Selection on the joint actions of pairs leads to divergent adaptation and coadaptation of care-giving parents during pre-hatching care

The joint actions of animals in partnerships or social groups evolve under both natural selection from the wider environment and social selection imposed by other members of the pair or group. We used experimental evolution to investigate how jointly expressed actions evolve upon exposure to a new environmental challenge. Our work focused on the evolution of carrion nest preparation by pairs of burying beetles Nicrophorus vespilloides, a joint activity undertaken by the pair but typically led by the male. In previous work, we found that carrion nest preparation evolved to be faster in experimental populations without post-hatching care (No Care: NC lines) than with post-hatching care (Full Care: FC lines). Here, we investigate how this joint activity evolved. After 15 generations of experimental evolution, we created heterotypic pairs (NC females with FC males and NC males with FC females) and compared their carrion nest making with homotypic NC and FC pairs. We found that pairs with NC males prepared the nest more rapidly than pairs with FC males, regardless of the female's line of origin. We discuss how social coadaptations within pairs or groups could act as a post-mating barrier to gene flow.

Brood size, food availability, and body size affects male care decisions and offspring performance

Parental care strategies do not only vary greatly across species, but also within species there can be substantial between- and within-individual variation in parental care behavior. To better understand the evolution of care strategies, it is crucial to determine how and when parents modify their behavior in response to internal as well as environmental factors. Here, we investigated the effect of brood size, resource size and an individual's quality on care strategies of uniparental males and examined the downstream consequences on offspring performance in the burying beetle Nicrophorus vespilloides. Burying beetles breed on small vertebrate cadavers and, on average, males invest much less in care than females. Nevertheless, we found that uniparentally caring males were responsive to their social and non-social environment and adjusted the amount as well as the type of care to the size of the brood, the size of the cadaver and their own body size. Additionally, we show that the care strategies affected offspring performance. Specifically, males that cared longer had larger and more surviving larvae. Our results add to our understanding of plastic parenting strategies by showing that even the sex that provides less care can evolve a very flexible care behavior.

Parent-offspring conflict and its outcome under uni-and biparental care

Conflicts over parental investment are predicted to be common among family members, especially between parents and their offspring. Parent-offspring conflict has been studied in many brood-caring organisms, but whether its outcome is closer to the parental or offspring optimum is usually unknown, as is whether the presence of a second parent, a caring male partner, can affect the outcome. Here, we manipulated the initial brood size of single and paired female burying beetles to examine how many offspring are necessary to maintain parental care in the current brood. We found that mothers continued to invest in small broods even if their reproductive output would have been higher if they had discontinued their care and produced a second brood instead. Consequently, our data suggests that the offspring have the upper hand in the conflict. However, our results further show that paired females laid a second egg clutch more often and produced more offspring than single females, suggesting that the presence of a male partner shifts the conflict outcome towards the parental optimum. This latter result not only is a novel aspect of parent-offspring theory, but also represents an additional factor that might explain the evolution of biparental care.

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.

Access to Resources Shapes Sex Differences Between Caring Parents

In species where both parents cooperate to care for their joint offspring, one sex often provides more care than the other. The magnitude of such sex differences often varies both between and within species and may depend on environmental conditions, such as access to resources, predation risk and interspecific competition. Here we investigated the impact of one such environmental variable – access to resources for breeding – on the magnitude of sex differences in parental care in the burying beetle Nicrophorus vespilloides. This species breeds on the carcasses of small vertebrates, which are the sole food source for parents and offspring during breeding. We manipulated access to resources by providing pairs with mouse carcasses from a broad mass range (3.65–26.15 g). We then monitored subsequent effects on the duration and amount of care provided by males and females, male and female food consumption and weight change during breeding, and larval traits related to offspring performance. We found that males increased their duration of care as carcass mass increased, while females remained with the brood until it had completed its development irrespective of carcass mass. There were thus more pronounced sex differences in parental care when parents had access to fewer resources for breeding. Overall, our findings show that sex differences between caring parents vary depending on access to resources during breeding. The finding that males extended their duration of care on larger carcasses suggests that access to more resources leads to a shift toward more cooperation between caring parents.

