Helpers increase food abundance in the territory of a cooperatively breeding fish
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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.
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.
KeywordsCichlid Cooperative foraging Group living Neolamprologus obscurus Helper effect Helping behavior
We thank all Hasli members, especially Jon Andreja Nuotclà for fruitful discussions. We also thank Tetsumi Takahashi, Masaya Morita, Kazutaka Ota, Michio Hori, and the staff of the Lake Tanganyika Research Unit, Mpulungu, Zambia, especially Harris Phiri, Danny Sinyinza, Taylor Banda, Ruben Shapola, and Henry Simpembwa for supporting our studies in the field. We appreciate Michio Hori for valuable advice on examining stomach contents of the cichlids, and Vera Schluessel and two anonymous reviewers for valuable comments on an earlier version of the manuscript. We are grateful to the organizers of the 12th Topical Meeting of the Ethological Society for encouraging us to submit this paper.
This work was financially supported by JSPS KAKENHI (25304017, 23570033 and 4501) to MK. During manuscript preparation, HT was funded by a SNF grant (31003A_166470) to JGF.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interests.
The study was carried out in the field in agreement with and approved by the Zambian Department of Fisheries: Ministry of Agriculture and Cooperatives. Data collection were in accordance with the current laws of the Republic of Zambia and followed the ASAB/ABS (2012) guidelines for the treatment of animals in behavioural research and teaching.
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