Starving the competition: a proximate cause of reproductive skew in burying beetles (Nicrophorus vespilloides)

Detection of reproductive trade-offs is influenced by resource availability and maintenance: an experimental study in the burying beetle (Nicrophorus vespilloides)

Abstract

Life-history theory predicts trade-offs between investment in current versus future reproduction. However, many studies find no or even positive correlations among these traits. The absence of the trade-off may result from resource availability, as it influences resource allocation to different traits. In addition, since large amounts of resources require additional effort in processing, resource maintenance may affect the detection of reproductive trade-offs. Here, we carried out two breeding attempts to assess the effects of resource availability and maintenance on reproductive trade-offs for both sexes in the burying beetle (Nicrophorus vespilloides). In the first breeding attempt, we simultaneously manipulated carcass size (small versus large) and carcass preparation (non-prepared versus prepared). In the second breeding attempt, we provided parents with same-sized, non-prepared carcasses. For both breeding attempts, we monitored the main and interactive effects of carcass size and carcass preparation on parental effort and reproductive outcome. In the first breeding attempt, males gained more weight and provided more care as carcass size increased, whereas females gained more weight but did not change their care. In addition, when breeding on non-prepared versus prepared carcasses, both parents provided more care and gained more weight. In the second breeding attempt, with increased investment for the first breeding, parents did not show decreased investment for the second brood, vice versa. In contrast, males breeding on large or non-prepared carcasses gained more weight during the first breeding attempt, then provided more care in subsequent reproduction. There were no differences in subsequent female care among different treatments.

Significance statement

Resource availability and resource maintenance may affect the detection of reproductive trade-offs. Here, we simultaneously manipulated carcass size and carcass preparation to assess these effects. This is the first time that the effects of resource availability and resource maintenance on reproductive trade-offs have been separated in burying beetles. Our findings suggest that despite the increased costs of parental care and resource maintenance, parents breeding on large or non-prepared carcasses gained more benefits in terms of increased body weight by staying longer and feeding more from the carcasses. Such benefits gained during the first breeding attempt offset the costs of current reproduction to some degree and masked the reproductive trade-off between current and future reproduction in terms of parental care. These findings enhance the understanding of the effects of resource availability and maintenance on reproductive trade-offs.

Sex-specific influence of communal breeding experience on parenting performance and fitness in a burying beetle

Communal breeding, wherein multiple conspecifics live and reproduce together, may generate short-term benefits in terms of defence and reproduction. However, its carry-over effects remain unclear. We experimentally tested the effects of communal breeding on parental care and reproduction in burying beetles (Nicrophorus vespilloides), which use carcasses as breeding resources and provide parental care to offspring. We subjected individuals to communal or non-communal breeding (i.e. pair breeding) during their first breeding event and to non-communal breeding during their second breeding event. We measured the parental care of individuals and of groups and the reproductive success of groups during both breeding events. In communal groups, large individuals became dominant and largely monopolized the carcass, whereas small individuals (i.e. subordinates) had restricted access to the carcass. At the first breeding event, large males in communal groups spent more time providing care than large males in non-communal groups, whereas such an effect was not observed for large females and small individuals. Reproductive successes were similar in communal and non-communal groups, indicating no short-term benefits of communal breeding in terms of reproduction. Compared with males from non-communal groups, males originating from communal groups produced a larger size of brood during their second breeding event, whereas such an effect was not observed for females. Our results demonstrate the sex-specific effects of communal breeding experience on parenting performance and fitness.

Social context shapes cognitive abilities: associative memories are modulated by fight outcome and social isolation in the crab Neohelice granulata

Cognitive abilities of an animal can be influenced by distinct social experiences. However, the extent of this modulation has not been addressed in different learning scenarios: are all tasks similarly affected by social experiences? In the present study, we analyzed the effect of social dominance in aversive and appetitive memory processes in the crab Neohelice granulata. In addition, we studied the influence of social isolation on memory ability. Social dominance experiments consisted of an agonistic phase immediately followed by a memory phase. During the agonistic phase, matched pairs of male crabs were staged in 10-min encounters and the dominant or subordinate condition of each member of the dyad was determined. During the memory phase, crabs were trained to acquire aversive or appetitive memory and tested 24 h later. Results showed that the agonistic encounter can modulate long-term memory according to the dominance condition in such a way that memory retention of subordinates results higher than their respective dominant. Remarkably, this result was found for both aversive and appetitive memory tasks. In addition, we found that isolated animals showed no memory retention when compared with animals that remained grouped. Altogether this work emphasizes the importance of social context as a modulator of cognitive abilities.

