Sexual cannibalism, the process by which some female creatures eat their male counterparts after mating, seems quite unpleasant but has been noted at least 30 species, including those in three orders of insects and in orders of arachnids, amphipods, copepods, and gastropods. Under certain conditions, it's theoretically evolutionarily beneficial for the male to sacrifice himself for the good of his offspring. But recent research is showing sexual cannibalism may be the exception, rather than the rule, even in certain species long thought famous for it.
Insect Reproduction Can Be Messy Business
Several scientific models exist as to why sexual cannibalism occurs and just how willing the participants are. The goal of any species is to procreate and pass on genes to another generation. Some species of insects, like the male praying mantis, are a bit more willing to do whatever it takes to achieve this goal. At least in captivity and in environments where the female is concerned about limited nutritional resources, the male is sacrificed. But in the wild, this appears to happen only 5 percent to 31 percent of the time. In fact, only one species of the 2,000 species of mantis displays cannibalism 100 percent of the time, and only because it is a necessity for the species' physical mating process. Cannibalizing the male allows the female more mating time and greater chance of fertilization; ultimately the female and her offspring benefit from the vital nutrients provided.
In another example of unique cannibalizing behavior, the male sagebrush cricket allows the female to feed on his hind wings. She dines on the oozing wounds during copulation but allows the male to survive for future matings. Like the mantis, this behavior does not always occur and is dependent on the female's nutritional needs. If resources are limited, females are much more likely to seek out virgin males with intact wings. But if nutritive resources are abundant, the female is indifferent and will more willingly mate with non-virgin males -- and show no further cannibalistic behavior.
The Bachelor Midge
The male bachelor midge seems to suffer more than most during copulation, but he is usually rewarded for his suffering -- even if he never knows it. During mating, the female sucks the blood from the male, causing his genitals to break off inside her. What seems gruesome is actually of evolutionary benefit, because it prevents other males from impregnating the female. Ideally, his sacrifice guarantees his offspring. A similar practice of broken male genitalia occurs in the white widow spider; however, his life and often his fertility are spared, suggesting the ultimate sacrifice isn't the only way this technique can pose an evolutionary advantage.
Scorpions and Spiders
Arachnids, which are not insects but share the insect phylum Athropoda, provide a different look at sexual cannibalism. For example, the female wolf spider occasionally engages in sexual cannibalism but only, it seems, after other options have been exhausted. When food-limited, she would much rather seek out a cricket to munch on than a conspecific male. The black widow female spider, however, has evolved to always have the option to cannibalize her mate -- in order to do so, he must place himself right between her fangs. But again, in only one species of widow is cannibalism the rule, and often the behavior is not seen in the wild. Among vertebrates, the female giant green anaconda mates in a ball of males and will eat one of them to sustain her pregnancy.
The Cannibalistic Necessity
Sexual cannibalism seems to occur more through necessity than preference. Environmental conditions -- most critically, food limitations -- significantly change the habits of insects, spiders and others. Therefore, studying these creatures in captivity and unnatural stressful conditions can significantly skew reality. But with more than 100 million insect species known to exist today, attempting to identify the extent to which sexual cannibalism occurs is a daunting and ongoing task.
- University of California, Berkeley: The Evolution of Sexual Cannibalism
- Journal of Arachnology: Frequency and Consequences of Damage to Male Copulatory Organs in a Widow Spider
- Springer: Males Make Poor Meals: a Comparison of Nutrient Extraction During Sexual Cannibalism and Predation
- PBS: Meet the Munchers
- Burke Museum: Just Plain Weird Stories
- JSTOR: The Natural Selection of Sexual Cannibalism
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