The highly social cichlid fish Neolamprologus pulcher, endemic to Lake Tanganyika in Africa, live in social groups consisting of a dominant breeding pair and between 1 and 15 subordinates of both sexes that perform brood care, territory defence and maintenance. Subordinates are often distantly related or unrelated to the dominants.
Cooperative breeding of this kind has puzzled evolutionary biologists for a long time as it is costly and often does not generate obvious fitness benefits to subordinates. In the case of N. pulcher, the main benefit for subordinates to stay in a territory of dominant breeders seems to be the protection gained against predators provided by the large group members.
Previously, it was assumed that male subordinates never achieved paternity but the Bristol researchers suspected that, due to low relatedness between dominants and subordinates, mature male subordinates would attempt to father offspring and that achieving paternity would increase their helping behaviour.
The team, led by Dr Rick Bruintjes, tested this theory by studying groups of cichlids at Kasakalawe Point, Zambia and found that while dominant females were the mothers of 99.7 per cent of all offspring, the dominant males only sired 88.8 per cent. Subordinate females did not participate in reproduction, but male subordinates successfully gained paternity in 27.8 per cent of all clutches.
Furthermore, subordinate males that sired offspring defended more rigorously against egg predators compared to similar males that did not sire offspring, and they also tended to stay closer to the breeding shelter.
The study shows that the cooperative behaviour of the subordinate male fish has a direct fitness benefit for them – that is, producing their own offspring – as well as such indirect benefits as safety.
Dr Bruintjes said: "This is the first evidence in a cooperatively breeding fish species that the helping effort of male subordinates may depend on obtained paternity, which stresses the need to consider direct fitness benefits in evolutionary studies of helping behaviour."
Paper
'Paternity of subordinates raises cooperative effort in cichlids' by Rick Bruintjes, Danielle Bonfils, Dik Heg and Michael Taborsky in PLoS ONE
Journal
PLoS ONE