Sex differences in the plasticity of life history in response to social environment

Predicting how social environment affects life history variation is critical to understanding if, and when, selection favors alternative life history development, especially in systems in which social interactions change over time or space. Although sexual selection theory predicts that males and females should respond differently to variation in the social environment, few studies have examined the responses of both male and female phenotypes to the same gradient of social environment. In this study, we used a livebearing fish to determine how males and females altered their life histories in response to variation in the social environment during development. We found that both males and females delayed maturity and attained larger sizes when their social environment included adults, in contrast to developing in juvenile-only environments. The magnitude of this effect differed substantially between the sexes. The common pattern of response in the sexes suggested that life history trade-offs, rather than sexual selection, is responsible for these changes in life history expression. These effects make the relationship between genotype and phenotype depend strongly on the environment experienced by each individual. These results indicate that social environment is an important driver of life history variation in sailfin mollies and can be at least as important as abiotic effects.

Heightened perception of competition hastens courtship

When animals use costly labile display or signal traits to display to the opposite sex, they face complex decisions regarding the degree and timing of their investment in separate instances of trait expression. Such decisions may be informed by not only the focal individual’s condition (or pool of available resources) but also aspects of the social environment, such as perceptions of same-sex competition or the quality of available mates. However, the relative importance of these factors to investment decisions remains unclear. Here, we use manipulations of condition (through dietary nutrition), recent social environment (exposure to a silenced male, nonsilenced male, female, or isolation), and female mating history (single or multiple male) to test how quickly male decorated crickets (Gryllodes sigillatus) decide to begin courting an available female. We find that males that were previously housed with nonsilenced males started courting the female earlier than other males. Females only mounted males after courtship began. Our results suggest a strong effect of the perception of competition on the decision to invest resources in sexual signaling behavior and that females might exert directional selection on its timing.

Distance assessment of detours by jumping spiders

To take an indirect route (detour) in order to reach a specific target requires complex cognitive processes. Yet more demanding, from the cognitive point of view, is when the goal is only visible at the beginning of the detour. In spiders from the family Salticidae, vision is a key sensory modality mediating navigation and prey search. Their acute vision allows them to perform complicated detours, possibly as a consequence of the multitude of potential routes in their typically complex 3-dimensional habitats. We used a 4-route choice test, in which routes differed in being either short or long and in the presence or absence of a lure of a prey item, to investigate route assessment in 2 salticid species, Trite planiceps and Marpissa marina. Although both species showed evidence of motivation to follow lured-routes, judging by the number of times they re-oriented toward them while detouring, we found that Trite chose short routes in preference to long routes, but did not prefer the lured-routes. In contrast, Marpissa exhibited random route choice, although it oriented toward lured-routes more often than control routes (lure absent). Our results suggest that decision-making processes about which route to take occurs before embarking on a route, but this is cognitively challenging. Spiders exhibited cognitive limitations in which the lack of visibility of the goal affected success. However, the severity of cognitive limitations depended on species. We suggest that variability in spatial ability across the Salticidae may be related to the habitat complexity inhabited by each species.

Constraints Imposed by a Natural Landscape Override Offspring Fitness Effects to Shape Oviposition Decisions in Wild Forked Fungus Beetles

Oviposition site decisions often maximize offspring fitness, but costs constraining choice can cause females to oviposit in poor developmental environments. It is unclear whether these constraints cumulatively outweigh offspring fitness to determine oviposition decisions in wild populations. Understanding how constraints shape oviposition in natural landscapes is a critical step toward revealing how maternal behavior influences fundamental phenomena like the evolution of specialization and the use of sink environments. Here, we used a genetic capture-recapture technique to reconstruct the oviposition decisions of individual females in a natural metapopulation of a beetle (Bolitotherus cornutus) that oviposits on three fungus species. We measured larval fitness-related traits (mass, development time, survival) on each fungus and compared the oviposition preferences of females in laboratory versus field tests. Larval fitness differed substantially among fungi, and females preferred a high-quality (high larval fitness) fungus in laboratory trials. However, females frequently laid eggs on the lowest-quality fungus in the wild. They preferred high-quality fungi when moving between oviposition sites, but this preference disappeared as the distance between sites increased and was inconsistent between study plots. Our results suggest that constraints on oviposition preferences in natural landscapes are sufficiently large to drive oviposition in poor developmental environments even when offspring fitness consequences are severe.

