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 observed among mantids in captivity, 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][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."[6]
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.[7]
[edit] Advantages
Since sexual cannibalism occurs in multiple species across different taxa, it is evident that there must be some evolutionary benefit to it. However, it is not understood whether this falls solely under the providence of natural selection, or whether sexual selection is somehow involved. When looking at sexual cannibalism through the viewpoint of natural selection, the trait will be beneficial if by allowing himself to be consumed, the male increases his genetic contribution to the next generation. There have been a number of proposals [8] as to how this happens, but two main ones are:
* Nutrition – It has been shown that sexual cannibalism may have arisen solely from foraging considerations.[9] By allowing himself to be eaten, males may be able to provide sustenance for the female, increasing both female fecundity and his own fitness.
* Increased Mating Time – Some males, by allowing themselves to be consumed, can increase the amount of time they spend transferring sperm to the female, and so fertilize more eggs.[10]
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 as a mate. Cannibalism and mating, however, are not mutually exclusive behaviors, and, many times, the male is devoured after intercourse.[11]
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][12]
There are three difficulties with this hypothesis, however:[13]
* 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.
[edit] Male strategies
According to a 2006 The American Naturalist article about Tenodera aridifolia sinensis:
...male mantids actively assess variation in risk and change their behavior to reduce the chance of being cannibalized...Males are clearly not complicit, and the act of sexual cannibalism in praying mantids is an example of extreme conflict between the sexes. [14]
Males of sexually cannibalistic arthropod 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][4][15] 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.[16] 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.[17]
While males of certain species are complicit in sexual cannibalism, that behavior is not held as a model for all sexual cannibalism. Even in certain 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][18]
[edit] Role reversal
In at least one documented case of what is still termed sexual cannibalism, males sometimes devour females:
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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