Cooperative personalities and social niche specialization in female meerkats

Pregnancy reduces concurrent pup care behaviour in meerkats, generating differences between dominant and subordinate females

In some mammals, and particularly in cooperative breeding ones, successive bouts of reproduction can overlap so that a female is often pregnant while still nurturing dependent young from her previous litter. Such an overlap requires females to divide their energetic budget between two reproductive activities, and pregnancy costs would consequently be expected to reduce investment in concurrent offspring care. However, explicit evidence for such reductions is scarce, and the potential effects they may have on work division in cooperative breeders have not been explored. Using 25 years of data on reproduction and cooperative behaviour in wild Kalahari meerkats, supplemented with field experiments, we investigated whether pregnancy reduces contributions to cooperative pup care behaviours, including babysitting, provisioning and raised guarding. We also explored whether pregnancy, which is more frequent in dominants than subordinates, could account for the reduced contributions of dominants to the cooperative pup care behaviours. We found that pregnancy, particularly at late stages of gestation, reduces contributions to cooperative pup care; that these reductions are eliminated when the food available to pregnant females is experimentally supplemented; and that pregnancy effects accounted for differences between dominants and subordinates in two of the three cooperative behaviours examined (pup provisioning and raised guarding but not babysitting). By linking pregnancy costs with reductions in concurrent pup care, our findings illuminate a trade-off between investment in successive, overlapping bouts of reproduction. They also suggest that some of the differences in cooperative behaviour between dominant and subordinate females in cooperative breeding mammals can be a direct consequence of differences in their breeding frequency.

The social evolution of individual differences: Future directions for a comparative science of personality in social behavior

Personality is essential for understanding the evolution of cooperation and conflict in behavior. However, personality science remains disconnected from the field of social evolution, limiting our ability to explain how personality and plasticity shape phenotypic adaptation in social behavior. Researchers also lack an integrative framework for comparing personality in the contextualized and multifaceted behaviors central to social interactions among humans and other animals. Here we address these challenges by developing a social evolutionary approach to personality, synthesizing theory, methods, and organizing questions in the study of individuality and sociality in behavior. We critically review current measurement practices and introduce social reaction norm models for comparative research on the evolution of personality in social environments. These models demonstrate that social plasticity affects the heritable variance of personality, and that individual differences in social plasticity can further modify the rate and direction of adaptive social evolution. Future empirical studies of frequency- and density-dependent social selection on personality are crucial for further developing this framework and testing adaptive theory of social niche specialization.

Hormones do not maketh the mole-rat: No steroid hormone signatures of subordinate behavioral phenotypes

In some cooperatively breeding groups, individuals have distinct behavioral characteristics that are often stable and predictable across time. However, in others, as in the eusocial naked mole-rat, evidence for behavioral phenotypes is ambiguous. Here, we study whether the naked mole-rat can be divided into discrete phenotypes and if circulating hormone concentrations underpin these differences. Naked mole-rat colonies consist of a single breeding female and large numbers of non-reproductive subordinates that in some cases can exceed several hundred in a colony. The subordinates can potentially be divided into soldiers, who defend the colony; workers, who maintain it; and dispersers, who want to leave it. We established six colonies de novo, tracked them over three years, and assessed the behavior and hormone concentrations of the subordinates. We found that soldiers tended to be from earlier litters and were higher ranked compared to workers, whereas dispersers were distributed throughout litters and rankings. There was no difference in estradiol, testosterone, or dehydroepiandrosterone (DHEA) concentrations among phenotypes. Progesterone concentrations were higher in soldiers, but this difference appeared to be driven by a few individuals. Principal component analysis demonstrated that soldiers separated into a discrete category relative to workers/dispersers, with the highest ranked loadings being age, body mass, and testosterone concentrations. However, the higher testosterone in soldiers was correlated with large body size instead of strictly behavioral phenotype. Workers and dispersers have more overlap with each other and no hormonal differences. Thus the behavioral variation in subordinate naked mole-rats is likely not driven by circulating steroid hormone concentrations, but rather it may stem from alternative neural and/or neuroendocrine mechanisms.

