Reproductive control via eviction (but not the threat of eviction) in banded mongooses

Group augmentation underlies the evolution of complex sociality in the face of environmental instability

Although kin selection is assumed to underlie the evolution of sociality, many vertebrates-including nearly half of all cooperatively breeding birds-form groups that also include unrelated individuals. Theory predicts that despite reducing kin structure, immigration of unrelated individuals into groups can provide direct, group augmentation benefits, particularly when offspring recruitment is insufficient for group persistence. Using population dynamic modeling and analysis of long-term data, we provide clear empirical evidence of group augmentation benefits favoring the evolution and maintenance of complex societies with low kin structure and multiple reproductives. We show that in the superb starling (Lamprotornis superbus)-a plural cooperative breeder that forms large groups with multiple breeding pairs, and related and unrelated nonbreeders of both sexes-offspring recruitment alone cannot prevent group extinction, especially in smaller groups. Further, smaller groups, which stand to benefit more from immigration, exhibit lower reproductive skew for immigrants, suggesting that reproductive opportunities as joining incentives lead to plural breeding. Yet, despite a greater likelihood of becoming a breeder in smaller groups, immigrants are more likely to join larger groups where they experience increased survivorship and greater reproductive success as breeders. Moreover, immigrants form additional breeding pairs, increasing future offspring recruitment into the group and guarding against complete reproductive failure in the face of environmental instability and high nest predation. Thus, plural breeding likely evolves because the benefits of group augmentation by immigrants generate a positive feedback loop that maintains societies with low and mixed kinship, large group sizes, and multiple reproductives.

Urban landscapes increase dispersal, gene flow, and pathogen transmission potential in banded mongoose (Mungos mungo) in northern Botswana

Disease transmission can be strongly influenced by the manner in which conspecifics are connected across a landscape and the effects of land use upon these dynamics. In northern Botswana, the territorial and group-living banded mongoose (Mungos mungo) lives across urban and natural landscapes and is infected with a novel Mycobacterium tuberculosis complex pathogen, M. mungi. Using microsatellite markers amplified from DNA derived from banded mongoose fecal and tissue samples (n = 168), we evaluated population genetic structure, individual dispersal, and gene flow for 12 troops. Genetic structure was detectable and moderately strong across groups (F ST = 0.086), with K = 7 being the best-supported number of genetic clusters. Indications of admixture in certain troops suggest formation of new groups through recent fusion events. Differentiation was higher for troops inhabiting natural areas (F ST = 0.102) than for troops in urban landscapes (F ST = 0.081). While this suggests increased levels of gene flow between urban-dwelling troops, the inclusion of a smaller number of study troops from natural land types may have influenced these findings. Of those individuals confirmed infected with M. mungi, the majority (73%, n = 11) were assigned to their natal group which is consistent with previous observations linking lower levels of dispersal with infection. Twenty-one probable dispersing individuals were identified, with all suspected migrants originating from troops within the urban landscape. Findings suggest that urbanized landscapes may increase gene flow and dispersal behavior with a concomitant increase in the risk of pathogen spread. As urban landscapes expand, there is an increasing need to understand how land use and pathogen infection may change wildlife behavior and disease transmission potential.

A veil of ignorance can promote fairness in a mammal society

Rawls argued that fairness in human societies can be achieved if decisions about the distribution of societal rewards are made from behind a veil of ignorance, which obscures the personal gains that result. Whether ignorance promotes fairness in animal societies, that is, the distribution of resources to reduce inequality, is unknown. Here we show experimentally that cooperatively breeding banded mongooses, acting from behind a veil of ignorance over kinship, allocate postnatal care in a way that reduces inequality among offspring, in the manner predicted by a Rawlsian model of cooperation. In this society synchronized reproduction leaves adults in a group ignorant of the individual parentage of their communal young. We provisioned half of the mothers in each mongoose group during pregnancy, leaving the other half as matched controls, thus increasing inequality among mothers and increasing the amount of variation in offspring birth weight in communal litters. After birth, fed mothers provided extra care to the offspring of unfed mothers, not their own young, which levelled up initial size inequalities among the offspring and equalized their survival to adulthood. Our findings suggest that a classic idea of moral philosophy also applies to the evolution of cooperation in biological systems.

Obscuring knowledge of personal gains from individuals can theoretically maintain fairness in a cooperative group. Experiments show that wild, cooperatively breeding banded mongooses uncertain of kinship allocate postnatal care in a way that reduces inequality among offspring, suggesting a classic idea of moral philosophy can apply in biological systems.

Network-level consequences of outgroup threats in banded mongooses: Grooming and aggression between the sexes

Animal groups are heterogeneous assemblages of individuals with differing fitness interests, which may lead to internal conflict over investment in group territorial defence. Differences between individuals may lead to different behavioural responses to intergroup conflict, particularly between the sexes. These potential impacts have been little studied. We used social network analysis to investigate the impact of simulated intergroup conflicts on social relationships in groups of wild banded mongooses Mungos mungo, in which intergroup fights are more costly for males than females. We predicted that social cohesion (specifically male-to-male and female-to-male grooming) would increase after conflict, and aggression would decrease, to minimize conflict between the sexes. Simulated intergroup conflicts were performed by exposing banded mongoose groups to scents, 'war cry' playbacks, and live intruders from a rival group. All grooming and aggression interactions between individuals were recorded, and grooming and aggression social networks were created for the 2 days preceding a simulated intergroup conflict (pre-conflict network) and the 2 days after (post-conflict network). We found no evidence of an increase in social cohesion after simulated conflicts, measured as grooming eigenvector centrality. Male-to-male, male-to-female and female-to-male grooming strength decreased after simulated intrusions compared to female-to-female grooming strength. However, male-female aggression decreased in intrusion trials compared to other interaction types, consistent with the hypothesis that intergroup encounters reduce the level of intragroup conflict between males and females. Males were more affected socially by intergroup encounters than females, which may be because they are investing in defence rather than internal relationships. Focusing on individual relationship changes, using social network analysis, can reveal changes in the directionality of behaviour in response to intergroup encounters, and highlight how individual responses to conflict may scale up to affect social networks and, potentially, group performance. This study highlights the importance of studying both group-level behaviours and individual relationships to more fully understand responses to intergroup encounters.

