triggered by own offspring experience.

Position in the laying order has sex-specific consequences for reproductive success in adult black-headed gulls

Mothers who produce multiple offspring within one reproductive attempt often allocate resources differentially; some maternally derived substances are preferentially allocated to last-produced offspring and others to first-produced offspring. The combined effect of these different allocation regimes on the overall fitness of offspring produced early or late in the sequence is not well understood, partly because production order is often coupled with birth order, making it difficult-to-separate effects of pre-natal maternal allocation from those of post-natal social environments. In addition, very little is known about the influence of laying order on fitness in later life. In this study, we used a semi-natural captive colony of black-headed gulls to test whether an offspring's position in the laying order affected its early-life survival and later-life reproductive success, independent of its hatching order. Later-laid eggs were less likely to hatch, but among those that did, survival to adulthood was greater than that of first-laid eggs. In adulthood, the laying order of females did not affect their likelihood of breeding in the colony, but male offspring hatched from last-laid eggs were significantly less likely to gain a breeding position than earlier-laid males. In contrast, later-laid female parents hatched lower proportions of their clutches than first-laid females, but hatching success was unrelated to the laying order of male parents. Our results indicate that gull mothers induce complex and sex-specific effects on both the early survival of their offspring and on long-term reproductive success through laying order effects among eggs of the same breeding attempt.

Mesocosm experiment reveals scale dependence of movement tendencies in sticklebacks

Habitat fragmentation can have negative impacts on migratory organisms that rely on the functional connectivity between growing and breeding grounds. Quantifying the population-level phenotypic consequences of such fragmentation requires fine-scaled tracking of individual behaviour and movements across relevant scales. Here we make use of a natural experiment where some populations of 'migrant' three-spined sticklebacks (Gasterosteus aculeatus) became 'residents', following habitat fragmentation five decades ago. To test whether residents have a lower movement tendency than migrants, we developed a novel experimental platform that allows the automated tracking of individual movements via RFID technology in a semi-natural mesocosm where spatio-temporal scales and environmental conditions can be manipulated. We found that residents moved significantly less than migrants at large but not at small spatial scale. This pattern was consistent across time and contexts (water flow and group size). Our study substantiates prior literature on rapid phenotypic divergence in sticklebacks in response to human-induced isolation and highlights the importance of observing behaviour in ecologically relevant set-ups that bridge the gap between laboratory and field studies.

Maternally-transferred thyroid hormones and life-history variation in birds

In vertebrates, thyroid hormones (THs) play an important role in the regulation of growth, development, metabolism, photoperiodic responses and migration. Maternally transferred THs are important for normal early phase embryonic development when embryos are not able to produce endogenous THs. Previous studies have shown that variation in maternal THs within the physiological range can influence offspring phenotype.

Given the essential functions of maternal THs in development and metabolism, THs may be a mediator of life‐history variation across species.

We tested the hypothesis that differences in life histories are associated with differences in maternal TH transfer across species. Using birds as a model, we specifically tested whether maternally transferred yolk THs covary with migratory status, developmental mode and traits related to pace‐of‐life (e.g. basal metabolic rate, maximum life span).

We collected un‐incubated eggs (n = 1–21 eggs per species, median = 7) from 34 wild and captive bird species across 17 families and six orders to measure yolk THs [both triiodothyronine (T3) and thyroxine (T4)], compiled life‐history trait data from the literature and used Bayesian phylogenetic mixed models to test our hypotheses.

Our models indicated that both concentrations and total amounts of the two main forms of THs (T3 and T4) were higher in the eggs of migratory species compared to resident species, and total amounts were higher in the eggs of precocial species, which have longer prenatal developmental periods, than in those of altricial species. However, maternal yolk THs did not show clear associations with pace‐of‐life‐related traits, such as fecundity, basal metabolic rate or maximum life span.

We quantified interspecific variation in maternal yolk THs in birds, and our findings suggest higher maternal TH transfer is associated with the precocial mode of development and migratory status. Whether maternal THs represent a part of the mechanism underlying the evolution of precocial development and migration or a consequence of such life histories is currently unclear. We therefore encourage further studies to explore the physiological mechanisms and evolutionary processes underlying these patterns.

Androgen Elevation Accelerates Reproductive Senescence in Three-Spined Stickleback

Costs of reproduction shape the life-history evolution of investment in current and future reproduction and thereby aging. Androgens have been proposed to regulate the physiology governing these investments. Furthermore, androgens are hypothesized to play a central role in carotenoid-dependent sexual signaling, regulating how much carotenoids are diverted to ornamentation and away from somatic maintenance, increasing oxidative stress, and accelerating aging. We investigated these relationships in male three-spined stickleback in which we elevated 11-ketotestosterone and supplied vitamin E, an antioxidant, in a 2 × 2 design. Androgen elevation shortened the time stickleback maintained reproductive activities. We suspect that this effect is caused by 11-ketotestosterone stimulating investment in current reproduction, but we detected no evidence for this in our measurements of reproductive effort: nest building, body composition, and breeding coloration. Carotenoid-dependent coloration was even slightly decreased by 11-ketotestosterone elevation and was left unaffected by vitamin E. Red coloration correlated with life expectancy and reproductive capacity in a quadratic manner, suggesting overinvestment of the individuals exhibiting the reddest bellies. In contrast, blue iris color showed a negative relationship with survival, suggesting physiological costs of producing this aspect of nuptial coloration. In conclusion, our results support the hypothesis that androgens regulate investment in current versus future reproduction, yet the precise mechanisms remain elusive. The quadratic relationships between sexual signal expression and aspects of quality have wider consequences for how we view sexual selection on ornamentation and its relationship with aging.

Habitat fragmentation induces rapid divergence of migratory and isolated sticklebacks

The adaptive capacity of many organisms is seriously challenged by human-imposed environmental change, which currently happens at unprecedented rates and magnitudes. For migratory fish, habitat fragmentation is a major challenge that can compromise their survival and reproduction. Therefore, it is important to study if fish populations can adapt to such modifications of their habitat. Here, we study whether originally anadromous three-spined stickleback populations (Gasterosteus aculeatus; “migrants”) changed in behavior and morphology in response to human-induced isolation. We made use of a natural field-experiment, where the construction of pumping stations and sluices in the 1970s unintendedly created replicates of land-locked stickleback populations (“resident”) in the Netherlands. For two years, we systematically tested populations of residents and migrants for differences in morphology and behavioral traits (activity, aggressiveness, exploration, boldness, and shoaling) in lab-based assays. We detected differences between migrant and resident populations in virtually all phenotypic traits studied: compared with the ancestral migrants, residents were smaller in size, had fewer and smaller plates and were significantly more active, aggressive, exploratory and bolder, and shoaled less. Despite large ecological differences between 2018 and 2019, results were largely consistent across the two years. Our study shows that human-induced environmental change has led to the rapid and consistent morphological and behavioral divergence of stickleback populations in about 50 generations. Such changes may be adaptive but this remains to be tested.

