Evolutionary ecology. Cycles of species replacement emerge from locally induced maternal effects on offspring behavior in a passerine bird

Developmental programming by prenatal sounds: insights into possible mechanisms

In recent years, the impact of prenatal sound on development, notably for programming individual phenotypes for postnatal conditions, has increasingly been revealed. However, the mechanisms through which sound affects physiology and development remain mostly unexplored. Here, I gather evidence from neurobiology, developmental biology, cellular biology and bioacoustics to identify the most plausible modes of action of sound on developing embryos. First, revealing often-unsuspected plasticity, I discuss how prenatal sound may shape auditory system development and determine individuals' later capacity to receive acoustic information. I also consider the impact of hormones, including thyroid hormones, glucocorticoids and androgen, on auditory plasticity. Second, I review what is known about sound transduction to other - non-auditory - brain regions, and its potential to input on classical developmental programming pathways. Namely, the auditory pathway has direct anatomical and functional connectivity to the hippocampus, amygdala and/or hypothalamus, in mammals, birds and anurans. Sound can thus trigger both immediate and delayed responses in these limbic regions, which are specific to the acoustic stimulus and its biological relevance. Third, beyond the brain, I briefly consider the possibility for sound to directly affect cellular functioning, based on evidence in earless organisms (e.g. plants) and cell cultures. Together, the multi-disciplinary evidence gathered here shows that the brain is wired to allow multiple physiological and developmental effects of sound. Overall, there are many unexplored, but possible, pathways for sound to impact even primitive or immature organisms. Throughout, I identify the most promising research avenues for unravelling the processes of acoustic developmental programming.

Interspecific behavioural interference and range dynamics: current insights and future directions

Novel biotic interactions in shifting communities play a key role in determining the ability of species' ranges to track suitable habitat. To date, the impact of biotic interactions on range dynamics have predominantly been studied in the context of interactions between different trophic levels or, to a lesser extent, exploitative competition between species of the same trophic level. Yet, both theory and a growing number of empirical studies show that interspecific behavioural interference, such as interspecific territorial and mating interactions, can slow down range expansions, preclude coexistence, or drive local extinction, even in the absence of resource competition. We conducted a systematic review of the current empirical research into the consequences of interspecific behavioural interference on range dynamics. Our findings demonstrate there is abundant evidence that behavioural interference by one species can impact the spatial distribution of another. Furthermore, we identify several gaps where more empirical work is needed to test predictions from theory robustly. Finally, we outline several avenues for future research, providing suggestions for how interspecific behavioural interference could be incorporated into existing scientific frameworks for understanding how biotic interactions influence range expansions, such as species distribution models, to build a stronger understanding of the potential consequences of behavioural interference on the outcome of future range dynamics.

Eco-Evolutionary Consequences of Dispersal Syndromes during Colonization in a Passerine Bird

AbstractIn most animal species, dispersing individuals possess phenotypic attributes that mitigate the costs of colonization and/or increase settlement success in new areas (dispersal syndromes). This phenotypic integration likely affects population dynamics and the direction of selection, but data are lacking for natural populations. Using an approach that combines population dynamics, quantitative genetics, and phenotypic selection analyses, we reveal the existence of dispersal syndromes in a pied flycatcher (Ficedula hypoleuca) population in the Netherlands: immigrants were larger, tended to have darker plumage, bred earlier, and produced larger clutches than local recruits, and some of these traits were genetically correlated. Over time, the phenotypic profile of the population gradually changed: each generation advanced arrival and breeding and exhibited longer wings as a result of direct and indirect selection on these correlated traits. Although phenotypic attributes of immigrants were favored by selection during the early phase of colonization, observed phenotypic changes were similar for immigrants and local recruits. We propose that immigrants facilitated initial population establishment but that temporal changes likely resulted from climate change-induced large-scale selection. This study highlights that newly established populations are of nonrandom composition and that phenotypic architecture affects evolutionary population trajectories.

Incorporating neurological and behavioral mechanisms of sociality into predator-prey models

Consumer-resource population models drive progress in predicting and understanding predation. However, they are often built by averaging the foraging outcomes of individuals to estimate per capita functional responses (functions that describe predation rate). Reliance on per-capita functional responses rests on the assumption that that individuals forage independently without affecting each other. Undermining this assumption, extensive behavioral neuroscience research has made clear that facilitative and antagonistic interactions among conspecifics frequently alter foraging through interference competition and persistent neurophysiological changes. For example, repeated social defeats dysregulates rodent hypothalamic signaling, modulating appetite. In behavioral ecology, similar mechanisms are studied under the concept of dominance hierarchies. Neurological and behavioral changes in response to conspecifics undoubtedly play some sort of role in the foraging of populations, but modern predator-prey theory does not explicitly include them. Here we describe how some modern approaches to population modeling might account for this. Further, we propose that spatial predator-prey models can be modified to describe plastic changes in foraging behavior driven by intraspecific interaction, namely individuals switching between patches or plastic strategies to avoid competition. Extensive neurological and behavioral ecology research suggests that interactions among conspecifics help shape populations' functional responses. Modeling interdependent functional responses woven together by behavioral and neurological mechanisms may thus be indispensable in predicting the outcome of consumer-resource interactions across systems.

Beyond genotype-phenotype maps: Toward a phenotype-centered perspective on evolution

Evolutionary biology is paying increasing attention to the mechanisms that enable phenotypic plasticity, evolvability, and extra-genetic inheritance. Yet, there is a concern that these phenomena remain insufficiently integrated within evolutionary theory. Understanding their evolutionary implications would require focusing on phenotypes and their variation, but this does not always fit well with the prevalent genetic representation of evolution that screens off developmental mechanisms. Here, we instead use development as a starting point, and represent it in a way that allows genetic, environmental and epigenetic sources of phenotypic variation to be independent. We show why this representation helps to understand the evolutionary consequences of both genetic and non-genetic phenotype determinants, and discuss how this approach can instigate future areas of empirical and theoretical research.

