Egg density and salinity influence filial cannibalism in common gobies

Abstract

Filial cannibalism, i.e. the consumption of own offspring, has fascinated animal ecologists for many decades but is still not fully understood. Often assumed to happen primarily due to energetic needs of the cannibalizing parents, we here address a more recent notion that suggests an interplay between egg density, salinity, egg infections and filial cannibalism in fish. Previous evidence indicates that (a) filial cannibalism may be related to egg density that (b) egg pathogens such as water moulds spread more easily on high density clutches and are (c) generally suppressed in high salinity conditions and that (d) parents selectively cannibalize infected eggs, suggesting cannibalism to maximise in high density clutches in low salinity as a response to egg infections. We thus tested if egg density, salinity and their interaction directly affect filial cannibalism using the common goby (Pomatoschistus microps) as a model system. We additionally recorded male brood care behaviour and weight to account for other potentially salinity-related effects. While males unexpectedly cannibalized more eggs in low density instead of high density clutches, we found that egg consumption was higher in low salinity conditions in agreement with our prediction. Neither male behaviour nor metabolism did adequately explain this finding, indicating that variation in filial cannibalism under different environmental conditions such as salinity may indeed be driven by a differential prevalence of egg infections. During parental care, parents may forgo taking care of their offspring but rather invest in future reproduction. Filial cannibalism, the consumption of one’s own offspring, has often been considered to occur for energetic reasons or as a strategy to reduce investment in less valuable offspring. In this study we specifically test how male brood care behaviour, i.e. egg fanning and filial cannibalism is affected by salinity and egg density (how tightly females decide to place their eggs) in a small marine fish. Our experimental approach is to assess environmental influences and to show their relevance for the evolution and persistence of such a seemingly maladaptive behavioural trait with drastic consequences for fish reproduction. We conclude that variation in filial cannibalism under different environmental conditions such as salinity may indeed be driven by a differential prevalence of egg infections.