Evolution and development of sex differences in cooperative behavior in meerkats

Individual contributions to territory defence in a cooperative breeder: weighing up the benefits and costs

While investment in territory defence is expected to be influenced by its benefits, the additional role that costs may play is rarely considered. Here, we quantify both benefits and costs of repelling prospecting males in cooperative meerkats, and demonstrate that both are required to explain the substantial variation in individual contributions to the defence observed. Males benefit more from repelling prospectors than females, as males may lose dominance and be expelled during intrusions. Accordingly, males invest the most in repelling prospectors. We also show that males experience an associated cost in the form of reduced weight gain and, as such, heavier males contribute more to chasing prospectors. Finally, we show evidence of a cost not restricted to individuals engaged in chasing: both males and females reduce their contributions to feeding dependent pups when prospectors are present, resulting in a reduction in pup weight gain in this context. Males appear to adjust their contributions to chasing in light of this cost, chasing at lower rates when their group contains dependent young. Our findings support the view that investment in cooperative behaviours can be attributed to benefits and costs, and highlight the additional importance of considering trade-offs in investment between cooperative behaviours.

Kin selection, not group augmentation, predicts helping in an obligate cooperatively breeding bird

Kin selection theory has been the central model for understanding the evolution of cooperative breeding, where non-breeders help bear the cost of rearing young. Recently, the dominance of this idea has been questioned; particularly in obligate cooperative breeders where breeding without help is uncommon and seldom successful. In such systems, the direct benefits gained through augmenting current group size have been hypothesized to provide a tractable alternative (or addition) to kin selection. However, clear empirical tests of the opposing predictions are lacking. Here, we provide convincing evidence to suggest that kin selection and not group augmentation accounts for decisions of whether, where and how often to help in an obligate cooperative breeder, the chestnut-crowned babbler (Pomatostomus ruficeps). We found no evidence that group members base helping decisions on the size of breeding units available in their social group, despite both correlational and experimental data showing substantial variation in the degree to which helpers affect productivity in units of different size. By contrast, 98 per cent of group members with kin present helped, 100 per cent directed their care towards the most related brood in the social group, and those rearing half/full-sibs helped approximately three times harder than those rearing less/non-related broods. We conclude that kin selection plays a central role in the maintenance of cooperative breeding in this species, despite the apparent importance of living in large groups.

Reproductive rate, not dominance status, affects fecal glucocorticoid levels in breeding female meerkats

Glucocorticoid hormones (GCs) have been studied intensively to understand the associations between physiological stress and reproductive skew in animal societies. However, we have little appreciation of the range of either natural levels within and among individuals, or the associations among dominance status, reproductive rate and GCs levels during breeding. To address these shortcomings, we examined variation in fecal glucocorticoid metabolites (fGC) during breeding periods in free-ranging female meerkats (Suricata suricatta) over 11 years. The vast majority of variation in fGC levels was found within breeding events by the same female (~87%), with the remaining variation arising among breeding events and among females. Concentrations of fGC generally tripled as pregnancy progressed. However, females with a high reproductive rate, defined as those conceiving within a month following parturition (mean = 9 days postpartum), showed significant reductions in fGC in the final 2 weeks before parturition. Despite these reductions, females with a high reproductive rate had higher fGC levels at conception of the following litter than those breeding at a low rate. After controlling for the higher reproductive rate of dominants, we found no association between levels of fGC and either age or dominance status. Our results suggest that one should be cautious about interpreting associations between dominance status, reproductive skew and GCs levels, without knowledge of the natural variation in GCs levels within and among females.

