Biomechanical adaptations enable phoretic mite species to occupy distinct spatial niches on host burying beetles.

Niche theory predicts that ecologically similar species coexist by minimizing interspecific competition through niche partitioning. Therefore, understanding the mechanisms of niche partitioning is essential for predicting interactions and coexistence between competing organisms. Here, we study two phoretic mite species, Poecilochirus carabi and Macrocheles nataliae that coexist on the same host burying beetle Nicrophorus vespilloides and use it to 'hitchhike' between reproductive sites. Field observations revealed clear spatial partitioning between species in distinct host body parts. Poecilochirus carabi preferred the ventral side of the thorax, whereas M. nataliae were exclusively found ventrally at the hairy base of the abdomen. Experimental manipulations of mite density showed that each species preferred these body parts, largely regardless of the density of the other mite species on the host beetle. Force measurements indicated that this spatial distribution is mediated by biomechanical adaptations, because each mite species required more force to be removed from their preferred location on the beetle. While P. carabi attached with large adhesive pads to the smooth thorax cuticle, M. nataliae gripped abdominal setae with their chelicerae. Our results show that specialist biomechanical adaptations for attachment can mediate spatial niche partitioning among species sharing the same host.

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.

Evolutionary change in the construction of the nursery environment when parents are prevented from caring for their young directly.

Parental care can be partitioned into traits that involve direct engagement with offspring and traits that are expressed as an extended phenotype and influence the developmental environment, such as constructing a nursery. Here, we use experimental evolution to test whether parents can evolve modifications in nursery construction when they are experimentally prevented from supplying care directly to offspring. We exposed replicate experimental populations of burying beetles (Nicrophorus vespilloides) to different regimes of posthatching care by allowing larvae to develop in the presence (Full Care) or absence of parents (No Care). After only 13 generations of experimental evolution, we found an adaptive evolutionary increase in the pace at which parents in the No Care populations converted a dead body into a carrion nest for larvae. Cross-fostering experiments further revealed that No Care larvae performed better on a carrion nest prepared by No Care parents than did Full Care larvae. We conclude that parents construct the nursery environment in relation to their effectiveness at supplying care directly, after offspring are born. When direct care is prevented entirely, they evolve to make compensatory adjustments to the nursery in which their young will develop. The rapid evolutionary change observed in our experiments suggests there is considerable standing genetic variation for parental care traits in natural burying beetle populations-for reasons that remain unclear.

Parental care results in a greater mutation load, for which it is also a phenotypic antidote.

Benevolent social behaviours, such as parental care, are thought to enable mildly deleterious mutations to persist. We tested this prediction experimentally using the burying beetle Nicrophorus vespilloides, an insect with biparental care. For 20 generations, we allowed replicate experimental burying beetle populations to evolve either with post-hatching care ('Full Care' populations) or without it ('No Care' populations). We then established new lineages, seeded from these experimental populations, which we inbred to assess their mutation load. Outbred lineages served as controls. We also tested whether the deleterious effects of a greater mutation load could be concealed by parental care by allowing half the lineages to receive post-hatching care, while half did not. We found that inbred lineages from the Full Care populations went extinct more quickly than inbred lineages from the No Care populations-but only when offspring received no post-hatching care. We infer that Full Care lineages carried a greater mutation load, but that the associated deleterious effects on fitness could be overcome if larvae received parental care. We suggest that the increased mutation load caused by parental care increases a population's dependence upon care. This could explain why care is seldom lost once it has evolved.

An evolutionary switch from sibling rivalry to sibling cooperation, caused by a sustained loss of parental care.

Sibling rivalry is commonplace within animal families, yet offspring can also work together to promote each other's fitness. Here we show that the extent of parental care can determine whether siblings evolve to compete or to cooperate. Our experiments focus on the burying beetle Nicrophorus vespilloides, which naturally provides variable levels of care to its larvae. We evolved replicate populations of burying beetles under two different regimes of parental care: Some populations were allowed to supply posthatching care to their young (Full Care), while others were not (No Care). After 22 generations of experimental evolution, we found that No Care larvae had evolved to be more cooperative, whereas Full Care larvae were more competitive. Greater levels of cooperation among larvae compensated for the fitness costs caused by parental absence, whereas parental care fully compensated for the fitness costs of sibling rivalry. We dissected the evolutionary mechanisms underlying these responses by measuring indirect genetic effects (IGEs) that occur when different sibling social environments induce the expression of more cooperative (or more competitive) behavior in focal larvae. We found that indirect genetic effects create a tipping point in the evolution of larval social behavior. Once the majority of offspring in a brood start to express cooperative (or competitive) behavior, they induce greater levels of cooperation (or competition) in their siblings. The resulting positive feedback loops rapidly lock larvae into evolving greater levels of cooperation in the absence of parental care and greater levels of rivalry when parents provide care.

