Why do some animals eat their partners after sex?

The practice of sexual cannibalism may sound like something out of a horror novel, but in fact, it occurs in several species of animal behavior. It refers to eating all or part of a mate during courtship or mating. This behavior seems like an evolutionary puzzle: Why would evolution favor behaviors that lead to the death of a mate or oneself?

Some invertebrates, including arachnids, insects, and cephalopods, engage in sexual cannibalism after mating. In some cases it is rare within a species, while in others it is common. A study of redback spiders (Latrodectus hasselti) found that during matings, 65 percent of male spiders were eaten by the female. In an observation of octopuses, a female octopus mated with her mate 13 times before eating it. Scientists believe the frequency of sexual cannibalism also differs between wild and captive populations within the same species.

Generally speaking, sexual cannibalism is when a female eats a male. How males respond to this interaction varies between species. Male redback spiders, for example, appear to sacrifice themselves voluntarily; that is, after mating, the male positions himself over the female's mouthparts, making it easier for the female to eat him. In contrast, the male giant Asian mantis, known as the membrane-winged mantis, appears to go to great lengths to avoid being eaten. It jumps on the female's back from a great distance and positions itself at an angle where an attack is unlikely. Why don't redback spiders do this?

This is a question faced by evolutionary biologists, who think about why a certain behavior occurs by studying how it contributes to an individual's reproductive success. For the act of mating, the costs and benefits differ between males and females; the female typically benefits by investing a large amount of energy and resources into a limited number of large gametes (i.e., her eggs), while the male typically benefits by investing a large amount of energy and resources to spread as many abundant gametes (i.e. his sperm) as possible. ) as much as possible. However, in the case of sexual cannibalism, the high cost to the individual male is obvious, but the evolutionary benefits to him and his mate are less obvious.

For a female, eating the male may give her extra nutrients and energy that can be passed on to her offspring. Studies have found that females are more likely to eat males when they are hungry, and in some cases, females that eat males lay heavier eggs and produce more offspring.

In contrast, the male that is eaten may benefit in several ways, despite the ultimate cost. Consider that approximately 80% of male redback spiders die without a chance to mate and produce offspring. In this case, the 65% chance of dying during mating may be worth the risk from an evolutionary fitness perspective. Additionally, a male sacrificing his body to his mate may cause a female to produce a greater number of offspring or stronger offspring that have a higher chance of survival, ensuring that their genes are passed on to future generations. Some studies suggest that male size may be important, with larger males providing more food than smaller males.

Additionally, sexual cannibalism may increase a male's chances of successfully fertilizing a female's eggs simply because the female spends more time with him. In some cases, extending the duration of mating can transfer more sperm, thereby increasing the chance of fertilization. However, extended mating can occur through other means and does not necessarily result in females eating males. In some species of flies and other insects, other wedding gifts, including regurgitated food, salivary secretions, and selected body parts, have been shown to prolong mating. Additionally, sexual cannibalism may reduce sperm competition between the male that sacrifices itself and other males that emerge later. Some studies suggest that females temporarily refrain from mating after eating their male partners, so subsequent males are less likely to be involved in fertilizing the female's eggs.

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