Praying mantis (sphodromantis gastrica)

Why do some animals eat their mates after sex?

The practice of sexual cannibalism may sound like a piece of apocalyptic fantasy, but in fact it occurs in the behavioral repertoire of many animals. Refers to eating all or part of a partner during courtship or sexual intercourse. This behavior can seem like an evolutionary conundrum: Why would evolution favor behavior that results in the death of its mate or itself?

Sexual cannibalism that occurs after mating has been documented in many invertebrates, including spiders, insects, and cephalopods. In some cases, it is rare within a species, while in others it is very common. Study of red-backed spiders (Latrodectus haselty) found that males are eaten by females in 65 percent of matings. In the octopus observation, a female ate her mate but only after mating with him 13 times. Scientists believe that the frequency of sexual cannibalism also differs between wild and captive populations within a single species.

In general, sexual cannibalism involves a female eating a male. The male’s response to this interaction can vary widely between species. For example, the male red-backed spider appears to voluntarily sacrifice itself; That is, it puts itself at the parts of the female’s mouth after mating, which makes it easier for her to eat it. In contrast, the male Asian giant mantis (i.e. praying mantis of the species membranous Herodula) seems to do everything in his power to avoid being eaten; It hops onto the female’s back from a distance and positions itself at an angle that makes attack less likely. Why doesn’t the red-backed spider do this?

This is a question for evolutionary biologists who study why a behavior occurs by examining how much it benefits an individual’s reproductive success. For mating behaviours, the costs and benefits differ between males and females; Females usually benefit by investing a lot of energy and resources in a limited number of large gametes (i.e. their eggs), while males usually benefit by investing their energy and resources in propagating as many abundant small gametes (i.e. their sperm) as possible. ) as possible. However, in terms of sexual cannibalism, the high cost to an individual male is clear, but the evolutionary benefits to him and his mate are not as clear.

For females, consumption of the male likely provides her with additional nutritional and energy benefits that can be passed on to the offspring. Studies have found that females are more likely to eat males when they are hungry, and in some cases, females that eat males go on to produce heavier eggs with more numerous offspring.

In contrast, the eaten males may benefit in several ways though paying the ultimate price. Keep in mind that about 80 percent of male redback spiders die without having a chance to mate and produce offspring. Under these conditions, a 65% chance of mating death might be worth the risk from an evolutionary fitness perspective. Sacrificing the male’s body for his mate can also translate into the female producing more numerous or more robust offspring that have a greater chance of survival, ensuring that his genes pass on to later generations. Some studies suggest that male body 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, prolonged mating allows more sperm to be transferred, which increases the chances of fertilization. However, prolonged mating can occur in other ways, and this need not result in the female eating the male. In some species of flies and other insects, other nuptial gifts (including regurgitated food, salivary secretions, and selected body parts) have been shown to increase mating duration. Furthermore, sexual cannibalism may reduce sperm competition between the self-immolating male and other males who emerge later; Some studies have shown that females do not mate for a period after eating a male partner, so other males who follow are less likely to participate in the fertilization of the female’s eggs.

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