Large group size yields group stability in the cooperatively breeding cichlid Neolamprologus pulcher

Abstract

Group size has been shown to positively influence survival of group members in many cooperatively breeding vertebrates, including the Lake Tanganyika cichlid Neolamprologus pulcher, suggesting Allee effects. However, long-term data are scarce to test how these survival differences translate into changes in group extinction risk, group size and composition. We show in a field study of 117 groups from six different colonies (three from two populations each), that group size critically influences these parameters between years. Within one year, 34% of the groups went extinct. Group size correlated positively between years and large groups did not go extinct. The latter were more likely to contain small helpers the subsequent year, which is a cumulative measure of the previous months' reproductive success. Finally, there was a tendency that large groups were more likely to contain a breeding male and female still a year after the first check. The breeder male size, breeder female size, and largest helper size did not influence these parameters, and also did not correlate with the sizes of these categories of fish after one year. This suggests that group size, and not the body size or fighting ability of group members, was the critical variable determining the success of groups. In total, seven groups had fused with other groups between years. To our knowledge, this is the first study showing long-term benefits of large group size in a cooperatively breeding fish. We discuss the importance of differential survival and dispersal of group members for the demonstrated group size effects.

Kinship reduces alloparental care in cooperative cichlids where helpers pay-to-stay

“…Our study generally confirms that kinship is an important component in cooperative systems, but remarkably relatedness may have effects contrary to predictions from conventional kin-selection theory, if coercion by dominants is involved. It is important to note here that group augmentation is likely to generate fitness benefits of helping in this species as well 24,48 , but due to our experimental design this mechanism cannot explain the differences found between the experimental treatments in this study. Rather, dominants and unrelated subordinates seem to trade different commodities reciprocally against each other, thereby generating fitness benefits to both.…”

“…N. pulcher (Fig. 1) is endemic to Lake Tanganyika and breeds cooperatively in groups of a dominant pair and typically 1 to 25 subordinates of different size classes 24,53,54 . Kin recognition has been demonstrated 55 and both related and unrelated subordinates participate in brood care of dominants' broods by cleaning and fanning the eggs 19 , in territory defence against predators, space competitors and conspecific intruders 19,29,53,54 , and in territory maintenance (for example, digging out shelters 47,53 ).…”

“…Direct fitness benefits to subordinates can be accrued for instance by group augmentation, if the survival of group members increases with group size 21 and the costs of helping behaviour are outweighed by the benefits of a magnified group size [22][23][24][25] . In addition, subordinates may benefit from group membership without group augmentation, for instance because this provides them with access to limited resources or enhances protection from predators 19 .…”

First- and second-order sociality determine survival and reproduction in cooperative cichlids

“…Group size, for instance, can affect a group's success and the fitness of group members [10,11]. Larger groups have been shown to yield higher survival [12,13], higher reproductive output [14,15] and better foraging success [16,17] to group members in a wide range of animals.…”

The relationship between social status, behaviour, growth and steroids in male helpers and breeders of a cooperatively breeding cichlid

“…The delicate social balance between breeders and helpers in N. pulcher is maintained because the benefits of group living outweigh the costs of competition among group members (Taborsky, 1984(Taborsky, , 1985Dierkes et al, 1999;Brouwer et al, 2005;Heg et al, 2005). N. pulcher is an excellent model system to study the complex relationships between social structure, behaviour and hormone levels, because it allows experimental manipulations of social structure under controlled conditions (Taborsky, 1984(Taborsky, , 1985Heg et al, 2004b;.…”