Dominant female meerkats do not use aggression to elevate work rates of helpers in response to increased brood demand

In cooperatively breeding animals, in which nonbreeding subordinates assist in rearing offspring born to dominants, breeders and helpers may be in conflict over their respective contributions to offspring care and selection may favour breeders that use aggression to elevate the work rates of helpers. We tested the prediction that dominant female meerkats, Suricata suricatta, should increase aggression towards subordinates when the need for help is higher, by playing back recordings of pup begging calls to simulate increased need for help. Second, we tested the prediction that dominants should reduce aggression when subordinates help more, by playing back recordings of feeding calls to simulate elevated pup provisioning rates by subordinates. Neither of the two playback experiments affected rates of aggressive interactions between breeding females and helpers. Instead, breeding females increased their own level of pup provisioning in response to increased pup begging. Hence, our results do not support a role of aggression in regulating helping behaviour in meerkats, but suggest that pup provisioning can be explained by direct and/or indirect benefits derived from helping. As yet, firm evidence that breeders use aggression to promote helping by subordinates in cooperative animal societies remains elusive.

Individual variation and the resolution of conflict over parental care in penduline tits

Eurasian penduline tits (Remiz pendulinus) have an unusually diverse breeding system consisting of frequent male and female polygamy, and uniparental care by the male or the female. Intriguingly, 30 to 40 per cent of all nests are deserted by both parents. To understand the evolution of this diverse breeding system and frequent clutch desertion, we use 6 years of field data to derive fitness expectations for males and females depending on whether or not they care for their offspring. The resulting payoff matrix corresponds to an asymmetric Snowdrift Game with two alternative evolutionarily stable strategies (ESSs): female-only and male-only care. This, however, does not explain the polymorphism in care strategies and frequent biparental desertion, because theory predicts that one of the two ESSs should have spread to fixation. Using a bootstrapping approach, we demonstrate that taking account of individual variation in payoffs explains the patterns of care better than a model based on the average population payoff matrix. In particular, a model incorporating differences in male attractiveness closely predicts the observed frequencies of male and female desertion. Our work highlights the need for a new generation of individual-based evolutionary game-theoretic models.

Progressive parenting behavior in wild golden lion tamarins

Young primates in the family Callitrichidae (the marmosets and tamarins) receive extensive and relatively prolonged care from adults. Of particular note, callitrichid young are routinely provisioned until well after weaning by parents and helpers, which is in stark contrast to typical juvenile primates, who must acquire most of their food independently once they are weaned. Adults of some callitrichid species produce a specialized vocalization that encourages immature group members to take proffered food from the caller. Here, I report that wild adult golden lion tamarins (Leontopithecus rosalia) not only used this food-offering call to encourage young, mobile offspring to approach and take captured prey from them, but as the young began to spend significant time foraging for themselves and to acquire prey by independent means, the frequency of these vocalizations in the context of food transfer declined. Adults then began to use food-offering calls in a novel context: to direct juveniles to foraging sites that contained hidden prey that the adults had found but not captured. During the period of these most frequent adult-directed prey captures, the independent prey-capture success rates of juveniles improved. Thus, adults modified their provisioning behavior in a progressive developmentally sensitive manner that may have facilitated learning how to find food. I hypothesize that as a result of these demonstrations by adults, juveniles either may be encouraged to continue foraging despite low return rates or to learn the properties of productive prey-foraging substrates in a complex environment.

The shadow of the future affects cooperation in a cleaner fish

Humans show great flexibility in adjusting their levels of cooperation to account for current and future circumstances. For example, levels of cooperation are higher if there is more competition at the level of the whole population than with interacting partners and when individuals are likely to gain social prestige. Humans also show the capacity to increase current levels of cooperation to account for future payoffs if it is likely that repeated interactions will occur with the same partner (known as 'the Shadow of the Future'). Here, we provide the first evidence for this capacity in a non-human animal, the cleaner fish Labroides bicolor. L. bicolor individuals show uneven frequency of use of different areas within a large home range, which should in turn affect the delay between repeated interactions with individual reef fish 'clients'. In areas where the frequency of clients encountering cleaners is higher, cleaners are more likely to experience future costs of cheating, so future payoffs are of more concern for current decisions. In line with this, we found a negative correlation between cheating and the frequency of clients encountering cleaners in L. bicolor home ranges.