Adaptation to monogamy influences parental care but not mating behavior in the burying beetle, Nicrophorus vespilloides

The mating system is expected to have an important influence on the evolution of mating and parenting behaviors. Although many studies have used experimental evolution to examine how mating behaviors evolve under different mating systems, this approach has seldom been used to study the evolution of parental care. We used experimental evolution to test whether adaptation to different mating systems involves changes in mating and parenting behaviors in populations of the burying beetle, Nicrophorus vespilloides. We maintained populations under monogamy or promiscuity for six generations. This manipulation had an immediate impact on reproductive performance and adult survival. Compared to monogamy, promiscuity reduced brood size and adult (particularly male) survival during breeding. After six generations of experimental evolution, there was no divergence between monogamous and promiscuous populations in mating behaviors. Parents from the promiscuous populations (especially males) displayed less care than parents from the monogamous populations. Our results are consistent with the hypothesis that male care will increase with the certainty of paternity. However, it appears that this change is not associated with a concurrent change in mating behaviors.

Ecological Transitions in Grouping Benefits Explain the Paradox of Environmental Quality and Sociality

Both benign and harsh environments promote the evolution of sociality. This paradox-societies occur in environments of such contrasting quality-may be explained by the different types of benefits that individuals receive from grouping: resource defense benefits that derive from group-defended critical resources versus collective action benefits that result from social cooperation among group members. Here, we investigate cooperative behavior in the burying beetle Nicrophorus nepalensis along an elevational gradient where environmental quality (climate and competition) varies with altitude. We show that climate (temperature) and competition (both intra- and interspecific) independently and synergistically influence sociality via different grouping benefits that vary along the gradient. At low elevations where interspecific competition for resources is intense, groups gain from the collective action benefit of increased interspecific competitive ability. In contrast, pairs have higher fitness at intermediate elevations where intraspecific competition for resources is greatest because resource defense is the key grouping benefit. However, groups and pairs have similar fitness at high elevations, suggesting that there is no grouping benefit in such physiologically challenging environments. Our results demonstrate that sociality is favored for different reasons under a range of environmental conditions, perhaps explaining why animal societies occur in environments of such contrasting quality.

Maternity uncertainty in cobreeding beetles: females lay more and larger eggs and provide less care

Cobreeding, which occurs when multiple females breed together, is likely to be associated with uncertainty over maternity of offspring in a joint brood, preventing females from directing resources towards their own offspring. Cobreeding females may respond to such uncertainty by shifting their investment towards the stages of offspring development when they are certain of maternity and away from those stages where uncertainty is greater. Here we examined how uncertainty of maternity influences investment decisions of cobreeding females by comparing cobreeding females and females breeding alone in the burying beetle, Nicrophorus vespilloides. In this species, females sometimes breed together on a single carcass but females cannot recognize their own offspring. We found that cobreeding females shifted investment towards the egg stage of offspring development by laying more and larger eggs than females breeding alone. Furthermore, cobreeding females reduced their investment to post-hatching care of larvae by spending less time providing care than females breeding alone. We show that females respond to the presence of another female by shifting allocation towards egg laying and away from post-hatching care, thereby directing resources to their own offspring. Our results demonstrate that responses to parentage uncertainty are not restricted to males, but that, unlike males, females respond by shifting their investment to different components of reproduction within a single breeding attempt. Such flexibility may allow females to cope with maternity uncertainly as well as a variety of other social or physical challenges.