Contribution of hippocampal BDNF/CREB signaling pathway and gut microbiota to emotional behavior impairment induced by chronic unpredictable mild stress during pregnancy in rats offspring

Background

Numerous studies have shown that exposure to prenatal maternal stress (PMS) is associated with various psychopathological outcomes of offspring. The accumulating evidence linking bacteria in the gut and neurons in the brain (the microbiota-gut-brain axis) has been aconsensus; however, there is a lack of research on the involvement mechanism of gut microbiota in the regulation of the BDNF/CREB signaling pathway in the hippocampus of prenatally stressed offspring.

Methods

Pregnant rats were subjected to chronic unpredictable mild stress (CUMS) to establish the prenatal maternal stress model. The body weight was measured and the behavioral changes were recorded. Offspring were tested to determine emotional state using sucrose preference test (SPT), open-field test (OFT) and suspended tail test (STT). Gut microbiota was evaluated by sequencing the microbial 16S rRNA V3-V4 region, and the interactive analysis of bacterial community structure and diversity was carried out. The expression of hippocampal BDNF, TrkB and CREB mRNA and proteins were respectively measured using RT-PCR and Western blotting.

Results

Prenatal maternal stress increased maternal plasma corticosterone levels, slowed maternal weight gain and caused depression-like behaviors (all P < 0.05). In offspring, prenatal maternal stress increased plasma corticosterone levels (P < 0.05) and emotional behavior changes (depression-like state) were observed (P < 0.05). The species abundance, diversity and composition of the offspring's gut microbiota changed after the maternal stress during pregnancy (P < 0.05). Compared with the control group's offspring, the species abundance of Lactobacillaceae was dropped, while the abundance of the Muribaculaceae species abundance was risen. Concurrent, changes in the hippocampal structure of the offspring and decreases in expression of BDNF/CREB signaling were noted (P < 0.05).

Conclusions

Prenatal maternal stress leads to high corticosterone status and abnormal emotion behavior of offspring, which may be associated with the abnormal BDNF/CREB signaling in hippocampus of offspring caused by the change of gut microbiota composition.

Unravelling the causes and consequences of dispersal syndromes in a wild passerine

Evidence accumulates that dispersal is correlated with individual behavioural phenotype (dispersal syndrome). The evolutionary causes and consequences of such covariation depend on the degree of plasticity versus inheritance of the traits, which requires challenging experiments to implement in mobile organisms. Here, we combine a forced dispersal experiment, natural colonization and longitudinal data to establish if dispersal and aggression levels are integrated and to test their adaptive nature in pied flycatchers (Ficedula hypoleuca). We found that (forced) dispersers behaved more aggressively in their first breeding year after dispersal and decreased their aggression in following years. Strength of dispersal syndrome and direction of fecundity selection on aggression in newly colonized areas varied between years. We propose that the net benefits of aggression for dispersers increase under harsh conditions (e.g. low food abundance). This hypothesis now warrants further testing. Overall, this study provides unprecedented experimental evidence that dispersal syndromes can be remodelled via adaptive plasticity depending on the individuals' local breeding experience and/or year-specific ecological conditions. It highlights the importance of individual behavioural variation in population dynamics.

Maternal stress and the maternal microbiome have sex-specific effects on offspring development and aggressive behavior in Siberian hamsters (Phodopus sungorus)

The gut microbiome, a community of commensal, symbiotic and pathogenic bacteria, fungi, and viruses, interacts with many physiological systems to affect behavior. Prenatal experiences, including exposure to maternal stress and different maternal microbiomes, are important sources of organismal variation that can affect offspring development. These physiological systems do not act in isolation and can have long-term effects on offspring development and behavior. Here we investigated the interactive effects of maternal stress and manipulations of the maternal microbiome on offspring development and social behavior using Siberian hamsters, Phodopus sungorus. We exposed pregnant females to either a social stressor, antibiotics, both the social stressor and antibiotics, or no treatment (i.e., control) over the duration of their pregnancy and quantified male and female offspring growth, gut microbiome composition and diversity, stress-induced cortisol concentrations, and social behavior. Maternal antibiotic exposure altered the gut microbial communities of male and female offspring. Maternal treatment also had sex-specific effects on aspects of offspring development and aggressive behavior. Female offspring produced by stressed mothers were more aggressive than other female offspring. Female, but not male, offspring produced by mothers exposed to the combined treatment displayed low levels of aggression, suggesting that alteration of the maternal microbiome attenuated the effects of prenatal stress in a sex-specific manner. Maternal treatment did not affect non-aggressive behavior in offspring. Collectively, our study offers insight into how maternal systems can interact to affect offspring in sex-specific ways and highlights the important role of the maternal microbiome in mediating offspring development and behavior.

Nest predation risk and deposition of yolk steroids in a cavity nesting songbird: an experimental test

Maternal hormones can shape offspring development and increase survival when predation risk is elevated. In songbirds, yolk androgens influence offspring growth and begging behaviors which can help mitigate offspring predation risk in the nest. Other steroids may also be important for responding to nest predation risk, but non-androgen steroids have been poorly studied. We used a nest predator playback experiment and liquid chromatography with tandem mass spectrometry (LC-MS-MS) to assess whether nest predation risk influences deposition of 10 yolk steroids. We found no clear evidence that yolk androgen deposition changed when perception of nest predation risk was experimentally increased. However, elevated nest predation risk led to decreased yolk progesterone deposition. Overall, our results suggest yolk progesterone may be more important than yolk androgens in responses to offspring predation risk and highlight new avenues for research.