No evidence for increased fitness of offspring from multigenerational effects of parental size or natal carcass size in the burying beetle Nicrophorus marginatus

Multigenerational effects (often called maternal effects) are components of the offspring phenotype that result from the parental phenotype and the parental environment as opposed to heritable genetic effects. Multigenerational effects are widespread in nature and are often studied because of their potentially important effects on offspring traits. Although multigenerational effects are commonly observed, few studies have addressed whether they affect offspring fitness. In this study we assess the effect of potential multigenerational effects of parental body size and natal carcass size on lifetime fitness in the burying beetle, Nicrophorus marginatus (Coleoptera; Silphidae). Lifespan, total number of offspring, and number of offspring in the first reproductive bout were not significantly related to parental body size or natal carcass size. However, current carcass size used for reproduction was a significant predictor for lifetime number of offspring and number of offspring in the first brood. We find no evidence that multigenerational effects from larger parents or larger natal carcasses contribute to increased fitness of offspring.

Constrained flexibility of parental cooperation limits adaptive responses to harsh conditions

Parental care is predicted to evolve to mitigate harsh environments, thus adaptive plasticity of care may be an important response to our climate crisis. In biparental species, fitness costs may be reduced by resolving conflict and enhancing cooperation among partners. We investigated this prediction with the burying beetle, Nicrophorus orbicollis, by exposing them to contrasting benign and harsh thermal environments. Despite measurable fitness costs under the harsh environment, sexual conflict persisted in the form of sex-specific social plasticity. That is, females provided equivalent care with or without males, whereas males with partners deserted earlier and reduced provisioning effort. The interaction of social condition and thermal environment did not explain variation in individual behavior, failing to support a temperature-mediated shift from conflict to cooperation. Examining selection gradients and splines on cumulative care revealed a likely explanation for these patterns. Contrary to predictions, increased care did not enhance offspring performance under stress. Rather, different components of care were under different selection regimes, with optimization constrained due to lack of coordination between parents. We suggest that the potential for parenting to ameliorate the effects of our climate crisis may depend on the sex-specific evolutionary drivers of parental care, and that this may be best reflected in components of care.

Burying Beetle Parents Adaptively Manipulate Information Broadcast from a Microbial Community

AbstractMicrobial volatiles provide essential information for animals, which compete to detect, respond to, and perhaps control this information. Burying beetle parents have the opportunity to influence microbially derived semiochemicals, because they monopolize a small carcass for their family, repairing feeding holes and applying exudates that alter the microbial community. To study adaptive manipulation of microbial cues, we integrated mechanistic and functional approaches. We contrasted gas chromatography-mass spectrometry (GC-MS) volatile profiles from carcasses that were or were not prepared by a resident pair of Nicrophorus orbicollis. Methyl thiocyanate (MeSCN), the primary attractant for burying beetles seeking a fresh carcass, was reduced 20-fold by carcass preparation, while dimethyl trisulfide (DMTS), which deters breeding beetles, was increased 20-fold. These results suggest that parental care serves to make previously public information more private (crypsis, MeSCN) and to disinform rivals with a deterrent (DMTS). Functional tests in the field demonstrated that carcass preparation reduced discovery and use by congeners (threefold) as well as by dipteran rivals. Because microbes and their chemicals influence nearly every aspect of animal ecology, animal manipulation of microbial cues may be as widespread as manipulation of their own signals.