Ecological and social constraints combine to promote evolution of non-breeding strategies in clownfish

Individuals that forgo their own reproduction in animal societies represent an evolutionary paradox because it is not immediately apparent how natural selection can preserve the genes that underlie non-breeding strategies. Cooperative breeding theory provides a solution to the paradox: non-breeders benefit by helping relatives and/or inheriting breeding positions; non-breeders do not disperse to breed elsewhere because of ecological constraints. However, the question of why non-breeders do not contest to breed within their group has rarely been addressed. Here, we use a wild population of clownfish (Amphiprion percula), where non-breeders wait peacefully for years to inherit breeding positions, to show non-breeders will disperse when ecological constraints (risk of mortality during dispersal) are experimentally weakened. In addition, we show non-breeders will contest when social constraints (risk of eviction during contest) are experimentally relaxed. Our results show it is the combination of ecological and social constraints that promote the evolution of non-breeding strategies. The findings highlight parallels between, and potential for fruitful exchange between, cooperative breeding theory and economic bargaining theory: individuals will forgo their own reproduction and wait peacefully to inherit breeding positions (engage in cooperative options) when there are harsh ecological constraints (poor outside options) and harsh social constraints (poor inside options).

Early-life manipulation of cortisol and its receptor alters stress axis programming and social competence

In many vertebrate species, early social experience generates long-term effects on later life social behaviour. These effects are accompanied by persistent modifications in the expression of genes implicated in the stress axis. It is unknown, however, whether stress axis programming can affect the development of social competence, and if so, by which mechanism(s). Here, we used pharmacological manipulations to persistently reprogramme the hypothalamic-pituitary-interrenal axis of juvenile cooperatively breeding cichlids, Neolamprologus pulcher. During the first two months of life, juveniles were repeatedly treated with cortisol, mifepristone or control treatments. Three months after the last manipulation, we tested for treatment effects on (i) social competence, (ii) the expression of genes coding for corticotropin-releasing factor ( crf), glucocorticoid receptor ( gr1) and mineralocorticoid receptor ( mr) in the telencephalon and hypothalamus and (iii) cortisol levels. Social competence in a social challenge was reduced in cortisol-treated juveniles, which is in accordance with previous work applying early-life manipulations using different social experiences. During early life, both cortisol and mifepristone treatments induced a persistent downregulation of crf and upregulation of mr in the telencephalon. We suggest that these persistent changes in stress gene expression may represent an effective physiological mechanism for coping with stress. This article is part of the theme issue 'Developing differences: early-life effects and evolutionary medicine'.

Resource heterogeneity and the evolution of public goods cooperation

Heterogeneity in resources is a ubiquitous feature of natural landscapes affecting many aspects of biology. However, the effect of environmental heterogeneity on the evolution of cooperation has been less well studied. Here, using a mixture of theory and experiments measuring siderophore production by the bacterium Pseudomonas aeruginosa as a model for public goods based cooperation, we explore the effect of heterogeneity in resource availability. We show that cooperation in metapopulations that were spatially heterogeneous in terms of resources can be maintained at a higher level than in homogeneous metapopulations of the same average resource value. The results can be explained by a positive covariance between fitness of cooperators, population size, and local resource availability, which allowed cooperators to have a disproportionate advantage within the heterogeneous metapopulations. These results suggest that natural environmental variation may help to maintain cooperation.

Spontaneous abortion as a response to reproductive conflict in the banded mongoose

When breeding females compete for limited resources, the intensity of this reproductive conflict can determine whether the fitness benefits of current reproductive effort exceed the potential costs to survival and future fertility. In group-living species, reproductive competition can occur through post-natal competition among the offspring of co-breeding females. Spontaneous abortion could be a response to such competition, allowing females to curtail reproductive expenditure on offspring that are unlikely to survive and to conserve resources for future breeding opportunities. We tested this hypothesis using long-term data on banded mongooses, Mungos mungo, in which multiple females within a group give birth synchronously to a communal litter that is cared for by other group members. As predicted, abortions were more likely during dry periods when food is scarce, and in breeding attempts with more intense reproductive competition. Within breeding events, younger, lighter females carrying smaller fetuses were more likely to abort, particularly those that were also of lower rank. Our results suggest that abortion may be a means by which disadvantaged females conserve resources for future breeding attempts in more benign conditions, and highlight that female reproductive competition may be resolved long before the production of offspring.