Reproductive value and the evolution of altruism

Altruism is favored by natural selection provided that it delivers sufficient benefits to relatives. An altruist's valuation of her relatives depends upon the extent to which they carry copies of her genes - relatedness - and also on the extent to which they are able to transmit their own genes to future generations - reproductive value. However, although relatedness has received a great deal of attention with regard to altruism, reproductive value has been surprisingly neglected. We review how reproductive value modulates patterns of altruism in relation to individual differences in age, sex, and general condition, and discuss how social partners may manipulate each other's reproductive value to incentivize altruism. This topic presents opportunities for tight interplay between theoretical and empirical research.

Breeder aggression does not predict current or future cooperative group formation in a cooperatively breeding bird

In cooperatively breeding species, subordinates forgo reproduction to assist breeders in raising offspring. When cooperative breeding is facultative, breeders from the same population may differ in whether they are assisted by non-breeding helpers. Predation risk is a major source of nest failure and assistance during nest defense is often an overlooked, yet important, way helpers assist breeders. A breeder's aggressive response to a nest predator could have important implications for whether they form cooperatively breeding groups. We investigated the hypothesis that breeder aggression towards a nest predator is related to current and future helper recruitment. We tested the prediction that less aggressive breeders were more likely to form cooperative groups, which could occur if these breeders benefit from helper assistance during nest defense. We also considered the possibility that more aggressive breeders were more likely to form cooperative groups. We assessed the effects of partnerships and tested whether aggression exhibited by breeding partners was correlated. We conducted this work in the facultative, cooperatively breeding brown-headed nuthatch (Sitta pusilla). We measured breeder aggression in response to a taxidermy model of a nest predator to determine whether breeders' aggression correlated with their current or future helper recruitment. We found no evidence of a sex difference in aggression among breeders and aggression scores of breeding partners were not significantly correlated. Aggression scores for both breeding males and breeding females were unrelated to whether they formed cooperative groups in the current year. We followed most of the breeding males, though not breeding females, across years and found that breeding males' aggression scores were unrelated to helper recruitment the following year. Our results suggest that breeders' responses to nest predators are not related to cooperative group formation in this species and that males and females showed comparable levels of aggression towards a nest predator.

Maternal effects and the outcome of interspecific competition

Maternal environmental effects create lagged population responses to past environments. Although they are ubiquitous and vary in expression across taxa, it remains unclear if and how their presence alters competitive interactions in ecological communities.Here, we use a discrete-time competition model to simulate how maternal effects alter competitive dynamics in fluctuating and constant environments. Further, we explore how omitting maternal effects alter estimates of known model parameters from observational time series data.Our simulations demonstrate that (i) maternal effects change competitive outcomes, regardless of whether competitors otherwise interact neutrally or exhibit non-neutral competitive differences, (ii) the consequences of maternal effects for competitive outcomes are mediated by the temporal structure of environmental variation, (iii) even in constant conditions, competitive outcomes are influenced by species' maternal effects strategies, and (iv) in observational time series data, omitting maternal effects reduces variation explained by models and biases parameter estimates, including competition coefficients.Our findings demonstrate that the ecological consequences of maternal effects hinge on the competitive environment. Evolutionary biologists have long recognized that maternal effects can be an important but often overlooked strategy buffering populations from environmental change. We suggest that maternal effects are similarly critical to ecology and call for research into maternal effects as drivers of dynamics in populations and communities.

Acoustic developmental programming: a mechanistic and evolutionary framework

Conditions experienced prenatally, by modulating developmental processes, have lifelong effects on individual phenotypes and fitness, ultimately influencing population dynamics. In addition to maternal biochemical cues, prenatal sound is emerging as a potent alternative source of information to direct embryonic development. Recent evidence suggests that prenatal acoustic signals can program individual phenotypes for predicted postnatal environmental conditions, which improves fitness. Across taxonomic groups, embryos have now been shown to have immediate adaptive responses to external sounds and vibrations, and direct developmental effects of sound and noise are increasingly found. Establishing the full developmental, ecological, and evolutionary impact of early soundscapes will reveal how embryos interact with the external world, and potentially transform our understanding of developmental plasticity and adaptation to changing environments.

The call of the wild: using non-model systems to investigate microbiome-behaviour relationships

On and within most sites across an animal's body live complex communities of microorganisms. These microorganisms perform a variety of important functions for their hosts, including communicating with the brain, immune system and endocrine axes to mediate physiological processes and affect individual behaviour. Microbiome research has primarily focused on the functions of the microbiome within the gastrointestinal tract (gut microbiome) using biomedically relevant laboratory species (i.e. model organisms). These studies have identified important connections between the gut microbiome and host immune, neuroendocrine and nervous systems, as well as how these connections, in turn, influence host behaviour and health. Recently, the field has expanded beyond traditional model systems as it has become apparent that the microbiome can drive differences in behaviour and diet, play a fundamental role in host fitness and influence community-scale dynamics in wild populations. In this Review, we highlight the value of conducting hypothesis-driven research in non-model organisms and the benefits of a comparative approach that assesses patterns across different species or taxa. Using social behaviour as an intellectual framework, we review the bidirectional relationship between the gut microbiome and host behaviour, and identify understudied mechanisms by which these effects may be mediated.

Maternally derived hormones, neurosteroids and the development of behaviour

In a wide range of taxa, there is evidence that mothers adaptively shape the development of offspring behaviour by exposing them to steroids. These maternal effects have major implications for fitness because, by shaping early development, they can permanently alter how offspring interact with their environment. However, theory on parent-offspring conflict and recent physiological studies showing that embryos rapidly metabolize maternal steroids have placed doubt on the adaptive significance of these hormone-mediated maternal effects. Reconciling these disparate perspectives requires a mechanistic understanding of the pathways by which maternal steroids can influence neural development. Here, we highlight recent advances in developmental neurobiology and psychiatric pharmacology to show that maternal steroid metabolites can have direct neuro-modulatory effects potentially shaping the development of neural circuitry underlying ecologically relevant behavioural traits. The recognition that maternal steroids can act through a neurosteroid pathway has critical implications for our understanding of the ecology and evolution of steroid-based maternal effects. Overall, compared to the classic view, a neurosteroid mechanism may reduce the evolutionary lability of hormone-mediated maternal effects owing to increased pleiotropic constraints and frequently influence long-term behavioural phenotypes in offspring.