Early-life effects on body size in each sex interact to determine reproductive success in the burying beetle Nicrophorus vespilloides

Early-life conditions have been shown to have a profound effect on an animal's body size and fecundity across diverse taxa. However, less is known about how early-life effects on fecundity within each sex interact to determine reproductive success. We used experiments with burying beetles Nicrophorus vespilloides to analyse this problem. The nutritional conditions experienced by burying beetles in early life are a key determinant of adult body size in both sexes, and adult body size in turn influences male reproductive tactics. In previous work, we showed that smaller males are more effective than larger males at stimulating virgin female fecundity. In this study, we manipulated male and female body size by restricting access to food in early development. We then conducted breeding assays, in which small and large females were mated sequentially with small and large males, and then allowed to raise offspring without paternal care. We tested whether large females, which are potentially more fecund, laid even more eggs when mated with small males. We found no evidence to support this prediction. Instead, we detected only a weak non-significant trend in the predicted direction and no equivalent trend in the number of larvae produced. However, we did find that larvae attained a greater mass by the end of development when their mother was large and mated with a small male first. We suggest that large females might have evolved counter-measures that prevent exploitation by small fecundity-stimulating males, including partial filial cannibalism. By eating surplus larvae during reproduction, larger females would leave more of the carrion for their offspring to consume. This could explain why their surviving larvae are able to attain a greater mass by the time they complete their development.

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.

Conflict within species determines the value of a mutualism between species

Mutually beneficial interactions between species play a key role in maintaining biodiversity and ecosystem function. Nevertheless, such mutualisms can erode into antagonistic interactions. One explanation is that the fitness costs and benefits of interacting with a partner species vary among individuals. However, it is unclear why such variation exists. Here, we demonstrate that social behavior within species plays an important, though hitherto overlooked, role in determining the relative fitness to be gained from interacting with a second species. By combining laboratory experiments with field observations, we report that conflict within burying beetles Nicrophorus vespilloides influences the fitness that can be gained from interacting with the mite Poecilochirus carabi. Beetles transport these mites to carrion, upon which both species breed. We show that mites help beetles win intraspecific contests for this scarce resource: mites raise beetle body temperature, which enhances beetle competitive prowess. However, mites confer this benefit only upon smaller beetles, which are otherwise condemned by their size to lose contests for carrion. Larger beetles need no assistance to win a carcass and then lose reproductive success when breeding alongside mites. Thus, the extent of mutualism is dependent on an individual's inability to compete successfully and singlehandedly with conspecifics. Mutualisms degrade into antagonism when interactions with a partner species start to yield a net fitness loss, rather than a net fitness gain. This study suggests that interactions with conspecifics determine where this tipping point lies.

Pheromones Regulating Reproduction in Subsocial Beetles: Insights with References to Eusocial Insects

Beetles have evolved diverse strategies to cope with environmental challenges. Although parents of the vast majority of beetle species do not take care of their offspring, there are some species, in which parents provide elaborate post-hatching care and remain temporarily associated with their offspring to defend them from competitors or to provision them with food. Usually, socially induced reproductive "control" is a core feature of eusocial societies, but here we highlight that already in small family groups, socially induced reproductive regulation can play a fundamental role. By discussing the family life of burying beetles, we illustrate the mechanisms behind such a reproductive "control" and show that - similar to eusocial insects - pheromones can be an important regulating factor. However, apart from burying beetles, our knowledge of pheromones or other signals mediating reproductive regulation is surprisingly rudimentary for social beetles. More data are required to broaden our currently patchy picture.

Physiology modulates social flexibility and collective behaviour in equids and other large ungulates

Though morphologically very similar, equids across the extant species occupy ecological niches that are surprisingly non-overlapping. Occupancy of these distinct niches appears related to subtle physiological and behavioural adaptations which, in turn, correspond to significant differences in the social behaviours and emergent social systems characterizing the different species. Although instances of intraspecific behavioural variation in equids demonstrate that the same body plan can support a range of social structures, each of these morphologically similar species generally shows robust fidelity to its evolved social system. The pattern suggests a subtle relationship between physiological phenotypes and behavioural flexibility. While environmental conditions can vary widely within relatively short temporal or spatial scales, physiological changes and changes to the behaviours that regulate physiological processes, are constrained to longer cycles of adaptation. Physiology is then the limiting variable in the interaction between ecological variation and behavioural and socio-structural flexibility. Behavioural and socio-structural flexibility, in turn, will generate important feedbacks that will govern physiological function, thus creating a coupled web of interactions that can lead to changes in individual and collective behaviour. Longitudinal studies of equid and other large-bodied ungulate populations under environmental stress, such as those discussed here, may offer the best opportunities for researchers to examine, in real time, the interplay between individual behavioural plasticity, socio-structural flexibility, and the physiological and genetic changes that together produce adaptive change.This article is part of the themed issue 'Physiological determinants of social behaviour in animals'.

