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Sexual Cannibalism
Cannibalism is a behavior seen throughout many species where a member of one species consumes another member of an identical species Cannibalism in a sexual setting refers to the female’s consumption of her male mate, prior, during, or proceeding copulation. It is a phenomenon characterized primarily by members of arachnid orders, and few insect orders. 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 will cannibalize 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

Male Adaptive Behaviors
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 will produce adaptations in the other. 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. 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.

Altered Sexual Approach
Multiple methods of sexual approaches have evolved in cannibalistic species as a result of sexual cannibalism. 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. 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. The male alters his approach through the utilization of the surrounding windy conditions, thus the risk of facing cannibalization is reduced

Mate Guarding
Sexual cannibalism has impaired the ability of the orb-weaving spider, N. fenestrata, to perform mate guarding. If a male successfully mates with a female, he then exhibits mate guarding, inhibiting the female from re-mating, thus ensuring his paternity and eliminating sperm competition. Guarding can refer to the blockage of female genital openings to prevent further insertion of a competing male pedipalps, or physical guarding from potential mates. Guarding can decrease female re-mating by fifty percent. Males who experience genital mutilation can sometimes exhibit the “gloves off” hypothesis which states that a male’s body weight and his endurance are inversely proportional. Thus when a male’s body weight decreases substantially, his endurance increases as a result, allowing him to guard his female mate with increased efficiency.

Mate Binding
Mate binding refers to a pre-copulatory courtship behavior where the male deposits silk onto the abdomen of the female while simultaneously massaging her in order to reduce her aggressive behavior. This action allows for initial and subsequent copulatory bouts. While both chemical and tactile cues are important factors for reducing cannibalistic behaviors, the latter functions as a resource to calm the female, exhibited in the orb-weaver spider species, Nephila pilipes. Additional hypotheses suggest that male silk contains pheromones which seduce the female into submission. However, silk deposits are not necessary for successful copulation. The primary factor in successful subsequent copulation lies in the tactile communication between the male and female spider that results in female acceptance of the male. The male mounts the posterior portion of the female’s abdomen, while rubbing his spinnerets on her abdomen during his attempt at copulation. Mate binding was not necessary for the initiation of copulation in the golden orb-weaving spider, except when the female was resistant to mating. Subsequent copulatory bouts are imperative for the male’s ability to copulate due to prolonged sperm transfer, therefore increasing his probability of paternity

Courtship Displays
Courtship displays in sexually cannibalistic spiders are imperative in order to ensure the female is less aggressive. Additional courtship displays include pre-copulatory dances such as those observed in the Australian redback spider, and vibrant male coloration morphologies which function as female attraction mechanisms, as seen in the peacock spider, Maratus volans. Nuptial gifts play a vital role in safe copulation for males in some species. Males present meals to the female to facilitate opportunistic mating while the female is distracted. Subsequent improvements in male adaptive mating success includes web reduction, as seen in the Western black widow, Latrodectus hesperus. Once mating occurs, the males destroy a large portion of the females web to discourage the female from future mating, thus reducing polyandry, which has been observed in the Australian redback spider, Latrodectus hasselti

Male-Induced Cataleptic State
In some species of spiders, such as Agelenopsis aperta, the male induces a passive state in the female prior to copulation It has been hypothesized that the cause of this “quiescent” state is male’s massaging of the female’s abdomen, following male vibratory signals on the web. The female enters a passive state, and the male’s risk of facing cannibalism is reduced. This state is most likely induced as a result of a male volatile pheromone. The chemical structure of the pheromone utilized by the male A. aperta is currently unknown; however, physical contact is not necessary for the induced passive state. Eunuch males, or males with partially or fully removed palps, are unable to induce the passive state on females from a distance, but can induce quiescence upon physical contact with the female; this suggests that the pheromone produced is potentially related to sperm production, since the male inserts sperm from his pedipalps, structures which are removed in eunuchs. This adaptation has most likely evolved in response to the overly aggressive nature of female spiders.

Costs and Benefits of Sexual Cannibalism for Males
The physiological impacts of cannibalism on male fitness include his inability to father any offspring if he is unable to mate with a female. There are males in species of arachnids, such as N. plumipes, that sire more offspring if the male is cannibalized after or during mating; copulation is prolonged and sperm transfer is increased. In the species of orb-weaving spider, Argiope arantia, males prefer short copulation duration upon the first palp insertion in order to avoid cannibalism. Upon the second insertion, however, the male remains inserted in the female. The male exhibits a “programmed death” to function as a full-body genital plug. This causes it to become increasingly difficult for the female to remove him from her genital openings, discouraging her from mating with other males. An additional benefit to cannibalization is the idea that a well-fed female is less likely to mate again. If the female has no desire to mate again, the male who has already mated with her has his paternity ensured.

Genital Mutilation
In spiders, genital mutilation involves the male using his two pedipalps to plug the female’s copulatory organs, located on either side of her abdomen. This behavior is seen in sexually cannibalistic spiders, causing them to exhibit the “eunuch phenomenon”. This phenomenon describes males with partially or fully mutilated pedipalps resulting from copulation with a female. The male can ensure his paternity if the female’s copulatory organs are blocked, discouraging sperm competition and mating with other males. Plugging the female’s genitalia with palps decreases re-mating with a different male by 75%. Some males, as in the orb-weaving spider, Argiope arantia, have been found to spontaneously die within fifteen minutes of their second copulation with a female 13. The male dies while his pedipalps are still intact with the female, as well as still swollen from copulation. By this “programmed death” the male is able to utilize his entire body as a genital plug for the female, causing it to be much more difficult for her to remove him from her copulatory ducts

Male Self-Sacrifice
Male reproductive success can be determined by their number of fathered offspring, and monogyny is seen quite often in sexually cannibalistic species. Males are willing to sacrifice themselves, or lose their reproductive organs in order to ensure their paternity from one mating instance. Whether it is by spontaneous programmed death, or the male catapulting into the mouth of the female, these self-sacrificing males will die in order for prolonged copulation to occur. Males of many of these species cannot replenish sperm stores, therefore they must exhibit these extreme behaviors in order to ensure sperm transfer and fathered offspring during their one and only mating instance. An example of such behavior can be seen in the Australian redback spider. The males of this species “somersault” into the mouths of the female after copulation has occurred, which has been shown to increase paternity by sixty five percent when compared to males that are not cannibalized. A majority of males in this species are likely to die on the search for a mate, so the male must sacrifice himself as an offering if it means prolonged copulation and doubled paternity. It is seen in many species that cannibalized males can mate for longer, thus have longer sperm transfers.

Monogamy
Males in these mating systems are generally monogamous, if not bigynous, and have sexually evolved accordingly. Since males of these cannibalistic species have adapted to the extreme mating system, and usually mate only once with a polyandrous female, they are considered monogynous.