Context-dependent social benefits drive cooperative predator defense in a bird

Understanding the major evolutionary transition from solitary individuals to complex societies is hampered by incomplete insight into the drivers of living in cooperative groups.^1-3 This may be because the benefits of sociality can derive from group living itself (e.g., dilution of predation risk),^4,5 or depend on social context (e.g., kin or potential mates represent beneficial group members).^6-8 Cooperative breeders, where non-breeding subordinates assist breeders, have provided important insights into the drivers of cooperation, but comprehensive assessment of diverse potential benefits has been hindered by a prevailing focus on benefits deriving from raising offspring.^9-11 We propose a novel paradigm to tease apart different benefits by comparing cooperative responses to predators threatening dependent young and adult group members according to their value for the responding individual. Applying this approach in purple-crowned fairy-wrens, Malurus coronatus, we show that non-breeding subordinates are more responsive to nest predators-a threat to offspring-when their probability of inheriting a breeding position is greater-irrespective of group size, relatedness to offspring, or opportunity to showcase individual quality to potential mates. This suggests that offspring defense is modulated according to the benefits of raising future helpers. Conversely, when predators pose a threat to adults, responsiveness depends on social context: subordinates respond more often when kin or potential mates are under threat, or when group members are associated with mutualistic social bonds, indirect genetic benefits, and future reproductive benefits.^9,12,13 Our results demonstrate that direct and kin-selected benefits of sociality are context dependent, and highlight the importance of predation risk in driving complex sociality.

Constrained flexibility of parental cooperation limits adaptive responses to harsh conditions

Parental care is predicted to evolve to mitigate harsh environments, thus adaptive plasticity of care may be an important response to our climate crisis. In biparental species, fitness costs may be reduced by resolving conflict and enhancing cooperation among partners. We investigated this prediction with the burying beetle, Nicrophorus orbicollis, by exposing them to contrasting benign and harsh thermal environments. Despite measurable fitness costs under the harsh environment, sexual conflict persisted in the form of sex-specific social plasticity. That is, females provided equivalent care with or without males, whereas males with partners deserted earlier and reduced provisioning effort. The interaction of social condition and thermal environment did not explain variation in individual behavior, failing to support a temperature-mediated shift from conflict to cooperation. Examining selection gradients and splines on cumulative care revealed a likely explanation for these patterns. Contrary to predictions, increased care did not enhance offspring performance under stress. Rather, different components of care were under different selection regimes, with optimization constrained due to lack of coordination between parents. We suggest that the potential for parenting to ameliorate the effects of our climate crisis may depend on the sex-specific evolutionary drivers of parental care, and that this may be best reflected in components of care.

Hard-working helpers contribute to long breeder lifespans in cooperative birds

In many species that raise young in cooperative groups, breeders live an exceptionally long time despite high investment in offspring production. How is this possible given the expected trade-off between survival and reproduction? One possibility is that breeders extend their lifespans by outsourcing parental care to non-reproductive group members. Having help lightens breeder workloads and the energy that is saved can be allocated to survival instead. We tested this hypothesis using phylogenetic meta-analysis across 23 cooperatively breeding bird species. We found that breeders with helpers had higher rates of annual survival than those without helpers (8% on average). Increased breeder survival was correlated with reduced investment in feeding offspring, which in turn depended on the proportion of feeding provided by helpers. Helpers had similar effects on female and male breeder survival. Our results indicate that one of the secrets to a long life is reduced investment in parental care. This appears to be a unique feature of cooperative societies with hard-working helpers. This article is part of the theme issue 'Ageing and sociality: why, when and how does sociality change ageing patterns?'

Group size increases inequality in cooperative behaviour

In cooperatively breeding species where rearing effort is shared among multiple group members, increases in group size typically reduce average per capita contributions to offspring care by all group members (load-lightening) but it is not known how changes in group size affect the distribution of workload among group members. The socioeconomic collective action theory suggests that, in larger groups, the incentives for free riding are stronger, leading to greater inequalities in work division among group members. Here, we use the Gini index to measure inequality at the group level in the contributions of helpers to three different cooperative behaviours (babysitting, pup-provisioning and raised guarding) in groups of varying size in wild Kalahari meerkats (Suricata suricatta). In larger groups, inequality in helpers' contributions to cooperative activities and the frequency of free riding both increased. Elevated levels of inequality were generated partly as a result of increased differences in contributions to cooperative activities between helpers in different sex and age categories in larger groups. After controlling for the positive effect of group size on total provisioning, increasing levels of inequality in contributions were associated with reductions in total pup-provisioning conducted by the group. Reductions in total pup-provisioning were, in turn, associated with reductions in the growth and survival of pups (but pup growth and survival were not directly affected by inequality in provisioning). Our results support the prediction of collective action theory described above and show how the Gini index can be used to investigate the distribution of cooperative behaviour within the group.