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.

Phoretic Poecilochirus mites specialize on their burying beetle hosts

Recurring species interactions can cause species to adapt to each other. Specialization will increase the fitness of symbionts in the coevolved association but may reduce the flexibility of symbiont choice as it will often decrease fitness in interactions with other than the main symbiont species. We analyzed the fitness interactions between a complex of two cryptic mite species and their sympatric burying beetle hosts in a European population. Poecilochirus mites (Mesostigmata, Parasitidae) are phoretic on burying beetles and reproduce alongside beetles, while these care for their offspring at vertebrate carcasses. While Poecilochirus carabi is typically found on Nicrophorus vespilloides beetles, P. necrophori is associated with N. vespillo. It has long been known that the mites discriminate between the two beetle species, but the fitness consequences of this choice remained unknown. We experimentally associated both mite species with both beetle species and found that mite fitness suffered when mites reproduced alongside a nonpreferred host. In turn, there is evidence that one of the beetle species is better able to cope with the mite species they are typically associated with. The overall fitness effect of mites on beetles was negative in our laboratory experiments. The Poecilochirus mites studied here are thus specialized competitors or parasites of burying beetles.

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.

Volatiles Emitted by Calling Males of Burying Beetles and Ptomascopus morio (Coleoptera: Silphidae: Nicrophorinae) Are Biogenetically Related

In burying beetles, Nicrophorus spp. (Coleoptera: Silphidae: Nicrophorinae) mate finding is mediated by male produced volatile compounds. To date, pheromone components of only two species have been identified. In an attempt to better understand the evolution of male pheromone signaling in burying beetles, we investigated the male released volatiles of ten Nicrophorus species and one closely related nicrophorine species, Ptomascopus mori. Volatiles emitted by calling males were collected in the laboratory by means of solid phase micro extraction and analyzed using gas chromatography coupled with mass spectrometry. Identified volatiles included short chain esters of 4-methylcarboxylic acids, terpenoids, and some other aliphatic compounds. The long-range volatile signals of the burying beetle species included in this study are blends of two to seven components. We found that methyl or ethyl esters of 4-methylheptanoic acid and 4-methyloctanoic acid are produced by eight of the ten investigated Nicrophorus species. These esters may play a key role in chemical communication. Their widespread occurrence suggests that these compounds did not evolve recently, but appeared relatively early in the phylogeny of the genus. Although Ptomascopus is considered the sister genus of Nicrophorus, P. morio males do not produce any of the Nicrophorus compounds, but release 3-methylalkan-2-ones, which are absent in Nicrophorus. A better understanding of the evolution of burying beetle pheromones, however, will only be possible once more species have been studied.

Allometry and morphometrics of clypeal membrane size and shape in Nicrophorus (Coleoptera: Silphidae)

Contests between same-sex opponents over resources necessary for reproduction, as well interactions used to discern mate quality, often involve exaggerated traits wherein large individuals have disproportionately larger traits. This positive allometric scaling of weapons or signals facilitates communication during social interactions by accentuating body size differences between individuals. Typically, males carry these exaggerated traits, as males must compete over limited female gametes. However, in Nicrophorus beetles both males and females engage in physical contests over the vertebrate carcasses they need to provision and raise offspring. Male and female Nicrophorus beetles have extended clypeal membranes directly above their mandibles, which could serve as signals. We investigated the scaling relationships between clypeal membrane size and shape and body size for five species of North American burying beetle to determine whether clypeal membranes contain exaggerated body size information. We found that clypeal membranes for both sexes of all species scaled positively with body size (slope > 1). Three of the five species also displayed sexual dimorphism in aspects of clypeal membrane size and shape allometry despite lack of dimorphism in body size. In two dimorphic species, small male clypeal membranes were statistically indistinguishable from the female form. We conclude that colored clypeal membranes in Nicrophorus beetles do contain exaggerated body size information. Observed patterns of dimorphism suggest that males sometimes experience stronger selection on marking size and shape, which might be explained by life history differences among species.