Live long and prosper: durable benefits of early-life care in banded mongooses

Kin selection theory defines the conditions for which altruism or 'helping' can be favoured by natural selection. Tests of this theory in cooperatively breeding animals have focused on the short-term benefits to the recipients of help, such as improved growth or survival to adulthood. However, research on early-life effects suggests that there may be more durable, lifelong fitness impacts to the recipients of help, which in theory should strengthen selection for helping. Here, we show in cooperatively breeding banded mongooses ( Mungos mungo) that care received in the first 3 months of life has lifelong fitness benefits for both male and female recipients. In this species, adult helpers called 'escorts' form exclusive one-to-one caring relationships with specific pups (not their own offspring), allowing us to isolate the effects of being escorted on later reproduction and survival. Pups that were more closely escorted were heavier at sexual maturity, which was associated with higher lifetime reproductive success for both sexes. Moreover, for female offspring, lifetime reproductive success increased with the level of escorting received per se, over and above any effect on body mass. Our results suggest that early-life social care has durable benefits to offspring of both sexes in this species. Given the well-established developmental effects of early-life care in laboratory animals and humans, we suggest that similar effects are likely to be widespread in social animals more generally. We discuss some of the implications of durable fitness benefits for the evolution of intergenerational helping in cooperative animal societies, including humans. This article is part of the theme issue 'Developing differences: early-life effects and evolutionary medicine'.

Reproductive control via the threat of eviction in the clown anemonefish

In social groups, high reproductive skew is predicted to arise when the reproductive output of a group is limited, and dominant individuals can suppress subordinate reproductive efforts. Reproductive suppression is often assumed to occur via overt aggression or the threat of eviction. It is unclear, however, whether the threat of eviction alone is sufficient to induce reproductive restraint by subordinates. Here, we test two assumptions of the restraint model of reproductive skew by investigating whether resource limitation generates reproductive competition and whether the threat of eviction leads to reproductive restraint in the clown anemonefish Amphiprion percula First, we use a feeding experiment to test whether reproduction is resource limited, which would create an incentive for the dominant pair to suppress subordinate reproduction. We show that the number of eggs laid increased in the population over the study period, but the per cent increase in fed groups was more than twice that in unfed groups (205% and 78%, respectively). Second, we use an eviction experiment to test whether the dominant pair evicts mature subordinates, which would create an incentive for the subordinates to forgo reproduction. We show that mature subordinates are seven times more likely to be evicted than immature subordinates of the same size. In summary, we provide experimental support for the assumptions of the restraint model by showing that resource limitation creates reproductive competition and a credible threat of eviction helps explain why subordinates forego reproduction. Transactional models of reproductive skew may apply well to this and other simple systems.

Sex-biased dispersal: a review of the theory

Dispersal is ubiquitous throughout the tree of life: factors selecting for dispersal include kin competition, inbreeding avoidance and spatiotemporal variation in resources or habitat suitability. These factors differ in whether they promote male and female dispersal equally strongly, and often selection on dispersal of one sex depends on how much the other disperses. For example, for inbreeding avoidance it can be sufficient that one sex disperses away from the natal site. Attempts to understand sex‐specific dispersal evolution have created a rich body of theoretical literature, which we review here. We highlight an interesting gap between empirical and theoretical literature. The former associates different patterns of sex‐biased dispersal with mating systems, such as female‐biased dispersal in monogamous birds and male‐biased dispersal in polygynous mammals. The predominant explanation is traceable back to Greenwood's (1980) ideas of how successful philopatric or dispersing individuals are at gaining mates or the resources required to attract them. Theory, however, has developed surprisingly independently of these ideas: models typically track how immigration and emigration change relatedness patterns and alter competition for limiting resources. The limiting resources are often considered sexually distinct, with breeding sites and fertilizable females limiting reproductive success for females and males, respectively. We show that the link between mating system and sex‐biased dispersal is far from resolved: there are studies showing that mating systems matter, but the oft‐stated association between polygyny and male‐biased dispersal is not a straightforward theoretical expectation. Here, an important understudied factor is the extent to which movement is interpretable as an extension of mate‐searching (e.g. are matings possible en route or do they only happen after settling in new habitat – or can females perhaps move with stored sperm). We also point out other new directions for bridging the gap between empirical and theoretical studies: there is a need to build Greenwood's influential yet verbal explanation into formal models, which also includes the possibility that an individual benefits from mobility as it leads to fitness gains in more than one final breeding location (a possibility not present in models with a very rigid deme structure). The order of life‐cycle events is likewise important, as this impacts whether a departing individual leaves behind important resources for its female or male kin, or perhaps both, in the case of partially overlapping resource use.

Subordinate females in the cooperatively breeding Seychelles warbler obtain direct benefits by joining unrelated groups

In many cooperatively breeding animals, a combination of ecological constraints and benefits of philopatry favours offspring taking a subordinate position on the natal territory instead of dispersing to breed independently. However, in many species individuals disperse to a subordinate position in a non-natal group ("subordinate between-group" dispersal), despite losing the kin-selected and nepotistic benefits of remaining in the natal group. It is unclear which social, genetic and ecological factors drive between-group dispersal. We aim to elucidate the adaptive significance of subordinate between-group dispersal by examining which factors promote such dispersal, whether subordinates gain improved ecological and social conditions by joining a non-natal group, and whether between-group dispersal results in increased lifetime reproductive success and survival. Using a long-term dataset on the cooperatively breeding Seychelles warbler (Acrocephalus sechellensis), we investigated how a suite of proximate factors (food availability, group composition, age and sex of focal individuals, population density) promote subordinate between-group dispersal by comparing such dispersers with subordinates that dispersed to a dominant position or became floaters. We then analysed whether subordinates that moved to a dominant or non-natal subordinate position, or became floaters, gained improved conditions relative to the natal territory and compared fitness components between the three dispersal strategies. We show that individuals that joined another group as non-natal subordinates were mainly female and that, similar to floating, between-group dispersal was associated with social and demographic factors that constrained dispersal to an independent breeding position. Between-group dispersal was not driven by improved ecological or social conditions in the new territory and did not result in higher survival. Instead, between-group dispersing females often became cobreeders, obtaining maternity in the new territory, and were likely to inherit the territory in the future, leading to higher lifetime reproductive success compared to females that floated. Males never reproduced as subordinates, which may be one explanation why subordinate between-group dispersal by males is rare. Our results suggest that subordinate between-group dispersal is used by females to obtain reproductive benefits when options to disperse to an independent breeding position are limited. This provides important insight into the additional strategies that individuals can use to obtain reproductive benefits.

