Size-dependent task specialization in a cooperative cichlid in response to experimental variation of demand

Many social species show some form of task specialization among group members. In cooperative vertebrates, this usually does not coincide with morphological differentiation. In species with indeterminate growth, however, size differences between subordinates may promote task specialization in the performance of helping behaviours, depending on size-specific capabilities, opportunities and cost/ benefit ratios. In the cooperatively breeding cichlid fish Neolamprologus pulcher, helpers perform different helping tasks. To examine the effects of helper size, which reflects age, on cooperative behaviour in the field, we experimentally presented egg predators that do not threaten helper survival. In the egg predator treatment, large and small helpers increased defence and all helpers roamed further from the breeding shelter, presumably to keep potential egg predators at a safe distance from the brood. Additionally, small helpers defended more against egg predators presented close to the breeding shelter than large helpers, whereas the latter put more effort into digging experimentally added sand out of the breeding shelter. Our results show size(age)-specific task specialization of helpers in a cooperatively breeding vertebrate, which is reminiscent of eusocial insects.