Development and application of 14 microsatellite markers in the burying beetle Nicrophorus vespilloides reveals population genetic differentiation at local spatial scales

Burying beetles (genus Nicrophorus) are relatively rare among insects in providing sophisticated parental care. Consequently, they have become model species in research analysing social evolution, the evolution of parental care and mating systems. We used the recently published N. vespilloides genome and transcriptome to develop microsatellite markers. Specifically, we developed 14 polymorphic markers with five to 13 alleles per locus and used them to investigate levels of genetic differentiation in four south Cambridgeshire (UK) populations of N. vespilloides, separated by 21 km at most. The markers revealed significant genetic structuring among populations (global F_ST = 0.023) with all but one of the pairwise comparisons among populations being significant. The single exception was the comparison between the two closest populations, which are approximately 2.5 km apart. In general, the microsatellite markers showed lower observed heterozygosity than expected. We infer that there is limited dispersal between populations and potentially also some inbreeding within them and suggest that this may be due to habitat fragmentation. We discuss these results in the context of recent laboratory experiments on inbreeding and beetle flight.

Selection on an antagonistic behavioral trait can drive rapid genital coevolution in the burying beetle, Nicrophorus vespilloides

Male and female genital morphology varies widely across many taxa, and even among populations. Disentangling potential sources of selection on genital morphology is problematic because each sex is predicted to respond to adaptations in the other due to reproductive conflicts of interest. To test how variation in this sexual conflict trait relates to variation in genital morphology we used our previously developed artificial selection lines for high and low repeated mating rates. We selected for high and low repeated mating rates using monogamous pairings to eliminate contemporaneous female choice and male-male competition. Male and female genital shape responded rapidly to selection on repeated mating rate. High and low mating rate lines diverged from control lines after only 10 generations of selection. We also detected significant patterns of male and female genital shape coevolution among selection regimes. We argue that because our selection lines differ in sexual conflict, these results support the hypothesis that sexually antagonistic coevolution can drive the rapid divergence of genital morphology. The greatest divergence in morphology corresponded with lines in which the resolution of sexual conflict over mating rate was biased in favor of male interests.

Interspecific interactions change the outcome of sexual conflict over prehatching parental investment in the burying beetle Nicrophorus vespilloides

Sexual conflict arises when the optimal reproductive strategy differs for males and females. It is associated with every reproductive stage, yet few studies have considered how the outcome may be changed by interactions with other species. Here, we show that phoretic mites Poecilochirus carabi change the outcome of sexual conflict over the supply of prehatching parental investment in the burying beetle Nicrophorus vespilloides. Burying beetles require a small dead vertebrate for reproduction, which they prepare by shaving it, rolling up the flesh, and burying it. When pairs were given a medium-sized mouse to prepare (13-16 g), mites changed how the costs of reproduction were divided between the sexes, with males then sustaining greater costs than females. We found no equivalent difference when pairs prepared larger or smaller carcasses. Thus, our experiment shows that the outcome of sexual conflict over prehatching parental investment is changed by interactions with other species during reproduction.

A hormone-related female anti-aphrodisiac signals temporary infertility and causes sexual abstinence to synchronize parental care

The high energetic demand of parental care requires parents to direct their resources towards the support of existing offspring rather than investing into the production of additional young. However, how such a resource flow is channelled appropriately is poorly understood. In this study, we provide the first comprehensive analysis of the physiological mechanisms coordinating parental and mating effort in an insect exhibiting biparental care. We show a hormone-mediated infertility in female burying beetles during the time the current offspring is needy and report that this temporary infertility is communicated via a pheromone to the male partner, where it inhibits copulation. A shared pathway of hormone and pheromone system ensures the reliability of the anti-aphrodisiac. Female infertility and male sexual abstinence provide for the concerted investment of parental resources into the existing developing young. Our study thus contributes to our deeper understanding of the mechanisms underlying adaptive parental decisions.