Structure and function in mammalian societies

Traditional interpretations of the evolution of animal societies have suggested that their structure is a consequence of attempts by individuals to maximize their inclusive fitness within constraints imposed by their social and physical environments. In contrast, some recent re-interpretations have argued that many aspects of social organization should be interpreted as group-level adaptations maintained by selection operating between groups or populations. Here, I review our current understanding of the evolution of mammalian societies, focusing, in particular, on the evolution of reproductive strategies in societies where one dominant female monopolizes reproduction in each group and her offspring are reared by other group members. Recent studies of the life histories of females in these species show that dispersing females often have little chance of establishing new breeding groups and so are likely to maximize their inclusive fitness by helping related dominants to rear their offspring. As in eusocial insects, increasing group size can lead to a progressive divergence in the selection pressures operating on breeders and helpers and to increasing specialization in their behaviour and life histories. As yet, there is little need to invoke group-level adaptations in order to account for the behaviour of individuals or the structure of mammalian groups.

Maternal effects in cooperative breeders: from hymenopterans to humans

The environment that an offspring experiences during its development can have lifelong consequences for its morphology, anatomy, physiology and behaviour that are strong enough to span generations. One aspect of an offspring's environment that can have particularly pronounced and long-lasting effects is that provided by its parent(s) (maternal effects). Some disciplines in biology have been quicker to appreciate maternal effects than others, and some organisms provide better model systems for understanding the causes and consequences of the maternal environment for ecology and evolution than others. One field in which maternal effects has been poorly represented, and yet is likely to represent a particularly fruitful area for research, is the field of cooperative breeding (i.e. systems where offspring are reared by carers in addition to parent(s)). Here, we attempt to illustrate the scope of cooperative breeding systems for maternal effects research and, conversely, highlight the importance of maternal effects research for understanding cooperative breeding systems. To this end, we first outline why mothers will commonly benefit from affecting the phenotype of their offspring in cooperative breeding systems, present potential strategies that mothers could employ in order to do so and offer predictions regarding the circumstances under which different types of maternal effects might be expected. Second, we highlight why a neglect of maternal strategies and the effects that they have on their offspring could lead to miscalculations of helper/worker fitness gains and a misunderstanding of the factors selecting for the evolution and maintenance of cooperative breeding. Finally, we introduce the possibility that maternal effects could have significant consequences for our understanding of both the evolutionary origins of cooperative breeding and the rise of social complexity in cooperative systems.

Sex differences in dispersal and the evolution of helping and harming

In this article, we explore the impact of sex-biased dispersal on local relatedness and on selection for helping and harming behavior among males and females. We show that in a patch-structured population, when there is a marked sex bias in dispersal, selection will almost always favor harming behavior among individuals of the sex more prone to dispersal. This result holds regardless of the effects of mating skew or overlapping generations. Selection may well also favor helping behavior among individuals of the philopatric sex, particularly if there is generational overlap, but this is less likely to occur if individuals of the philopatric sex compete more intensely for fewer breeding opportunities. In this last case, if generational overlap is low and mating skew pronounced, the result may be selection for harming behavior among both males and females. In general, the rate of dispersal and the level of relatedness among individuals of one sex do not reliably predict their level of helping or harming behavior; selection on either males or females depends on the dispersal of both sexes.

Sex differences in responsiveness to begging in a cooperative mammal

In species where young are provisioned by both parents, males commonly contribute less to parental care than females, and are less responsive to variation in begging rates. Similar differences in the care of young occur among adults in cooperative breeders, but fewer studies have investigated whether these are associated with differences in responsiveness. Here, we present results from a playback experiment investigating responsiveness to begging in the meerkat (Suricata suricatta), a cooperatively breeding mammal. Although increased begging rate raised the feeding rate of adults of both sexes, there was no consistent tendency for females to be more responsive than males. However, when we examined changes in the proportion of food items found that were fed to pups (generosity), we found that females were more responsive than males to increased begging rate. These results can be explained in terms of sex differences in dispersal: in meerkats, females are philopatric and receive considerable benefits from investing in young, both directly, by increasing group size, and indirectly, by recruiting helpers if they inherit the breeding position. In addition, they emphasize that generosity provides a more sensitive measure of responsiveness to begging than feeding rate, as it accounts for variation in foraging success.