The great Indian joint families of free-ranging dogs

Cooperative breeding is an excellent example of cooperation in social groups. Domestic dogs have evolved from cooperatively hunting and breeding ancestors but have adapted to a facultatively social scavenging lifestyle on streets, and solitary living in human homes. Pets typically breed and reproduce under human supervision, but free-ranging dogs can provide insights into the natural breeding ecology of dogs. We conducted a five year-long field based behavioural study on parental care of free-ranging dogs in India. 23 mother-litter units, belonging to 15 groups were observed, which revealed the presence of widespread allo-parenting by both adult males and females. While all the females were known to be related to the pups receiving care, the relatedness with the males could not be determined. Hence, we coined the term "putative father" for caregiving males. Allomothers provided significantly less care than the mothers, but the putative fathers showed comparable levels of care with the mothers. Mothers invested more effort in nursing and allogrooming, while the putative fathers played and protected more. Our observations provide support for both the "benefit-of-philopatry" and "assured fitness returns" hypotheses. Free-ranging dogs are not cooperative breeders like wolves but are rather communal breeders; their breeding biology bearing interesting similarities with the human joint family system. This breeding strategy is likely to have played an important role in increasing pup survival in a stochastic environment and helping to adapt to living among humans during the domestication of dogs.

Intragroup competition predicts individual foraging specialisation in a group-living mammal

Individual foraging specialisation has important ecological implications, but its causes in group-living species are unclear. One of the major consequences of group living is increased intragroup competition for resources. Foraging theory predicts that with increased competition, individuals should add new prey items to their diet, widening their foraging niche ('optimal foraging hypothesis'). However, classic competition theory suggests the opposite: that increased competition leads to niche partitioning and greater individual foraging specialisation ('niche partitioning hypothesis'). We tested these opposing predictions in wild, group-living banded mongooses (Mungos mungo), using stable isotope analysis of banded mongoose whiskers to quantify individual and group foraging niche. Individual foraging niche size declined with increasing group size, despite all groups having a similar overall niche size. Our findings support the prediction that competition promotes niche partitioning within social groups and suggest that individual foraging specialisation may play an important role in the formation of stable social groupings.

Sex differences in the drivers of reproductive skew in a cooperative breeder

Many cooperatively breeding societies are characterized by high reproductive skew, such that some socially dominant individuals breed, while socially subordinate individuals provide help. Inbreeding avoidance serves as a source of reproductive skew in many high-skew societies, but few empirical studies have examined sources of skew operating alongside inbreeding avoidance or compared individual attempts to reproduce (reproductive competition) with individual reproductive success. Here, we use long-term genetic and observational data to examine factors affecting reproductive skew in the high-skew cooperatively breeding southern pied babbler (Turdoides bicolor). When subordinates can breed, skew remains high, suggesting factors additional to inbreeding avoidance drive skew. Subordinate females are more likely to compete to breed when older or when ecological constraints on dispersal are high, but heavy subordinate females are more likely to successfully breed. Subordinate males are more likely to compete when they are older, during high ecological constraints, or when they are related to the dominant male, but only the presence of within-group unrelated subordinate females predicts subordinate male breeding success. Reproductive skew is not driven by reproductive effort, but by forces such as intrinsic physical limitations and intrasexual conflict (for females) or female mate choice, male mate-guarding and potentially reproductive restraint (for males). Ecological conditions or "outside options" affect the occurrence of reproductive conflict, supporting predictions of recent synthetic skew models. Inbreeding avoidance together with competition for access to reproduction may generate high skew in animal societies, and disparate processes may be operating to maintain male vs. female reproductive skew in the same species.

Adult sex ratios and their implications for cooperative breeding in birds

Cooperative breeding is a form of breeding system where in addition to a core breeding pair, one or more usually non-breeding individuals provide offspring care. Cooperative breeding is widespread in birds, but its origin and maintenance in contemporary populations are debated. Although deviations in adult sex ratio (ASR, the proportion of males in the adult population) have been hypothesized to influence the occurrence of cooperative breeding because of the resulting surplus of one sex and limited availability of breeding partners, this hypothesis has not been tested across a wide range of taxa. By using data from 188 bird species and phylogenetically controlled analyses, we show that cooperatively breeding species have more male-biased ASRs than non-cooperative species. Importantly, ASR predicts helper sex ratio: in species with more male-biased ASR, helper sex ratio is also more male biased. We also show that offspring sex ratios do not predict ASRs, so that the skewed ASRs emerge during the period when individuals aim to obtain a breeding position or later during adulthood. In line with this result, we found that ASR (among both cooperatively and non-cooperatively breeding species) is inversely related to sex bias in dispersal distance, suggesting that the cost of dispersal is more severe for the further-dispersing sex. As females usually disperse further in birds, this explains the generally male-biased ASR, and in combination with benefits of philopatry for males, this probably explains why ASR is more biased in cooperatively breeding species. Taken together, our results suggest that a sex bias in helping in cooperatively breeding species relates to biased ASRs. We propose that this relationship is driven by sex-specific costs and benefits of dispersal and helping, as well as other demographic factors. Future phylogenetic comparative and experimental work is needed to establish how this relationship emerges.This article is part of the themed issue 'Adult sex ratios and reproductive decisions: a critical re-examination of sex differences in human and animal societies'.

