Helpers increase food abundance in the territory of a cooperatively breeding fish

In cooperatively breeding or eusocial animals, increasing resources such as food is a major task of brood care helpers or workers. While such food acquisition has been shown in several animal taxa, evidence is absent in fishes. Here, we provide the first evidence of increased food abundance caused by helpers in a cooperatively breeding fish. Helpers of the cichlid Neolamprologus obscurus excavate cavities by digging sand from under stones, which serve as shelter for the group members. We test whether these cavities additionally increase the abundance of benthic invertebrates in the territory. Stomach content analyses of wild-caught fish revealed that benthic invertebrates pose the main food resource of N. obscurus. Experimental assessments of daily benthic invertebrate immigration into artificial cavities demonstrate a significant increase in invertebrate prey abundance according to the size of the excavated stone area. Finally, by applying correlational and experimental approaches in the field, we show that helpers play a crucial role in the maintenance of the excavated cavities. In combination, these results demonstrate that helpers increase the abundance of benthic invertebrates inside the territory of breeders in N. obscurus. Our results provide the first evidence of increased food abundance through helpers in fishes. Such foraging system may resemble those described in other species living in highly social groups, and appears to be a ubiquitous mechanism underpinning the maintenance of complex societies in animals. SIGNIFICANCE STATEMENT: Evidence of elaborate food acquisition such as farming or trap building is only known from a limited number of animal taxa. The cichlid Neolamprologus obscurus is a highly social fish, where all group members create and maintain cavities under stones, which serve as shelters. These fish feed on benthic invertebrates, which hide inside such cavities during daytime. We show that the cavities of N. obscurus additionally increase the food abundance in their territory. Behavioral observation and experiment in the field revealed that group members increase the excavated cavities in their territory, and food abundance increases according to the size of excavated cavities. Our results provide the first evidence of food acquisition by group members in fishes.