Sexual cannibalism
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- 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 a conspecific (same species) male before, during, or after copulation.[1]
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.[3]
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[edit] 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][2][4][5]
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."[3]
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.[3]
[edit] 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 fecundity.
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.[6]
Because the specifics of what sexual cannibalism provides varies according to species, its evolutionary origins are obscure:
...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][7]
[edit] Male strategies
Evidence for male complicity 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][8]
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 male spiders, particularly nursery web spiders, bring the female a diversionary meal and attempt to complete copulation before the female completes the offering. Some spiders and mantids delay their courtship approach until a female catches another prey item.[1][4][9]
[edit] References
- ^ a b c d e f g h [1] The Evolution of Sexual Cannibalism
- ^ a b [2] Sexual cannibalism, mate choice, and sperm competition in praying mantids
- ^ a b c [3] "This Can’t Be Love, The Curious Case of Sexual Cannibalism", New York Times, September 5, 2006
- ^ a b Crump, Marty. Headless Males Make Great Lovers & Other Unusual Natural Histories, with illustrations by Alan Crump, University of Chicago Press, 2005, ISBN: 0-226-12199-2
- ^ Elgar, M.A. and B.J. Crespi. 1992. "Ecology and evolution of cannibalism". Pages 1-12 in M.A. Elgar and B.J. Crespi, editors. Cannibalism: ecology and evolution among diverse taxa. Oxford University Press, Oxford.
- ^ [4] "Sexual cannibalism and mate choice decisions in wolf spiders: influence of male size and secondary sexual characters", Department of Biological Sciences, University of Cincinnati, published online 11 November 2004;
- ^ Thornhill, R. 1976. "Sexual selection and paternal investment in insects." The American Naturalist 110:153-163.
- ^ Forster, L.M. 1985. "Target discrimination in jumping spiders". Pages 249-273 in F.G. Barth, editor. Neurobiology of arachnids. Springer-Verlag, New York, New York.
- ^ [5] Sierwald, P. (1997) "Phylogenetic analysis of Pisaurine nursery web spiders, with revisions of Tetragonophthalma and Perenethis (Araneae, Lycosidea, Pisauridae)". The Journal of Arachnology 25 :361–407 (1997)
