Low incidence of sibling cannibalism among brood parasitic cuckoo catfish embryos

Brood parasites have demanding needs of host resources. Brood parasitic offspring are highly competitive and frequently cause the failure of host broods and the survival of a single parasitic offspring. Accordingly, virulent brood parasites lay single eggs in host nests to minimize multiple parasitism and ensuing sibling competition. In the cuckoo catfish (Synodontis multipunctatus), which parasitise mouthbrooding cichlid fishes in Lake Tanganyika, the modes of host and parasite oviposition lead to frequent cases of multiple parasitism. We experimentally tested the prediction that multiple parasitism leads to frequent siblicide. Cuckoo catfish embryos prey upon host offspring to obtain nourishment during their 3-week development in the host buccal cavity and may also consume conspecific siblings. The potential benefits of siblicide in the system are, therefore, twofold: to decrease competition for limited resources (i.e. host brood with rich yolk sacs) and to directly obtain nourishment by consuming rivals. We found that sibling cannibalism indeed provided measurable benefits in terms of increased growth of the cannibals, but sibling cannibalism was rare and typically occurred only when all host offspring had been consumed. This implies that cannibalism in the cuckoo catfish embryos emerges to mitigate starvation rather than eliminate sibling competition. 

Methods
Experimental dataset. Data collected in laboratory, as described in the paper. Dataset is composed of body size and survival, and associated treatment data.