Exploration speed in captivity predicts foraging tactics and diet in free-living red knots

Variation in foraging tactics and diet is usually attributed to differences in morphology, experience and prey availability. Recently, consistent individual differences in behaviour (personality) have been shown to be associated with foraging strategies. Bolder or more exploratory individuals are predicted to have a faster pace-of-life and offset the costs of moving more or in risky areas, with higher energetic gains by encountering profitable foraging opportunities and prey. However, the relationship between personality, foraging and diet is poorly understood. We investigated how exploratory behaviour in red knots Calidris canutus is associated with foraging tactics and diet by combining laboratory experiments, field observations and stable isotope analysis. First, we developed a mobile experimental arena to measure exploration speed in controlled settings. We validated the method by repeated testing of individuals over time and contexts. This setup allowed us to measure exploratory personality at the field site, eliminating the need to bring birds into captivity for long periods of time. After releasing birds within days of their capture, we asked whether exploration speed was associated with differences in foraging tactics and diet in the wild. We found that tactile foraging red knots mainly caught hard-shelled prey that are buried in the sediment, whereas visual foraging knots only captured soft preys located close to or on the surface. We also found that faster explorers showed a higher percentage of visual foraging than slower explorers. By contrast, morphology (bill length and gizzard size) had no significant effect on foraging tactics. Diet analysis based on δ¹⁵ N and δ¹³ C stable isotope values of plasma and red blood cells confirmed our field observations with slower explorers mainly consumed hard-shelled prey while faster explorers consumed more soft than hard-shelled prey. Our results show that foraging tactics and diet are associated with a personality trait, independent of morphological differences. We discuss how consistent behaviour might develop early in life through positive feedbacks between foraging tactics, prey type and foraging efficiency.

Is imitational learning a driving factor for the population bias in human hand preference?

Lateral preference is a widespread organizational principle in human and nonhuman animals. In humans, the most apparent lateralized trait (handedness) is unique in the animal kingdom because of a very pronounced bias towards right-handedness on a population level. In this study, based on previous experiments, we test the hypothesis that this bias was-among other factors-shaped by evolution through the facilitation of social learning. We exposed 134 subjects to footage of right- or left-handed knot making and analyzed whether concordant handedness between instructor and student facilitated quicker and more successful imitation. We used a set of nautical knots of different difficulty levels in order to test whether the potential effect of concordance became stronger with increasing knot difficulty. For all three performance measures (time until correct completion, number of attempts needed and correct imitation), we found hand congruency and difficulty level to be significant predictors but not the interaction of the two. We conclude that concordance of handedness between teacher and student of a motor skill enhances the speed and accuracy of imitation, which may have been a beneficial trait for selection to act upon, thereby shaping the human population bias in handedness.

Testing the Darwinian function of lateralization. Does separation of workload between brain hemispheres increase cognitive performance?

Brain lateralization is a fundamental aspect of the organization of brain and behavior in the animal kingdom, begging the question about its Darwinian function. We tested the possibility that lateralization enhances cognitive performance in single- and dual-tasks. Previous studies reported mixed results on this topic and only a handful of studies have measured functional brain lateralization and performance independently and simultaneously. We therefore examined a possible positive effect of the strength and direction of lateralization on two demanding cognitive tasks: A visuospatial task (mental rotation MR), and a language task (word generation WG), executed either as a singletask or as dual-task. Participants (n = 72) performed these tasks while their single-task brain lateralization was assessed with functional Transcranial Doppler for both tasks. From these measurements we determined strength and direction of lateralization for both tasks and the individual pattern of lateralization (contralateral or ipsilateral) was derived. These factors, along with sex, were used in a GLM analysis to determine if they predicted the respective performance measure of the tasks. We found that for MR there was a significant medium effect of direction of lateralization on performance with better performance in left-lateralized (atypical) participants (partial eta squared 0.061; p = .039). After correction for outliers, there was a significant effect for strength (p = .049). For the dual-task, there was a significant positive medium effect of strength of lateralization on performance (partial eta squared 0.062; p = .038, respectively) No other association between direction or strength in either tests were found. We conclude that there is no evidence for hemispheric crowding, and that strength of lateralization may be a factor that contributes to the evolutionary selection of functional brain lateralization. Pattern of lateralization does not, explaining the large inter-individual variation in these traits.

Who listens to mother? A whole-family perspective on the evolution of maternal hormone allocation

Maternal effects, or the influence of maternal environment and phenotype on offspring phenotype, may allow mothers to fine-tune their offspring's developmental trajectory and resulting phenotype sometimes long after the offspring has reached independence. However, maternal effects on offspring phenotype do not evolve in isolation, but rather within the context of a family unit, where the separate and often conflicting evolutionary interests of mothers, fathers and offspring are all at play. While intrafamilial conflicts are routinely invoked to explain other components of reproductive strategy, remarkably little is known about how intrafamilial conflicts influence maternal effects. We argue that much of the considerable variation in the relationship between maternally derived hormones, nutrients and other compounds and the resulting offspring phenotype might be explained by the presence of conflicting selection pressures on different family members. In this review, we examine the existing literature on maternal hormone allocation as a case study for maternal effects more broadly, and explore new hypotheses that arise when we consider current findings within a framework that explicitly incorporates the different evolutionary interests of the mother, her offspring and other family members. Specifically, we hypothesise that the relationship between maternal hormone allocation and offspring phenotype depends on a mother's ability to manipulate the signals she sends to offspring, the ability of family members to be plastic in their response to those signals and the capacity for the phenotypes and strategies of various family members to interact and influence one another on both behavioural and evolutionary timescales. We also provide suggestions for experimental, comparative and theoretical work that may be instrumental in testing these hypotheses. In particular, we highlight that manipulating the level of information available to different family members may reveal important insights into when and to what extent maternal hormones influence offspring development. We conclude that the evolution of maternal hormone allocation is likely to be shaped by the conflicting fitness optima of mothers, fathers and offspring, and that the outcome of this conflict depends on the relative balance of power between family members. Extending our hypotheses to incorporate interactions between family members, as well as more complex social groups and a wider range of taxa, may provide exciting new developments in the fields of endocrinology and maternal effects.

The importance of understanding function and evolution

Ocklenburg et al. (2020, Laterality 2020: Entering the next decade. Laterality) provided the field of laterality research with a stimulating research perspective for the coming decade, based on the current state of the art in both animal and human laterality research. Although this is paper takes many different approaches of laterality into account, we emphasize that the eco-evolutionary approach needs more attention. This concerns the question why organisms are lateralized in the first place, in other words, how does lateralization enhance the Darwinian fitness of the individual. We argue that laterality can be distinguished along four dimensions, and that each of them requires different ultimate explanations. Studying these functional and evolutionary explanations requires the development of ecologically relevant tests, adapted to the species at hand. It also requires experimental manipulation of laterality, testing the effect in (semi)-natural conditions on fitness parameters. Tools for such manipulation of laterality urgently require a better understanding of the developmental plasticity of lateralization, extending the field of evo-devo to that of eco-evo-devo. We also warn against seeing the minority in the distribution of direction or strength of lateralization as being a pathology as such minorities in biology can often be explained as adaptations by natural frequency dependent selection.