Niche construction through a Goldilocks principle maximizes fitness for a nest-sharing brood parasite.

Generalist brood parasites that share nests with host nestlings can optimize resource acquisition from host parents by balancing the benefits that host nest-mates provide, including attracting increased provisions to the nest, against the costs of competing with the same host young over foster parental resources. However, it is unclear how parasitic chicks cope when faced with more nest-mates than are optimal for their survival upon hatching. We suggest that, in the obligate brood parasitic brown-headed cowbird (Molothrus ater), chicks use a niche construction strategy and reduce larger, more competitive host broods to maximize the parasites' survival to fledging. We experimentally altered brood sizes to test for Goldilocks principle patterns (i.e. a 'just right' intermediate brood size) of cowbird survival in nests of prothonotary warbler (Protonotaria citrea) hosts. We found that intermediate brood sizes of two host nestlings maximized cowbird fledging success relative to 0 or 4 host nest-mates at hatching. Specifically, cowbird nestlings lowered host brood sizes towards this optimum when placed in broods with more host nestlings. The results suggest that cowbirds reduce, but do not eliminate, host broods as a niche construction mechanism to improve their own probability of survival.

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.

A weapons-testes trade-off in males is amplified in female traits.

Sexually selected weapons are assumed to trade off with traits related to ejaculates, such as testes. However, remarkably little is known about what governs resource allocation and why trade-offs are found in some cases and not others. Often-used models depict competitive allocation occurring within the functional grouping of traits (e.g. reproduction); however, other factors including tissue expense and developmental timing may influence allocation. Experimental comparisons of investment across the sexes have the potential to illuminate allocation rules, because the sexes do not always use traits for the same functions. Here, we capitalize upon a species where females have weapons-testes homologues. We report that a documented trade-off in investment between hind-limb weapons and testes in leaf-footed cactus bugs, Narnia femorata, is even more pronounced in female hind limbs and ovaries. Female hind limbs in this species do not share the clear reproductive function of male hind limbs; therefore, this trade-off spans trait functional groups. Such patterns of investment suggest that future studies of reproductive trade-offs should consider factors such as tissue expense and developmental timing.

Indole: An evolutionarily conserved influencer of behavior across kingdoms.

Indole is a key environmental cue that is used by many organisms. Based on its biochemistry, we suggest indole is used so universally, and by such different organisms, because it derives from the metabolism of tryptophan, a resource essential for many species yet rare in nature. These properties make it a valuable, environmental cue for resources almost universally important for promoting fitness. We then describe how indole is used to coordinate actions within organisms, to influence the behavior of conspecifics and can even be used to change the behavior of species that belong to other kingdoms. Drawing on the evolutionary framework that has been developed for understanding animal communication, we show how this is diversely achieved by indole acting as a cue, a manipulative signal, and an honest signal, as well as how indole can be used synergistically to amplify information conveyed by other molecules. Clarifying these distinct functions of indole identifies patterns that transcend different kingdoms of organisms.

Adaptive evolution of synchronous egg-hatching in compensation for the loss of parental care.

Interactions among siblings are finely balanced between rivalry and cooperation, but the factors that tip the balance towards cooperation are incompletely understood. Previous observations of insect species suggest that (i) sibling cooperation is more likely when siblings hatch at the same time, and (ii) this is more common when parents provide little to no care. In this paper, we tested these ideas experimentally with the burying beetle, Nicrophorus vespilloides Burying beetles convert the body of a small dead vertebrate into an edible nest for their larvae, and provision and guard their young after hatching. In our first experiment, we simulated synchronous or asynchronous hatching by adding larvae at different intervals to the carrion-breeding resource. We found that 'synchronously' hatched broods survived better than 'asynchronously' hatched broods, probably because 'synchronous hatching' generated larger teams of larvae, that together worked more effectively to penetrate the carrion nest and feed upon it. In our second experiment, we measured the synchronicity of hatching in experimental populations that had evolved for 22 generations without any post-hatching care, and control populations that had evolved in parallel with post-hatching care. We found that larvae were more likely to hatch earlier, and at the same time as their broodmates, in the experimental populations that evolved without post-hatching care. We suggest that synchronous hatching enables offspring to help each other when parents are not present to provide care. However, we also suggest that greater levels of cooperation among siblings cannot compensate fully for the loss of parental care.