The influence of social-grouping on territorial defense behavior in the black-crested titmouse (Baeolophus atricristatus)

Abstract

Status signals have evolved for individuals to avoid energetic and physical costs of resource defense. These signals reflect an individual’s competitive ability and therefore influence competitors’ decisions on how to invest in a fight. We hypothesized that the response of receivers to status signals will depend on the social context. During territorial defense, group members may provide support to a territory owner by participating in defense. We investigated whether the presence of juveniles—who group together with territorial males—alters the territorial male’s attack decisions and level of aggression in the black-crested titmouse (Baeolophus atricristatus). Crest-length in this species functions as status signal. We simultaneously presented two taxidermic male models in a territory: one with an unmanipulated crest and one with a modified shortened crest. Models were presented to males that had resident juveniles cohabiting on their territory, and to males without juveniles. During intrusions, juveniles actively defended against the simulated intruders by approaching and sometimes attacking. The presence of juveniles affected how territorial males responded to the status signals of the intruders: when juveniles were present, males were more likely to first attack the model with the unmanipulated crest (i.e., longer, and more threatening), compared to males residing without juveniles. This suggests that juvenile support alters the risk-taking decision of the territorial male. To our knowledge, this is the first indication that behavioral responses to a status signal depends on the presence of supportive group members.

Significance statement

Status signals can indicate relative quality of animals and can therefore be used to evaluate a competitor when deciding whether or not to fight over resources. The black-crested titmouse has been shown to use its crest length as a status signal during fights over food. In our study, we assessed if this status signal is also used in territorial defense, by conducting an experiment where we presented two taxidermic male models with different crest sizes to a territorial male. We also investigated whether juvenile presence influenced which model was attacked. In trials where juveniles were present, territorial males attacked the longer crested model significantly more often than in trials where territorial males were alone. This suggests that the presence of juveniles, which help the male defend the territory, allows the male to attack the more aggressive-appearing intruder.

Predator defense is shaped by risk, brood value and social group benefits in a cooperative breeder

Predation is a major cause of mortality and nest failure in birds. Cooperative predator defense can enhance nest success and adult survival but, because it is inherently risky, dynamic risk assessment theory predicts that individuals modify defense behavior according to the risk posed by the predator. Parental investment theory, on the other hand, predicts that reproductive payoffs (brood value) determine investment in nest defense. We propose that, in cooperative breeders, fitness benefits deriving from the survival of other group members may additionally influence defense behavior (social group benefits theory). We tested predictions of these theories in the cooperatively breeding purple-crowned fairy-wren, Malurus coronatus, where brood value is higher for breeders, but social group benefits more important for helpers. We recorded experimentally induced individual defense behaviors in response to predator models presented near nests, representing differing levels of threat to nests and adults. As predicted, 1) individuals engaged in less risky defenses when encountering a more dangerous predator (dynamic risk assessment theory); 2) individuals defended older broods more often, and breeders defended more than helpers (parental investment theory); and 3) helpers were more likely to respond to a predator of adults (social group benefits theory). Our findings highlight that predator defense in cooperative breeders is complex, shaped by the combination of immediate risk and multiple benefits.

Experimentally induced antipredator responses are mediated by social and environmental factors

Nest predation is a common cause of reproductive failure for many bird species, and various antipredator defense behaviors have evolved to reduce the risk of nest predation. However, trade-offs between current reproductive duties and future reproduction often limit the parent's ability to respond to nest predation risk. Individual responses to experimentally increased nest predation risk can give insights into these trade-offs. Here, we investigate whether social and ecological factors affect individual responses to predation risk by experimentally manipulating the risk of nest predation using taxidermic mounts in the cooperative breeding Seychelles warbler (Acrocephalus sechellensis). Our results show that dominant females, but not males, alarm called more often when they confront a nest predator model alone than when they do so with a partner, and that individuals that confront a predator together attacked more than those that did so alone. Dominant males increased their antipredator defense by spending more time nest guarding after a presentation with a nest predator, compared with a nonpredator control, but no such effect was found for females, who did not increase the time spent incubating. In contrast to incubation by females, nest guarding responses by dominant males depended on the presence of other group members and food availability. These results suggest that while female investment in incubation is always high and not dependent on social and ecological conditions, males have a lower initial investment, which allows them to respond to sudden changes in nest predation risk.