Kin discrimination via odour in the cooperatively breeding banded mongoose

Kin discrimination is often beneficial for group-living animals as it aids in inbreeding avoidance and providing nepotistic help. In mammals, the use of olfactory cues in kin discrimination is widespread and may occur through learning the scents of individuals that are likely to be relatives, or by assessing genetic relatedness directly through assessing odour similarity (phenotype matching). We use scent presentations to investigate these possibilities in a wild population of the banded mongoose Mungos mungo, a cooperative breeder in which inbreeding risk is high and females breed communally, disrupting behavioural cues to kinship. We find that adults show heightened behavioural responses to unfamiliar (extra-group) scents than to familiar (within-group) scents. Interestingly, we found that responses to familiar odours, but not unfamiliar odours, varied with relatedness. This suggests that banded mongooses are either able to use an effective behavioural rule to identify likely relatives from within their group, or that phenotype matching is used in the context of within-group kin recognition but not extra-group kin recognition. In other cooperative breeders, familiarity is used within the group and phenotype matching may be used to identify unfamiliar kin. However, for the banded mongoose this pattern may be reversed, most likely due to their unusual breeding system which disrupts within-group behavioural cues to kinship.

Rapid juvenile hormone downregulation in subordinate wasp queens facilitates stable cooperation

In many cooperatively breeding animals, subordinate group members have lower reproductive capacity than dominant group members. Theory suggests subordinates may downregulate their reproductive capacity because dominants punish subordinates who maintain high fertility. However, there is little direct experimental evidence that dominants cause physiological suppression in subordinates. Here, we experimentally test how social interactions influence subordinate reproductive hormones in Polistes dominula paper wasps. Polistes dominula queens commonly found nests in cooperative groups where the dominant queen is more fertile than the subordinate queen. In this study, we randomly assigned wasps to cooperative groups, assessed dominance behaviour during group formation, then measured levels of juvenile hormone (JH), a hormone that mediates Polistes fertility. Within three hours, lowest ranking subordinates had less JH than dominants or solitary controls, indicating that group formation caused rapid JH reduction in low-ranking subordinates. In a second experiment, we measured the behavioural consequences of experimentally increasing subordinate JH. Subordinates with high JH-titres received significantly more aggression than control subordinates or subordinates from groups where the dominant's JH was increased. These results suggest that dominants aggressively punished subordinates who attempted to maintain high fertility. Low-ranked subordinates may rapidly downregulate reproductive capacity to reduce costly social interactions with dominants. Rapid modulation of subordinate reproductive physiology may be an important adaptation to facilitate the formation of stable, cooperative groups.

Biased escorts: offspring sex, not relatedness explains alloparental care patterns in a cooperative breeder

Kin selection theory predicts that animals should direct costly care where inclusive fitness gains are highest. Individuals may achieve this by directing care at closer relatives, yet evidence for such discrimination in vertebrates is equivocal. We investigated patterns of cooperative care in banded mongooses, where communal litters are raised by adult 'escorts' who form exclusive caring relationships with individual pups. We found no evidence that escorts and pups assort by parentage or relatedness. However, the time males spent escorting increased with increasing relatedness to the other group members, and to the pup they had paired with. Thus, we found no effect of relatedness in partner choice, but (in males) increasing helping effort with relatedness once partner choices had been made. Unexpectedly, the results showed clear assortment by sex, with female carers being more likely to tend to female pups, and male carers to male pups. This sex-specific assortment in helping behaviour has potential lifelong impacts on individual development and may impact the future size and composition of natal groups and dispersing cohorts. Where relatedness between helpers and recipients is already high, individuals may be better off choosing partners using other predictors of the costs and benefits of cooperation, without the need for possibly costly within-group kin discrimination.

Hormonal correlates of development and natal dispersal in wild female owl monkeys (Aotus azarae) of Argentina

Pair-living and socially monogamous primates typically do not reproduce before dispersing. It is currently unclear whether this reproductive suppression is due to endocrine or behavioral mechanisms. Cooperatively breeding taxa, like callitrichids, may forego reproduction in natal groups because they reap inclusive fitness benefits and/or they are avoiding inbreeding. However, neither of these benefits of delayed reproduction appear to adequately explain the lack of reproduction prior to leaving the natal group in pair-living monogamous species. In this study, we determined whether wild Azara's owl monkeys (Aotus azarae) in the Argentinean Chaco establish reproductive maturity prior to dispersing. We utilized 635 fecal extracts to characterize reproductive hormone profiles of 11 wild juvenile and subadult females using enzyme immunoassays. Subadult females showed hormone profiles indicative of ovulatory cycling and had mean PdG and E1G concentrations approximately five times higher than juveniles. Contrary to expectations from the inbreeding avoidance hypothesis, female owl monkeys do not delay puberty, but rather commence ovarian cycling while residing in their natal group. Still, subadults appear to have a period during which they experience irregular, non-conceptive cycles prior to reproducing. Commencing these irregular cycles in the natal group may allow them to develop a state of suspended readiness, which could be essential to securing a mate, while avoiding costs of ranging solitarily. Our results indicate that reproductive suppression in female owl monkeys is not due to endocrine suppression. We suggest that adults likely use behavioral mechanisms to prevent subadults from reproducing with unrelated adult males in their natal group.