Strategies for managing rival bacterial communities: Lessons from burying beetles.

The role of bacteria in animal development, ecology and evolution is increasingly well understood, yet little is known of how animal behaviour affects bacterial communities. Animals that benefit from defending a key resource from microbial competitors are likely to evolve behaviours to control or manipulate the animal's associated external microbiota. We describe four possible mechanisms by which animals could gain a competitive edge by disrupting a rival bacterial community: "weeding," "seeding," "replanting" and "preserving." By combining detailed behavioural observations with molecular and bioinformatic analyses, we then test which of these mechanisms best explains how burying beetles, Nicrophorus vespilloides, manipulate the bacterial communities on their carcass breeding resource. Burying beetles are a suitable species to study how animals manage external microbiota because reproduction revolves around a small vertebrate carcass. Parents shave a carcass and apply antimicrobial exudates on its surface, shaping it into an edible nest for their offspring. We compared bacterial communities in mice carcasses that were either fresh, prepared by beetles or unprepared but buried underground for the same length of time. We also analysed bacterial communities in the burying beetle's gut, during and after breeding, to understand whether beetles could be "seeding" the carcass with particular microbes. We show that burying beetles do not "preserve" the carcass by reducing bacterial load, as is commonly supposed. Instead, our results suggest they "seed" the carcass with bacterial groups which are part of the Nicrophorus core microbiome. They may also "replant" other bacteria from the carcass gut onto the surface of their carrion nest. Both these processes may lead to the observed increase in bacterial load on the carcass surface in the presence of beetles. Beetles may also "weed" the bacterial community by eliminating some groups of bacteria on the carcass, perhaps through the production of antimicrobials themselves. Whether these alterations to the bacterial community are adaptive from the beetle's perspective, or are simply a by-product of the way in which the beetles prepare the carcass for reproduction, remains to be determined in future work. In general, our work suggests that animals might use more sophisticated techniques for attacking and disrupting rival microbial communities than is currently appreciated.

Adaptation to a novel family environment involves both apparent and cryptic phenotypic changes.

Cryptic evolution occurs when evolutionary change is masked by concurrent environmental change. In most cases, evolutionary changes in the phenotype are masked by changing abiotic factors. However, evolutionary change in one trait might also be masked by evolutionary change in another trait, a phenomenon referred to as evolutionary environmental deterioration. Nevertheless, detecting this second type of cryptic evolution is challenging and there are few compelling examples. Here, we describe a likely case of evolutionary environmental deterioration occurring in experimental burying beetle (Nicrophorus vespilloides) populations that are adapting to a novel social environment that lacks post-hatching parental care. We found that populations rapidly adapted to the removal of post-hatching parental care. This adaptation involved clear increases in breeding success and larval density (number of dispersing larvae produced per gram of breeding carcass), which in turn masked a concurrent increase in the mean larval mass across generations. This cryptic increase in larval mass was accomplished through a change in the reaction norm that relates mean larval mass to larval density. Our results suggest that cryptic evolution might be commonplace in animal families, because evolving trophic and social interactions can potentially mask evolutionary change in other traits, like body size.

Using Experimental Evolution to Study Adaptations for Life within the Family.

Parents of many species provision their young, and the extent of parental provisioning constitutes a major component of the offspring's social environment. Thus, a change in parental provisioning can alter selection on offspring, resulting in the coevolution of parental and offspring traits. Although this reasoning is central to our evolutionary understanding of family life, there is little direct evidence that selection by parents causes evolutionary change in their offspring. Here we use experimental evolution to examine how populations of burying beetles adapt to a change in posthatching parental provisioning. We measured the performance of larvae descended from lab populations that had been maintained with and without posthatching parental care (Full Care and No Care populations). We found that adaptation to the absence of posthatching care led to rapid and consistent changes in larval survival in the absence of care. Specifically, larvae from No Care populations had higher survival in the absence of care than larvae from Full Care populations. Other measures of larval performance, such as the ability of larvae to consume a breeding carcass and larval mass at dispersal, did not differ between the Full Care and No Care populations. Nevertheless, our results show that populations can adapt rapidly to a change in the extent of parental care and that experimental evolution can be used to study such adaptation.