Parents are faced with the dilemma whether to invest in their current offspring, or potential future young. Here, Engel et al. show that nutritionally-dependent young induce temporary infertility in female burying beetles, which in turn is signalled to the male partner via a hormone-related anti-aphrodisiac.

Interspecific Interactions and the Scope for Parent-Offspring Conflict: High Mite Density Temporarily Changes the Trade-Off between Offspring Size and Number in the Burying Beetle, Nicrophorus vespilloides

Parents have a limited amount of resources to invest in reproduction and commonly trade-off how much they invest in offspring size (or quality) versus brood size. A negative relationship between offspring size and number has been shown in numerous taxa and it underpins evolutionary conflicts of interest between parents and their young. For example, previous work on vertebrates shows that selection favours mothers that produce more offspring, at the expense of individual offspring size, yet favours offspring that have relatively few siblings and therefore attain a greater size at independence. Here we analyse how this trade-off is temporarily affected by stochastic variation in the intensity of interspecific interactions. We examined the effect of the mite Poecilochirus carabi on the relationship between offspring size and number in the burying beetle, Nicrophorus vespilloides. We manipulated the initial number of mites in the reproductive event (by introducing either no mites, 4 mites, 10 mites, or 16 mites), and assessed the effect on the brood. We found a similar trade-off between offspring size and number in all treatments, except in the '16 mite' treatment where the correlation between offspring number and size flattened considerably. This effect arose because larvae in small broods failed to attain a high mass by dispersal. Our results show that variation in the intensity of interspecific interactions can temporarily change the strength of the trade-off between offspring size and number. In this study, high densities of mites prevented individual offspring from attaining their optimal weight, thus potentially temporarily biasing the outcome of parent-offspring conflict in favour of parents.

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.

Interspecific interactions explain variation in the duration of paternal care in the burying beetle

Why is there so much variation within species in the extent to which males contribute to offspring care? Answers to this question commonly focus on intraspecific sources of variation in the relative costs and benefits of supplying paternal investment. With experiments in the laboratory on the burying beetle, Nicrophorus vespilloides, and its phoretic mite Poecilochirus carabi, we investigated whether interactions with a second species might also account for intraspecific variation in the extent of paternal care, and whether this variation is due to adaptation or constraint. In our first experiment we bred beetles in the presence or absence of phoretic mites, using a breeding box that mimicked natural conditions by allowing parents to leave the breeding attempt at a time of their choosing. We found that males abandoned their brood sooner when breeding alongside mites than when breeding in their absence. Female patterns of care were unchanged by the mites. Nevertheless, in this experiment, no correlates of beetle fitness were affected by the presence of the mites during reproduction (neither paternal life span after reproduction nor brood size or average larval mass). In a second experiment, we again bred beetles with or without mites but this time we prevented parents from abandoning the brood. This time we found that both parents and the brood suffered fitness costs when breeding alongside mites, compared with families breeding in the absence of mites. We conclude that males adaptively reduce their contributions to care when mites are present, so as to defend their offspring's fitness and their own residual fitness. Interspecific interactions thus account for intraspecific variation in the duration of paternal care.

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The extent of paternal care varies greatly within species.

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Some of this variation might be due to interspecific interactions.

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We investigated how mites influence paternal care in the burying beetle.

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We found that males leave their brood earlier when mites are present.

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We show that this is adaptive because it enhances offspring fitness.