Pregnancy is detected via odour in a wild cooperative breeder

Among mammals, scent has long been known to encode oestrus; however, in many species, detecting pregnancy may also be important in terms of both competition and mate-choice. Here, we show, through odour presentation experiments, that pregnancy is discernible via scent by both sexes in the cooperatively breeding banded mongoose, Mungos mungo Males spent more time investigating and were more likely to scent mark the odours of non-pregnant females, compared to pregnant females. Females showed increased levels of scent marking when odours were of the same reproductive state as themselves. These results present the first direct demonstration that pregnancy is detectable via scent in wild cooperative breeders. Detecting pregnancy may be particularly important in cooperative breeders as, in addition to the competition between males for receptive mates, there is also intense competition between females for access to alloparental care. Consequently, dominant females benefit from targeting reproductive suppression towards subordinates that represent direct threats, such as pregnant females.

Explaining negative kin discrimination in a cooperative mammal society

Kin selection theory predicts that, where kin discrimination is possible, animals should typically act more favorably toward closer genetic relatives and direct aggression toward less closely related individuals. Contrary to this prediction, we present data from an 18-y study of wild banded mongooses, Mungos mungo, showing that females that are more closely related to dominant individuals are specifically targeted for forcible eviction from the group, often suffering severe injury, and sometimes death, as a result. This pattern cannot be explained by inbreeding avoidance or as a response to more intense local competition among kin. Instead, we use game theory to show that such negative kin discrimination can be explained by selection for unrelated targets to invest more effort in resisting eviction. Consistent with our model, negative kin discrimination is restricted to eviction attempts of older females capable of resistance; dominants exhibit no kin discrimination when attempting to evict younger females, nor do they discriminate between more closely or less closely related young when carrying out infanticidal attacks on vulnerable infants who cannot defend themselves. We suggest that in contexts where recipients of selfish acts are capable of resistance, the usual prediction of positive kin discrimination can be reversed. Kin selection theory, as an explanation for social behavior, can benefit from much greater exploration of sequential social interactions.

Lifetime fitness consequences of early-life ecological hardship in a wild mammal population

Early-life ecological conditions have major effects on survival and reproduction. Numerous studies in wild systems show fitness benefits of good quality early-life ecological conditions ("silver-spoon" effects). Recently, however, some studies have reported that poor-quality early-life ecological conditions are associated with later-life fitness advantages and that the effect of early-life conditions can be sex-specific. Furthermore, few studies have investigated the effect of the variability of early-life ecological conditions on later-life fitness. Here, we test how the mean and variability of early-life ecological conditions affect the longevity and reproduction of males and females using 14 years of data on wild banded mongooses (Mungos mungo). Males that experienced highly variable ecological conditions during development lived longer and had greater lifetime fitness, while those that experienced poor early-life conditions lived longer but at a cost of reduced fertility. In females, there were no such effects. Our study suggests that exposure to more variable environments in early life can result in lifetime fitness benefits, whereas differences in the mean early-life conditions experienced mediate a life-history trade-off between survival and reproduction. It also demonstrates how early-life ecological conditions can produce different selection pressures on males and females.

Reproductive competition triggers mass eviction in cooperative banded mongooses

In many vertebrate societies, forced eviction of group members is an important determinant of population structure, but little is known about what triggers eviction. Three main explanations are: (i) the reproductive competition hypothesis, (ii) the coercion of cooperation hypothesis, and (iii) the adaptive forced dispersal hypothesis. The last hypothesis proposes that dominant individuals use eviction as an adaptive strategy to propagate copies of their alleles through a highly structured population. We tested these hypotheses as explanations for eviction in cooperatively breeding banded mongooses (Mungos mungo), using a 16-year dataset on life history, behaviour and relatedness. In this species, groups of females, or mixed-sex groups, are periodically evicted en masse. Our evidence suggests that reproductive competition is the main ultimate trigger for eviction for both sexes. We find little evidence that mass eviction is used to coerce helping, or as a mechanism to force dispersal of relatives into the population. Eviction of females changes the landscape of reproductive competition for remaining males, which may explain why males are evicted alongside females. Our results show that the consequences of resolving within-group conflict resonate through groups and populations to affect population structure, with important implications for social evolution.

Elevated glucocorticoid concentrations during gestation predict reduced reproductive success in subordinate female banded mongooses

Dominant females in social species have been hypothesized to reduce the reproductive success of their subordinates by inducing elevated circulating glucocorticoid (GC) concentrations. However, this ‘stress-related suppression' hypothesis has received little support in cooperatively breeding species, despite evident reproductive skews among females. We tested this hypothesis in the banded mongoose (Mungos mungo), a cooperative mammal in which multiple females conceive and carry to term in each communal breeding attempt. As predicted, lower ranked females had lower reproductive success, even among females that carried to term. While there were no rank-related differences in faecal glucocorticoid (fGC) concentrations prior to gestation or in the first trimester, lower ranked females had significantly higher fGC concentrations than higher ranked females in the second and third trimesters. Finally, females with higher fGC concentrations during the third trimester lost a greater proportion of their gestated young prior to their emergence from the burrow. Together, our results are consistent with a role for rank-related maternal stress in generating reproductive skew among females in this cooperative breeder. While studies of reproductive skew frequently consider the possibility that rank-related stress reduces the conception rates of subordinates, our findings highlight the possibility of detrimental effects on reproductive outcomes even after pregnancies have become established.