Temporal consistency and ecological validity of personality structure in common marmosets (Callithrix jacchus): A unifying field and laboratory approach

Personality in animals has been extensively researched in recent decades. Temporal consistency of behaviors is almost always part of the personality definition and is usually explored in several different testing sessions or observation periods. However, it is still unclear whether the obtained personality constructs are stable across several years, which might be especially important for long-living animals, such as primates. Further, little is known on whether the personality structures obtained in the laboratory reflect the structures obtained under ecologically relevant conditions in the wild. Therefore, we conducted a battery of personality tests on common marmosets (Callithrix jacchus) (N = 27), compared it with a test battery conducted 4 years beforehand on a subset of animals in captivity (N = 13) and ran an adapted version under field conditions at Baracuhy Biological Field Station, Brazil (N = 18). Under captive conditions, we found a remarkably similar personality structure across 4 testing years. Further, we found high long-term temporal consistency in the first two personality components, Boldness and Exploration; however, monkeys that changed their social (i.e., breeding) status between the two testing periods showed a significant increase in Boldness scores. Under field conditions, we found a somewhat similar personality structure as compared to the laboratory, which to some extent corroborates ecological validity of our personality test design. Nevertheless, whether the structure in the wild is suppressed or expanded in comparison to captivity, and in which way the social setting influences personality structure, should be further explored. Taken together, our results contribute to the discussion about the reliability and ecological validity of personality structures in nonhuman primates.

Testosterone as a mediator of the tradeoff between cooperation and competition in the context of cooperative reproductive behaviors

Behavioral tradeoffs occur when the expression of one behavior detracts from the expression of another. Understanding the proximate mediators of behavioral tradeoffs is important as these tradeoffs can act as potential constraints on evolutionary responses to selection. Here, we describe the tradeoff between cooperation and competition faced by species that exhibit cooperative reproductive behaviors and propose that testosterone is a key hormonal mediator of the tradeoff. Cooperative reproductive behaviors occur when multiple individuals coordinate their efforts to gain a reproductive advantage over other individuals and/or those individuals attempting to reproduce in absence of cooperation. We propose that testosterone, a sex steroid known to mediate a number of physiological and behavioral actions associated with reproductive competition, is involved in mediating the tradeoff between cooperation and competition. To support this proposition, we first describe the importance of individual variation in behavior to the evolution of cooperative behaviors. We then describe how proximate mechanisms represent a prominent source of individual variation in social behaviors and highlight evidence suggesting testosterone mediates variation in cooperative behaviors. Two case studies in which the relationship between testosterone and cooperative behaviors have been investigated in detail are then summarized. Throughout we highlight the importance of studying individual variation to understand the mechanistic basis of behaviors, behavioral tradeoffs, and the evolution of cooperative reproductive behaviors more broadly.

Effects of trap confinement on personality measurements in two terrestrial rodents

In recent years, consistent individual differences in behavior, or personalities, have been a topic of increasing interest as researchers strive to understand and predict the responses of individuals and populations to anthropogenic changes. Behavioral studies in wild populations often require that animals are live trapped before behavioral observation can occur, and this is especially true in studies investigating animal personalities. However, it is unknown whether the amount of time confined to a live trap may regulate the behavior of trapped individuals. Specifically, if the duration of trap confinement directly influences behavior, then by obtaining wild animals through live trapping we may be confounding the very measurements of greatest interest. To investigate whether the duration of trap confinement influences the behavior of trapped individuals, we performed a study on two small mammal species, focusing specifically on personality traits. We positioned high-definition trail cameras facing Longworth small mammal traps in the field to observe capture events and record the time of capture. We then measured personality in captured deer mice (Peromyscus maniculatus) and southern red-backed voles (Myodes gapperi) using three standardized tests, and through linear and generalized linear models we found that the time an animal had spent confined to a trap before testing did not affect 86% of behaviors exhibited. Our results showed two weak behavioral effects of confinement duration on boldness and docility resulting from an interaction between the duration of confinement and whether or not an individual was naïve to trapping. Our results suggest that behavioral measurements of wild, trapped small mammals are not determined by the time spent confined to a trap. However, researchers should use caution and consider whether an animal is naïve to trapping during analysis since habituation to the live trap may play a role in the effects of confinement duration on behavior.

