Broodcare or Egg Cannibalism by Parents and Helpers in Neolamprologus brichardi (Poll 1986) (Pisces: Cichlidae): a Study on Behavioural Mechanisms

Nepotism mediates enforced cooperation in asymmetric negotiations

In cooperative societies, group members typically exchange different commodities among each other, which involves an incessant negotiation process. How is the conflict of fitness interests resolved in this continual bargaining process between unequal partners, so that maintaining the cooperative interaction is the best option for all parties involved? Theory predicts that relatedness between group members may alleviate the conflict of fitness interests, thereby promoting the evolution of cooperation. To evaluate the relative importance of relatedness and direct fitness effects in the negotiation process, we experimentally manipulated both the relatedness and mutual behavioral responses of dominant breeders and subordinate helpers in the cooperatively breeding cichlid fish Neolamprologus pulcher. Results show that coercion by breeders is crucial for the performance of alloparental egg care by helpers, but that kinship significantly decreases the need for coercion as predicted by theory. This illustrates the relative importance of kinship and enforcement in the bargaining process.

Galanin expression varies with parental care and social status in a wild cooperatively breeding fish

As many busy parents will attest, caring for young often comes at the expense of having time to feed and care for oneself. Galanin is a neuropeptide that regulates food intake and modulates parental care; however, the relative importance of galanin in the regulation of feeding versus caring by parents has never been evaluated before under naturalistic settings. Here, we assessed how expression of the galanin system varied in two brain regions, the hypothalamus (which regulates feeding) and the preoptic area (which modulates social behaviours including care) in a wild cichlid fish, Neolamprologus pulcher. Females with young had higher hypothalamic expression of galanin receptor 1a, and the highest expression of galanin and galanin receptor 1a was observed in females that foraged the least. However, expression of five other feeding-related neuropeptides did not change while females were caring for young suggesting that changes in the hypothalamic galanin system may not have been directly related to changes in food intake. The preoptic galanin system was unaffected by the presence of young, but preoptic galanin expression was higher in dominant females (which are aggressive, regularly reproduce and care for young) compared to subordinate females (which are submissive, rarely reproduce but often help care for young). Additionally, preoptic galanin expression was higher in fish that performed more territory defense. Overall, our results indicate that galanin has brain-region-specific roles in modulating both parental care and social status in wild animals.

Galanin and prolactin expression in relation to parental care in two sympatric cichlid species from Lake Tanganyika

Our understanding of the hormonal mechanisms underlying parental care mainly stems from research on species with uniparental care. Far less is known about the physiological changes underlying motherhood and fatherhood in biparental caring species. Here, using two biparental caring cichlid species (Neolamprologus caudopunctatus and Neolamprologus pulcher), we explored the relative gene-expression levels of two genes implicated in the control of parental care, galanin (gal) and prolactin (prl). We investigated whole brain gene expression levels in both, male and female caring parents, as well as in non-caring individuals of both species. Caring males had higher prl and gal mRNA levels compared to caring females in both fish species. Expression of gal was highest when young were mobile and the need for parental defense was greatest and gal was lowest during the more stationary egg tending phase in N. caudopunctatus. The onset of parenthood was associated with lower expression of prl and higher expression of gal in N. pulcher, but this pattern was not observed in N. caudopunctatus. Our study demonstrates that gal gene expression is correlated with changes in parental care in two biparental cichlid species and extends both knowledge and taxonomic coverage of the possible neurogenetic mechanisms underlying parental care.

How individual and relative size affect participation in territorial defense and cortisol levels in a social fish

For many species, behaviors such as territory defense and parental care are energetically costly, but are nonetheless can provide substantial fitness gains. In systems in which both parents provide parental care, each of the parents benefits from exhibiting (or having their partner exhibit) these behaviors. However, in many cases, costs and benefits differ between parents due to factors such as size or sex. Different intruder types may also impose different costs on parents. Predatory intruders might consume offspring, whereas conspecifics might threaten the social status of a parent, or provide benefits as a potential group joiner or mate. Responses to these intrusions may also be associated with variation in individual stress responses. We investigated associations among male and female sizes, and the interaction between these, with defense against conspecific and heterospecific territorial intruders by members of successfully breeding pairs in the cichlid fish Neolamprologus pulcher. We also investigated whether cortisol levels were associated with size or participation in territory defense because each may be a cause or consequence of individual variation in the stress response. We found that females paired with large males performed fewer defensive behaviors than females paired with smaller males. Males paired with relatively large females had higher baseline cortisol levels than those paired with smaller females. Collectively, individual characteristics such as size have consequences for each individual's behavior, and also influence the behavior, and endocrine state of social partners.