Manipulation of Prenatal Thyroid Hormones Does Not Affect Growth or Physiology in Nestling Pied Flycatchers

Hormones transferred from mothers to their offspring are thought to be a tool for mothers to prepare their progeny for expected environmental conditions, thus increasing fitness. Thyroid hormones (THs) are crucial across vertebrates for embryonic and postnatal development and metabolism. Yet yolk THs have mostly been ignored in the context of hormone-mediated maternal effects. In addition, the few studies on maternal THs have yielded contrasting results that could be attributed to either species or environmental differences. In this study, we experimentally elevated yolk THs (within the natural range) in a wild population of a migratory passerine, the European pied flycatcher (Ficedula hypoleuca), and assessed the effects on hatching success, nestling survival, growth, and oxidative status (lipid peroxidation, antioxidant enzyme activity, and oxidative balance). We also sought to compare our results with those of a closely related species, the collared flycatcher (Ficedula albicolis), that has strong ecological and life-history similarities with our species. We found no effects of yolk THs on any of the responses measured. We could detect only a weak trend on growth: elevated yolk THs tended to increase growth during the second week after hatching. Our results contradict the findings of previous studies, including those of the collared flycatcher. However, differences in fledging success and nestling growth between both species in the same year suggest a context-dependent influence of the treatment. This study should stimulate more research on maternal effects mediated by THs and their potential context-dependent effects.

Testing different forms of regulation of yolk thyroid hormone transfer in pied flycatchers

Hormones transferred from mothers to their offspring are considered a maternal tool to prepare progeny for expected environmental conditions, increasing maternal and offspring fitness. To flexibly influence offspring, mothers should be able to transmit the hormonal signals independent of their own hormonal status. However, the ability to regulate hormone transfer to the next generation is under debate. We studied the transfer of thyroid hormones (THs) to eggs in a bird model. We elevated thyroxine (T4, the prohormone for the biologically active triiodothyronine, T3) during egg laying using T4 implants in females of a wild population of pied flycatchers (Ficedula hypoleuca), and measured the resulting plasma and yolk T4 and T3 levels. We found an increase in plasma and yolk T4 and no change in plasma or yolk T3 concentration, leading to a decrease in yolk T3/T4 ratio in response to the T4 treatment. The yolk T3/T4 ratio was similar to the plasma ratio in females during the yolking phase. This suggests that mothers are not able to regulate TH transfer to yolk but may regulate the T4 to T3 conversion to avoid potential costs of elevated exposure to the active hormone to herself and to her progeny. The absence of regulation in hormone transfer to eggs is in contrast to our predictions. Future studies on deiodinase activity that converts T4 to T3 in maternal and embryonic tissues may help our understanding of how mothers regulate circulating THs during breeding, as well as the embryos' role in converting maternal T4 to its biologically active T3 form during development.

Temporally consistent species differences in parasite infection but no evidence for rapid parasite-mediated speciation in Lake Victoria cichlid fish

Parasites may have strong eco‐evolutionary interactions with their hosts. Consequently, they may contribute to host diversification. The radiation of cichlid fish in Lake Victoria provides a good model to study the role of parasites in the early stages of speciation. We investigated patterns of macroparasite infection in a community of 17 sympatric cichlids from a recent radiation and 2 older species from 2 nonradiating lineages, to explore the opportunity for parasite‐mediated speciation. Host species had different parasite infection profiles, which were only partially explained by ecological factors (diet, water depth). This may indicate that differences in infection are not simply the result of differences in exposure, but that hosts evolved species‐specific resistance, consistent with parasite‐mediated divergent selection. Infection was similar between sampling years, indicating that the direction of parasite‐mediated selection is stable through time. We morphologically identified 6 Cichlidogyrus species, a gill parasite that is considered a good candidate for driving parasite‐mediated speciation, because it is host species‐specific and has radiated elsewhere in Africa. Species composition of Cichlidogyrus infection was similar among the most closely related host species (members of the Lake Victoria radiation), but two more distantly related species (belonging to nonradiating sister lineages) showed distinct infection profiles. This is inconsistent with a role for Cichlidogyrus in the early stages of divergence. To conclude, we find significant interspecific variation in parasite infection profiles, which is temporally consistent. We found no evidence that Cichlidogyrus‐mediated selection contributes to the early stages of speciation. Instead, our findings indicate that species differences in infection accumulate after speciation.

Testing sensory drive speciation in cichlid fish: Linking light conditions to opsin expression, opsin genotype and female mate preference

Ecological speciation is facilitated when divergent adaptation has direct effects on selective mating. Divergent sensory adaptation could generate such direct effects, by mediating both ecological performance and mate selection. In aquatic environments, light attenuation creates distinct photic environments, generating divergent selection on visual systems. Consequently, divergent sensory drive has been implicated in the diversification of several fish species. Here, we experimentally test whether divergent visual adaptation explains the divergence of mate preferences in Haplochromine cichlids. Blue and red Pundamilia co‐occur across south‐eastern Lake Victoria. They inhabit different photic conditions and have distinct visual system properties. Previously, we documented that rearing fish under different light conditions influences female preference for blue versus red males. Here, we examine to what extent variation in female mate preference can be explained by variation in visual system properties, testing the causal link between visual perception and preference. We find that our experimental light manipulations influence opsin expression, suggesting a potential role for phenotypic plasticity in optimizing visual performance. However, variation in opsin expression does not explain species differences in female preference. Instead, female preference covaries with allelic variation in the long‐wavelength‐sensitive opsin gene (LWS), when assessed under broad‐spectrum light. Taken together, our study presents evidence for environmental plasticity in opsin expression and confirms the important role of colour perception in shaping female mate preferences in Pundamilia. However, it does not constitute unequivocal evidence for the direct effects of visual adaptation on assortative mating.

The relationship between male social status, ejaculate and circulating testosterone concentration and female yolk androgen transfer in red junglefowl (Gallus gallus)

Several studies show that avian females prefer males based on their secondary sexual ornaments and dominance status. We tested in red junglefowl (Gallus gallus) how comb size affected the result of fighting and how the dominance status related to testosterone concentrations in their circulation and ejaculates. We subsequently tested how social status was related to female reproductive investment, including yolk hormone transfer. We found that after a fight 1) winners increased plasma T and decreased ejaculates T whereas losers' T remained unchanged, and 2) plasma T of winners was higher but ejaculates T was lower than those of losers. We argued those are consistent with the different reproductive strategies of dominant and subordinate males. Furthermore, in line with offspring sex-dependent growth patterns females transferred significantly more androstenedione to female than male embryos when mated with winners, while doing the opposite when mated with losers. We concluded therefore that female reproductive investment was affected by both partner quality and embryo sex. The results indicate that male quality influences sex-specific maternal investment, which could be mediated by ejaculate testosterone concentration.

