Cannibalism of young is related to low paternity and nest take-overs in an intertidal fish

Parental care is costly, and theory suggests that caregivers should reduce parental investment or even stop caring altogether when the costs of caring are too high or the benefits too low. Brood cannibalism is one tactic by which parents can divert investment away from current offspring and towards potentially higher-quality future offspring, but the various selective factors underlying partial brood cannibalism and their relative importance remain poorly understood. Here we used the plainfin midshipman fish, Porichthys notatus, to concurrently examine three hypotheses for partial brood cannibalism and test whether cannibalism increases when (1) parental body condition is low, (2) brood sizes are large and/or (3) brood paternity is low. To investigate these predictions, we combine multiyear, multisite field data with genetic paternity testing and show that partial brood cannibalism is not related to low parental body condition or to large brood sizes, but rather is linked to low paternity. In particular, males that had taken over nests from other males, and were thus unrelated to the broods present in the new nests, consumed the largest number of young (∼15 or more eaten at a time). Our data also suggest that the consumption of only a few young (∼1–2 at a time) appears to be governed by other factors that are not clearly related to paternity. Overall, we highlight the utility of concurrently testing multiple hypotheses for partial brood cannibalism within the same system to better understand this otherwise puzzling behaviour.