Genetic population structure and relatedness in the narrow-striped mongoose (Mungotictis decemlineata), a social Malagasy carnivore with sexual segregation

Information on the genetic structure of animal populations can allow inferences about mechanisms shaping their social organization, dispersal, and mating system. The mongooses (Herpestidae) include some of the best-studied mammalian systems in this respect, but much less is known about their closest relatives, the Malagasy carnivores (Eupleridae), even though some of them exhibit unusual association patterns. We investigated the genetic structure of the Malagasy narrow-striped mongoose (Mungotictis decemlineata), a small forest-dwelling gregarious carnivore exhibiting sexual segregation. Based on mtDNA and microsatellite analyses, we determined population-wide haplotype structure and sex-specific and within-group relatedness. Furthermore, we analyzed parentage and sibship relationships and the level of reproductive skew. We found a matrilinear population structure, with several neighboring female units sharing identical haplotypes. Within-group female relatedness was significantly higher than expected by chance in the majority of units. Haplotype diversity of males was significantly higher than in females, indicating male-biased dispersal. Relatedness within the majority of male associations did not differ from random, not proving any kin-directed benefits of male sociality in this case. We found indications for a mildly promiscuous mating system without monopolization of females by males, and low levels of reproductive skew in both sexes based on parentages of emergent young. Low relatedness within breeding pairs confirmed immigration by males and suggested similarities with patterns in social mongooses, providing a starting point for further investigations of mate choice and female control of reproduction and the connected behavioral mechanisms. Our study contributes to the understanding of the determinants of male sociality in carnivores as well as the mechanisms of female competition in species with small social units.

Eco-evolutionary dynamics of social dilemmas

Social dilemmas are an integral part of social interactions. Cooperative actions, ranging from secreting extra-cellular products in microbial populations to donating blood in humans, are costly to the actor and hence create an incentive to shirk and avoid the costs. Nevertheless, cooperation is ubiquitous in nature. Both costs and benefits often depend non-linearly on the number and types of individuals involved-as captured by idioms such as 'too many cooks spoil the broth' where additional contributions are discounted, or 'two heads are better than one' where cooperators synergistically enhance the group benefit. Interaction group sizes may depend on the size of the population and hence on ecological processes. This results in feedback mechanisms between ecological and evolutionary processes, which jointly affect and determine the evolutionary trajectory. Only recently combined eco-evolutionary processes became experimentally tractable in microbial social dilemmas. Here we analyse the evolutionary dynamics of non-linear social dilemmas in settings where the population fluctuates in size and the environment changes over time. In particular, cooperation is often supported and maintained at high densities through ecological fluctuations. Moreover, we find that the combination of the two processes routinely reveals highly complex dynamics, which suggests common occurrence in nature.

Variable ecological conditions promote male helping by changing banded mongoose group composition

Ecological conditions are expected to have an important influence on individuals' investment in cooperative care. However, the nature of their effects is unclear: both favorable and unfavorable conditions have been found to promote helping behavior. Recent studies provide a possible explanation for these conflicting results by suggesting that increased ecological variability, rather than changes in mean conditions, promote cooperative care. However, no study has tested whether increased ecological variability promotes individual-level helping behavior or the mechanisms involved. We test this hypothesis in a long-term study population of the cooperatively breeding banded mongoose, Mungos mungo, using 14 years of behavioral and meteorological data to explore how the mean and variability of ecological conditions influence individual behavior, body condition, and survival. Female body condition was more sensitive to changes in rainfall leading to poorer female survival and pronounced male-biased group compositions after periods of high rainfall variability. After such periods, older males invested more in helping behavior, potentially because they had fewer mating opportunities. These results provide the first empirical evidence for increased individual helping effort in more variable ecological conditions and suggest this arises because of individual differences in the effect of ecological conditions on body condition and survival, and the knock-on effect on social group composition. Individual differences in sensitivity to environmental variability, and the impacts this has on the internal structure and composition of animal groups, can exert a strong influence on the evolution and maintenance of social behaviors, such as cooperative care.

Comparing pre- and post-copulatory mate competition using social network analysis in wild crickets

In many animals, males compete for fertilizations both before and after mating. But do males specialize in 1 type of competition? And do physical fights between males lead to less competition between their ejaculates within females? We studied competitions between wild crickets by building networks of interactions. We found that males that had more fights were more likely to meet in sperm competition, suggesting that evolution will not favor specialists in one of the 2 types of competition.

Twitter: @DFofFreedom

Sexual selection results from variation in success at multiple stages in the mating process, including competition before and after mating. The relationship between these forms of competition, such as whether they trade-off or reinforce one another, influences the role of sexual selection in evolution. However, the relationship between these 2 forms of competition is rarely quantified in the wild. We used video cameras to observe competition among male field crickets and their matings in the wild. We characterized pre- and post-copulatory competition as 2 networks of competing individuals. Social network analysis then allowed us to determine 1) the effectiveness of precopulatory competition for avoiding postcopulatory competition, 2) the potential for divergent mating strategies, and 3) whether increased postcopulatory competition reduces the apparent reproductive benefits of male promiscuity. We found 1) limited effectiveness of precopulatory competition for avoiding postcopulatory competition; 2) males do not specifically engage in only 1 type of competition; and 3) promiscuous individuals tend to mate with each other, which will tend to reduce variance in reproductive success in the population and highlights the trade-off inherent in mate guarding. Our results provide novel insights into the works of sexual competition in the wild. Furthermore, our study demonstrates the utility of using network analyses to study competitive interactions, even in species lacking obvious social structure.