Socially cued seminal fluid gene expression mediates responses in ejaculate quality to sperm competition risk

There is considerable evidence that males will increase the number of sperm ejaculated in response to sperm competition risk. However, whether they have the capacity to adjust seminal fluid components of the ejaculate has received less attention. Male crickets (Teleogryllus oceanicus) have been shown to adjust the viability of sperm in their ejaculate in response to sperm competition risk. Here we show that socially mediated plasticity in sperm viability is probably due, at least in part, to male adjustments in the protein composition of the seminal fluid. Seven seminal fluid protein genes were found to have an increased expression in males exposed to rival calls. Increased expression of these genes was correlated with increased sperm viability in whole ejaculates, and gene knockdown confirmed that at least one of these proteins promotes sperm viability. Our results lend support for recent theoretical models that predict complex responses in male allocation to seminal fluid composition in response to sperm competition risk.

You are what you eat: diet shapes body composition, personality and behavioural stability

BACKGROUND

Behavioural phenotypes vary within and among individuals. While early-life experiences have repeatedly been proposed to underpin interactions between these two hierarchical levels, the environmental factors causing such effects remain under-studied. We tested whether an individual's diet affected both its body composition, average behaviour (thereby causing among-individual variation or 'personality') and within-individual variability in behaviour and body weight (thereby causing among-individual differences in residual within-individual variance or 'stability'), using the Southern field cricket Gryllus bimaculatus as a model. We further asked whether effects of diet on the expression of these variance components were sex-specific.

METHODS

Manipulating both juvenile and adult diet in a full factorial design, individuals were put, in each life-stage, on a diet that was either relatively high in carbohydrates or relatively high in protein. We subsequently measured the expression of multiple behavioural (exploration, aggression and mating activity) and morphological traits (body weight and lipid mass) during adulthood.

RESULTS

Dietary history affected both average phenotype and level of within-individual variability: males raised as juveniles on high-protein diets were heavier, more aggressive, more active during mating, and behaviourally less stable, than conspecifics raised on high-carbohydrate diets. Females preferred more protein in their diet compared to males, and dietary history affected average phenotype and within-individual variability in a sex-specific manner: individuals raised on high-protein diets were behaviourally less stable in their aggressiveness but this effect was only present in males. Diet also influenced individual differences in male body weight, but within-individual variance in female body weight.

DISCUSSION

This study thereby provides experimental evidence that dietary history explains both heterogeneous residual within-individual variance (i.e., individual variation in 'behavioural stability') and individual differences in average behaviour (i.e., 'personality'), though dietary effects were notably trait-specific. These findings call for future studies integrating proximate and ultimate perspectives on the role of diet in the evolution of repeatedly expressed traits, such as behaviour and body weight.

Australian black field crickets show changes in neural gene expression associated with socially-induced morphological, life-history, and behavioral plasticity

BACKGROUND

Ecological and evolutionary model organisms have provided extensive insight into the ecological triggers, adaptive benefits, and evolution of life-history driven developmental plasticity. Despite this, we still have a poor understanding of the underlying genetic changes that occur during shifts towards different developmental trajectories. The goal of this study is to determine whether we can identify underlying gene expression patterns that can describe the different life-history trajectories individuals follow in response to social cues of competition. To do this, we use the Australian black field cricket (Teleogryllus commodus), a species with sex-specific developmental trajectories moderated by the density and quality of calls heard during immaturity. In this study, we manipulated the social information males and females could hear by rearing individuals in either calling or silent treatments. We next used RNA-Seq to develop a reference transcriptome to study changes in brain gene expression at two points prior to sexual maturation.

RESULTS

We show accelerated development in both sexes when exposed to calling; changes were also seen in growth, lifespan, and reproductive effort. Functional relationships between genes and phenotypes were apparent from ontological enrichment analysis. We demonstrate that increased investment towards traits such as growth and reproductive effort were often associated with the expression of a greater number of genes with similar effect, thus providing a suite of candidate genes for future research in this and other invertebrate organisms.