Multiple Environmental Stressors Induce an Adaptive Maternal Effect

AbstractEvolution of adaptation requires predictability and recurrence of functional contexts. Yet organisms live in multifaceted environments that are dynamic and ever changing, making it difficult to understand how complex adaptations evolve. This problem is particularly apparent in the evolution of adaptive maternal effects, which are often assumed to require reliable and discrete cues that predict conditions in the offspring environment. One resolution to this problem is if adaptive maternal effects evolve through preexisting, generalized maternal pathways that respond to many cues and also influence offspring development. Here, we assess whether an adaptive maternal effect in western bluebirds is influenced by maternal stress pathways across multiple challenging environments. Combining 18 years of hormone sampling across diverse environmental contexts with an experimental manipulation of the competitive environment, we show that multiple environmental factors influenced maternal corticosterone levels, which, in turn, influenced a maternal effect on aggression of sons in adulthood. Together, these results support the idea that multiple stressors can induce a known maternal effect in this system. More generally, they suggest that activation of general pathways, such as the hypothalamic-pituitary-adrenal axis, may simplify and facilitate the evolution of adaptive maternal effects by integrating variable environmental conditions into preexisting maternal physiological systems.

Between a rock and a hard place: adaptive sensing and site-specific dispersal

Environmental variability can lead to dispersal: why stay put if it is better elsewhere? Without clues about local conditions, the optimal strategy is often to disperse a set fraction of offspring. Many habitats contain environmentally differing sub-habitats. Is it adaptive for individuals to sense in which sub-habitat they find themselves, using environmental clues, and respond plastically by altering the dispersal rates? This appears to be done by some plants which produce dimorphic seeds with differential dispersal properties in response to ambient temperature. Here we develop a mathematical model to show, that in highly variable environments, not only does sensing promote plasticity of dispersal morph ratio, individuals who can sense their sub-habitat and respond in this way have an adaptive advantage over those who cannot. With a rise in environmental variability due to climate change, our understanding of how natural populations persist and respond to changes has become crucially important.

Recent biological invasion shapes species recognition and aggressive behaviour in a native species: A behavioural experiment using robots in the field

Invasive species are a world-wide threat to biodiversity. Yet, our understanding of biological invasions remains incomplete, partly due to the difficulty of tracking and studying behavioural interactions in recently created species interactions. We tested whether the interactions between the recently introduced invasive lizard Anolis cristatellus and the native Anolis oculatus in Dominica have led to changes in species recognition and aggressive behaviour of the native species. The use of realistic robots allowed us to test the behavioural response of 131 A. oculatus males towards relevant and controlled conspecific versus heterospecific stimuli, directly in the field and in two contexts (allopatry vs. sympatry). Our results show that species recognition evolved prior to sympatry in A. oculatus. Moreover, interspecific competition resulted in an increase in the time spent displaying and a divergence in the aggressive behaviour of the native species towards conspecifics versus heterospecifics. Inherent species recognition and higher aggressive behaviour may limit species coexistence as they are expected to favour A. oculatus during territorial interactions with A. cristatellus. While more studies are needed to understand the causes of these behavioural shifts and their consequences on long-term species coexistence, the present study highlights the role of behaviour as a first response to interspecific interactions.

Maternal effects and urbanization: Variation of yolk androgens and immunoglobulin in city and forest blackbirds

Wildlife inhabiting urban environments exhibit drastic changes in morphology, physiology, and behavior. It has often been argued that these phenotypic responses could be the result of micro-evolutionary changes following the urbanization process. However, other mechanisms such as phenotypic plasticity, maternal effects, and developmental plasticity could be involved as well. To address maternal effects as potential mechanisms, we compared maternal hormone and antibody concentrations in eggs between city and forest populations of European blackbirds (Turdus merula), a widely distributed species for which previous research demonstrated differences in behavioral and physiological traits. We measured egg and yolk mass, yolk concentrations of androgens (androstenedione [A4], testosterone [T], 5α-dihydrotestosterone [5α-DHT], and immunoglobulins [IgY]) and related them to population, clutch size, laying order, embryo sex, and progress of breeding season. We show (a) earlier onset of laying in the city than forest population, but similar egg and clutch size; (b) higher overall yolk androgen concentrations in the forest than the city population (sex-dependent for T); (c) greater among-female variation of yolk T and 5α-DHT concentrations in the forest than city population, but similar within-clutch variation; (d) similar IgY concentrations with a seasonal decline in both populations; and (e) population-specific positive (city) or negative (forest) association of yolk A4 and T with IgY concentrations. Our results are consistent with the hypotheses that hormone-mediated maternal effects contribute to differences in behavioral and physiological traits between city and forest individuals and that yolk androgen and immunoglobulin levels can exhibit population-specific relationships rather than trade-off against each other.

Differences in perceived predation risk associated with variation in relative size of extra-pair and within-pair offspring

Extra-pair paternity (EPP) is a widespread phenomenon in birds. Researchers have long hypothesized that EPP must confer a fitness advantage to extra-pair offspring (EPO), but empirical support for this hypothesis is definitively mixed. This could be because genetic benefits of EPP only exist in a subset of environmental contexts to which a population is exposed. From 2013 to 2015, we manipulated perceived predator density in a population of tree swallows (Tachycineta bicolor) breeding in New York to see whether fitness outcomes of extra-pair and within-pair offspring (WPO) varied with predation risk. In nests that had been exposed to predators, EPO were larger, longer-winged and heavier than WPO. In nonpredator nests, WPO tended to be larger, longer-winged and heavier than EPO, though the effect was nonsignificant. We found no differences in age, morphology or stress physiology between extra-pair and within-pair sires from the same nest, suggesting that additive genetic benefits cannot fully explain the differences in nestling size that we observed. The lack of an effect of predator exposure on survival or glucocorticoid stress physiology of EPO and WPO further suggests that observed size differences do not reflect more general variation in intrinsic genetic quality. Instead, we suggest that size differences may have arisen through differential investment into EPO and WPO by females, perhaps because EPO and WPO represent different reproductive strategies, with each type of nestling conferring a fitness advantage in specific ecological contexts.