Group-size-dependent punishment of idle subordinates in a cooperative breeder where helpers pay to stay

In cooperative breeding systems, dominant breeders sometimes tolerate unrelated individuals even if they inflict costs on the dominants. According to the 'pay-to-stay' hypothesis, (i) subordinates can outweigh these costs by providing help and (ii) dominants should be able to enforce help by punishing subordinates that provide insufficient help. This requires that dominants can monitor helping and can recognize group members individually. In a field experiment, we tested whether cooperatively breeding cichlid Neolamprologus pulcher subordinates increase their help after a forced 'idle' period, how other group members respond to a previously idle helper, and how helper behaviour and group responses depend on group size. Previously, idle helpers increased their submissiveness and received more aggression than control helpers, suggesting that punishment occurred to enforce help. Subordinates in small groups increased their help more than those in large groups, despite receiving less aggression. When subordinates were temporarily removed, dominants in small groups were more likely to evict returning subordinates. Our results suggest that only in small groups do helpers face a latent threat of punishment by breeders as predicted by the pay-to-stay hypothesis. In large groups, cognitive constraints may prevent breeders from tracking the behaviour of a large number of helpers.

Suppressing subordinate reproduction provides benefits to dominants in cooperative societies of meerkats

In many animal societies, a small proportion of dominant females monopolize reproduction by actively suppressing subordinates. Theory assumes that this is because subordinate reproduction depresses the fitness of dominants, yet the effect of subordinate reproduction on dominant behaviour and reproductive success has never been directly assessed. Here, we describe the consequences of experimentally preventing subordinate breeding in 12 groups of wild meerkats (Suricata suricatta) for three breeding attempts, using contraceptive injections. When subordinates are prevented from breeding, dominants are less aggressive towards subordinates and evict them less often, leading to a higher ratio of helpers to dependent pups, and increased provisioning of the dominant's pups by subordinate females. When subordinate breeding is suppressed, dominants also show improved foraging efficiency, gain more weight during pregnancy and produce heavier pups, which grow faster. These results confirm the benefits of suppression to dominants, and help explain the evolution of singular breeding in vertebrate societies.

Policing of reproduction by hidden threats in a cooperative mammal

The evolution of cooperation in animal and human societies is associated with mechanisms to suppress individual selfishness. In insect societies, queens and workers enforce cooperation by "policing" selfish reproduction by workers. Insect policing typically takes the form of damage limitation after individuals have carried out selfish acts (such as laying eggs). In contrast, human policing is based on the use of threats that deter individuals from acting selfishly in the first place, minimizing the need for damage limitation. Policing by threat could in principle be used to enforce reproductive suppression in animal societies, but testing this idea requires an experimental approach to simulate reproductive transgression and provoke out-of-equilibrium behavior. We carried out an experiment of this kind on a wild population of cooperatively breeding banded mongooses (Mungos mungo) in Uganda. In this species, each group contains multiple female breeders that give birth to a communal litter, usually on the same day. In a 7-y experiment we used contraceptive injections to manipulate the distribution of maternity within groups, triggering hidden threats of infanticide. Our data suggest that older, socially dominant females use the threat of infanticide to deter selfish reproduction by younger females, but that females can escape the threat of infanticide by synchronizing birth to the same day as older females. Our study shows that reproduction in animal societies can be profoundly influenced by threats that remain hidden until they are triggered experimentally. Coercion may thus extend well beyond the systems in which acts of infanticide are common.

Resolving social conflict among females without overt aggression

Members of animal societies compete over resources and reproduction, but the extent to which such conflicts of interest are resolved peacefully (without recourse to costly or wasteful acts of aggression) varies widely. Here, we describe two theoretical mechanisms that can help to understand variation in the incidence of overt behavioural conflict: (i) destruction competition and (ii) the use of threats. The two mechanisms make different assumptions about the degree to which competitors are socially sensitive (responsive to real-time changes in the behaviour of their social partners). In each case, we discuss how the model assumptions relate to biological reality and highlight the genetic, ecological and informational factors that are likely to promote peaceful conflict resolution, drawing on empirical examples. We suggest that, relative to males, reproductive conflict among females may be more frequently resolved peacefully through threats of punishment, rather than overt acts of punishment, because (i) offspring are more costly to produce for females and (ii) reproduction is more difficult to conceal. The main need now is for empirical work to test whether the mechanisms described here can indeed explain how social conflict can be resolved without overt aggression.

Intra-sexual selection in cooperative mammals and birds: why are females not bigger and better armed?

In cooperatively breeding mammals and birds, intra-sexual reproductive competition among females may often render variance in reproductive success higher among females than males, leading to the prediction that intra-sexual selection in such species may have yielded the differential exaggeration of competitive traits among females. However, evidence to date suggests that female-biased reproductive variance in such species is rarely accompanied by female-biased sexual dimorphisms. We illustrate the problem with data from wild Damaraland mole-rat, Fukomys damarensis, societies: the variance in lifetime reproductive success among females appears to be higher than that among males, yet males grow faster, are much heavier as adults and sport larger skulls and incisors (the weapons used for fighting) for their body lengths than females, suggesting that intra-sexual selection has nevertheless acted more strongly on the competitive traits of males. We then consider potentially general mechanisms that could explain these disparities by tempering the relative intensity of selection for competitive trait exaggeration among females in cooperative breeders. Key among these may be interactions with kin selection that could nevertheless render the variance in inclusive fitness lower among females than males, and fundamental aspects of the reproductive biology of females that may leave reproductive conflict among females more readily resolved without overt physical contests.