Sexual Cannibalism
Sexual cannibalism is when a female cannibalizes her male mate prior to, during, or after copulation.
It is a phenomenon characterized primarily by members of arachnid orders as well as several insect orders.[2] The adaptive foraging hypothesis,[3] aggressive spillover hypothesis[4] and mistaken identity hypothesis[5] are several hypotheses that have been proposed to explain how sexual cannibalism evolved. This behavior is believed to have evolved as a manifestation of sexual conflict, occurring when the reproductive interests of males and females differ.
In many of the species that exhibit sexual cannibalism, the female cannibalizes the male upon detection. Females of cannibalistic species are generally hostile and unwilling to mate; thus many males of these species have developed adaptive behaviors to counteract female aggression.
adaptive foraging hypothesis
The adaptive foraging hypothesis is a proposed pre-copulatory explanation in which females assess the nutritional value of a male compared to the male's value as a mate.[12] Starving females are usually in poor physical condition and are therefore more likely to cannibalize a male than mate with him.[13]
Aggressive spillover hypothesis
The aggressive spillover hypothesis suggests that the more aggressive a female is concerning prey, the more likely the female is to cannibalize a potential mate. The decision of a female to cannibalize a male is not defined by the nutritional value or genetic advantage (courtship dances, male aggressiveness, and large body size) of males but instead depends strictly on her aggressive state.
Mate choice
Females use mate choice to reject unwanted and unfit males by cannibalizing them.[23][24] Mate choice often correlates size with fitness level; smaller males tend to be less aggressive and display low level of fitness; smaller males are therefore eaten more often because of their undesirable traits.[23] Males perform elaborate courtship dances to display fitness and genetic advantage.[25] Female orb-web spiders (Nephilengys livida) tend to cannibalize males displaying less aggressive behavior and mate with males displaying more aggressive behavior, showing a preference for this trait,[18] which, along with large body size, displays high male quality and genetic advantage.[18][26]
Indirect mate choice can be witnessed in fishing spiders, Dolomedes fimbriatus, where females do not discriminate against smaller body size, attacking males of all sizes. Females had lower success rates cannibalizing large males, which managed to escape where smaller males could not.[4] It was shown that males with desirable traits (large body size, high aggression, and long courtship dances) had longer copulation duration than males with undesirable traits.[18][26] In A. keyserlingi and Nephila edulis females allow longer copulation duration and a second copulation for smaller males.[27] The gravity hypothesis suggests that some species of spiders may favor smaller body sizes because it enables them to climb up plants more efficiently and find a mate faster.[28] Also smaller males may be favored because they hatch and mature faster, giving them a direct advantage in finding and mating with a female.[29] In Latrodectus revivensis females tend to limit copulation duration for small males and deny them of a second copulation, showing preference for larger body size.[26] Another form of mate choice is the genetic bet-hedging hypothesis in which a female consumes males to prevent them from exploiting her.[30] It is not beneficial for a female exploited by multiple males because it may result in prey theft, reduction in web, and reduced time of foraging.[31] Sexual cannibalism might have promoted the evolution of some behavioral and morphological traits exhibited by spiders today.Mistaken identity hypothesis
The mistaken identity hypothesis suggests that sexual cannibalism occurs when females fail to identify males that try to court.[5] This hypothesis suggests that a cannibalistic female attacks and consumes the male without the knowledge of mate quality. In pre-copulatory sexual cannibalism, mistaken identity can be seen when a female does not allow the male to perform the courtship dance and engages in attack.[14] There is no conclusive evidence for this hypothesis because scientists struggle to distinguish between mistaken identity and the other hypotheses (aggressive spillover, adaptive foraging, and mate choice).[32]
Male adaptive behaviors
In some cases, sexual cannibalism may characterize an extreme form of male monogamy, in which the male sacrifices itself to the female. Males may gain reproductive success from being cannibalized by either providing nutrients to the female (indirectly to the offspring), or through enhancing the probability that their sperm is used to fertilize the female's eggs.[33] Although sexual cannibalism is fairly common in spiders, male self-sacrifice has only been reported in six genera of araneoid spiders. However, much of the evidence for male complicity in such cannibalistic behavior may be anecdotal, and has not been replicated in experimental and behavioral studies.[34]
Members of cannibalistic species have adapted different mating tactics as a mechanism for escaping the cannibalistic tendencies of their female counterparts. Current theory suggests antagonistic co-evolution has occurred, where adaptations seen in one sex produce adaptations in the other.[8] Adaptations consist of: courtship displays, opportunistic mating tactics, and mate binding.
Opportunistic mating
The risk of cannibalism becomes greatly reduced when opportunistic mating is practiced.[8] Opportunistic mating has been characterized in numerous orb-weaving spider species, such as Nephila fenestrata, where the male spider waits until the female is feeding or distracted, and then proceeds with copulation; this greatly reduces the chances of cannibalization. This distraction can be facilitated by the male's presentation of nuptial gifts, where they provide a distracting meal for the female in order to prolong copulation and increase paternity.[8]
Altered sexual approach
Multiple methods of sexual approaches have evolved in cannibalistic species as a result of sexual cannibalism.[35] The mechanism by which the male approaches the female is imperative for his survival. If the female is unable to detect his presence, the male is less likely to face cannibalization. This is evident in the mantid species,Tenodera aridifolia, where the male alters his approach utilizing the surrounding windy conditions. The male attempts to avoid detection by approaching the female when the wind impairs her ability to hear him.[36] In the praying mantid species, Pseudomantis albofimbrata, the males approach the female either from a "slow mounting from the rear" or a "slow approach from the front" position to remain undetected.[35] The male alters his approach through the utilization of the surrounding windy conditions, thus the risk of facing cannibalization is reduced [35]