Revisiting mechanisms and functions of prenatal hormone-mediated maternal effects using avian species as a model

Maternal effects can adaptively modulate offspring developmental trajectories in variable but predictable environments. Hormone synthesis is sensitive to environmental factors, and maternal hormones are thus a powerful mechanism to transfer environmental cues to the next generation. Birds have become a key model for the study of hormone-mediated maternal effects because the embryo develops outside the mother's body, facilitating the measurement and manipulation of prenatal hormone exposure. At the same time, birds are excellent models for the integration of both proximate and ultimate approaches, which is key to a better understanding of the evolution of hormone-mediated maternal effects. Over the past two decades, a surge of studies on hormone-mediated maternal effects has revealed an increasing number of discrepancies. In this review, we discuss the role of the environment, genetic factors and social interactions in causing these discrepancies and provide a framework to resolve them. We also explore the largely neglected role of the embryo in modulating the maternal signal, as well as costs and benefits of hormone transfer and expression for the different family members. We conclude by highlighting fruitful avenues for future research that have opened up thanks to new theoretical insights and technical advances in the field. This article is part of the theme issue 'Developing differences: early-life effects and evolutionary medicine'.

Visual adaptation and microhabitat choice in Lake Victoria cichlid fish

When different genotypes choose different habitats to better match their phenotypes, genetic differentiation within a population may be promoted. Mating within those habitats may subsequently contribute to reproductive isolation. In cichlid fish, visual adaptation to alternative visual environments is hypothesized to contribute to speciation. Here, we investigated whether variation in visual sensitivity causes different visual habitat preferences, using two closely related cichlid species that occur at different but overlapping water depths in Lake Victoria and that differ in visual perception (Pundamilia spp.). In addition to species differences, we explored potential effects of visual plasticity, by rearing fish in two different light conditions: broad-spectrum (mimicking shallow water) and red-shifted (mimicking deeper waters). Contrary to expectations, fish did not prefer the light environment that mimicked their typical natural habitat. Instead, we found an overall preference for the broad-spectrum environment. We also found a transient influence of the rearing condition, indicating that the assessment of microhabitat preference requires repeated testing to control for familiarity effects. Together, our results show that cichlid fish exert visual habitat preference but do not support straightforward visual habitat matching.

Avian yolk androgens are metabolized instead of taken up by the embryo during the first days of incubation

Several studies show effects of yolk androgens in avian eggs on the phenotype of the offspring. Yolk hormone concentrations decline strongly already in the first few days of incubation. Although early embryonic uptake of yolk androgens is suggested by the presence of radioactivity in the embryo when eggs are injected with radiolabelled androgens, these studies do not verify the chemical identity of radioactive compound(s), while it is known that these androgens can be metabolized substantially. By using stable isotope labelled testosterone and androstenedione in combination with mass spectrometry, enabling verification of the exact molecular identity of labelled compounds in the embryo, we found that after five days of incubation the androgens are not taken up by the embryo. Yet their concentrations in the entire yolk-albumen homogenates decline strongly, even when corrected for dilution by albumen and water. Our results indicate metabolism of maternal androgens, very likely to 5β-androstane-3α,17β-diol, etiocholanolone, and their conjugated forms. The results imply that the effects of increased exposure of the embryo to maternal androgens either take place before this early conversion or are mediated by these metabolites with a so far unknown function, opening new avenues for understanding hormone mediated maternal effects in vertebrates.

Early embryonic modification of maternal hormones differs systematically among embryos of different laying order: A study in birds

Vertebrate embryos are exposed to maternal hormones that can profoundly affect their later phenotype. Although it is known that the embryo can metabolize these maternal hormones, the metabolic outcomes, their quantitative dynamics and timing are poorly understood. Moreover, it is unknown whether embryos can adjust their metabolic activity to, for example, hormones or other maternal signals. We studied the dynamics of maternal steroids in fertilized and unfertilized rock pigeon eggs during early incubation. Embryos of this species are naturally exposed to different amounts of maternal steroids in the egg according to their laying position, which provides a natural context to study differential embryonic regulation of the maternal signals. We used mass spectrometric analyses to map changes in the androgen and estrogen pathways of conversion. We show that the active hormones are heavily metabolized only in fertilized eggs, with a corresponding increase in supposedly less potent metabolites already within one-fourth of total incubation period. Interestingly, the rate of androgen metabolism was different between embryos in different laying positions. The results also warrant a re-interpretation of the timing of hormone mediated maternal effects and the role of the supposedly biologically inactive metabolites. Furthermore, the results also provide a potential solution as to how the embryo can prevent maternal steroids in the egg from interfering with its sexual differentiation processes as we show that the embryo can metabolize most of the maternal steroids before sexual differentiation starts.

Prenatal and pubertal testosterone affect brain lateralization

After decades of research, the influence of prenatal testosterone on brain lateralization is still elusive, whereas the influence of pubertal testosterone on functional brain lateralization has not been investigated, although there is increasing evidence that testosterone affects the brain in puberty. We performed a longitudinal study, investigating the relationship between prenatal testosterone concentrations in amniotic fluid, pubertal testosterone concentrations in saliva, and brain lateralization (measured with functional Transcranial Doppler ultrasonography (fTCD)) of the Mental Rotation, Chimeric Faces and Word Generation tasks. Thirty boys and 30 girls participated in this study at the age of 15 years. For boys, we found a significant interaction effect between prenatal and pubertal testosterone on lateralization of Mental Rotation and Chimeric Faces. In the boys with low prenatal testosterone levels, pubertal testosterone was positively related to the strength of lateralization in the right hemisphere, while in the boys with high prenatal testosterone levels, pubertal testosterone was negatively related to the strength of lateralization. For Word Generation, pubertal testosterone was negatively related to the strength of lateralization in the left hemisphere in boys. For girls, we did not find any significant effects, possibly because their pubertal testosterone levels were in many cases below quantification limit. To conclude, prenatal and pubertal testosterone affect lateralization in a task-specific way. Our findings cannot be explained by simple models of prenatal testosterone affecting brain lateralization in a similar way for all tasks. We discuss alternative models involving age dependent effects of testosterone, with a role for androgen receptor distribution and efficiency.