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.

Divergence of developmental trajectories is triggered interactively by early social and ecological experience in a cooperative breeder

Cooperative breeders feature the highest level of social complexity among vertebrates. Environmental constraints foster the evolution of this form of social organization, selecting for both well-developed social and ecological competences. Cooperative breeders pursue one of two alternative social trajectories: delaying reproduction to care for the offspring of dominant breeders or dispersing early to breed independently. It is yet unclear which ecological and social triggers determine the choice between these alternatives and whether diverging developmental trajectories exist in cooperative vertebrates predisposing them to dispersal or philopatry. Here we experimentally reared juveniles of cooperatively breeding cichlid fish by varying the social environment and simulated predation threat in a two-by-two factorial long-term experiment. First, we show that individuals develop specialized behavioral competences, originating already in the early postnatal phase. Second, these specializations predisposed individuals to pursue different developmental trajectories and either to disperse early or to extend philopatry in adulthood. Thus, our results contrast with the proposition that social specializations in early ontogeny should be restricted to eusocial species. Importantly, social and ecological triggers were both required for the generation of divergent life histories. Our results thus confirm recent predictions from theoretical models that organisms should combine relevant information from different environmental cues to develop integrated phenotypes.

A test of male infanticide as a reproductive tactic in a cichlid fish

Infanticide and offspring cannibalism are taxonomically widespread phenomena. In some group-living species, a new dominant individual taking over a group can benefit from infanticide if doing so induces potential mates to become reproductively available sooner. Despite widespread observations of infanticide (i.e. egg cannibalism) among fishes, no study has investigated whether egg cannibalism occurs in fishes as a result of group takeovers, or how this type of cannibalism might be adaptive. Using the cooperatively breeding cichlid, Neolamprologus pulcher, we tested whether new unrelated males entering the dominant position in a social group were more likely to cannibalize eggs, and whether such cannibalism would shorten the interval until the female's next spawning. Females spawned again sooner if their broods were removed than if they were cared for. Egg cannibalism occurred frequently after a group takeover event, and was rarer if the original male remained with the group. While dominant breeder females were initially highly aggressive towards newcomer males that took over the group, the degree of resistance depended on relative body size differences between the new pair and, ultimately, female aggression did not prevent egg cannibalism. Egg cannibalism, however, did not shorten the duration until subsequent spawning, or increase fecundity during subsequent breeding in our laboratory setting. Our results show that infanticide as mediated through group takeovers is a taxonomically widespread behaviour.

Social status influences responses to unfamiliar conspecifics in a cooperatively breeding fish

In group living animals, individuals may visit other groups. The costs and benefits of such visits for the members of a group will depend on the attributes and intentions of the visitor, and the social status of responding group members. Using wild groups of the cooperatively breeding cichlid fish (Neolamprologus pulcher), we compared group member responses to unfamiliar ‘visiting’ conspecifics in control groups and in experimentally manipulated groups from which a subordinate the same size and sex as the visitor was removed. High-ranking fish were less aggressive towards visitors in removal groups than in control groups; low-ranking subordinates were more aggressive in the removal treatment. High-ranking females and subordinates the same size and sex as the visitor responded most aggressively toward the visitor in control groups. These results suggest that visitors are perceived as potential group joiners, and that such visits impose different costs and benefits on current group members.

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.

Group size adjustment to ecological demand in a cooperative breeder

Environmental factors can determine which group size will maximize the fitness of group members. This is particularly important in cooperative breeders, where group members often serve different purposes. Experimental studies are yet lacking to check whether ecologically mediated need for help will change the propensity of dominant group members to accept immigrants. Here, we manipulated the perceived risk of predation for dominant breeders of the cooperatively breeding cichlid fish Neolamprologus pulcher to test their response to unrelated and previously unknown immigrants. Potential immigrants were more readily accepted if groups were exposed to fish predators or egg predators than to herbivorous fish or control situations lacking predation risk. Our data are consistent with both risk dilution and helping effects. Egg predators were presented before spawning, which might suggest that the fish adjust acceptance rates also to a potential future threat. Dominant group members of N. pulcher apparently consider both present and future need of help based on ecological demand. This suggests that acceptance of immigrants and, more generally, tolerance of group members on demand could be a widespread response to ecological conditions in cooperatively breeding animals.