Behaviorally Induced Camouflage: A New Mechanism of Avian Egg Protection.

When animals potentially occupy diverse microhabitats, how can camouflage be achieved? Here we combine descriptive and experimental methods to uncover a novel form of phenotypic plasticity in the camouflage of bird eggs that may be present in other avian taxa. Soil from the bare substrate adheres to the blue-footed booby's (Sula nebouxii's) pale eggs, which parents manipulate both under and on top of their webs. Analysis of digital images confirmed that dirtiness increases progressively during the first 16 days of the incubation period, making eggs more similar to the nest substrate. Observations of 3,668 single-egg clutches showed that the probability of egg loss declines progressively over the same time frame and then remains low for the rest of the 41-day incubation period. An experiment showed that when chicken eggs are soiled and exposed in artificial booby nests, they are less likely to be taken by Heermann's gulls (Larus heermanni) than clean eggs.

Friend or foe: inter-specific interactions and conflicts of interest within the family.

Interactions between species can vary from mutually beneficial to evolutionarily neutral to antagonistic, even when the same two species are involved. Similarly, social interactions between members of the same species can lie on a spectrum from conflict to cooperation.The aim of the present study was to investigate whether variation in the two types of social behaviour are interconnected. Is the fitness of the various classes of social partner within species (such as parent and offspring, or male and female) differently affected by interactions with a second species? Moreover, can inter-specific interactions influence the outcome of social interactions within species?The present experiments focus on the interactions between the burying beetle Nicrophorus vespilloides Herbst and the phoretic mite Poecilochirus carabi G. Canestrini & R. Canestrini. The approach was to measure the fitness of burying beetle mothers, fathers, and offspring after reproduction, which took place either in the presence or absence of mites.We found that male, female, and larval burying beetles derive contrasting fitness costs and benefits from their interactions with the mite, despite sharing a common family environment. From the mite's perspective, its relationship with the burying beetle can, therefore, be simultaneously antagonistic, neutral, and possibly even mutualistic, depending on the particular family member involved. We also found that mites can potentially change the outcome of evolutionary conflicts within the family.We conclude that inter-specific interactions can explain some of the variation in social interactions seen within species. It is further suggested that intra-specific interactions might contribute to variation in the outcome of interactions between species.

Competition among host-specific lineages of Poecilochirus carabi mites influences the extent of co-adaptation with their Nicrophorus vespilloides burying beetle hosts.

Reciprocal selection between symbiotic organisms and their hosts can generate variations in local adaptation between them. Symbionts often form species complexes with lineages partially adapted to various hosts. However, it is unclear how interactions among these lineages influences geographic variation in the extent of host-symbiont local adaptation. We addressed this shortcoming with experiments on burying beetles Nicrophorus vespilloides and their specialist phoretic mite Poecilochirus carabi in two adjacent woodlands. Burying beetles transport these mites to vertebrate carrion upon which they both reproduce. P. carabi appears to be a species complex, with distinct lineages that specialise on breeding alongside different Nicrophorus species. We found that in one wood (Gamlingay Woods), N. vespilloides carries a mixture of mite lineages, with each lineage corresponding to one of the four Nicrophorus species that inhabits this wood. However, two burying beetle species coexist in neighbouring Waresley Woods and here N. vespilloides predominantly carries the mite lineage that favours N. vespilloides. Mite lineage mixing alters the degree of local adaptation for both N. vespilloides and the P. carabi mites, affecting reproductive success variably across different woodlands. In Gamlingay, mite lineage mixing reduced N. vespilloides reproductive success, while experimentally purifying mites lineage enhanced it. The near pure lineage of vespilloides mites negligibly affected Waresley N. vespilloides. Mite reproductive success varied with host specificity: Gamlingay mites had greatest reproductive success on Gamlingay beetles, and performed less well with Waresley beetles. By contrast, Waresley mites had consistent reproductive success, regardless of beetle's woodland of origin. We conclude that there is some evidence that N. vespilloides and its specific mite lineage have coadapted. However, neither N. vespilloides nor its mite lineage adapted to breed alongside other mite lineages. This, we suggest, causes variation between Waresley and Gaminglay Woods in the extent of local adaptation between N. vespilloides beetles and their P. carabi mites.