Maternal thyroid hormones enhance hatching success but decrease nestling body mass in the rock pigeon (Columba livia)

Thyroid hormones (THs) - triiodothyronine (T3) and thyroxine (T4) - are essential for embryonic development in vertebrates. All vertebrate embryos are exposed to THs from maternal origin. As maternal TH levels are known to be essential to embryonic development, the natural variation of maternal THs probably represents a pathway of maternal effects that can modify offspring phenotype. However, potential fitness consequences of variation of maternal TH exposure within the normal physiological range and without confounding effects of the mother have never been experimentally investigated. We experimentally manipulated the levels of yolk T3 and T4 within the physiological range in a species in which the embryo develops outside the mother's body, the Rock Pigeon (Columba livia) eggs. Making use of the natural difference of yolk testosterone between the two eggs of pigeon clutches, we were also able to investigate the potential interaction between THs and testosterone. Elevated yolk TH levels enhanced embryonic development and hatching success, and reduced body mass but not tarsus length between day 14 and fledging. The yolk hormones increased plasma T4 concentrations in females but reduced it in males, in line with the effect on metabolic rate at hatching. Plasma concentrations of T3 and testosterone were not significantly affected. The effects of treatment did not differ between eggs with high or low testosterone levels. Our data indicate that natural variation in maternal yolk TH levels affects offspring phenotype and embryonic survival, potentially influencing maternal and chick fitness.

Coadaptation of offspring begging and parental provisioning: A role for prenatal maternal effects?

Recent studies on birds have shown that offspring begging and parental provisioning covary at the phenotypic level, which is thought to reflect genetic correlations. However, prenatal maternal factors, like yolk testosterone, may also facilitate parent-offspring coadaptation via their effects on offspring begging and development. In fact, maternal effects are thought to adjust offspring phenotype to the environmental conditions they will experience after birth, which are in turn strongly dependent on the levels of parental provisioning. Using cross-fostering experiments in canaries, we tested the role of maternal effects on parent-offspring coadaptation from two different approaches. First, we analyzed whether females deposit yolk testosterone in relation to their own or their partner's prospective parental provisioning, measured as the rate of parental feeding to foster nestlings. Second, we investigated whether females deposit yolk testosterone in relation to costs they incurred when raising a previous brood, as this likely impinges on their capacity to provide parental care in the near future. However, from the results of both experiments we have no evidence that canary females deposit yolk testosterone in order to match offspring begging to the levels of care they and/or their partners provide. We therefore found no evidence that yolk testosterone facilitates parent-offspring coadaptation. In addition, our results suggest that the functional consequences of yolk testosterone deposition may relate to hatching asynchrony since it primarily varied with egg laying order.

Temperature-induced variation in yolk androgen and thyroid hormone levels in avian eggs

Global warming has substantially changed the environment, but the mechanisms to cope with these changes in animals, including the role of maternal effects, are poorly understood. Maternal effects via hormones deposited in eggs, have important environment-dependent effects on offspring development and fitness: thus females are expected to adjust these hormones to the environment, such as the ambient temperature. Longer-term temperature variation could function as a cue, predicting chick rearing conditions to which yolk hormone levels are adjusted, while short-term temperature variation during egg formation may causally affect hormone transfer to eggs. We studied the effects of ambient temperature on yolk androgens (testosterone and androstenedione) and thyroid hormones (thyroxine and triiodothyronine) in great tits (Parus major) using data from unmanipulated clutches from a wild population and from aviary birds (ad libitum food) exposed to different experimental temperature treatments during five years. Both in the wild and in captivity, longer-term pre-laying ambient temperature was not associated with clutch mean yolk hormone levels, while the way androstenedione and thyroxine levels varied across the laying sequence did associate with pre-laying temperature in the wild. Yolk testosterone levels were positively correlated with short-term temperature (during yolk formation) changes within clutches in both wild and captivity. We also report, for the first time in a wild bird, that yolk thyroxine levels correlated with a key environmental factor: thyroxine levels were negatively correlated with ambient temperature during egg formation. Thus, yolk hormone levels, especially testosterone, seem to be causally affected by ambient temperature. These short-term effects might reflect physiological changes in females with changes in ambient temperature. The adaptive value of the variation with ambient temperatures pre-laying or during egg formation should be studied with hormone manipulations in different thermal environments.

Heritable variation in maternally derived yolk androgens, thyroid hormones and immune factors

Maternal reproductive investment can critically influence offspring phenotype, and thus these maternal effects are expected to be under strong natural selection. Knowledge on the extent of heritable variation in the physiological mechanisms underlying maternal effects is however limited. In birds, resource allocation to eggs is a key mechanism for mothers to affect their offspring and different components of the egg may or may not be independently adjusted. We studied the heritability of egg components and their genetic and phenotypic covariation in great tits (Parus major), using captive-bred full siblings of wild origin. Egg mass, testosterone (T) and androstenedione (A4) hormone concentrations showed moderate heritability, in agreement with earlier findings. Interestingly, yolk triiodothyronine hormone (T3), but not its precursor, thyroxine hormone (T4), concentration was heritable. An immune factor, albumen lysozyme, showed moderate heritability, but yolk immunoglobulins (IgY) did not. The genetic correlation estimates were moderate but statistically nonsignificant; a trend for a positive genetic correlation was found between A4 and egg mass, T and lysozyme and IgY and lysozyme, respectively. Interestingly, phenotypic correlations were found only between A4 and T, and T4 and T3, respectively. Given that these egg components are associated with fitness-related traits in the offspring (and mother), and that we show that some components are heritable, it opens the possibility that natural selection may shape the rate and direction of phenotypic change via egg composition.

Effects of experimentally sustained elevated testosterone on incubation behaviour and reproductive success in female great tits (Parus major)

In many seasonally breeding birds, female and male testosterone (T) levels peak at the start of the breeding season, coinciding with pair bonding and nesting activities. Shortly after the onset of egg laying, T levels slowly decline to baseline levels in both sexes, but more rapidly so in females. During this period, T in males may still function to facilitate territorial behaviour, mate guarding and extra pair copulations, either via short lasting peaks or elevated basal levels of the hormone. In some species, however, males become insensitive to increased T after the onset of egg laying. It has been postulated that in these species bi-parental care is essential for offspring survival, as T is known to inhibit paternal care. However, only very few studies have analysed this for females. As females are heavily involved in parental care, they too might become insensitive to T after egg laying. Alternatively, because territorial defence, mate guarding and extra pair copulations are expected to be less important for females than for males, they may not have had the need to evolve a mechanism to become insensitive to T during the period of maternal care, because their natural T levels are never elevated during this part of the breeding season anyway. We tested these alternative hypotheses in female great tits (Parus major). Male great tits have previously been shown to be insensitive to T after egg laying with regard to nestling feeding behaviour (but not song rate). When females had started nest building, we experimentally elevated their T levels up to the nestling feeding phase, and measured incubation behaviour (only females incubate) and reproductive success. T did not significantly affect nest building or egg laying behaviour, although egg laying tended to be delayed in T females. Females with experimentally enhanced T maintained lower temperature during incubation but did not spend less time incubating. This might explain the reduced hatching success of their eggs, smaller brood size and lower number of fledglings we found in this study. As in this species T-dependent behaviour by females during the phase of parental care is not needed, the results support the hypothesis that in this species the need for selection in favour of T-insensitivity did not occur.