Red-eared slider hatchlings (Trachemys scripta) show a seasonal shift in behavioral types

Correlated and repeatable patterns of behavior, termed behavioral types, can affect individual fitness. The most advantageous behavioral type may differ across predictable environments (e.g., seasonally), and maternally mediated effects may match hatchling behavior to the environment. We measured righting response, an indicator of behavioral type, of juvenile red-eared slider turtles (Trachemys scripta) emerging from early and late season clutches to understand if the production of behavioral types differs across the nesting season. There was a significant effect of season, with early season hatchlings righting more quickly than late season hatchlings, and we explored two potential underlying mechanisms, maternal estrogens and maternal investment (e.g., yolk allocation). We dosed early season eggs with an estrogen mixture to mimic late season eggs and assayed hatchling righting response, but found no significant effect of this maternal effect. We assessed maternal investment by measuring egg, hatchling, and residual yolk masses. We found a seasonal pattern in yolk allocation, where early season eggs have more yolk than late season eggs. Early season hatchlings used more yolk for growth rather than maintenance of existing tissues, resulting in larger hatchlings. Interestingly, across both seasons, hatchlings that received less maternal yolk appeared to be more efficient at converting yolk to tissue, but we found no direct correlation with righting behavior. We demonstrate that the prevalence of behavioral types varies across the nesting season, creating correlated suites of seasonal phenotypes in turtle hatchlings, but it appears that neither maternal estrogens or investment in yolk directly underlie this shift in behavior.

How differing modes of non-genetic inheritance affect population viability in fluctuating environments

Different modes of non-genetic inheritance are expected to affect population persistence in fluctuating environments. We here analyse Caenorhabditis elegans density-independent per capita growth rate time series on 36 populations experiencing six controlled sequences of challenging oxygen level fluctuations across 60 generations, and parameterise competing models of non-genetic inheritance in order to explain observed dynamics. Our analysis shows that phenotypic plasticity and anticipatory maternal effects are sufficient to explain growth rate dynamics, but that a carryover model where 'epigenetic' memory is imperfectly transmitted and might be reset at each generation is a better fit to the data. We further find that this epigenetic memory is asymmetric since it is kept for longer when populations are exposed to the more challenging environment. Our analysis suggests that population persistence in fluctuating environments depends on the non-genetic inheritance of phenotypes whose expression is regulated across multiple generations.

Intergenerational fitness effects of the early life environment in a wild rodent

The early life environment can have profound, long-lasting effects on an individual's fitness. For example, early life quality might (a) positively associate with fitness (a silver spoon effect), (b) stimulate a predictive adaptive response (by adjusting the phenotype to the quality of the environment to maximize fitness) or (c) be obscured by subsequent plasticity. Potentially, the effects of the early life environment can persist beyond one generation, though the intergenerational plasticity on fitness traits of a subsequent generation is unclear. To study both intra- and intergenerational effects of the early life environment, we exposed a first generation of bank voles to two early life stimuli (variation in food and social environment) in a controlled environment. To assess possible intra-generational effects, the reproductive success of female individuals was investigated by placing them in large outdoor enclosures in two different, ecologically relevant environments (population densities). Resulting offspring were raised in the same population densities where they were conceived and their growth was recorded. When adult, half of the offspring were transferred to opposite population densities to evaluate their winter survival, a crucial fitness trait for bank voles. Our setup allowed us to assess: (a) do early life population density cues elicit an intra-generational adaptive response, that is a higher reproductive success when the density matches the early life cues and (b) can early life stimuli of one generation elicit an intergenerational adaptive response in their offspring, that is a higher growth and winter survival when the density matches the early life cues of their mother. Our results show that the early life environment directly affects the phenotype and reproductive success of the focal generation, but adaptive responses are only evident in the offspring. Growth of the offspring is maintained only when the environment matches their mother's early life environment. Furthermore, winter survival of offspring also tended to be higher in high population densities if their mothers experienced an competitive early life. These results show that the early life environment can contribute to maintain high fitness in challenging environments, but not necessarily in the generation experiencing the early life cues.

The development of individual differences in cooperative behaviour: maternal glucocorticoid hormones alter helping behaviour of offspring in wild meerkats

The phenotype of parents can have long-lasting effects on the development of offspring as well as on their behaviour, physiology and morphology as adults. In some cases, these changes may increase offspring fitness but, in others, they can elevate parental fitness at a cost to the fitness of their offspring. We show that in Kalahari meerkats ( Suricata suricatta), the circulating glucocorticoid (GC) hormones of pregnant females affect the growth and cooperative behaviour of their offspring. We performed a 3-year experiment in wild meerkats to test the hypothesis that GC-mediated maternal effects reduce the potential for offspring to reproduce directly and therefore cause them to exhibit more cooperative behaviour. Daughters (but not sons) born to mothers treated with cortisol during pregnancy grew more slowly early in life and exhibited significantly more of two types of cooperative behaviour (pup rearing and feeding) once they were adults compared to offspring from control mothers. They also had lower measures of GCs as they aged, which could explain the observed increases in cooperative behaviour. Because early life growth is a crucial determinant of fitness in female meerkats, our results indicate that GC-mediated maternal effects may reduce the fitness of offspring, but may elevate parental fitness as a consequence of increasing the cooperative behaviour of their daughters. This article is part of the theme issue 'Developing differences: early-life effects and evolutionary medicine'.