Variation in contributions to teaching by meerkats

Recent evidence from cooperative insect, bird and mammal societies has challenged the assumption that teaching is restricted to humans. However, little is known about the factors affecting the degree to which individuals in such societies contribute to teaching. Here, I examine variation in contributions to teaching in meerkats, where older group members teach pups to handle difficult prey. I show that investment in teaching varies with characteristics of pups, helpers, groups and ecological conditions. Although prior experience in caring for pups did not significantly influence teaching behaviour, younger helpers, which were still investing in growth, contributed less to teaching than older individuals. This suggests that, in common with other cooperative activities, contributions to teaching vary with the costs experienced by individual group members. However, in contrast to other forms of helping in meerkats, I detected no effects of nutritional state on teaching, suggesting that it carries relatively low costs. In species where individuals can potentially gain direct or indirect fitness benefits from facilitating learning in others, low costs divided among multiple group members may help tip the balance towards selection for teaching.

Strategic adjustment of begging effort by banded mongoose pups

Variation in the intensity of conspicuous displays raises three basic questions: (i) the relationship between internal state and display intensity, (ii) the relationship between display intensity and receiver response, and (iii) the effect of variation in receiver responsiveness on signaller behaviour. Here, I investigate the interaction between pups and helpers in the communally breeding banded mongoose (Mungos mungo), where each pup forms an exclusive relationship with a single adult helper (termed its 'escort'). By experimentally manipulating pup need, I demonstrate that changes in begging rate correspond to changes in short-term need. The data then suggest that escorts in good condition may be more responsive to increased begging and that pups associating with them increase their begging more than do pups paired with escorts in poor condition. Escorts also appear more responsive to increased begging by female pups, and female pups increase their begging more than do male pups. These results suggest that banded mongoose pups may strategically adjust their investment in begging in relation to variation in the expected pay-off. I argue that such adjustment is likely to be a general phenomenon: wherever there is variation in responsiveness to signals, signallers will be selected to identify different categories of receiver and adjust their signals in order to maximize the pay-offs. Therefore, differences in signal intensity may be as much a product of context as an indication of variation in individual phenotypic or genotypic state.

Female-mediated causes and consequences of status change in a social fish

In highly social species, dominant individuals often monopolize reproduction, resulting in reproductive investment that is status dependent. Yet, for subordinates, who typically invest less in reproduction, social status can change and opportunities to ascend to dominant social positions are presented suddenly, requiring abrupt changes in behaviour and physiology. In this study, we examined male reproductive anatomy, physiology and behaviour following experimental manipulations of social status in the cooperatively breeding cichlid fish, Neolamprologus pulcher. This unusual fish species lives in permanent social groups composed of a dominant breeding pair and 1-20 subordinates that form a linear social dominance hierarchy. By removing male breeders, we created 18 breeding vacancies and thus provided an opportunity for subordinate males to ascend in status. Dominant females play an important role in regulating status change, as males successfully ascended to breeder status only when they were slightly larger than the female breeder in their social group. Ascending males rapidly assumed behavioural dominance, demonstrated elevated gonadal investment and androgen concentrations compared with males remaining socially subordinate. Interestingly, to increase gonadal investment ascending males appeared to temporarily restrain somatic growth. These results highlight the complex interactions between social status, reproductive physiology and group dynamics, and underscore a convergent pattern of reproductive investment among highly social, cooperative species.

Sex-linked neuroanatomical basis of human altruistic cooperativeness

Human altruistic cooperativeness, one of the most important components of our highly organized society, is along with a greatly enlarged brain relative to body size a spectacular outlier in the animal world. The "social-brain hypothesis" suggests that human brain expansion reflects an increased necessity for information processing to create social reciprocity and cooperation in our complex society. The present study showed that the young adult females (n = 66) showed greater Cooperativeness as well as larger relative global and regional gray matter volumes (GMVs) than the matched males (n = 89), particularly in the social-brain regions including bilateral posterior inferior frontal and left anterior medial prefrontal cortices. Moreover, in females, higher cooperativeness was tightly coupled with the larger relative total GMV and more specifically with the regional GMV in most of the regions revealing larger in female sex-dimorphism. The global and most of regional correlations between GMV and Cooperativeness were significantly specific to female. These results suggest that sexually dimorphic factors may affect the neurodevelopment of these "social-brain" regions, leading to higher cooperativeness in females. The present findings may also have an implication for the pathophysiology of autism; characterized by severe dysfunction in social reciprocity, abnormalities in social-brain, and disproportionately low probability in females.

