Increases in glucocorticoids are sufficient but not necessary to increase cooperative burrowing in Damaraland mole-rats.

Despite widespread interest in the evolution of cooperative behaviour, the physiological mechanisms shaping their expression remain elusive. We tested the hypothesis that glucocorticoid (GC) hormones affect cooperative behaviour using captive Damaraland mole-rats (Fukomys damarensis), a cooperatively breeding mammal. Within groups, individuals routinely contribute to public goods that include foraging tunnels, which provide all group members access to the tubers of desert plants they feed on, communal food stores and nests. We found that experimental increases in glucocorticoid concentration (GCc) in non-breeding female helpers led them to be active for longer and to burrow more while active, raising their daily contributions to burrowing, but not food carrying or nest building. However, experimentally induced increases in burrowing did not lead to elevated GCc in helpers of both sexes. These results suggest that heightened GCc may stimulate some cooperative behaviours that are energetically demanding (a characteristic shared by many types of cooperative activities across species) but that the cooperative behaviours affected by GCc can also be regulated by other mechanisms.

Morphological and genomic shifts in mole-rat 'queens' increase fecundity but reduce skeletal integrity.

In some mammals and many social insects, highly cooperative societies are characterized by reproductive division of labor, in which breeders and nonbreeders become behaviorally and morphologically distinct. While differences in behavior and growth between breeders and nonbreeders have been extensively described, little is known of their molecular underpinnings. Here, we investigate the consequences of breeding for skeletal morphology and gene regulation in highly cooperative Damaraland mole-rats. By experimentally assigning breeding 'queen' status versus nonbreeder status to age-matched littermates, we confirm that queens experience vertebral growth that likely confers advantages to fecundity. However, they also upregulate bone resorption pathways and show reductions in femoral mass, which predicts increased vulnerability to fracture. Together, our results show that, as in eusocial insects, reproductive division of labor in mole-rats leads to gene regulatory rewiring and extensive morphological plasticity. However, in mole-rats, concentrated reproduction is also accompanied by costs to bone strength.

Some social animals are highly cooperative creatures that live in tight-knit colonies. Bees and ants are perhaps the most well-known examples of social insects, while Damaraland mole-rats and naked mole-rats, two rodent species found in southern and eastern Africa, are among the most cooperative mammal species. In these colony-forming animals, only one or a few females reproduce and these fertile females are frequently referred to as "queens". When an animal becomes a queen, her body shape can change dramatically to support the demands of high fertility and frequent reproduction. The molecular basis of such changes has been well-described in social insects. However, they are poorly understood in mammals. To address this knowledge gap, Johnston et al. studied how transitioning to queen status affects bone growth and structural integrity in Damaraland mole-rats, as well as body shape and size. The experiments compared non-breeding female mole-rats with other adult females recently paired with a male to become the sole breeder of a new colony. Johnston et al. also used bone-derived cells grown in the laboratory to assess underlying gene regulatory changes in new queen mole-rats. Johnston et al. showed that transitioning to the role of queen leads to a cascade of skeletal changes accompanied by shifts in the regulation of genetic pathways linked to bone growth. Queen mole-rats show accelerated growth in the spinal column of their lower back. These bones are called lumbar vertebrae and this likely allows them to have larger litters. However, queen mole-rats also lose bone growth potential in their leg bones and develop thinner thigh bones, which may increase the risk of bone fracture. Therefore, unlike highly social insects, mole-rats do not seem to have escaped the physical costs of intensive reproduction. This work adds to our understanding of the genes and physical traits that have evolved to support cooperative behaviour in social animals, including differences between insects and mammals. It also shows, with a striking example, how an animal's genome responds to social cues to produce a diverse range of traits that reflect their designated social role.

Oxidative costs of cooperation in cooperatively breeding Damaraland mole-rats.