Superior stimulation of female fecundity by subordinate males provides a mechanism for telegony

When females mate promiscuously, rival males compete to fertilise the ova. In theory, a male can increase his success at siring offspring by inducing the female to lay more eggs, as well as by producing more competitive sperm. Here we report that the evolutionary consequences of fecundity stimulation extend beyond rival males, by experimentally uncovering effects on offspring. With experiments on the burying beetle Nicrophorus vespilloides, we show that smaller subordinate males are better able to stimulate female fecundity than larger, dominant males. Furthermore dominant males also benefit from the greater fecundity induced by smaller males, and so gain from the female's earlier promiscuity ‐ just as predicted by theory. By inducing females to produce more offspring on a limited resource, smaller males cause each larva to be smaller, even those they do not sire themselves. Fecundity stimulation thus promotes the non‐genetic inheritance of offspring body size, and provides a mechanism for telegony.

Male reproductive suppression: not a social affair

In the animal kingdom there are countless strategies via which males optimize their reproductive success when faced with male–male competition. These male strategies typically fall into two main categories: pre- and post-copulatory competition. Within these 2 categories, a set of behaviors, referred to as reproductive suppression, is known to cause inhibition of reproductive physiology and/or reproductive behavior in an otherwise fertile individual. What becomes evident when considering examples of reproductive suppression is that these strategies conventionally encompass reproductive interference strategies that occur between members of a hierarchical social group. However, mechanisms aimed at impairing a competitor’s reproductive output are also present in non-social animals. Yet, current thinking emphasizes the importance of sociality as the primary driving force of reproductive suppression. Therefore, the question arises as to whether there is an actual difference between reproductive suppression strategies in social animals and equivalent pre-copulatory competition strategies in non-social animals. In this perspective paper we explore a broad taxonomic range of species whose individuals do not repeatedly interact with the same individuals in networks and yet, depress the fitness of rivals. Examples like alteration of male reproductive physiology, female mimicry, rival spermatophore destruction, and cementing the rival’s genital region in non-social animals, highlight that male pre-copulatory reproductive suppression and male pre-copulatory competition overlap. Finally, we highlight that a distinction between male reproductive interference in animals with and without a social hierarchy might obscure important similarities and does not help to elucidate why different proximate mechanisms evolved. We therefore emphasize that male reproductive suppression need not be restricted to social animals.

Female house wrens value the nest cavity more than exclusive access to males during conflicts with female intruders

Individuals should fight hardest when they stand to lose the most. Whereas males frequently compete for fertile females, females more often compete for high quality males, male care, or resources required to breed. We asked whether established, territorial female house wrens (Troglodytes aedon) challenged by simulated female intruders fight as if they place more value on retaining (1) their nesting cavity or (2) exclusive access to other benefits offered by males. We randomly assigned house wren pairs to receive one or three nest boxes and then assayed female aggression. The relative costs to losing differed between box treatments. For one-box females, the risk of losing the cavity and territory was higher. For three-box females, the risk of losing the cavity may be lower because intruders may be able to settle as secondary females in the supplemental boxes. In this situation, females would lose exclusive access to males and their territories but would still retain the male’s assistance rearing offspring since male house wrens favour their oldest brood. We found that one-box females were significantly more aggressive. This response may be adaptive, as females that switched territories between broods were significantly more likely to lose their entire nest prior to hatching than females that retained the same territory. We interpret our results to mean that female house wrens value the nest cavity more than other benefits from exclusive access to males and their territories. This work contributes to a body of evidence that females often compete for resources required to breed.

Why does it take two to tango? Lifetime fitness consequences of parental care in a burying beetle

In species that require parental care, each parent can either care for their offspring or leave them in the care of the other parent. For each parent this creates three possible parental care strategies: biparental care, uniparental (male or female) care, and uniparental desertion by either the male or female. The burying beetle, Nicrophorus orbicollis, typically exhibits biparental care of offspring, and thus provides a unique system that allows us to compare the fitness benefits of these parental care strategies in an unconfounded way. In this study, we assess the lifetime fitness of biparental care, uniparental care, and uniparental desertion strategies in both male and female N. orbicollis. Specifically, we tested for increased fitness of the biparental care strategy compared to uniparental care strategies. Second, we test for equality of fitness between uniparental care and uniparental desertion strategies. Surprisingly, biparental care yields lower lifetime fitness for both parents compared to the other two strategies. Also, uniparental care and uniparental desertion strategies yielded equal fitness. The evolution of biparental care in this system is not consistent with the expectation of a mutual fitness benefit. We discuss other potential explanations for the evolution of biparental care in this system.

