Sexual cannibalism

This article is about arthropod behavior; for the human psychological disorder see sexually motivated cannibalism

Sexual cannibalism is a special case of cannibalism in which a female organism kills and consumes male of the same species before, during, or after copulation. Rarely, these roles are reversed.^[1][2]  

The New York Times provides this lurid description:

A male mantis approaches a female, flapping his wings and swaying his abdomen. Leaping on her back, he begins to mate. And quite often, she tears off his head. The female mantis devours the head of the still-mating male and then moves on to the rest of his body. [...] If you put a pair together and come back later, you’ll just find the wings of the male and no other evidence he was ever there [...] Sexual cannibalism has fascinated biologists ever since Darwin.^[4]

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Prevalence

Although other forms of cannibalism are widespread in the animal kingdom, sexual cannibalism has been documented only in arachnids, insects and amphipods although anecdotal evidence suggests its existence in gastropods and copepods as well.^[1]

Despite its overall rarity, sexual cannibalism is common in many families of spiders and scorpions, and can have important effects on population size and sex ratio. Among insects, sexual cannibalism is a nearly universal habit among mantids but is otherwise rare. In most species in which it occurs, sexual cannibalism is related to the larger size of the female due to sexual dimorphism.^[1][3][5][6]

Some scientists have downplayed the significance of sexual cannibalism. Stephen Jay Gould argued that sexual cannibalism was too rare to be significant and said biologists had become "overzealous about the power and range of selection by trying to attribute every significant form and behavior to its direct action."^[7]

Subsequent research contradicted this opinion and shows that for some sexually cannibalistic species, males are a significant food source for females. One study estimated that 63 percent of the diet of female Chinese mantids are the males of the species.^[4]

Advantages

There are two obvious advantages to sexual cannibalism:

• Natural selection - Any male who succumbs to the female's attempts to devour him before mating occurs fails to pass on his DNA. This is only applicable in species in which the male is not fully complicit in his demise.
• Nutrition - Any male eaten by a female, whether he has mated with her or not, provides a female with nutritional benefits which might increase the quantity or quality of her offspring.

Premating cannibalism has been considered as a foraging decision, a by-product of natural selection for aggressiveness and even a case of mistaken identity. Some sexual cannibalism might be a matter of the female regarding her suitor as more acceptable as a meal than a mate. But cannibalism and mating are not mutually exclusive behaviors and many times the male is devoured post-coitus.^[8]

...sexual cannibalism takes many different forms with respect to the role and behavior of each sex, the potential benefit to each sex, and the timing in the courtship/copulatory sequence [...] Due to these differences, researchers have proposed many pathways for its evolution, often invoking contradicting sets of selective forces acting on different sexes and different species. With several conflicting models and an increasing number of empirical studies attempting to explain its origin and maintenance in insects and arachnids, the evolution of sexual cannibalism remains a subject of debate.^[1]

Reproductive strategies of males and females often differ, resulting in asymmetric costs of time and energy between the sexes. As an adaptive female strategy, sexual cannibalism is easy to understand. It is male complicity that has long been the focus of inquiry into the evolution of sexual cannibalism.^[1]

By becoming food for the stronger sex, males forfeit any future mating opportunities. Sexual cannibalism is favored by natural selection when it significantly increases the viable eggs fertilized by the cannibalized male's sperm. This model only applies to situations in which cannibalism occurs after sperm transfer. For males, sexual cannibalism may have evolved as an extreme form of paternal investment. If a male's sacrifice significantly increases the quality or quantity of his offspring, sexual cannibalism could be an adaptive male strategy rather than the result of a conflict of interest between the sexes.^[1][9]

There are three difficulties with this hypothesis, however:^[10]

• Males are prevented from further matings
• Males often try to avoid being eaten, suggesting it is not advantageous to them
• Males are usually eaten before they can mate

Together this evidence may suggest a case of sexual conflict, however it still remains unclear if this is the case.

Male strategies

  Males of sexually cannibalistic species use diverse strategies to decrease their chances of being cannibalized. Male scorpions sometimes sting females while depositing their spermatophore. Male black widows and crab spiders often restrain females in silk prior to copulation. Some spiders have specialized jaws that hold open the jaws of females during copulation. Others preferentially mate with females in ecdysis (that is, while the female is moulting) when cannibalism is physically impossible. Some spiders and mantids delay their courtship approach until a female catches another prey item.^[1][5][11] Some male spiders, particularly nursery web spiders, bring the female a diversionary meal.

The males in some species are very small compared to the female. Female golden orb-web spiders are over 20 times as heavy as males.^[12] It is suggested this is the result of small spiders being more agile and able to play 'hide and seek' in Darwin's The Descent of Man, and Selection in Relation to Sex.^[13]

Evidence for male complicity in their own cannibalism is anecdotal and has not been borne out by experimental and behavioral research. Even in species in which cannibalism is known to increase the number and/or viability of offspring (including mantids, black widow spiders, jumping spiders, and scorpions) males approach females cautiously and retreat quickly after copulation. In the sexually cannibalistic black widow spider Latrodectus mactans, when males survive copulation they often fertilize multiple females.^[1][14]

Role reversal

Two types of sexual cannibalism, differing in the sex of the victim, were found among heterosexual pairs of the parasitic isopod Ichthyoxenus fushanensis...In one type, categorized as sexual cannibalism, the male was consumed by the female before or after mating. In the other, reversed type, the female was eaten by her mate during or after breeding. Both types of cannibalism occurred during the breeding season...Because an individual of I. fushanensis undergoes protandrous sex change, the cannibalistic behavior could not have evolved in response to selection on either the male or female sexuality. Rather, both types of cannibalism may be regarded as the result of competition between paired individuals, which appears to be a by-product in the evolution of a reproductive strategy rather than a consequence of sexual selection.^[2]

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