Effects of experimentally manipulated yolk thyroid hormone levels on offspring development in a wild bird species

Maternal effects are a crucial mechanism in a wide array of taxa to generate phenotypic variation, thereby affecting offspring development and fitness. Maternally derived thyroid hormones (THs) are known to be essential for offspring development in mammalian and fish models, but have been largely neglected in avian studies, especially in respect to natural variation and an ecological context. We studied, for the first time in a wild species and population, the effects of maternally derived THs on offspring development, behavior, physiology and fitness-related traits by experimental elevation of thyroxine and triiodothyronine in ovo within the physiological range in great tits (Parus major). We found that elevated yolk TH levels had a sex-specific effect on growth, increasing male and decreasing female growth, relative to controls, and this effect was similar throughout the nestling period. Hatching or fledging success, motor coordination behavior, stress reactivity and resting metabolic rate were not affected by the TH treatment. We conclude that natural variation in maternally derived THs may affect some offspring traits in a wild species. As this is the first study on yolk thyroid hormones in a wild species and population, more such studies are needed to investigate its effects on pre-hatching development, and juvenile and adult fitness before generalizations on the importance of maternally derived yolk thyroid hormones can be made. However, this opens a new, interesting avenue for further research in the field of hormone mediated maternal effects.

Maternal adjustment or constraint: differential effects of food availability on maternal deposition of macro-nutrients, steroids and thyroid hormones in rock pigeon eggs

In oviparous species like birds, eggs provide the direct environment in which embryos are developing. Mothers may adjust different egg components in different ways in reaction to environmental cues either to adjust offspring development or because of constraints. In this study, we investigated the effects of food quality and quantity before and during egg laying on three different aspects of egg quality: macro‐nutrients (egg and yolk mass), androgens (testosterone and androstenedione), and thyroid hormones (3,5,3′‐triiodothyronine, T3 and l‐thyroxine, T4), using the rock pigeon (Columba livia). As expected, egg and yolk mass were significantly reduced for the eggs laid under the poor‐food condition, indicating a maternal trade‐off between offspring and self in allocating important resources. We did not find any significant change in yolk testosterone or their within‐clutch pattern over the laying sequence. This is consistent with the fact that, in contrast with nutrients, these hormones are not costly to produce, but does not support the hypothesis that they play a role in adjusting brood size to food conditions. In contrast, we found that T3 levels were higher in the egg yolks under the poor‐food condition whereas the total T4 content was lower. This change could be related to the fact that iodine, the critical constituent of thyroid hormones, might be a limiting factor in the production of this hormone. Given the knowledge that food restriction usually lead to reduction of circulating T3 levels, our results suggested that avian mothers can independently regulate its concentrations in their eggs from their own circulation. The study demonstrates that environmentally induced maternal effects via the egg can be a result of a combination of constrained resources and unconstrained signals and that thyroid hormones might be an interesting case of both. Therefore, this hormone and the interplay of different maternal effects on the offspring phenotype deserve much more attention.

Introducing biological realism into the study of developmental plasticity in behaviour

There is increasing attention for integrating mechanistic and functional approaches to the study of (behavioural) development. As environments are mostly unstable, it is now often assumed that genetic parental information is in many cases not sufficient for offspring to become optimally adapted to the environment and that early environmental cues, either indirectly via the parents or from direct experience, are necessary to prepare them for a specific environment later in life. To study whether these early developmental processes are adaptive and through which mechanism, not only the early environmental cues but also how they impinge on the later-life environmental context has therefore to be taken into account when measuring the animal's performance. We first discuss at the conceptual level six ways in which interactions between influences of different time windows during development may act (consolidation, cumulative information gathering and priming, compensation, buffering, matching and mismatching, context dependent trait expression). In addition we discuss how different environmental factors during the same time window may interact in shaping the phenotype during development. Next we discuss the pros and cons of several experimental designs for testing these interaction effects, highlighting the necessity for full, reciprocal designs and the importance of adjusting the nature and time of manipulation to the animal's adaptive capacity. We then review support for the interaction effects from both theoretical models and animal experiments in different taxa. This demonstrates indeed the existence of interactions at multiple levels, including different environmental factors, different time windows and between generations. As a consequence, development is a life-long, environment-dependent process and therefore manipulating only the early environment without taking interaction effects with other and later environmental influences into account may lead to wrong conclusions and may also explain inconsistent results in the literature.

Influence of mate preference and laying order on maternal allocation in a monogamous parrot species with extreme hatching asynchrony

It is well established that in many avian species, prenatal maternal resource allocation varies both between and within clutches and may affect offspring fitness. Differential allocation of maternal resources, in terms of egg weight and yolk composition, may therefore allow the female to adjust brood reduction and to fine-tune reproductive investment in accordance with the expected fitness returns. The adaptive value of such maternal resource allocation is thought to be context-dependent as well as species-specific. We investigated the effects of female preference for her mate on the allocation of prenatal maternal resources in the budgerigar, Melopsittacus undulatus, a monogamous species of parrot that shows an extreme hatching asynchrony. We assessed mate preferences in a two-way preference test and allowed females two breeding rounds: one with the preferred and one with the non-preferred partner. We found no effect of preference on either latency to lay or clutch size, but females mated with the preferred partner laid eggs that contained significantly more yolk. Their eggs also contained significantly more androstenedione but not testosterone. Our results suggest that in this species, female preference may influence maternal resource allocation, and that the functional roles of each androgen in the yolk should be considered separately. In addition, we found a significant effect of laying order on egg and yolk weight as well as on yolk testosterone and androstenedione levels. These measures, however, did not change linearly with the laying order and render it unlikely that female budgerigars compensate for the extreme hatching asynchrony by adjusting within-clutch allocation of prenatal maternal resources.

An appraisal of how the vitamin A-redox hypothesis can maintain honesty of carotenoid-dependent signals

The vitamin A-redox hypothesis provides an explanation for honest signaling of phenotypic quality by carotenoid-dependent traits. A key aspect of the vitamin A-redox hypothesis, applicable to both yellow and red coloration, is the hypothesized negative feedback of tightly regulated Vitamin A plasma levels on the enzyme responsible for sequestering both Vitamin A and carotenoids from the gut. We performed a meta-analysis and find that vitamin A levels are positively related to carotenoid plasma levels (r = 0.50, P = 0.0002). On the basis of this finding and further theoretical considerations, we propose that the vitamin A-redox hypothesis is unlikely to explain carotenoid-dependent honest signaling.