Compensatory and additive helper effects in the cooperatively breeding Seychelles warbler (Acrocephalus sechellensis)

In cooperatively breeding species, care provided by helpers may affect the dominant breeders' investment trade-offs between current and future reproduction. By negatively compensating for such additional care, breeders can reduce costs of reproduction and improve their own chances of survival. Alternatively, helper care can be additive to that of dominants, increasing the fledging fitness of the current brood. However, the influence helpers have on brood care may be affected by group size and territory quality. Therefore, the impact of helping needs to be disentangled from other factors determining offspring investment before conclusive inferences about the effect of help on additive and compensatory care can be made. We used 20 years of provisioning data to investigate the effect of helping on provisioning rates in the facultative cooperatively breeding Seychelles warbler Acrocephalus sechellensis. Our extensive dataset allowed us to statistically disentangle the effects of helper presence, living in larger groups and different food availability. We show compensatory and additive care (i.e., partial compensation) in response to helper provisioning. Helpers lightened the provisioning load of the dominant male and female and increased total provisioning to nestlings. This was irrespective of group size or territory quality (food availability). Moreover, our results illustrate sex-specific variation in parental care over the course of the breeding event. We discriminate between temporal variation, group size, and territory quality processes affecting cooperative care and as such, gain further insight into the importance of these factors to the evolutionary maintenance of helping behavior.

The other facets of family life and their role in the evolution of animal sociality

Family life forms an integral part of the life history of species across the animal kingdom and plays a crucial role in the evolution of animal sociality. Our current understanding of family life, however, is almost exclusively based on studies that (i) focus on parental care and associated family interactions (such as those arising from sibling rivalry and parent-offspring conflict), and (ii) investigate these phenomena in the advanced family systems of mammals, birds, and eusocial insects. Here, we argue that these historical biases have fostered the neglect of key processes shaping social life in ancestral family systems, and thus profoundly hamper our understanding of the (early) evolution of family life. Based on a comprehensive survey of the literature, we first illustrate that the strong focus on parental care in advanced social systems has deflected scrutiny of other important social processes such as sibling cooperation, parent-offspring competition and offspring assistance. We then show that accounting for these neglected processes - and their changing role over time - could profoundly alter our understanding of the origin and subsequent evolution of family life. Finally, we outline how this 'diachronic' perspective on the evolution of family living provides novel insights into general processes driving the evolution of animal sociality. Overall, we infer that the explicit consideration of thus-far neglected facets of family life, together with their study across the whole diversity of family systems, are crucial to advance our understanding of the processes that shape the evolution of social life.

The oxidative costs of parental care in cooperative and pair-breeding African starlings

The cost of parental care has long been thought to favor the evolution of cooperative breeding, because breeders can provide reduced parental care when aided by alloparents. Oxidative stress-the imbalance between reactive oxygen species and neutralizing antioxidants-has been proposed to mediate the cost of parental care, though results from empirical studies remain equivocal. We measured changes in oxidative status during reproduction in cooperatively breeding superb starlings (Lamprotornis superbus) to gain insight into the relationships among breeding status, parental care, and oxidative stress. We also compared the oxidative cost of reproduction in the cooperatively breeding superb starling to that in a sympatric non-cooperatively breeding species, the greater blue-eared glossy starling (L. chalybaeus), to determine whether cooperatively breeding individuals face reduced oxidative costs of parental care relative to non-cooperatively breeding individuals. Breeders and alloparents of the cooperative species did not differ in oxidative status throughout a breeding attempt. However, individuals of the non-cooperative species incurred an increase in reactive oxygen metabolites proportionally to an individual's workload during offspring care. These findings suggest that non-cooperative starlings experience an oxidative cost of parental care, whereas cooperatively breeding starlings do not. It is possible that high nest predation risk and multi-brooding in the cooperatively breeding species may have favored reduced physiological costs of parental care more strongly compared to pair-breeding starlings. Reduced physiological costs of caring for young may thus represent a direct benefit that promotes cooperative breeding.