Relatedness and helping in fish: examining the theoretical predictions

Many studies have attempted to explain the evolution of cooperation, yet little attention has been paid to what factors control the amount or kind of cooperation performed. Kin selection theory suggests that more cooperation, or help, should be given by relatives. However, recent theory suggests that under specific ecological and demographic conditions, unrelated individuals must 'pay to stay' in the group and therefore may help more. We tested these contrasting predictions using the cooperatively breeding fish, Neolamprologus pulcher, and found that the degree of work effort by helpers depended on which helping behaviours were considered and on their level of relatedness to the breeding male or female. In the field, helpers unrelated to the breeding male performed more territory defence, while helpers unrelated to the breeding female contributed less to territory defence. In the laboratory, unrelated group members helped more. Our work demonstrates that a number of factors in addition to kinship shape cooperative investment patterns.

Unrelated helpers will not fully compensate for costs imposed on breeders when they pay to stay

Unrelated subordinates may invest in costly help to avoid being evicted from groups (the 'pay-to-stay' hypothesis). However, the effectiveness of eviction to enforce help should depend on its being applied accurately and on the costs it imposes on both dominants and subordinates. The relative cost of being evicted is a function of the population frequency of eviction when population growth is limited by density-dependent factors. We describe a stage-structured pay-to-stay model incorporating density-dependent population growth, costly eviction and occasional errors. Breeders demand some amount of help and evict subordinates that do not provide it. Helpers decide on the amount of help they will provide. The threat of eviction alone is sufficient to enforce helping. However, helping will not be favoured if helpers do not impose costs on breeders. The amount of help provided is less than the cost that subordinates impose upon breeders, when any help is provided. Thus, the net fitness effect of a helper under pay-to-stay alone is negative, even if it is investing in cooperative behaviour. Constraints on dispersal have no effect on the amount of help, although they may influence the tolerance threshold of breeders and group stability, depending on the mechanism of density dependence.

When to parasitize? A dynamic optimization model of reproductive strategies in a cooperative breeder

We consider a cooperatively breeding group and find the optimal pattern of reproductive parasitism by a subordinate helper as a function of its body size, and hence the share of reproduction obtained by the subordinate. We develop the model for the social system of the cooperatively breeding cichlid fish Neolamprologus pulcher but the general framework is also applicable to other cooperative systems. In addition to behaving cooperatively by sharing tasks, sexually mature male cichlid helpers may directly parasitize the reproduction of dominant breeders in the group. We investigate the relative influence of life history and behavioural variables including growth, parasitism capacity, future reproductive fitness benefits and costs, relatedness and expulsion risk on the optimal reproductive strategy of subordinates. In a detailed analysis of the parameter space we show that a male helper should base its decision to parasitize primarily on an increase in expulsion risk resulting from reproductive parasitism (punishment), intra-group relatedness and the parasitism capacity. If expulsion risk is high then helpers should not parasitize reproduction at medium body size but should parasitize either when small or large.

Behavioural time-energy budgets of cooperatively breeding Neolamprologus pulcher (Pisces: Cichlidae)

We estimated the energy expenditures resulting from helping behaviour in the cooperatively breeding cichlid N. pulcher by measuring the metabolic rates directly associated with specific social and territory maintenance behaviours of individual pair males and females, and their helpers, in a respirometer. In pair males, pair females and helpers, routine metabolism was raised on average 4.4, 3.8 and 3.6 times, respectively, during agonistic behaviour. Helpers spent 3.3 and 6.1 times routine metabolism, respectively, on submissive behaviour (tail quivering) and digging. These estimates of energy expenditure were combined with laboratory time budgets, obtained previously, to calculate behavioural time-energy budgets for pair members and helpers. Both groups spent on average 98.5% of total metabolism on routine and standard metabolism. With regard to the energy expended on specific behaviours, pair males invested almost exclusively in intrafamily agonistic behaviour, while pair females and helpers shared the investment in territory maintenance and direct brood care. The behavioural energy budget of helpers was strongly determined by their submissive behaviour. This serves to maintain the social status of the helpers within the family hierarchy and may therefore be regarded as 'paying for staying', which may also be true for direct brood care and other helper duties. We conclude that the substantial energy expenditures associated with helping behaviours are probably partly responsible for the reduced growth rates of helpers. This is the first study in which energy expenditures associated with specific helping behaviours have been measured in brood care helpers, and it provides the first estimate of total behavioural energy expenditure in a cooperatively breeding fish. (c) 1998 The Association for the Study of Animal Behaviour.