The role of variation and plasticity in parental care during the adaptive radiation of three-spine sticklebacks

Phenotypic plasticity might influence evolutionary processes such as adaptive radiations. Plasticity in parental care might be especially effective in facilitating adaptive radiations if it allows populations to persist in novel environments. Here, we test the hypothesis that behavioral plasticity by parents in response to predation risk facilitated the adaptive radiation of three-spine sticklebacks. We compared the behavior of fathers across multiple ancestral (marine) and derived (freshwater) stickleback populations that differ in time since establishment. We measured behavioral plasticity in fathers in response to a predator found only in freshwater environments, simulating conditions marine males experience when colonizing freshwater. The antipredator behavior of males from newly established freshwater populations was intermediate between marine populations and well-established freshwater populations. In contrast to our predictions, on average, there was greater behavioral plasticity in derived freshwater populations than in ancestral marine populations. However, we found greater individual variation in behavioral reaction norms in marine populations compared to well-established freshwater populations, with newly established freshwater populations intermediate. This suggests that standing variation in behavioral reaction norms within ancestral populations might provide different evolutionary trajectories, and illustrates how plasticity can contribute to adaptive radiations.

Life-history evolution under fluctuating density-dependent selection and the adaptive alignment of pace-of-life syndromes

We present a novel perspective on life-history evolution that combines recent theoretical advances in fluctuating density-dependent selection with the notion of pace-of-life syndromes (POLSs) in behavioural ecology. These ideas posit phenotypic co-variation in life-history, physiological, morphological and behavioural traits as a continuum from the highly fecund, short-lived, bold, aggressive and highly dispersive 'fast' types at one end of the POLS to the less fecund, long-lived, cautious, shy, plastic and socially responsive 'slow' types at the other. We propose that such variation in life histories and the associated individual differences in behaviour can be explained through their eco-evolutionary dynamics with population density - a single and ubiquitous selective factor that is present in all biological systems. Contrasting regimes of environmental stochasticity are expected to affect population density in time and space and create differing patterns of fluctuating density-dependent selection, which generates variation in fast versus slow life histories within and among populations. We therefore predict that a major axis of phenotypic co-variation in life-history, physiological, morphological and behavioural traits (i.e. the POLS) should align with these stochastic fluctuations in the multivariate fitness landscape created by variation in density-dependent selection. Phenotypic plasticity and/or genetic (co-)variation oriented along this major POLS axis are thus expected to facilitate rapid and adaptively integrated changes in various aspects of life histories within and among populations and/or species. The fluctuating density-dependent selection POLS framework presented here therefore provides a series of clear testable predictions, the investigation of which should further our fundamental understanding of life-history evolution and thus our ability to predict natural population dynamics.

Parental habituation to human disturbance over time reduces fear of humans in coyote offspring

A fundamental tenet of maternal effects assumes that maternal variance over time should have discordant consequences for offspring traits across litters. Yet, seldom are parents observed across multiple reproductive bouts, with few studies considering anthropogenic disturbances as an ecological driver of maternal effects. We observed captive coyote (Canis latrans) pairs over two successive litters to determine whether among‐litter differences in behavior (i.e., risk‐taking) and hormones (i.e., cortisol and testosterone) corresponded with parental plasticity in habituation. Thus, we explicitly test the hypothesis that accumulating experiences of anthropogenic disturbance reduces parental fear across reproductive bouts, which should have disparate phenotypic consequences for first‐ and second‐litter offspring. To quantify risk‐taking behavior, we used foraging assays from 5–15 weeks of age with a human observer present as a proxy for human disturbance. At 5, 10, and 15 weeks of age, we collected shaved hair to quantify pup hormone levels. We then used a quantitative genetic approach to estimate heritability, repeatability, and between‐trait correlations. We found that parents were riskier (i.e., foraged more frequently) with their second versus first litters, supporting our prediction that parents become increasingly habituated over time. Second‐litter pups were also less risk‐averse than their first‐litter siblings. Heritability for all traits did not differ from zero (0.001–0.018); however, we found moderate support for repeatability in all observed traits (r = 0.085–0.421). Lastly, we found evidence of positive phenotypic and cohort correlations among pup traits, implying that cohort identity (i.e., common environment) contributes to the development of phenotypic syndromes in coyote pups. Our results suggest that parental habituation may be an ecological cue for offspring to reduce their fear response, thus emphasizing the role of parental plasticity in shaping their pups’ behavioral and hormonal responses toward humans.

Parent-embryo acoustic communication: a specialised heat vocalisation allowing embryonic eavesdropping

Sound is arguably the external cue most accessible to embryos of many species, and as such may constitute an unrivalled source of early information. Recent evidence shows that prenatal sounds, similarly to maternal effects, may shape developmental trajectories. Establishing whether parental vocalisations are signals directed at embryos, or parental cues on which embryos eavesdrop, can elucidate whether parents or embryos control developmental outcomes. Prenatal exposure to a characteristic heat-related parental call was recently shown to alter zebra finch growth and fitness. Here, we test the ecological context of this behaviour in the wild, and assess the information value and specificity of this vocalisation for an embryonic audience. We show that wild zebra finches also produce this characteristic call, only at high temperatures. In addition, in the lab, we demonstrate experimentally that calling is specifically triggered by high air temperatures, can occur without an embryonic audience, and importantly, is predicted by individuals’ body mass. Overall, our findings reveal a specialised heat vocalisation that enables embryonic eavesdropping, by indicating high ambient temperatures, and parents’ capacity to cope with such conditions. This challenges the traditional view of embryos as passive agents of their development, and opens exciting research avenues on avian adaptation to extreme heat.