An evolutionary quantitative genetics model for phenotypic (co)variances under limited dispersal, with an application to socially synergistic traits

Darwinian evolution consists of the gradual transformation of heritable traits due to natural selection and the input of random variation by mutation. Here, we use a quantitative genetics approach to investigate the coevolution of multiple quantitative traits under selection, mutation, and limited dispersal. We track the dynamics of trait means and of variance-covariances between traits that experience frequency-dependent selection. Assuming a multivariate-normal trait distribution, we recover classical dynamics of quantitative genetics, as well as stability and evolutionary branching conditions of invasion analyses, except that due to limited dispersal, selection depends on indirect fitness effects and relatedness. In particular, correlational selection that associates different traits within-individuals depends on the fitness effects of such associations between-individuals. We find that these kin selection effects can be as relevant as pleiotropy for the evolution of correlation between traits. We illustrate this with an example of the coevolution of two social traits whose association within-individuals is costly but synergistically beneficial between-individuals. As dispersal becomes limited and relatedness increases, associations between-traits between-individuals become increasingly targeted by correlational selection. Consequently, the trait distribution goes from being bimodal with a negative correlation under panmixia to unimodal with a positive correlation under limited dispersal.

Social experiences shape behavioral individuality and within-individual stability

Individual repeatability characterizes many behaviors. Repeatable behavior may result from repeated social interactions among familiar group members, owing to adaptive social niche specialization. In the context of aggression, in species like field crickets, social niche specialization should also occur when individuals repeatedly interact with unfamiliar individuals. This would require the outcome of social interactions to have carry-over effects on fighting ability and aggressiveness in subsequent interactions, leading to long-term among-individual differentiation. To test this hypothesis, we randomly assigned freshly emerged adult males of the southern field cricket Gryllus bimaculatus to either a solitary or social treatment. In social treatment, males interacted with a same-sex partner but experienced a new partner every 3 days. After 3 weeks of treatment, we repeatedly subjected treated males to dyadic interactions to measure aggression. During this time, we also continuously measured the 3-daily rate of carbohydrate and protein consumption. Individual differentiation was considerably higher among males reared in the social versus solitary environment for aggressiveness but not for nutrient intake. Simultaneously, social experience led to lower within-individual stability (i.e., increased within-individual variance) in carbohydrate intake. Past social experiences, thus, shaped both behavioral individuality and stability. While previous research has emphasized behavioral individuality resulting from repeated interactions among familiar individuals, our study implies that behavioral individuality, in the context of aggression, may generally result from social interactions, whether with familiar or unfamiliar individuals. Our findings thus imply that social interactions may have a stronger effect on individual differentiation than previously appreciated.

Growth affects dispersal success in social mole-rats, but not the duration of philopatry

In naked mole-rats (Heterocephalus glaber), some non-breeding males show faster growth and are more likely to disperse than others. These differences have been suggested to be the result of a specialized developmental strategy leading to shorter philopatry and independent breeding, as opposed to extended philopatry as non-reproductive helpers. However, it is unclear whether fast-growing males disperse sooner than slow-growing males. An alternative explanation is that variation in quality between individuals causes high-quality individuals to grow quickly and maximize dispersal success without reducing philopatry. Here we show that in Damaraland mole-rats (Fukomys damarensis), males that subsequently disperse successfully grow faster than other non-reproductive males. This pattern is predicted by both hypotheses and does not discriminate between them. However, contrary to the suggestion that faster growth represents a developmental specialization for early dispersal, fast-growing and slow-growing males remained equally long in their natal groups. Our study provides no evidence for adaptive divergence in male development leading either to early dispersal or extended philopatry. Instead of representing specialized dispersers, fast-growing males of this species may be high-quality individuals.