Contests over reproductive resources in female roller beetles: Outcome predictors and sharing as an option

Fights among females are frequent, although less attention has been placed on them than on male fights. They arise when females compete for food, oviposition, mates, brooding sites, or access to resources which increase offspring survival. It has been shown that the outcome of female fights may be less predictable by asymmetries in resource holding power, than in male fights. Male roller beetles fight over food resources, food balls, needed for mating and nesting, and it has been show in some species that asymmetries in reproductive experience and resource holding power in terms of size predict fight outcome, including ties in which contenders cut and split the food ball. In this study, we tested the influence of asymmetries in reproductive status (experience) and body size on female fight outcome in the carrion roller beetle Canthon cyanellus cyanellus. As predicted, and as previously found for males of the same species, female reproductive status of both contenders and relative size predict fight outcome. Larger and reproductively experienced contenders have a higher probability of winning. Furthermore, ties are more likely in fights involving opposing asymmetries (vgr. Large reproductively naïve owner versus small reproductively experienced intruder). Also as predicted, food ball splitting is more likely to be started by the predicted loser. This mode of resource sharing may be the result of a fighting strategy in which the costs of continuing to fight are greater than the benefits of not splitting, if a fraction of the disputed resource is more than the minimum needed for the present reproductive needs, and reduces costs associated to a longer fight.

Adaptive collective foraging in groups with conflicting nutritional needs

Collective foraging, based on positive feedback and quorum responses, is believed to improve the foraging efficiency of animals. Nutritional models suggest that social information transfer increases the ability of foragers with closely aligned nutritional needs to find nutrients and maintain a balanced diet. However, whether or not collective foraging is adaptive in a heterogeneous group composed of individuals with differing nutritional needs is virtually unexplored. Here we develop an evolutionary agent-based model using concepts of nutritional ecology to address this knowledge gap. Our aim was to evaluate how collective foraging, mediated by social retention on foods, can improve nutrient balancing in individuals with different requirements. The model suggests that in groups where inter-individual nutritional needs are unimodally distributed, high levels of collective foraging yield optimal individual fitness by reducing search times that result from moving between nutritionally imbalanced foods. However, where nutritional needs are highly bimodal (e.g. where the requirements of males and females differ) collective foraging is selected against, leading to group fission. In this case, additional mechanisms such as assortative interactions can coevolve to allow collective foraging by subgroups of individuals with aligned requirements. Our findings indicate that collective foraging is an efficient strategy for nutrient regulation in animals inhabiting complex nutritional environments and exhibiting a range of social forms.

Social Network Analysis and Nutritional Behavior: An Integrated Modeling Approach

Animals have evolved complex foraging strategies to obtain a nutritionally balanced diet and associated fitness benefits. Recent research combining state-space models of nutritional geometry with agent-based models (ABMs), show how nutrient targeted foraging behavior can also influence animal social interactions, ultimately affecting collective dynamics and group structures. Here we demonstrate how social network analyses can be integrated into such a modeling framework and provide a practical analytical tool to compare experimental results with theory. We illustrate our approach by examining the case of nutritionally mediated dominance hierarchies. First we show how nutritionally explicit ABMs that simulate the emergence of dominance hierarchies can be used to generate social networks. Importantly the structural properties of our simulated networks bear similarities to dominance networks of real animals (where conflicts are not always directly related to nutrition). Finally, we demonstrate how metrics from social network analyses can be used to predict the fitness of agents in these simulated competitive environments. Our results highlight the potential importance of nutritional mechanisms in shaping dominance interactions in a wide range of social and ecological contexts. Nutrition likely influences social interactions in many species, and yet a theoretical framework for exploring these effects is currently lacking. Combining social network analyses with computational models from nutritional ecology may bridge this divide, representing a pragmatic approach for generating theoretical predictions for nutritional experiments.

