Group intrusions by a brood parasitic fish are competitive not cooperative

Brood parasites delegate all parental duties to unrelated hosts. Hosts resistance against brood parasitism is most effective during egg laying and is best countered by surreptitious oviposition. This may be aided through distraction of host attention by the male partner or a larger cooperative group. Cuckoo catfish (Synodontis multipunctatus) parasitize the broods of mouthbrooding cichlids, which collect their eggs immediately after oviposition. Cuckoo catfish must time their intrusion precisely, as the temporal window for parasitism lasts only a few seconds. As the cuckoo catfish typically intrude host spawning as a group, we tested whether groups of catfish distract spawning cichlid pairs more successfully than a single catfish pair, or whether group invasion represents competition for a rare resource. We found that larger catfish groups were not more effective in parasitism, as parasitism success by groups of three catfish pairs increased only proportionally to single catfish pairs. The number of eggs in host clutches decreased at high catfish abundance, apparently due to elevated cuckoo catfish predation on the eggs. Hence, group intrusions represent competitive rather than cooperative actions, with an increased cost to the host cichlid from greater egg predation by cuckoo catfish rather than social facilitation of brood parasitism.