Living in groups is a widespread phenomenon in many animal taxa. The reduction of predation risk is thought to be an important cause for the formation of groups. Consequently, grouping behavior is particularly pronounced during vulnerable life stages, i.e., as juveniles. However, group living does not only provide benefits but also imposes costs on group members, e.g., increased competition for food. Thus, benefits of grouping behavior might not be evident when predation risk is absent. The adaptive significance of living and also developing in a group independent from predation risk has received relatively little attention although this might have important implications on the evolution and maintenance of group living. The first aim of the present study was to examine whether the social environment affects juvenile performance in the cichlid fish Pelvicachromis taeniatus and, secondly, whether kinship affects social behavior. Kin selection theory predicts benefits from grouping with kin. Here, we demonstrate that juveniles reared in a group grow on average faster compared to juveniles reared in isolation under standardized laboratory conditions without predation risk. Furthermore, we found significant differences in social behavior between juveniles reared in a group and reared in isolation. Fish reared in isolation were significantly more aggressive and less willing to shoal than group-reared fish. As expected, genetic relatedness influenced social behavior in group-reared fish as well: dyads of juveniles consisting of kin showed increased group cohesiveness compared to non-kin dyads. We discuss the potential benefits of group living in general and living with kin in particular.

The social environment individuals are exposed to during ontogeny shapes social skills and social competence in group-living animals. Consequently, social deprivation has serious effects on behaviour and development in animals but little is known about its impact on cooperation. In this study, we examined the effect of social environment on cooperative predator inspection. Predator inspection behaviour is a complex behaviour, which is present in a variety of shoaling fish species. Often, two fish leave the safety of the group and inspect a potentially dangerous predator in order to gather information about the current predation risk. As predator inspection is highly risky, it is prone to conflicts and cheating. However, cooperation among individuals may reduce the individual predation risk. We investigated this complex social behaviour in juveniles of the cichlid fish Pelvicachromis taeniatus that were reared in two different social environments throughout development. Fish reared in a group inspected more often than isolation-reared fish and were more likely to cooperate, i.e. they conducted conjoint inspection of a predator. By contrast, isolation-reared fish were more likely to perform a single inspection without a companion. These results suggest an impairment of cooperative behaviour in isolation-reared fish most probably due to lack of social experience and resulting in lowered social skills needed in coordinated behaviour.

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Sexual selection is an important force in the evolution of body size. Both intersexual selection, that is, preference for large individuals, and intrasexual selection, that is, increased competitiveness of large individuals, are involved in this process. Furthermore, preferences based on body size of the choosing individual might also influence body size evolution. Here, we investigated male mate choice and competitiveness in relation to male body size in Pelvicachromis taeniatus, a size-dimorphic cichlid. In previous experiments, both sexes showed mating preferences for larger and genetically related individuals. First, we examined male inbreeding preferences based on olfactory cues. Males that highly varied in body size were given the choice between the odor of a familiar sister and the odor of an unfamiliar unrelated female that were presented in combination with a computeranimated image of a female P. taeniatus as a visual stimulus. Male preference for the odor of their sisters was correlated with male body size. Only larger males were choosy concerning related odors and preferred their sisters, whereas smaller males were unselective. Second, we showed that large males outcompete smaller males in contest over a breeding site. The extent of aggression was negatively correlated with the size difference between the 2 males. Variation in male choice may reflect an adaptive strategy: small, less competitive and less attractive males might avoid the risk of failing to mate at all by reducing choosiness. Consequently, only large competitive males should obtain the benefits of choice, which may further contribute to the selective advantages of large body size.

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