Parental effects and flight behaviour in the burying beetle, Nicrophorus vespilloides

Parents play a key role in determining the phenotype of their offspring. However, relatively few studies have investigated whether parents can change their offspring's behaviour in a sustained way that persists into adulthood. With experiments on the burying beetle, Nicrophorus vespilloides, we investigated how the developmental environment created by parents affects their offspring's wing morphology in adulthood, and the correlated effects on adult flight behaviour. Burying beetles exhibit complex biparental care, but offspring can survive without parental provisioning. By removing parents just prior to hatching, while holding the nutritional environment constant, we investigated the downstream consequences for offspring morphology and behaviour. Larvae that developed in the absence of their parents had relatively long and more slender wings than those that developed in their parents' presence. Flight mill tests revealed that flight performance was dependent on the presence of parents during development but not on wing shape. Our results demonstrate that parents have long-lasting effects on the behaviour of their offspring, by influencing the morphology and flight behaviour of their young even after they have matured into adults.

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We investigated parental influence on offspring's morphology and flight behaviour.

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Parental care quality affects wing shape allometries and flight performance.

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Wing shape and body size do not affect flight performances in a flight mill.

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Parental care quality affects offspring's morphology and behaviour into adulthood.

Parental effects alter the adaptive value of an adult behavioural trait

The parents' phenotype, or the environment they create for their young, can have long-lasting effects on their offspring, with profound evolutionary consequences. Yet, virtually no work has considered how such parental effects might change the adaptive value of behavioural traits expressed by offspring upon reaching adulthood. To address this problem, we combined experiments on burying beetles (Nicrophorus vespilloides) with theoretical modelling and focussed on one adult behavioural trait in particular: the supply of parental care. We manipulated the early-life environment and measured the fitness payoffs associated with the supply of parental care when larvae reached maturity. We found that (1) adults that received low levels of care as larvae were less successful at raising larger broods and suffered greater mortality as a result: they were low-quality parents. Furthermore, (2) high-quality males that raised offspring with low-quality females subsequently suffered greater mortality than brothers of equivalent quality, which reared larvae with higher quality females. Our analyses identify three general ways in which parental effects can change the adaptive value of an adult behavioural trait: by influencing the associated fitness benefits and costs; by consequently changing the evolutionary outcome of social interactions; and by modifying the evolutionarily stable expression of behavioural traits that are themselves parental effects.

DOI:http://dx.doi.org/10.7554/eLife.07340.001

The burying beetle is an unusual insect in that both the father and the mother take care of their young larvae. They do this by providing food in the form of a small dead animal, such as a mouse, from which they diligently remove any fur or feathers, and by defending both the food and the larvae from rivals. These actions reduce the fitness of the parents, which can be estimated by measuring by how long they survive after caring for their brood. They also increase the health of the larvae, as measured by how large the larvae are when they move away from the carcass to pupate.

Kilner et al. wanted to know how the parenting received by larvae affects their behaviour when they grow up and have their own offspring. Larvae were given varying amounts of care, ranging from none at all to five days (which is the typical length of the larval stage for burying beetles). Larvae that received little or no care grew up to become low-quality parents, whereas those that received lots of care became high-quality parents. A low-quality parent is, by definition, a parent that becomes less fit as a result of rearing offspring; a high-quality parent providing the same amount of care would not suffer such a large reduction in its fitness.

Each of the female beetles from this first experiment was then mated with a high-quality male and together they took care of their offspring. Kilner et al. observed that the fathers lived longer when they were paired with high-quality mothers than they did when they were paired with lower quality mothers. This happened because the lower quality mothers effectively exploited the fathers, forcing them to do more of the parenting. Although the males gained by raising healthy larvae, they paid a price by dying at a younger age.

Results from these insect experiments are not directly linked to human behaviour, but they might tell us why animals of other species are generally so careful to choose a mate that matches them in quality. In this way, they can avoid being exploited when the pair work together to raise young. In future, Kilner et al. will investigate how beetles adjust their parenting effort in response to the effort put in by their partner: can they estimate parental quality directly, or do they simply observe how much care the other partner is providing?