CONCLUSIONS

Our results provide interesting insight into the genomic underpinnings of developmental plasticity and highlight the potential of a genomic exploration of other evolutionary theories such as condition dependence and sex-specific developmental strategies.

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.

Sex differences in the effects of juvenile and adult diet on age-dependent reproductive effort

Sexual selection should cause sex differences in patterns of resource allocation. When current and future reproductive effort trade off, variation in resource acquisition might further cause sex differences in age-dependent investment, or in sensitivity to changes in resource availability over time. However, the nature and prevalence of sex differences in age-dependent investment remain unclear. We manipulated resource acquisition at juvenile and adult stages in decorated crickets, Gryllodes sigillatus, and assessed effects on sex-specific allocation to age-dependent reproductive effort (calling in males, fecundity in females) and longevity. We predicted that the resource and time demands of egg production would result in relatively consistent female strategies across treatments, whereas male investment should depend sharply on diet. Contrary to expectations, female age-dependent reproductive effort diverged substantially across treatments, with resource-limited females showing much lower and later investment in reproduction; the highest fecundity was associated with intermediate lifespans. In contrast, long-lived males always signalled more than short-lived males, and male age-dependent reproductive effort did not depend on diet. We found consistently positive covariance between male reproductive effort and lifespan, whereas diet altered this covariance in females, revealing sex differences in the benefits of allocation to longevity. Our results support sex-specific selection on allocation patterns, but also suggest a simpler alternative: males may use social feedback to make allocation decisions and preferentially store resources as energetic reserves in its absence. Increased calling effort with age therefore could be caused by gradual resource accumulation, heightened mortality risk over time, and a lack of feedback from available mates.

Prior mating success can affect allocation towards future sexual signaling in crickets

Fitness is often correlated with the expression level of a sexually selected trait. However, sexually selected traits are costly to express such that investment in their expression should be optimised to maximize their overall fitness gains. Social interactions, in the form of successful and unsuccessful matings, may offer males one type of feedback allowing them to gauge how to allocate their resources towards sexual signaling. Here we tested whether adult male black field crickets (Teleogryllus commodus) modify the extent of their calling effort (the sexually selected trait) in response to successful and unsuccessful matings with females. To examine the effect that mating interactions with females have on investment into sexual signaling, we monitored male calling effort after maturation and then provided males with a female at two points within their life, manipulating whether or not males were able to successfully mate each time. Our results demonstrate that males alter their investment towards sexual signaling in response to successful matings, but only if the experience occurs early in their life. Males that mated early decreased their calling effort sooner than males that were denied a mating. Our results demonstrate that social feedback in the form of successful and unsuccessful matings has the potential to alter the effort a male places towards sexual signaling.

How populations differentiate despite gene flow: sexual and natural selection drive phenotypic divergence within a land fish, the Pacific leaping blenny

Background

Divergence between populations in reproductively important features is often vital for speciation. Many studies attempt to identify the cause of population differentiation in phenotype through the study of a specific selection pressure. Holistic studies that consider the interaction of several contrasting forms of selection are more rare. Most studies also fail to consider the history of connectivity among populations and the potential for genetic drift or gene flow to facilitate or limit phenotypic divergence. We examined the interacting effects of natural selection, sexual selection and the history of connectivity on phenotypic differentiation among five populations of the Pacific leaping blenny (Alticus arnoldorum), a land fish endemic to the island of Guam.

Results

We found key differences among populations in two male ornaments—the size of a prominent head crest and conspicuousness of a coloured dorsal fin—that reflected a trade-off between the intensity of sexual selection (male biased sex ratios) and natural selection (exposure to predators). This differentiation in ornamentation has occurred despite evidence suggesting extensive gene flow among populations, which implies that the change in ornament expression has been recent (and potentially plastic).

Conclusions

Our study provides an early snapshot of divergence in reproductively important features that, regardless of whether it reflects genetic or plastic changes in phenotype, could ultimately form a reproductive barrier among populations.