Cannibalism of females by males before, during or immediately following sex has been attributed to misidentification of females, rejection of females as mates and prioritisation of feeding over reproduction. In the gammarid amphipod Gammarus zaddachi, males demonstrate that they have identified a female and accepted her as a suitable mate by engaging in precopula pairing behaviour. However, a male may later decide to eat the female after pairing with her. Laboratory experiments were performed in which survival of females in precopula pairs of G. zaddachi was monitored after their male partners had been subjected to starvation, herbivorous diets or diets containing animal matter. These indicate that the female is less likely to survive when she is abnormally small relative to the male, when the male is at risk of death by starvation or when the male's diet has lacked animal content. Decisions by males to forgo reproductive opportunities in favour of cannibalism, even after engaging in costly mate-guarding behaviour, emphasise the importance of carnivory in the trophic ecology of Gammarus species.