The effect of size and sex ratio experiences on reproductive competition in Nicrophorus vespilloides burying beetles in the wild

Male parents face a choice: should they invest more in caring for offspring or in attempting to mate with other females? The most profitable course depends on the intensity of competition for mates, which is likely to vary with the population sex ratio. However, the balance of pay‐offs may vary among individual males depending on their competitive prowess or attractiveness. We tested the prediction that sex ratio and size of the resource holding male provide cues regarding the level of mating competition prior to breeding and therefore influence the duration of a male's biparental caring in association with a female. Male burying beetles, Nicrophorus vespilloides were reared, post‐eclosion, in groups that differed in sex ratio. Experimental males were subsequently translocated to the wild, provided with a breeding resource (carcass) and filmed. We found no evidence that sex ratio cues prior to breeding affected future parental care behaviour but males that experienced male‐biased sex ratios took longer to attract wild mating partners. Smaller males attracted a higher proportion of females than did larger males, securing significantly more monogamous breeding associations as a result. Smaller males thus avoided competitive male–male encounters more often than larger males. This has potential benefits for their female partners who avoid both intrasexual competition and direct costs of higher mating frequency associated with competing males.

Drosophila females trade off good nutrition with high quality oviposition sites when choosing foods

Animals, from insects to human, select foods to regulate their acquisition of key nutrients in amounts and balances maximising fitness. In species where the nutrition of juveniles depends on parents, adults must make challenging foraging decisions that simultaneously address their own nutrient needs as well as those of the progeny. Here we examined how fruit flies Drosophila melanogaster, a species where individuals eat and lay eggs in decaying fruits, integrate feeding decisions (individual nutrition) and oviposition decisions (offspring nutrition) when foraging. Using cafeteria assays with artificial diets varying in concentrations and ratios of protein to carbohydrates, we show that Drosophila females exhibit complex foraging patterns, alternating between laying eggs on high carbohydrate foods and feeding on foods with different nutrient contents depending on their own nutritional state. Although larvae showed faster development on high protein foods, both survival and learning performances were higher on balanced foods. We suggest that the apparent mismatch between the oviposition preference of females for high carbohydrate foods and the high performances of larvae on balanced foods reflects a natural situation where high carbohydrate ripened fruits gradually enrich in proteinaceous yeast as they start rotting, thereby yielding optimal nutrition for the developing larvae. Our findings that animals with rudimentary parental care uncouple feeding and egg-laying decisions in order to balance their own diet and provide a nutritionally optimal environment to their progeny reveals unsuspected levels of complexity in the nutritional ecology of parent-offspring interactions.

Evolving nutritional strategies in the presence of competition: a geometric agent-based model

Access to nutrients is a key factor governing development, reproduction and ultimately fitness. Within social groups, contest-competition can fundamentally affect nutrient access, potentially leading to reproductive asymmetry among individuals. Previously, agent-based models have been combined with the Geometric Framework of nutrition to provide insight into how nutrition and social interactions affect one another. Here, we expand this modelling approach by incorporating evolutionary algorithms to explore how contest-competition over nutrient acquisition might affect the evolution of animal nutritional strategies. Specifically, we model tolerance of nutrient excesses and deficits when ingesting nutritionally imbalanced foods, which we term 'nutritional latitude'; a higher degree of nutritional latitude constitutes a higher tolerance of nutritional excess and deficit. Our results indicate that a transition between two alternative strategies occurs at moderate to high levels of competition. When competition is low, individuals display a low level of nutritional latitude and regularly switch foods in search of an optimum. When food is scarce and contest-competition is intense, high nutritional latitude appears optimal, and individuals continue to consume an imbalanced food for longer periods before attempting to switch to an alternative. However, the relative balance of nutrients within available foods also strongly influences at what levels of competition, if any, transitions between these two strategies occur. Our models imply that competition combined with reproductive skew in social groups can play a role in the evolution of diet breadth. We discuss how the integration of agent-based, nutritional and evolutionary modelling may be applied in future studies to further understand the evolution of nutritional strategies across social and ecological contexts.