Within cooperatively breeding societies, individuals adjust cooperative contributions to maximize indirect fitness and minimize direct fitness costs. Yet, little is known about the physiological costs of cooperation, which may be detrimental to direct fitness. Oxidative stress, the imbalance between reactive oxygen species (by-products of energy production) and antioxidant protection, may represent such a cost when cooperative behaviours are energetically demanding. Oxidative stress can lead to the accumulation of cellular damage, compromising survival and reproduction, thus mediating the trade-off between these competing life-history traits. Here, we experimentally increased energetically demanding cooperative contributions in captive Damaraland mole-rats (Fukomys damarensis). We quantified oxidative stress-related effects of increased cooperation on somatic and germline tissues, and the trade-off between them. Increased cooperative contributions induced oxidative stress in females and males, without increasing somatic damage. Males accumulated oxidative damage in their germline despite an increase in antioxidant defences. Finally, oxidative damage accumulation became biased towards the germline, while antioxidant protection remained biased towards the soma, suggesting that males favour the maintenance of somatic tissues (i.e. survival over reproduction). Our results show that heightened cooperative contributions can ultimately affect direct fitness through oxidative stress costs, which may represent a key selective pressure for the evolution of cooperation.

No task specialization among helpers in Damaraland mole-rats.

The specialization of individuals in specific behavioural tasks is often attributed either to irreversible differences in development, which generate functionally divergent cooperative phenotypes, or to age-related changes in the relative frequency with which individuals perform different cooperative activities; both of which are common in many insect caste systems. However, contrasts in cooperative behaviour can take other forms and, to date, few studies of cooperative behaviour in vertebrates have explored the effects of age, adult phenotype and early development on individual differences in cooperative behaviour in sufficient detail to discriminate between these alternatives. Here, we used multinomial models to quantify the extent of behavioural specialization within nonreproductive Damaraland mole-rats, Fukomys damarensis, at different ages. We showed that, although there were large differences between individuals in their contribution to cooperative activities, there was no evidence of individual specialization in cooperative activities that resembled the differences found in insect societies with distinct castes where individual contributions to different activities are negatively related to each other. Instead, individual differences in helping behaviour appeared to be the result of age-related changes in the extent to which individuals committed to all forms of helping. A similar pattern is observed in cooperatively breeding meerkats, Suricata suricatta, and there is no unequivocal evidence of caste differentiation in any cooperative vertebrate. The multinomial models we employed offer a powerful heuristic tool to explore task specialization and developmental divergence across social taxa and provide an analytical approach that may be useful in exploring the distribution of different forms of helping behaviour in other cooperative species.

Allo-parental care in Damaraland mole-rats is female biased and age dependent, though independent of testosterone levels.

In Damaraland mole-rats (Fukomys damarensis), non-breeding subordinates contribute to the care of offspring born to the breeding pair in their group by carrying and retrieving young to the nest. In social mole-rats and some cooperative breeders, dominant females show unusually high testosterone levels and it has been suggested that high testosterone levels may increase reproductive and aggressive behavior and reduce investment in allo-parental and parental care, generating age and state-dependent variation in behavior. Here we show that, in Damaraland mole-rats, allo-parental care in males and females is unaffected by experimental increases in testosterone levels. Pup carrying decreases with age of the non-breeding helper while the change in social status from non-breeder to breeder has contrasting effects in the two sexes. Female breeders were more likely than female non-breeders to carry pups but male breeders were less likely to carry pups than male non-breeders, increasing the sex bias in parental care compared to allo-parental care. Our results indicate that testosterone is unlikely to be an important regulator of allo-parental care in mole-rats.

Differences in cooperative behavior among Damaraland mole rats are consequences of an age-related polyethism.

In many cooperative breeders, the contributions of helpers to cooperative activities change with age, resulting in age-related polyethisms. In contrast, some studies of social mole rats (including naked mole rats, Heterocephalus glaber, and Damaraland mole rats, Fukomys damarensis) suggest that individual differences in cooperative behavior are the result of divergent developmental pathways, leading to discrete and permanent functional categories of helpers that resemble the caste systems found in eusocial insects. Here we show that, in Damaraland mole rats, individual contributions to cooperative behavior increase with age and are higher in fast-growing individuals. Individual contributions to different cooperative tasks are intercorrelated and repeatability of cooperative behavior is similar to that found in other cooperatively breeding vertebrates. Our data provide no evidence that nonreproductive individuals show divergent developmental pathways or specialize in particular tasks. Instead of representing a caste system, variation in the behavior of nonreproductive individuals in Damaraland mole rats closely resembles that found in other cooperatively breeding mammals and appears to be a consequence of age-related polyethism.