Multiple benefits of alloparental care in a fluctuating environment

Although cooperatively breeding vertebrates occur disproportionately in unpredictable environments, the underlying mechanism shaping this biogeographic pattern remains unclear. Cooperative breeding may buffer against harsh conditions (hard life hypothesis), or additionally allow for sustained breeding under benign conditions (temporal variability hypothesis). To distinguish between the hard life and temporal variability hypotheses, we investigated whether the number of alloparents at a nest increased reproductive success or load-lightening in superb starlings (Lamprotornis superbus), and whether these two types of benefits varied in harsh and benign years. We found that mothers experienced both types of benefits consistent with the temporal variability hypothesis, as larger contingents of alloparents increased the number of young fledged while simultaneously allowing mothers to reduce their provisioning rates under both harsh and benign rainfall conditions. By contrast, fathers experienced load-lightening only under benign rainfall conditions, suggesting that cooperative breeding may serve to take advantage of unpredictable benign breeding seasons when they do occur. Cooperative breeding in unpredictable environments may thus promote flexibility in offspring care behaviour, which could mitigate variability in the cost of raising young. Our results highlight the importance of considering how offspring care decisions vary among breeding roles and across fluctuating environmental conditions.

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.

Turn-taking in cooperative offspring care: by-product of individual provisioning behavior or active response rule?

Abstract

For individuals collaborating to rear offspring, effective organization of resource delivery is difficult because each carer benefits when the others provide a greater share of the total investment required. When investment is provided in discrete events, one possible solution is to adopt a turn-taking strategy whereby each individual reduces its contribution rate after investing, only increasing its rate again once another carer contributes. To test whether turn-taking occurs in a natural cooperative care system, here we use a continuous time Markov model to deduce the provisioning behavior of the chestnut-crowned babbler (Pomatostomus ruficeps), a cooperatively breeding Australian bird with variable number of carers. Our analysis suggests that turn-taking occurs across a range of group sizes (2–6), with individual birds being more likely to visit following other individuals than to make repeat visits. We show using a randomization test that some of this apparent turn-taking arises as a by-product of the distribution of individual inter-visit intervals (“passive” turn-taking) but that individuals also respond actively to the investment of others over and above this effect (“active” turn-taking). We conclude that turn-taking in babblers is a consequence of both their individual provisioning behavior and deliberate response rules, with the former effect arising through a minimum interval required to forage and travel to and from the nest. Our results reinforce the importance of considering fine-scale investment dynamics when studying parental care and suggest that behavioral rules such as turn-taking may be more common than previously thought.

Significance statement

Caring for offspring is a crucial stage in the life histories of many animals and often involves conflict as each carer typically benefits when others contribute a greater share of the work required. One way to resolve this conflict is to monitor when other carers contribute and adopt a simple “turn-taking” rule to ensure fairness, but natural parental care has rarely been studied in sufficient detail to identify such rules. Our study investigates whether cooperatively breeding chestnut-crowned babblers “take turns” delivering food to offspring, and (if so) whether this a deliberate strategy or simply a by-product of independent care behavior. We find that babblers indeed take turns and conclude that part of the observed turn-taking is due to deliberate responsiveness, with the rest arising from the species’ breeding ecology.

Electronic supplementary material

The online version of this article (10.1007/s00265-017-2391-4) contains supplementary material, which is available to authorized users.

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.

Cooperation and the common good

In this paper, we draw the attention of biologists to a result from the economic literature, which suggests that when individuals are engaged in a communal activity of benefit to all, selection may favour cooperative sharing of resources even among non-relatives. Provided that group members all invest some resources in the public good, they should refrain from conflict over the division of these resources. The reason is that, given diminishing returns on investment in public and private goods, claiming (or ceding) a greater share of total resources only leads to the actor (or its competitors) investing more in the public good, such that the marginal costs and benefits of investment remain in balance. This cancels out any individual benefits of resource competition. We illustrate how this idea may be applied in the context of biparental care, using a sequential game in which parents first compete with one another over resources, and then choose how to allocate the resources they each obtain to care of their joint young (public good) versus their own survival and future reproductive success (private good). We show that when the two parents both invest in care to some extent, they should refrain from any conflict over the division of resources. The same effect can also support asymmetric outcomes in which one parent competes for resources and invests in care, whereas the other does not invest but refrains from competition. The fact that the caring parent gains higher fitness pay-offs at these equilibria suggests that abandoning a partner is not always to the latter's detriment, when the potential for resource competition is taken into account, but may instead be of benefit to the 'abandoned' mate.