Natural selection acts in opposite ways on correlated hormonal mediators of prenatal maternal effects in a wild bird population

Maternal hormones are important mediators of prenatal maternal effects. Although many experimental studies have demonstrated their potency in shaping offspring phenotypes, we know remarkably little about their adaptive value. Using long-term data on a wild collared flycatcher (Ficedula albicollis) population, we show that natural selection acts in opposite ways on two maternally derived androgens, yolk androstenedione (A4) and yolk testosterone (T). High yolk A4 concentrations are associated with higher fitness, whereas high yolk T concentrations are associated with lower fitness. Natural selection thus favours females that produce eggs with high A4 and low T concentrations. Importantly, however, there exists a positive (non-genetic) correlation between A4 and T, which suggests that females are limited in their ability to reach this adaptive optimum. Thereby, these results provide strong evidence for an adaptive value of differential maternal androgen deposition, and a mechanistic explanation for the maintenance of variation in maternal investment in the wild.

Response of testosterone and corticosterone plasma levels to the challenge of sibling competition: a study in common terns

The hormonal response to social challenges has been widely studied, however, most work focused on adult behavior in a reproductive context although developing animals also encounter important social challenges early in life. We studied the relationship between acute sibling competition and plasma corticosterone (CORT) and testosterone (T) in common tern (Sterna hirundo) chicks, a species whose young compete for access to food by scramble interactions. Blood samples were taken in nests with two and only one single chick both immediately after a feeding bout and in non-challenged controls. We found that T levels were lower in siblings challenged by a feeding bout as compared to controls, which may be explained by the fact that T suppresses begging behavior and is only elevated in response to territorial intrusion but not sibling competition in a related species. Singletons had, corrected for body condition, generally lower CORT levels than siblings suggesting that growing up with siblings creates a competitive environment in which high CORT levels are sustained irrespective of a social challenge. CORT levels were also negatively correlated with body condition and were higher in males than in females. The latter may be related to sex-specific food requirements and susceptibility to stress. Our results suggest a possible suppressive effect of acute sibling competition on T secretion, and a positive effect on CORT levels by longer term sibling competition. The degree to which these dynamics are related to begging or aggression, or both, needs further experimental work.

Parents and early life environment affect behavioral development of laying hen chickens

Severe feather pecking (SFP) in commercial laying hens is a maladaptive behavior which is associated with anxiety traits. Many experimental studies have shown that stress in the parents can affect anxiety in the offspring, but until now these effects have been neglected in addressing the problem of SFP in commercially kept laying hens. We therefore studied whether parental stock (PS) affected the development of SFP and anxiety in their offspring. We used flocks from a brown and white genetic hybrid because genetic background can affect SFP and anxiety. As SFP can also be influenced by housing conditions on the rearing farm, we included effects of housing system and litter availability in the analysis. Forty-seven rearing flocks, originating from ten PS flocks were followed. Behavioral and physiological parameters related to anxiety and SFP were studied in the PS at 40 weeks of age and in the rearing flocks at one, five, ten and fifteen weeks of age. We found that PS had an effect on SFP at one week of age and on anxiety at one and five weeks of age. In the white hybrid, but not in the brown hybrid, high levels of maternal corticosterone, maternal feather damage and maternal whole-blood serotonin levels showed positive relations with offsprings' SFP at one week and offsprings' anxiety at one and five weeks of age. Disruption and limitation of litter supply at an early age on the rearing farms increased SFP, feather damage and fearfulness. These effects were most prominent in the brown hybrid. It appeared that hens from a brown hybrid are more affected by environmental conditions, while hens from a white hybrid were more strongly affected by parental effects. These results are important for designing measures to prevent the development of SFP, which may require a different approach in brown and white flocks.

Testosterone influences song behaviour and social dominance - but independent of prenatal yolk testosterone exposure

In the last two decades, maternally derived yolk androgens have been shown to significantly alter offspring development, and a number of these effects persist into adulthood. However, little is known about their underlying mechanisms. Mechanisms that have been suggested are changes in the endogenous androgen production post-hatching or changes in the sensitivity towards circulating androgens. We tested the effects of yolk testosterone on the plasma testosterone levels and the sensitivity to testosterone in 5months old male canaries that hatched from eggs that were either injected with testosterone (yT-males) or with a control solution (yC-males). Changes in sensitivity were investigated via the behavioural response to an experimental elevation of the plasma testosterone levels. We performed the experiment in fall (low endogenous testosterone production), focusing on testosterone dependent response traits (aggression and song). Before implantation, there was a non-significant trend that the plasma testosterone levels were lower in yT-males than in yC-males. Elevating the plasma testosterone concentrations increased aggressiveness, song bout length and similarity of repeated song elements (=consistency), with the latter likely being a consequence of testosterone-driven song crystallization. However, these effects were not different among yT- or yC-males in any of the parameters. Thus, our findings render it unlikely that changes in the sensitivity to testosterone post-hatching would form the main underlying mechanism of hormone-mediated maternal effects in birds. Further experiments are urgently needed in order to understand the nature of the phenotypic effects resulting from embryonic exposure to maternal yolk testosterone.

Maternal corticosterone elevation during egg formation in chickens (Gallus gallus domesticus) influences offspring traits, partly via prenatal undernutrition

The relationship between maternal stress during pregnancy in humans and the subsequent physical and mental health disorders in their children has inspired a wide array of studies on animal models. Almost all of these studies have used mammalian species, but more recently oviparous species in which the embryo develops outside the mother's body have received more attention. These new models facilitate disentangling of the underlying mechanism due to the accessibility of the prenatal environment, the egg. Studies in birds have found that maternal stress during egg formation induces phenotypic alterations in the offspring that hatch from these eggs. However, different offspring traits have been measured in different studies and potential underlying mechanisms are barely addressed. In this study we experimentally manipulated maternal corticosterone levels in laying hens. We found that mothers with experimentally elevated plasma corticosterone levels produced offspring that are smaller at hatching, less competitive, less fearful, have lower immunocompetence and higher plasma testosterone levels, as well as an alteration of visually guided behavioural lateralization. Earlier we have showed that eggs produced by these corticosterone treated mothers were lighter and contained lower concentrations of testosterone and progesterone in the yolk. While yolk hormones showed no correlation with any offspring traits, egg mass correlated positively with offspring's body mass from hatching until 10days of age and hatching mass correlated positively with the offspring's ability to compete for food, indicating that prenatal under nutrition might mediate some effects of maternal stress.

The fighting hypothesis in combat: how well does the fighting hypothesis explain human left-handed minorities?

The strong population bias in hand preference in favor of right-handedness seems to be a typical human trait. An elegant evolutionary hypothesis explaining this trait is the so-called fighting hypothesis that postulates that left-handedness is under frequency-dependent selection. The fighting hypothesis assumes that left-handers, being in the minority because of health issues, are still maintained in the population since they would have a greater chance of winning in fights than right-handers due to a surprise effect. This review critically evaluates the assumptions and evidence for this hypothesis and concludes that some evidence, although consistent with the fighting hypothesis, does not directly support it and may also be interpreted differently. Other supportive data are ambiguous or open for both statistical and theoretical criticism. We conclude that, presently, evidence for the fighting hypothesis is not particularly strong, but that there is little evidence to reject it either. The hypothesis thus remains an intuitively plausible explanation for the persistent left-hand preference in the population. We suggest alternative explanations and several ways forward for obtaining more crucial data for testing this frequently cited hypothesis.