A window on the past: male ornamental plumage reveals the quality of their early-life environment.

It is well established that the expression of many ornamental traits is dependent on the current condition of the bearer. However, conditions experienced in early life are also known to be important for an individual's subsequent fitness and therefore, directly or indirectly, for the fitness of their mate. Specifically, a recent hypothesis suggests that sexually selected traits might be sensitive to conditions experienced during early-life development and thereby function as honest indicators of developmental history. Whether this applies to colourful male plumage, however, is largely unknown. We tested this idea with a field experiment by manipulating neonatal nutrition in a sexually dichromatic passerine, the hihi (Notymystis cincta). We found that carotenoid supplementation increased nestling plasma carotenoid concentration, which was in turn correlated with increased yellow saturation in male breeding plumage after moulting. We also found that the post-moult luminance (lightness) of the white ear-tufts tended to be reduced in males that had received an all-round nutritional supplement as nestlings. Black breeding plumage was not affected by neonatal nutritional treatment. Although the mechanisms that generate colourful plumage are evidently diverse, our results show that at least some parts of this display are accurate indicators of environmental conditions during development.

A direct physiological trade-off between personal and social immunity.

Recent work shows that organisms possess two strategies of immune response: personal immunity, which defends an individual, and social immunity, which protects other individuals, such as kin. However, it is unclear how individuals divide their limited resources between protecting themselves and protecting others. Here, with experiments on female burying beetles, we challenged the personal immune system and measured subsequent investment in social immunity (antibacterial activity of the anal exudates). Our results show that increased investment in one aspect of personal immunity (wound repair) causes a temporary decrease in one aspect of the social immune response. Our experiments further show that by balancing investment in personal and social immunity in this way during one breeding attempt, females are able to defend their subsequent lifetime reproductive success. We discuss the nature of the physiological trade-off between personal and social immunity in species that differ in the degree of eusociality and coloniality, and suggest that it may also vary within species in relation to age and partner contributions to social immunity.

Parental care shapes the evolution of molecular genetic variation.

Cooperative social behaviors, such as parental care, have long been hypothesized to relax selection leading to the accumulation of genetic variation in populations. Although the idea has been discussed for decades, there has been relatively little experimental work to investigate how social behavior contributes to genetic variation in populations. Here, we investigate how parental care can shape molecular genetic variation in the subsocial insect, Nicrophorus vespilloides. Using whole-genome sequencing of populations that had evolved in the presence or absence of parental care for 30 generations, we show that parental care maintains levels of standing genetic variation. In contrast, under a harsh environment without care, strong directional selection caused a reduction in genetic variation. Furthermore, we show that adaptation to the loss of care is associated with genetic divergence between populations at loci related to stress, morphological development, and transcriptional regulation. These data reveal how social behavior is linked to the genetic processes that shape and maintain genetic diversity within populations, and provides rare empirical evidence for an old hypothesis.

The evolutionary demise of a social interaction: experimentally induced loss of traits involved in the supply and demand of care.

Phenotypic plasticity enables animals to adjust their behavior flexibly to their social environment-sometimes through the expression of adaptive traits that have not been exhibited for several generations. We investigated how long social adaptations can usefully persist when they are not routinely expressed, by using experimental evolution to document the loss of social traits associated with the supply and demand of parental care. We allowed populations of burying beetles Nicrophorus vespilloides to evolve in two different social environments for 48 generations in the lab. In "Full Care" populations, traits associated with the supply and demand of parental care were expressed at every generation, whereas in "No Care" populations we prevented expression of these traits experimentally. We then revived trait expression in the No Care populations at generations 24, 43, and 48 by allowing parents to supply post-hatching care and compared these social traits with those expressed by the Full Care populations. We found that offspring demands for care and male provision of care in the No Care populations were lost sooner than female provision of care. We suggest that this reflects differences in the strength of selection for the expression of alternative traits in offspring, males and females, which can enhance fitness when post-hatching care is disrupted.