When cooperation begets cooperation: the role of key individuals in galvanizing support

Life abounds with examples of conspecifics actively cooperating to a common end, despite conflicts of interest being expected concerning how much each individual should contribute. Mathematical models typically find that such conflict can be resolved by partial-response strategies, leading investors to contribute relatively equitably. Using a case study approach, we show that such model expectations can be contradicted in at least four disparate contexts: (i) bi-parental care; (ii) cooperative breeding; (iii) cooperative hunting; and (iv) human cooperation. We highlight that: (a) marked variation in contributions is commonplace; and (b) individuals can often respond positively rather than negatively to the contributions of others. Existing models have surprisingly limited power in explaining these phenomena. Here, we propose that, although among-individual variation in cooperative contributions will be influenced by differential costs and benefits, there is likely to be a strong genetic or epigenetic component. We then suggest that selection can maintain high investors (key individuals) when their contributions promote support by increasing the benefits and/or reducing the costs for others. Our intentions are to raise awareness in—and provide testable hypotheses of—two of the most poorly understood, yet integral, questions regarding cooperative ventures: why do individuals vary in their contributions and when does cooperation beget cooperation?

Manipulating carer number versus brood size: complementary but not equivalent ways of quantifying carer effects on offspring

Measuring the causal effects of increasing carer number on offspring success is required to understand the evolution of cooperative care systems. Here, we did so using 2 experimental techniques in the chestnut-crowned babbler from outback Australia. Both carer removal and brood size manipulations indicate causal effects of helpers on offspring food acquisition. However, the results were not equivalent, with nestlings receiving more food following brood size manipulations, even after controlling for similar carer to offspring ratios.

Experiments designed to quantify the effects of increasing numbers of carers on levels of offspring care are rare in cooperative breeding systems, where offspring are reared by individuals additional to the breeding pair. This paucity might stem from disagreement over the most appropriate manipulations necessary to elucidate these effects. Here, we perform both carer removal and brood enhancement experiments to test the effects of numbers of carers and carer:offspring ratios on provisioning rates in the cooperatively breeding chestnut-crowned babbler (Pomatostomus ruficeps). Removing carers caused linear reductions in overall brood provisioning rates. Further analyses failed to provide evidence that this effect was influenced by territory quality or disruption of group dynamics stemming from the removals. Likewise, adding nestlings to broods caused linear increases in brood provisioning rates, suggesting carers are responsive to increasing offspring demand. However, the 2 experiments did not generate quantitatively equivalent results: Each nestling received more food following brood size manipulation than carer removal, despite comparable carer:offspring ratios in each. Following an at-hatching split-design cross-fostering manipulation to break any links between prehatching maternal effects and posthatching begging patterns, we found that begging intensity increased in larger broods after controlling for metrics of hunger. These findings suggest that manipulation of brood size can, in itself, influence nestling provisioning rates when begging intensity is affected by scramble competition. We highlight that carer number and brood size manipulations are complimentary but not equivalent; adopting both can yield greater overall insight into carer effects in cooperative breeding systems.

Group augmentation and the evolution of cooperation

The group augmentation (GA) hypothesis states that if helpers in cooperatively breeding animals raise the reproductive success of the group, the benefits of living in a resulting larger group--improved survival or future reproductive success--favour the evolution of seemingly altruistic helping behaviour. The applicability of the GA hypothesis remains debatable, however, partly owing to the lack of a clear conceptual framework and a shortage of appropriate empirical studies. We conceptualise here the GA hypothesis and illustrate that benefits of GA can accrue via different evolutionary mechanisms that relate closely to well-supported general concepts of group living and cooperation. These benefits reflect several plausible explanations for the evolutionary maintenance of helping behaviour in cooperatively breeding animals.

The role of social environment on parental care: offspring benefit more from the presence of female than male helpers

Investment in offspring depends on the costs and benefits to the carer, which can vary with sex and social status. Investment also depends on the effort of others by allowing for compensation (load-lightening), with biparental care studies showing that this depends on the state and type of the other carer. By contrast, studies on cooperative breeders have solely focussed on the effects of group size rather than its composition (i.e. social environment). Here we propose and provide the first test of the 'Social Environment' hypothesis, that is, how the characteristics (here the sex) of other helpers present in the group affect parental care and how this in turn affects offspring fitness in cooperatively breeding red-winged fairy-wrens (Malurus elegans). Breeders provisioned nestlings at a higher rate than helpers, but there was no sex difference in provisioning rate. Compensation to increasing group size varied little with sex and status, but strongly depended on social environment. All group members reduced their provisioning rates in response to an increasing number of male (load-lightening), but not female helpers (additive care). As a result, nestlings received more food and grew faster in the presence of female helpers. The increased nestling growth did convey a fitness advantage due to a higher post-fledging survival to adulthood. Our study provides the first evidence that parental care can depend on social environment. This could be an important overlooked aspect to explain variation in parental care in cooperative breeders in general and in particular the enormous variation between the sexes, which we reveal in a literature overview.