DOI:http://dx.doi.org/10.7554/eLife.07340.002

Male burying beetles extend, not reduce, parental care duration when reproductive competition is high

Male parents spend less time caring than females in many species with biparental care. The traditional explanation for this pattern is that males have lower confidence of parentage, so they desert earlier in favour of pursuing other mating opportunities. However, one recent alternative hypothesis is that prolonged male parental care might also evolve if staying to care actively improves paternity. If this is the case, an increase in reproductive competition should be associated with increased paternal care. To test this prediction, we manipulated the level of reproductive competition experienced by burying beetles, Nicrophorus vespilloides (Herbst, 1783). We found that caregiving males stayed for longer and mated more frequently with their partner when reproductive competition was greater. Reproductive productivity did not increase when males extended care. Our findings provide support for the increased paternity hypothesis. Extended duration of parental care may be a male tactic both protecting investment (in the current brood) and maximizing paternity (in subsequent brood(s) via female stored sperm) even if this fails to maximize current reproductive productivity and creates conflict of interest with their mate via costs associated with increased mating frequency.

Behavioral plasticity and G × E of reproductive tactics in Nicrophorus vespilloides burying beetles

Phenotypic plasticity is important in the evolution of traits and facilitates adaptation to rapid environmental changes. However, variation in plasticity at the individual level, and the heritable basis underlying this plasticity is rarely quantified for behavioral traits. Alternative behavioral reproductive tactics are key components of mating systems but are not often considered within a phenotypic plasticity framework (i.e., as reaction norms). Here, using lines artificially selected for repeated mating rate, we test for genetic (G × E) sources of variation in reproductive behavior of male Nicrophorus vespilloides burying beetles (including signaling behavior), as well as the role of individual body size, in responsiveness to changes in social environment. The results show that body size influences the response of individuals' signaling behavior to changes in the social environment. Moreover, there was G × E underlying the responses of males to variation in the quality of social environment experienced (relative size of focal male compared to his rival). This shows that individual variation in plasticity and social sensitivity of signaling behavior can evolve in response to selection on investment in mating behavior, with males selected for high mating investment having greater social sensitivity.

Effects of resource variation during early life and adult social environment on contest outcomes in burying beetles: a context-dependent silver spoon strategy?

Good early nutritional conditions may confer a lasting fitness advantage over individuals suffering poor early conditions (a ‘silver spoon’ effect). Alternatively, if early conditions predict the likely adult environment, adaptive plastic responses might maximize individual performance when developmental and adult conditions match (environmental-matching effect). Here, we test for silver spoon and environmental-matching effects by manipulating the early nutritional environment of Nicrophorus vespilloides burying beetles. We manipulated nutrition during two specific early developmental windows: the larval environment and the post-eclosion environment. We then tested contest success in relation to variation in adult social environmental quality experienced (defined according to whether contest opponents were smaller (good environment) or larger (poor environment) than the focal individual). Variation in the larval environment influenced adult body size but not contest success per se for a given adult social environment experienced (an ‘indirect’ silver spoon effect). Variation in post-eclosion environment affected contest success dependent on the quality of the adult environment experienced (a context-dependent ‘direct’ silver spoon effect). By contrast, there was no evidence for environmental-matching. The results demonstrate the importance of social environmental context in determining how variation in nutrition in early life affects success as an adult.

Correlated evolution in parental care in females but not males in response to selection on paternity assurance behaviour

According to classical parental care theory males are expected to provide less parental care when offspring in a brood are less likely to be their own, but empirical evidence in support of this relationship is equivocal. Recent work predicts that social interactions between the sexes can modify co-evolution between traits involved in mating and parental care as a result of costs associated with these social interactions (i.e. sexual conflict). In burying beetles (Nicrophorus vespilloides), we use artificial selection on a paternity assurance trait, and crosses within and between selection lines, to show that selection acting on females, not males, can drive the co-evolution of paternity assurance traits and parental care. Males do not care more in response to selection on mating rate. Instead, patterns of parental care change as an indirect response to costs of mating for females.