The number of subordinates moderates intrasexual competition among males in cooperatively breeding meerkats

For dominant individuals in cooperatively breeding species, the presence of subordinates is associated with both benefits (i.e. increased reproductive output and other group-living benefits) and costs (i.e. intrasexual competition on reproduction). The biological market theory predicts that dominant individuals are tolerant to same-sex group members when there are only a few subordinates, so as to maximize their own reproductive success. We investigated factors affecting aggression by dominant males and submission by subordinate males for a cooperatively breeding mammal, meerkats, Suricata suricatta. In this species, reproductive conflict occurs between the dominant male and the non-offspring males. As predicted, the number of subordinates in a group was positively associated with the aggression frequency by the dominant male and with the submission frequency by the subordinate males. Relative to the aggression frequency against male offspring, the frequency of aggression against non-offspring males was comparable in small groups, but was higher in large groups. These results indicate that reproductive conflict is present between the dominant male and the non-offspring males but is moderated in groups with small numbers of subordinates. This study provides an empirical data agreeing with the biological market theory in the context of intrasexual competition in cooperatively breeding species.

Patterns of relatedness and parentage in an asocial, polyandrous striped hyena population

We investigated patterns of relatedness and reproduction in a population of striped hyenas in which individuals are behaviourally solitary but form polyandrous spatial groups consisting of one adult female and multiple adult males. Group-mate males were often close relatives, but were unrelated or distantly related in some cases, indicating that male coalitions are not strictly a result of philopatry or dispersal with cohorts of relatives. Most male-female pairs within spatial groups were unrelated or only distantly related. Considering patterns of relatedness between groups, relatedness was significantly higher among adult males living in non-neighbouring ranges than among neighbouring males. Mean relatedness among male-female dyads was highest for group-mates, but relatedness among non-neighbouring males and females was also significantly higher than among dyads of opposite-sex neighbours. Female-female relatedness also increased significantly with increasing geographic separation. These unusual and unexpected patterns may reflect selection to settle in a nonadjacent manner to reduce inbreeding and/or competition among relatives for resources (both sexes), or mates (males). Finally, resident males fathered the majority of the resident female's cubs, but extra-group paternity was likely in 31% of the cases examined, and multiple paternity was likely in half of the sampled litters.

Evolutionary Explanations for Cooperation

Natural selection favours genes that increase an organism's ability to survive and reproduce. This would appear to lead to a world dominated by selfish behaviour. However, cooperation can be found at all levels of biological organisation: genes cooperate in genomes, organelles cooperate to form eukaryotic cells, cells cooperate to make multicellular organisms, bacterial parasites cooperate to overcome host defences, animals breed cooperatively, and humans and insects cooperate to build societies. Over the last 40 years, biologists have developed a theoretical framework that can explain cooperation at all these levels. Here, we summarise this theory, illustrate how it may be applied to real organisms and discuss future directions.

Sex-biased preferential care in the cooperatively breeding Arabian babbler

In many social birds there are sex differences in dispersal patterns, with males commonly remaining in their natal group whereas females typically disperse at adolescence. Group members may therefore increase their fitness by preferentially caring for offspring of a particular sex according to social circumstances. Although previous studies have focussed on intragroup social factors that may affect preferential care, we propose that the relative size of neighbouring groups is of primary importance. Here we show that in the cooperatively breeding Arabian babbler (Turdoides squamiceps), parents preferentially feed male offspring when relative group size is small, and female offspring when group size is large. Unlike parents, helpers consistently favour young of the opposite sex to themselves, suggesting the risk of competition with members of the same sex for future breeding opportunities may override other considerations. These results emphasize the complexity of investment strategies in relation to social circumstances and the variable benefits of raising males vs. females in a species with sex-biased philopatry.