Maternal Trophic Status and Offpsring Phenotype in a Marine Invertebrate

Offspring size variation in relation to maternal size and season is characteristic of a range of species living in seasonal environments. Little is known about the proximate mechanisms explaining the links between maternally driven variation in offspring phenotypes, for instance when mothers have different diets depending on their size or the season. Here, we use stable isotopes techniques to quantify size dependent and seasonal variations in diet in mothers of shrimp Palaemon serratus and explore possible links between maternal diet and phenotype of embryos and freshly hatched larvae. We found that larger females, which occur more frequently in winter, produce larvae with higher carbon and nitrogen content as well as higher percent carbon, than smaller mothers collected in winter. In addition, isotopic composition suggest that larger mothers collected in winter, were feeding at a higher trophic level, or on an enriched prey pool compared with smaller mothers collected in summer. Overall, there seems to be a strong association between offspring size and maternal diet, mediated by maternal size and/or season.

HormoneBase, a population-level database of steroid hormone levels across vertebrates

Hormones are central regulators of organismal function and flexibility that mediate a diversity of phenotypic traits from early development through senescence. Yet despite these important roles, basic questions about how and why hormone systems vary within and across species remain unanswered. Here we describe HormoneBase, a database of circulating steroid hormone levels and their variation across vertebrates. This database aims to provide all available data on the mean, variation, and range of plasma glucocorticoids (both baseline and stress-induced) and androgens in free-living and un-manipulated adult vertebrates. HormoneBase (www.HormoneBase.org) currently includes >6,580 entries from 476 species, reported in 648 publications from 1967 to 2015, and unpublished datasets. Entries are associated with data on the species and population, sex, year and month of study, geographic coordinates, life history stage, method and latency of hormone sampling, and analysis technique. This novel resource could be used for analyses of the function and evolution of hormone systems, and the relationships between hormonal variation and a variety of processes including phenotypic variation, fitness, and species distributions.

Evolutionary genetics of personality in the Trinidadian guppy I: maternal and additive genetic effects across ontogeny

Among-individual variation in behaviour is a widespread phenomenon, with several frameworks developed to explain its existence. Maternal effects, which can have significant influence over evolutionary processes, are an understudied source of behavioural variation. Maternal effects are not necessarily static, however, since their importance can change over offspring ontogeny, typically declining with age relative to additive genetic effects. Here, using a quantitative genetics approach, we test the prediction that maternal effects will influence age-specific risk-taking behaviour in Trinidadian guppies, Poecilia reticulata. Individuals were subject to a single open-field trial as juveniles and up to four repeat trials as adults, with five traits indicative of risk-taking behaviour measured in each trial. We then partitioned phenotypic variance into additive genetic (V_A) and maternal identity (V_M) components, in addition to testing brood size and maternal weight as specific sources of maternal effects. We found that V_M had significant influence over juvenile traits, with very low V_A estimates. Whereas, in adults, all traits were significantly heritable, with little support for V_M. We also found a strong influence of maternal traits on juvenile behaviours as predicted, with significant, albeit smaller, effects found in adults. Maternal weight was heritable and itself subject to maternal effects. Thus, maternal weight is a likely source of maternal genetic effects that are expected to alter response to selection on personality in this system. More generally, our study highlights that while maternal effects can be an important source of personality variation, this varies over ontogeny of offspring.

Meta-analysis reveals weak associations between intrinsic state and personality

Individual differences in behaviour characterize humans and animals alike. A hot field in behavioural ecology asks why this variation in 'personality' evolved. Theory posits that selection favours the integration of 'intrinsic state' and behaviour. Metabolism, hormones, energetic reserves and structural size have particularly been proposed as states covarying with behaviour among-individuals, either genetically or through plasticity integration. We conducted a meta-analysis estimating the amount of among-individual variation in behaviour attributable to variation in state. Our literature search showed that only 22% of the studies claiming to estimate individual-level associations between state and behaviour actually did so. Our meta-analysis revealed that relatively aggressive, bold, explorative and/or active individuals had relatively high metabolic rates, hormone levels, body weights and/or body sizes. The proportion of among-individual variation common to state and behaviour was nevertheless small (approx. 5%). This means that (i) adaptive explanations involving intrinsic states fail to explain much individual variation in behaviour, (ii) empiricists should consider nonlinear, additive or interactive effects of (multiple) intrinsic states, (iii) explanations not involving intrinsic states might be important, or (iv) empirical tests of state-dependent personality theory were inappropriate. Our meta-analysis highlights the importance of feedback between empiricists and theoreticians in the study of adaptive behavioural variation.

Why and how the early-life environment affects development of coping behaviours

Understanding the ways in which individuals cope with threats, respond to challenges, make use of opportunities and mediate the harmful effects of their surroundings is important for predicting their ability to function in a rapidly changing world. Perhaps one of the most essential drivers of coping behaviour of adults is the environment experienced during their early-life development. Although the study of coping, defined as behaviours displayed in response to environmental challenges, has a long and rich research history in biology, recent literature has repeatedly pointed out that the processes through which coping behaviours develop in individuals are still largely unknown. In this review, we make a move towards integrating ultimate and proximate lines of coping behaviour research. After broadly defining coping behaviours (1), we review why, from an evolutionary perspective, the development of coping has become tightly linked to the early-life environment (2), which relevant developmental processes are most important in creating coping behaviours adjusted to the early-life environment (3), which influences have been shown to impact those developmental processes (4) and what the adaptive significance of intergenerational transmission of coping behaviours is, in the context of behavioural adaptations to a fast changing world (5). Important concepts such as effects of parents, habitat, nutrition, social group and stress are discussed using examples from empirical studies on mammals, fish, birds and other animals. In the discussion, we address important problems that arise when studying the development of coping behaviours and suggest solutions.

