Sexual cannibalism is when an animal, usually the female, cannibalizes its mate prior to, during, or after copulation. This trait is observed in many arachnid orders, carnivorous insects, several crustacean clades, gastropods, and some snake species. Several hypotheses to explain this seemingly paradoxical behavior have been proposed, including the adaptive foraging hypothesis, aggressive spillover hypothesis and mistaken identity hypothesis. 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 species that exhibit sexual cannibalism, the female consumes 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.
Prevalence Sexual cannibalism occurs among arachnids, amphipods and some carnivorous insects. Sexual cannibalism occurs more often among species with prominent sexual size dimorphism (SSD); extreme SSD likely drives this trait of sexual cannibalism in spiders. In insects, sexual cannibalism mostly occurs in mantises, some biting midges including serromyia femorata, some odonates, sagebush crickets, and some cockroaches. Although very rare in vertebrates, it also sometimes occurs in some anacondas among snakes, especially the green anaconda (Eunectes murinus), where females are larger than the males.
Proposed explanations
Different hypotheses have been proposed to explain sexual cannibalism, namely adaptive foraging, aggressive spillover, mate choice, and mistaken identity.
Adaptive foraging 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. Starving females are usually in poor physical condition and are therefore more likely to cannibalize a male than to mate with him. Among mantises, cannibalism by female Pseudomantis albofimbriata improves fecundity, overall growth, and body condition. A study on the Chinese mantis found that cannibalism occurred in up to 50% of matings. Partial sexual cannibalism occurs among sagebush crickets, where the female consumes the wings and spermatophore of the male to obtain nutrients for egg production. Among spiders, Dolomedes triton females in need of additional energy and nutrients for egg development choose to consume the closest nutritional source, even if this means cannibalizing a potential mate. In Agelenopsis pennsylvanica and Lycosa tarantula, a significant increase in fecundity, egg case size, hatching success, and survivorship of offspring has been observed when hungry females choose to cannibalize smaller males before copulating with larger, genetically superior males. This reproductive success was largely due to the increased energy uptake by cannibalizing males and investing that additional energy in the development of larger, higher-quality egg cases. In D. triton, post-copulatory sexual cannibalism was observed in the females that had a limited food source; these females copulated with the males and then cannibalized them. The adaptive foraging hypothesis has been criticized because males are considered poor meals when compared to crickets; however, recent findings discovered Hogna helluo males have nutrients crickets lack, including various proteins and lipids. In H. helluo, females have a higher protein diet when cannibalizing males than when consuming only house crickets. Further studies show that Argiope keyserlingi females with high-protein/low-lipid diets resulting from sexual cannibalism may produce eggs of greater egg energy density (yolk investment).
Aggressive spillover 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, & large body size) of males but instead depends strictly on her aggressive state. Aggression of the female is measured by latency (speed) of attack on prey. The faster the speed of attack and consumption of prey, the higher the aggressiveness level. Females displaying aggressive characteristics tend to grow larger than other females and display continuous cannibalistic behavior. Such behavior may drive away potential mates, reducing chances of mating. Aggressive behavior is less common in an environment that is female-biased, because there is more competition to mate with a male. In these female dominated environments, such aggressive behavior comes with the risk of scaring away potential mates. Males of the Pisaura mirabilis species feign death to avoid being cannibalized by a female prior to copulation. When males feign death, their success in reproduction depends on the level of aggressiveness the female displays. Research has shown that in the Nephilengys livida species, female aggressiveness had no effect on the likelihood of her cannibalizing a potential mate; male aggressiveness and male-male competition determined which male the female cannibalized. Males with aggressive characteristics were favored and had a higher chance of mating with a female.
Mate choice
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