The behavior of some animals eating their mates after sex, a phenomenon known as cannibalism, can be influenced by various factors including mate competition, energy conservation, and the survival of their genetic material. Here are several reasons why some animals might exhibit this behavior:

1. Mate Competition: In some species, males and females compete fiercely for the opportunity to mate. Sometimes, a dominant individual may consume its competitor to secure resources, such as food or territory, or to prevent the competitor from reproducing in the future.
2. Energy Conservation: After mating, animals may be in a weakened state, having expended energy on the reproductive act. Consuming the mate can be a way to replenish energy reserves, especially if the animal expects to encounter future mates infrequently.
3. Increasing Own Reproductive Success: In some cases, cannibalism after mating may be a way for an animal to ensure that it invests the minimum amount of energy in offspring that may not survive. By consuming the mate, the animal can potentially redirect resources towards more successful or future offspring.
4. Genetic Relatedness: In some species, mates are closely related, and by consuming the mate, the animal is in essence getting rid of genes that are too similar to its own. This can be particularly true in cases of inbreeding, where the benefits of genetic diversity are reduced.
5. Mate Bond Disruption: In some species, the act of cannibalism may be a way to break bonds between mates, which can be advantageous for preventing future mate guarding or to ensure a higher investment in future reproductive opportunities.
6. Learning and Conditioning: In certain situations, cannibalism may be learned or conditioned through experience. If an animal has previously benefited from consuming a mate (e.g., increased resources, reduced competition), it may be more likely to repeat this behavior.

It’s important to note that cannibalism is not the norm for animals and is typically a response to specific ecological pressures or evolutionary strategies. Many animal species have complex mating behaviors and strategies that can vary widely based on ecological conditions, mate availability, and genetic relatedness.