Associations between glucocorticoids and sociality across a continuum of vertebrate social behavior

The causes and consequences of individual differences in animal behavior and stress physiology are increasingly studied in wild animals, yet the possibility that stress physiology underlies individual variation in social behavior has received less attention. In this review, we bring together these study areas and focus on understanding how the activity of the vertebrate neuroendocrine stress axis (HPA-axis) may underlie individual differences in social behavior in wild animals. We first describe a continuum of vertebrate social behaviors spanning from initial social tendencies (proactive behavior) to social behavior occurring in reproductive contexts (parental care, sexual pair-bonding) and lastly to social behavior occurring in nonreproductive contexts (nonsexual bonding, group-level cooperation). We then perform a qualitative review of existing literature to address the correlative and causal association between measures of HPA-axis activity (glucocorticoid levels or GCs) and each of these types of social behavior. As expected, elevated HPA-axis activity can inhibit social behavior associated with initial social tendencies (approaching conspecifics) and reproduction. However, elevated HPA-axis activity may also enhance more elaborate social behavior outside of reproductive contexts, such as alloparental care behavior. In addition, the effect of GCs on social behavior can depend upon the sociality of the stressor (cause of increase in GCs) and the severity of stress (extent of increase in GCs). Our review shows that the while the associations between stress responses and sociality are diverse, the role of HPA-axis activity behind social behavior may shift toward more facilitating and less inhibiting in more social species, providing insight into how stress physiology and social systems may co-evolve.

Consistent cooperation in a cichlid fish is caused by maternal and developmental effects rather than heritable genetic variation

Studies on the evolution of cooperative behaviour are typically confined to understanding its adaptive value. It is equally essential, however, to understand its potential to evolve, requiring knowledge about the phenotypic consistency and genetic basis of cooperative behaviour. While previous observational studies reported considerably high heritabilities of helping behaviour in cooperatively breeding vertebrates, experimental studies disentangling the relevant genetic and non-genetic components of cooperative behaviour are lacking. In a half-sibling breeding experiment, we investigated the repeatability and heritability of three major helping behaviours performed by subordinates of the cooperatively breeding fish Neolamprologus pulcher To experimentally manipulate the amount of help needed in a territory, we raised the fish in two environments differing in egg predation risk. All three helping behaviours were significantly repeatable, but had very low heritabilities. The high within-individual consistencies were predominantly due to maternal and permanent environment effects. The perceived egg predation risk had no effect on helping, but social interactions significantly influenced helping propensities. Our results reveal that developmentally plastic adjustments of provided help to social context shape cooperative phenotypes, whereas heritable genetic variation plays a minor role.

Genetics and developmental biology of cooperation

Despite essential progress towards understanding the evolution of cooperative behaviour, we still lack detailed knowledge about its underlying molecular mechanisms, genetic basis, evolutionary dynamics and ontogeny. An international workshop "Genetics and Development of Cooperation," organized by the University of Bern (Switzerland), aimed at discussing the current progress in this research field and suggesting avenues for future research. This review uses the major themes of the meeting as a springboard to synthesize the concepts of genetic and nongenetic inheritance of cooperation, and to review a quantitative genetic framework that allows for the inclusion of indirect genetic effects. Furthermore, we argue that including nongenetic inheritance, such as transgenerational epigenetic effects, parental effects, ecological and cultural inheritance, provides a more nuanced view of the evolution of cooperation. We summarize those genes and molecular pathways in a range of species that seem promising candidates for mechanisms underlying cooperative behaviours. Concerning the neurobiological substrate of cooperation, we suggest three cognitive skills necessary for the ability to cooperate: (i) event memory, (ii) synchrony with others and (iii) responsiveness to others. Taking a closer look at the developmental trajectories that lead to the expression of cooperative behaviours, we discuss the dichotomy between early morphological specialization in social insects and more flexible behavioural specialization in cooperatively breeding vertebrates. Finally, we provide recommendations for which biological systems and species may be particularly suitable, which specific traits and parameters should be measured, what type of approaches should be followed, and which methods should be employed in studies of cooperation to better understand how cooperation evolves and manifests in nature.