Nutritional ecology beyond the individual: a conceptual framework for integrating nutrition and social interactions

Over recent years, modelling approaches from nutritional ecology (known as Nutritional Geometry) have been increasingly used to describe how animals and some other organisms select foods and eat them in appropriate amounts in order to maintain a balanced nutritional state maximising fitness. These nutritional strategies profoundly affect the physiology, behaviour and performance of individuals, which in turn impact their social interactions within groups and societies. Here, we present a conceptual framework to study the role of nutrition as a major ecological factor influencing the development and maintenance of social life. We first illustrate some of the mechanisms by which nutritional differences among individuals mediate social interactions in a broad range of species and ecological contexts. We then explain how studying individual- and collective-level nutrition in a common conceptual framework derived from Nutritional Geometry can bring new fundamental insights into the mechanisms and evolution of social interactions, using a combination of simulation models and manipulative experiments.

Modelling nutrition across organizational levels: from individuals to superorganisms

The Geometric Framework for nutrition has been increasingly used to describe how individual animals regulate their intake of multiple nutrients to maintain target physiological states maximizing growth and reproduction. However, only a few studies have considered the potential influences of the social context in which these nutritional decisions are made. Social insects, for instance, have evolved extreme levels of nutritional interdependence in which food collection, processing, storage and disposal are performed by different individuals with different nutritional needs. These social interactions considerably complicate nutrition and raise the question of how nutrient regulation is achieved at multiple organizational levels, by individuals and groups. Here, we explore the connections between individual- and collective-level nutrition by developing a modelling framework integrating concepts of nutritional geometry into individual-based models. Using this approach, we investigate how simple nutritional interactions between individuals can mediate a range of emergent collective-level phenomena in social arthropods (insects and spiders) and provide examples of novel and empirically testable predictions. We discuss how our approach could be expanded to a wider range of species and social systems.

Effects of age and experience on contest behavior in the burying beetle, Nicrophorus vespilloides

Aggression and likelihood of winning contests are expected to change as a male ages. We test this idea in burying beetles, a species which competes over small mammal carcasses as a breeding resource. We find that male size relative to his opponent is far more important in determining contest outcome than any effects of age or social experience.

Contest behavior forms an important part of reproductive investment. Life-history theory predicts that as individuals age and their residual reproductive value decreases, they should increase investment in contest behavior. However, other factors such as social experience may also be important in determining age-related variation in contest behavior. To understand how selection acts on contest behavior over an individual’s lifetime, it is therefore important to tease apart the effects of age per se from other factors that may vary with age. Here, we independently manipulate male age and social experience to examine their effects on male contest behavior in the burying beetle Nicrophorus vespilloides. We found that social experience, but not age, influenced male contest behavior but that these changes in behavior did not alter contest outcomes. Male size (relative to his opponent) was overwhelmingly the most important factor determining contest outcome. Our results suggest that in systems with high variation in fighting ability among males, there may be little opportunity for selection to act on factors that influence contest outcomes by altering motivation to win.

Intrasexual competition in females: evidence for sexual selection?

In spite of recent interest in sexual selection in females, debate exists over whether traits that influence female-female competition are sexually selected. This review uses female-female aggressive behavior as a model behavioral trait for understanding the evolutionary mechanisms promoting intrasexual competition, focusing especially on sexual selection. I employ a broad definition of sexual selection, whereby traits that influence competition for mates are sexually selected, whereas those that directly influence fecundity or offspring survival are naturally selected. Drawing examples from across animal taxa, including humans, I examine 4 predictions about female intrasexual competition based on the abundance of resources, the availability of males, and the direct or indirect benefits those males provide. These patterns reveal a key sex difference in sexual selection: Although females may compete for the number of mates, they appear to compete more so for access to high-quality mates that provide direct and indirect (genetic) benefits. As is the case in males, intrasexual selection in females also includes competition for essential resources required for access to mates. If mate quality affects the magnitude of mating success, then restricting sexual selection to competition for quantity of mates may ignore important components of fitness in females and underestimate the role of sexual selection in shaping female phenotype. In the future, understanding sex differences in sexual selection will require further exploration of the extent of mutual intrasexual competition and the incorporation of quality of mating success into the study of sexual selection in both sexes.