Factors influencing cannibalism in the plainfin midshipman fish

• We tracked the degree of offspring cannibalism in the plainfin midshipman fish.
• We examined changes in body condition and competition over the entire breeding season.
• Body condition was not correlated with patterns of cannibalism.
• Cannibalism was most prevalent during nest take-overs and when competition for mates was high.

Cannibalism of young is a common yet seemingly paradoxical phenomenon observed across a wide variety of taxa. Understanding this behaviour in the context of parental care remains a challenge for evolutionary biologists. A common adaptive explanation for the consumption of offspring is that it serves to increase the current or future reproductive success or survival of the cannibalistic parent by replenishing energy stores and facilitating continued care for any remaining young. Another explanation is that cannibalism may be a competitive response to cuckoldry or lowered certainty of parentage. We tested these ideas using the plainfin midshipman fish, Porichthys notatus, a species with an extended period of male-only parental care and documented offspring cannibalism. We found that the occurrence of cannibalism was not linked to the deterioration of body condition, but instead was most frequent during periods of high intrasexual competition and nest take-overs. Our results suggest that cannibalism is not driven by the energetic demands of parental care, but instead by competition among males for nests and females, and the resulting low paternity stemming from both nest take-overs and cuckoldry.