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.