Avoiding the misuse of BLUP in behavioural ecology

Having recognized that variation around the population-level "Golden Mean" of labile traits contains biologically meaningful information, behavioural ecologists have focused increasingly on exploring the causes and consequences of individual variation in behaviour. These are exciting new directions for the field, assisted in no small part by the adoption of mixed-effects modelling techniques that enable the partitioning of among- and within-individual behavioural variation. It has become commonplace to extract predictions of individual random effects from such models for use in subsequent analyses (for example, between a personality trait and other individual traits such as cognition, physiology, or fitness-related measures). However, these predictions are made with large amounts of error that is not carried forward, rendering further tests susceptible to spurious P values from these individual-level point estimates. We briefly summarize the problems with such statistical methods that are used regularly by behavioural ecologists, and highlight the robust solutions that exist within the mixed model framework, providing tutorials to aid in their implementation.

Exploration is dependent on reproductive state, not social state, in a cooperatively breeding bird

Personality is an intriguing phenomenon in populations because it constrains behavioral flexibility. One theory suggests that personality could be generated and maintained if dependent on asset protection. It is predicted that trade-offs with fitness expectations and survival probability encourage consistent behavioral differences among individuals (personality). Although not mutually exclusive, the social niche specialization hypothesis suggests that a group of individuals that repeatedly interact will develop personality to avoid costly social conflict. The point at which behavioral consistency originates in the social niche hypothesis is still unclear, with predictions for development after a change in social status. In the facultative cooperatively breeding Seychelles warbler (Acrocephalus sechellensis), residing on Cousin Island, breeding vacancies are limited and this forces individuals into different social roles. We used this system to test whether reproductive and social state predicted among-individual differences in exploration. We had 2 predictions. First, that an individual's start in life can predict personality, whereby young individuals with a good start to life (associated with early age reproduction and earlier onset survival senescence) are fast explorers, suggesting reproductive state-dependence. Second, that an individual's social status can predict personality, whereby dominant individuals will be fast explorers, suggesting that the behavior is social state-dependent. Neither of the behaviors was associated with social state and social state did not affect behavioral consistency. However, novel object exploration was associated with a proxy of reproductive state. Our results provide further support for state being a mechanism for generating individual differences in behavior.

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.

Individual and Group Performance Suffers from Social Niche Disruption

The social niche specialization hypothesis predicts that animal personalities emerge as a result of individuals occupying different social niches within a group. Here we track individual personality and performance and collective performance among groups of social spiders where we manipulated the familiarity of the group members. We show that individual personalities, as measured by consistent individual differences in boldness behavior, strengthen with increasing familiarity and that these personalities can be disrupted by a change in group membership. Changing group membership negatively impacted both individual and group performance. Individuals in less familiar groups lost weight, and these groups were less successful at performing vital collective tasks. These results provide a mechanism for the evolution of stable social groups by demonstrating that social niche reestablishment carries a steep cost for both individuals and groups. Social niche specialization may therefore provide a potential first step on the path toward more organized social systems.

Persistent social interactions beget more pronounced personalities in a desert-dwelling social spider

The social niche specialization hypothesis predicts that repeated social interactions will generate social niches within groups, thereby promoting consistent individual differences in behaviour. Current support for this hypothesis is mixed, probably because the importance of social niches is dependent upon the ecology of the species. We test whether repeated interactions among group mates generate consistent individual differences in boldness in the social spider, Stegodyphus dumicola. In support of the social niche specialization hypothesis, we found that consistent individual differences in boldness increased with longer group tenure. Interestingly, these differences took longer to appear than in previous work suggesting this species needs more persistent social interactions to shape its behaviour. Recently disturbed colonies were shyer than older colonies, possibly reflecting differences in predation risk. Our study emphasizes the importance of the social environment in generating animal personalities, but also suggests that the pattern of personality development can depend on subtle differences in species' ecologies.