Intra- and interspecific challenges modulate cortisol but not androgen levels in a year-round territorial damselfish.

Interactions between individuals of different species are commonplace in animal communities. Some behaviors displayed during these interspecific social interactions may be very similar to those displayed during intraspecific social interactions. However, whether functional analogies between intra- and interspecific behaviors translate at the proximate level into an overlap in their underlying endocrine mechanisms remains largely unknown. Because steroids both mediate social behaviors and respond to them, we approached this question by comparing the behavioral and steroid response of free-living dusky gregories (Stegastes nigricans) to standardized territorial intrusions (sTI) of either conspecific or heterospecific food competitors. Stegastes nigricans is a year-round territorial fish that 'cultivates' the algae on which it feeds and is highly aggressive to both intra- and interspecific intruders. Behavioral differences between intra- and interspecific aggressive responses to sTI were marginal, and sTI tests caused an increase in cortisol levels that was positively related with the levels of aggression. In contrast, androgen levels did not increase in response to sTI, yet they showed a positive relationship with agonistic behavior. These results parallel a pattern that was first described for year-round territorial bird species. Furthermore, they suggest that changes in endocrine-hormone levels during territoriality might be independent of the species that induces the territorial response.

Intra- and interspecific aggression do not modulate androgen levels in dusky gregories, yet male aggression is reduced by an androgen blocker.

Discussions about social behavior are generally limited to fitness effects of interactions occurring between conspecifics. However, many fitness relevant interactions take place between individuals belonging to different species. Our detailed knowledge about the role of hormones in intraspecific interactions provides a starting point to investigate how far interspecific interactions are governed by the same physiological mechanisms. Here, we carried out standardized resident-intruder (sRI) tests in the laboratory to investigate the relationship between androgens and both intra- and interspecific aggression in a year-round territorial coral reef fish, the dusky gregory, Stegastes nigricans. This damselfish species fiercely defend cultivated algal crops, used as a food source, against a broad array of species, mainly food competitors, and thus represent an ideal model system for comparisons of intra-and interspecific territorial aggression. In a first experiment, resident S. nigricans showed elevated territorial aggression against intra- and interspecific intruders, yet neither elicited a significant increase in androgen levels. However, in a second experiment where we treated residents with flutamide, an androgen receptor blocker, males but not females showed decreased aggression, both towards intra- and interspecific intruders. Thus androgens appear to affect aggression in a broader territorial context where species identity of the intruder appears to play no role. This supports the idea that the same hormonal mechanism may be relevant in intra- and interspecific interactions. We further propose that in such a case, where physiological mechanisms of behavioral responses are found to be context dependent, interspecific territorial aggression should be considered a social behavior.

Treatment with the glucocorticoid antagonist RU486 reduces cooperative cleaning visits of a common reef fish, the lined bristletooth.

Cooperation often involves a conflict of interest. This is particularly true in situations where one individual seeks out a service but cannot properly control the quality of the service given by the partner who would gain from defecting. An example is cleaning mutualism involving the bluestreak cleaner wrasse (Labroides dimidiatus) and its reef-fish 'clients'. These cleaners may reduce the stress experienced by their clients by removing parasites; however they occasionally cheat clients (i.e. defect) by eating mucus and other living tissues. Here we present experimental support for the hypothesis that stress responses increase the motivation for clients to seek out such risky asymmetric interactions. We manipulated the stress response by blocking glucocorticoid receptors with the antagonist RU486 in a species that is a regular visitor of cleaner fish, the lined bristletooth (Ctenochaetus striatus). Field observations 1 week after treatment with RU486 showed that antagonist treatment led to a reduction in cleaning duration compared to control treatment. This was not explained by a general effect on client behavior as intraspecific social behavior appeared unaffected. We propose that antagonist treatment reduced stress responses to the presence of ectoparasites, which in turn reduced the client's perception of benefits from seeking out cleaning interactions. The results demonstrate a hitherto overlooked variable role of stress and stress responses on cooperative behavior.