Effect of rapid modulation of circulating plasma testosterone concentration on begging, aggressive behavior and competition for food in black-headed gull (Larus ridibundus) chicks

Sibling competition mediated by begging behavior is extremely common in avian species and recent studies have highlighted the role of endogenous testosterone in regulating such phenomenon. However, current literature depicts an inconsistent pattern in altricial vs. semi-precocial species, with stimulating versus inhibitory effects of the hormone respectively. This is possibly due to a difference in the methodology of hormone treatment (short-term moderate dose versus a long-term stronger elevation, respectively) between the studies performed so far. In this study, we induced short-term moderate peaks in plasma testosterone levels, as applied in altricial bird species, and assessed the effects of our manipulation on begging, competitive and aggressive behavior in black-headed gull (Larus ridibundus) chicks, a semi-precocial species. Our results suggest that, unlike in altricial songbirds, temporary increase of plasma testosterone concentration suppresses begging and enhances aggressiveness towards intruders. However, it also increases aggression and the chances of getting priority while scrambling with nest mates to gain access to food. Thus, the inconsistencies in the hormonal control of begging behavior observed between altricial vs. semi-precocial birds seem real and perhaps related to species differences in complexity of the display and the nature of competition. These may be elucidated by future comparative studies.

Yolk concentrations of hormones and glucose and egg weight and egg dimensions in unincubated chicken eggs, in relation to egg sex and hen body weight

Birds can manipulate offspring sex ratio under natural and experimental conditions and maternal hormones have been shown to be involved in this process. Studies also provided evidence for the presence of sex specific concentrations of yolk hormones in avian eggs. These findings led to the suggestion that yolk hormones could influence genetic sex determination in birds. However, in previous studies, yolk hormone concentrations and egg sex were studied in incubated eggs, although incubation of the eggs and embryonic development can alter yolk hormone concentrations and measured sex ratio. This study is the first to determine a wide array of egg components and hen body weight in relation to the sex of the egg in unincubated eggs. Egg parameters studied were yolk concentrations of testosterone, estradiol, androstenedione, progesterone, dihydrotestosterone, and glucose, and egg weight and dimensions. In addition, we studied the associations among all measured parameters. Associations were found between a number of yolk hormones (progesterone associated with testosterone, estradiol and androstenedione; androstenedione with testosterone; dihydrotestosterone with estradiol and androstenedione) as well as between yolk testosterone and egg length and egg weight. There were no significant overall differences between male and female chicken eggs in any of the measured egg parameters. However, there were a few interactions such as the interaction of egg sex with dihydrotestosterone and with hen body weight which predicted estradiol levels and an interaction of estradiol levels with egg width for predicting sex of egg. Their biological relevance need, however, further study.

A test of genetic association among male nuptial coloration, female mating preference, and male aggression bias within a polymorphic population of cichlid fish

Both interand intrasexual selection have been implicated in the origin and maintenance of species-rich taxa with diverse sexual traits. Simultaneous disruptive selection by female mate choice and male-male competition can, in theory, lead to speciation without geographical isolation if both act on the same male trait. Female mate choice can generate discontinuities in gene flow, while male-male competition can generate negative frequency-dependent selection stabilizing the male trait polymorphism. Speciation may be facilitated when mating preference and/or aggression bias are physically linked to the trait they operate on. We tested for genetic associations among female mating preference, male aggression bias and male coloration in the Lake Victoria cichlid Pundamilia. We crossed females from a phenotypically variable population with males from both extreme ends of the phenotype distribution in the same population (blue or red). Male offspring of a red sire were significantly redder than males of a blue sire, indicating that intra-population variation in male coloration is heritable. We tested mating preferences of female offspring and aggression biases of male offspring using binary choice tests. There was no evidence for associations at the family level between female mating preferences and coloration of sires, but dam identity had a significant effect on female mate preference. Sons of the red sire directed significantly more aggression to red than blue males, whereas sons of the blue sire did not show any bias. There was a positive correlation among individuals between male aggression bias and body coloration, possibly due to pleiotropy or physical linkage, which could facilitate the maintenance of color polymorphism.

Within-clutch variation in yolk testosterone as an adaptive maternal effect to modulate avian sibling competition: evidence from a comparative study

In many species, embryos are exposed to maternal hormones in utero, in the egg, or in the seed. In birds, mothers deposit substantial testosterone into their eggs, which enhances competitive ability of offspring. These maternal testosterone concentrations vary systematically within clutches in different patterns and may enable mothers to adaptively fine-tune competitive hierarchies within broods. We performed a comparative analysis to investigate this hypothesis using a broad set of avian species. We expected species with small size differences among siblings (arising from small hatching asynchrony or slow growth rates) to aim for survival of the whole brood in good years and therefore compensate last-hatching eggs with relatively more testosterone. We expected species with large size differences among siblings (large hatching asynchrony or fast growth rates) to produce surplus young as insurance against failed offspring and to facilitate elimination of redundant surplus young by bestowing last-hatching eggs with relatively less testosterone. As predicted, we found that maternal testosterone compensation to last-hatching eggs is stronger when size differences among siblings become smaller. Maternal testosterone compensation to last-hatching eggs also correlated negatively with hatching asynchrony and growth rates. These findings provide evidence for correlated evolution of several maternal effects that together support different maternal reproductive strategies.

Maternal androgens increase sibling aggression, dominance, and competitive ability in the siblicidal black-legged kittiwake (Rissa tridactyla)

Animals and plants routinely produce more offspring than they can afford to rear. Mothers can favour certain young by conferring on them competitive advantages such as a leading position in the birth sequence, more resources or hormones. Avian mothers create hatching asynchrony within a clutch and at the same time bestow the eggs with different concentrations of androgens that may enhance or counteract the competitive advantage experienced by early-hatching “core” young. In siblicidal birds, core young assume a dominant social position in the nest due to their size advantage and when threatened with starvation fatally attack subdominant later-hatching “marginal” young. A role for maternal androgens in siblicidal aggression has frequently been suggested but never tested. We studied this in the facultatively siblicidal black-headed kittiwake. We found that marginal eggs contain higher instead of lower concentrations of androgens than core eggs. Surprisingly, exposure to experimentally elevated yolk androgens increased sibling aggression and dominance, even though in nature marginal eggs never produce dominant chicks. We propose the “adoption facilitation hypothesis” to explain this paradox. This cliff-nesting colonial species has a high adoption rate: ejected marginal kittiwake chicks frequently fall into other nests containing chicks of similar or smaller size and exposure to yolk androgens might help them integrate themselves into a foster nest.