Born at the right time? A conceptual framework linking reproduction, development, and settlement in reef fish

Parents are expected to make decisions about reproductive timing and investment that maximize their own fitness, even if this does not maximize the fitness of each individual offspring. When offspring survival is uncertain, selection typically favors iteroparity, which means that offspring born at some times can be disadvantaged, while others get lucky. The eventual fate of offspring may be further modified by their own decisions. Are fates of offspring set by birthdates (i.e., determined by parents), or can offspring improve upon the cards they've been dealt? If so, do we see adaptive plasticity in the developmental timing of offspring? We evaluate these questions for a coral reef fish (the sixbar wrasse, Thalassoma hardwicke) that is characterized by extreme iteroparity and flexible larval development. Specifically, we monitored larval settlement to 192 small reefs over 11 lunar months and found that most fish settled during new moons of a lunar cycle (consistent with preferential settlement on dark nights). Settlement was significantly lower than expected by chance during the full moon and last quarter of the lunar cycle (consistent with avoidance of bright nights). Survival after settlement was greatest for fish that settled during times of decreasing lunar illumination (from last quarter to new moon). Fish that settled on the last quarter of the lunar cycle were ~10% larger than fish that settled during other periods, suggesting larvae delay settlement to avoid the full moon. These results are consistent with a numerical model that predicts plasticity in larval development time that enables avoidance of settlement during bright periods. Collectively, our results suggest that fish with inauspicious birthdates may alter their developmental trajectories to settle at better times. We speculate that such interactions between parent and offspring strategies may reinforce the evolution of extreme iteroparity and drive population dynamics, by increasing the survival of offspring born at the "wrong" time by allowing them to avoid the riskiest times of settlement.

Causes and Consequences of Behavioral Interference between Species

Behavioral interference between species, such as territorial aggression, courtship, and mating, is widespread in animals. While aggressive and reproductive forms of interspecific interference have generally been studied separately, their many parallels and connections warrant a unified conceptual approach. Substantial evidence exists that aggressive and reproductive interference have pervasive effects on species coexistence, range limits, and evolutionary processes, including divergent and convergent forms of character displacement. Alien species invasions and climate change-induced range shifts result in novel interspecific interactions, heightening the importance of predicting the consequences of species interactions, and behavioral interference is a fundamental but neglected part of the equation. Here, we outline priorities for further theoretical and empirical research on the ecological and evolutionary consequences of behavioral interference.

Seasonal shifts in sex ratios are mediated by maternal effects and fluctuating incubation temperatures

Sex-specific maternal effects can be adaptive sources of phenotypic plasticity. Reptiles with temperature-dependent sex determination (TSD) are a powerful system to investigate such maternal effects because offspring phenotype, including sex, can be sensitive to maternal influences such as oestrogens and incubation temperatures.In red-eared slider turtles (Trachemys scripta), concentrations of maternally derived oestrogens and incubation temperatures increase across the nesting season; we wanted to determine if sex ratios shift in a seasonally concordant manner, creating the potential for sex-specific maternal effects, and to define the sex ratio reaction norms under fluctuating temperatures across the nesting season.Eggs from early and late season clutches were incubated under a range of thermally fluctuating temperatures, maternally derived oestradiol concentrations were quantified via radioimmunoassay, and hatchling sex was identified. We found that late season eggs had higher maternal oestrogen concentrations and were more likely to produce female hatchlings. The sex ratio reaction norm curves systematically varied with season, such that with even a slight increase in temperature (0.5°C), late season eggs produced up to 49% more females than early season eggs.We found a seasonal shift in sex ratios which creates the potential for sex-specific phenotypic matches across the nesting season driven by maternal effects. We also describe, for the first time, systematic variation in the sex ratio reaction norm curve within a single population in a species with TSD.

The effect of maternal glucocorticoid levels on juvenile docility in yellow-bellied marmots

Maternal effects can have significant and long-term consequences on offspring behavior and survival, while consistent individual differences (i.e., personality) can have profound impacts on individual fitness. Thus, both can influence population dynamics. However, the underlying mechanisms that determine variation in personality traits are poorly understood. Maternal effects are one potential mechanism that may explain personality variation. We capitalized on a long-term study of yellow-bellied marmots (Marmota flaviventris) to identify maternal effects on juvenile docility. To do so, we partitioned the variance in juvenile docility using a quantitative genetic modeling approach to isolate potential maternal effects. We also directly tested whether maternal stress, measured through fecal glucocorticoid metabolite levels during lactation of 82 mothers, was associated with offspring docility. Docility scores were estimated for 645 juveniles trapped between 2002 and 2012. We found an interaction between maternal glucocorticoid levels and dam age on juvenile docility. We also found significant maternal, litter, permanent environment, and year effects. These results suggest that a mother's life history stage interacts with stress to influence offspring personality. This early life influence can have long lasting effects on an individual's docility throughout life.

Maternal androgens in avian brood parasites and their hosts: Responses to parasitism and competition?

In the coevolutionary dynamic of avian brood parasites and their hosts, maternal (or transgenerational) effects have rarely been investigated. We examined the potential role of elevated yolk testosterone in eggs of the principal brood parasite in North America, the brown-headed cowbird, and three of its frequent host species. Elevated maternal androgens in eggs are a common maternal effect observed in many avian species when breeding conditions are unfavorable. These steroids accelerate embryo development, shorten incubation period, increase nestling growth rate, and enhance begging vigor, all traits that can increase the survival of offspring. We hypothesized that elevated maternal androgens in host eggs are a defense against brood parasitism. Our second hypothesis was that elevated maternal androgens in cowbird eggs are a defense against intra-specific competition. For host species, we found that elevated yolk testosterone was correlated with parasitized nests of small species, those whose nest success is most reduced by cowbird parasitism. For cowbirds, we found that elevated yolk testosterone was correlated with eggs in multiply-parasitized nests, which indicate intra-specific competition for nests due to high cowbird density. We propose experimental work to further examine the use of maternal effects by cowbirds and their hosts.