Adaptive Value

 

General Benefits and Costs of Sexual Cannibalism for Male vs. Female Mantids

The adaptive value of sexual cannibalism of the male mantid by the female mantid is expectedly different for each sex. For cannibalism to be adaptive, the possible benefits must on average outweigh the costs. In the case of the female, possible benefits from consuming the romantically interested male must outweigh possible costs including the preemption or interruption of sperm delivery and possible male inflicted injuries resulting from the cannibalistic attack (Prokop and Vaclav, 2005). For cannibalism to be advantageous for the male, the benefits, in the form of increased number of offspring, must similarly outweigh the cost of being unable to mate with any more females.

 

Photo showing surviving offspring emerging from ootheca.

Photo citation: http://www.restaurantwidow.com/images/mantis_1.jpg

 

Benefits and Costs for the Female

Overall it appears that the benefits the female reaps in terms of increased nutrients for survival and reproductive efforts do outweigh the potential costs of sexual cannibalism (Prete, 1999; Prokop and Vaclav, 2005). One study examining female fitness benefits resulting from eating male mates found that the cannibalistic Hierodula Membranacea (H. Membranacea) females tended to attain a greater maximum weight as compared to non-cannibalistic females. As female weight is directly associated with ootheca dry weight and egg weight, both of which in turn are associated with increased number of surviving offspring, there appear real fitness benefits resulting from male eating (Birkhead et al., 1988). These findings have also been shown to be true for a number of other mantid species including Pseudomantis albofimbriata, whose female fecundity is increased in response to cannibalism. Another study suggested that by eating the male, a female could increase her number of offspring by as much as 20% (Barry et al., 2008). Reproductive output seems to increase especially if the cannibalistic female has had limited access to food prior to eating her mate (Lawrence, 1992; Prete, 1999). Some researchers have suggested that cannibalism could further act as a form of female-female competition, whereby killing the male after copulation could prevent other females from mating and having as many offspring (Polis, 1981). Yet another hypothesis suggests that cannibalism is a beneficial method that a female can use to increase her own fecundity by increasing her nutritional status and preventing copulation with an inferior quality male by consuming him (Edmunds, 1975).

 

Benefits and Costs for the Male

The fact that many aspects of male sexual behavior have been selected to avoid cannibalism suggests that being eaten is not advantageous for the males (Prete, 1999). In addition, males can experience sexual/sperm competition—a fact that again indicates that sexual cannibalism is most likely not as advantageous for the males as it is for the females (Birkhead et al., 1988). Although overall it seems that males would do better to survive and mate again, males have developed adaptive methods of dealing with cannibalism. For example, males of many species can continue copulating for several hours after they have lost their head and part of their prothorax (Kumar, 1973; Prete, 1999). Moreover, in some species such as H. Membranacea, cannibalism leads to males fertilizing a larger number of ova as compared to non-cannibalized males. This higher fertilization occurs as decapitation removes cephalic inhibition of copulatory motor patterns, an occurrence that leads to an extended copulatory period and decapitated males performing more vigorous sexual movements (Kynaston et al., 1994; Prete, 1999). Additionally there is evidence that decapitation leads the males of species such as H. Membranacea to transfer their spermatophore to females faster than their non-cannibalized counterparts (Lawrence, 1992). Although these adaptive compensatory mechanisms of increasing male fitness in the face of cannibalism do exist, it is unlikely that males are able to increase their fitness as much as if they had been able to partake in multiple copulations.

 

 

Video showing decapitated male copulating with cannibalistic praying mantis female.

Video citation: http://www.youtube.com/v/E92Fj4rGUAc?fs=1&hl=en_US