Sexual coercion in animals is the use of violence, threats, harassment, and other tactics to help them forcefully copulate.[1] Such behavior has been compared to sexual assault, including rape, among humans.[2]
In nature, males and females usually differ in reproductive fitness optima.[3] Males generally prefer to maximize their number of offspring, and therefore their number of mates; females, on the other hand, tend to care more for their offspring and have fewer mates.[4] Because of this, there are generally more males available to mate at a given time, making females a limited resource.[4][5] This leads males to evolve aggressive mating behaviors which can help them acquire mates.[5]
Sexual coercion has been observed in many species, including mammals, birds, insects, and fish.[6] While sexual coercion does help increase male fitness, it is very often costly to females.[5] However, in spite of these costs, a possible benefit to the females is a chance to test the stamina of the males, so that only those with “good genes” will father their offspring.[7] Sexual coercion has been observed to have consequences, such as intersexual coevolution, speciation, and sexual dimorphism.[4][8]
Harassment/aggression
Harassment is a technique used by males of many species to force females to submit to mating.[9] It has been observed in numerous species, including mammals, birds, insects and fish.[6] Aggression and harassment have been documented in the males of guppies (Poecilia reticulata),[4] bottlenose dolphins (Tursiops aduncus), botos (Inia geoffrensis), dusky dolphins (Lagenorhynchus obscurus), Hector’s dolphins (Cephalorhynchus hectori), grizzly bears, polar bears, and ungulates.[10] It is also seen in Chinook salmon (Oncorhynchus tshawytscha),[6] red-spotted newts (Notophthalmus viridescens), and seed-eating true bugs (Neacoryphus spp.).[11] Furthermore, it is prevalent in spider monkeys,[1] wild Barbary macaques (Macaca sylvanus) and many other primates.[12]
In basically all major primate taxa, aggression is used by the dominant males when herding females and keeping them away from other males.[1] In hamadryas baboons, the males often bite the females’ necks and threaten them.[13] Wild chimpanzees can charge at females, shake branches, hit, slap, kick, pound, drag, and bite them. Orangutans are among the most forceful of mammals. Bornean orangutans (Pongo pygmaeus) exhibited aggression in almost 90 percent of their copulations, including when the females were not resisting.[14] A possible explanation for aggressive behaviors in primates is that it is a way for males to train females to be afraid of them and be more likely to surrender to future sexual advances.[1]
Intimidation
Males may also use more indirect techniques to mate with females, such as intimidation. While most female water striders (Gerridae) have their genitalia exposed, females of the water strider species Gerris gracilicornis have evolved a shield over their genitals. As a result, males cannot physically coerce females because mating is difficult unless the female exposes her genitalia. Therefore, males intimidate females into mating by attracting predators; they tap on the water’s surface and create ripples that catch the attention of predatory fish. From there, it is in the best interest of the female to mate, and as quickly as possible, to avoid being eaten by predators. Typical mating positions of water striders have the females on the bottom, closer to predators, so the risk of predation is much higher for them. Females succumb to copulation to get males to cease signaling to predators.[15]
Another indirect form of sexual coercion occurs in red-sided garter snakes, Thamnophis sirtalis parietalis. When males “court” females, they line their bodies up to the females’ and produce caudocephalic waves, which are a series of muscle contractions that travel through their bodies from tail to head. The exact reason for this behavior is unknown, but some studies show that it relates to stress. Females have nonrespiratory air sacs containing anoxic air, and the waving pushes this air into her lungs. The resulting stress causes her cloaca to open, and aids the male in inserting his hemipenis. The stronger and more frequent the caudocephalic waves and the closer the male’s cloaca to the female’s, the more likely the male is to mate successfully.[16]
Grasping and grappling
See also: Sexual conflict § Grasping
Males of certain species have evolved mating behaviors in which they forcefully attempt to mate with and inseminate females, often employing grasping techniques. These male grasping devices exist to increase the duration of copulation and restrict females from mating with other males. They are in some ways a form of mate guarding. While some males have evolved different types of modifications to aid in grasping, others just grab females and attempt to force copulation.
One type of grasping modification is spiny male genitalia. In seed beetles (Coleoptera: Bruchidae), males possess sclerotized spines on their genitalia. These spines are used during copulation to help overcome female resistance and penetrate into their copulatory duct. In addition to aiding penetration, these spines promote the passage of seminal fluids, and act as an anchor to keep the female from fleeing. Furthermore, spiny genitals can injure the females and make them less likely to remate.[8] Sepsidae fly males have modifications on their forelegs to help them grasp onto female wing bases. These modifications include cuticular outgrowths, indentations, and bristles, and males use them to secure themselves onto females after jumping on them. Once the males grab on, a struggle ensues akin to a rodeo, where males try to hold on while females violently shake them off.[7]
Another type of modification is found in male diving beetles (of the family Dytiscidae), who are equipped with suction cup structures on their front legs. They use these to grab passing females and attach to their dorsal surfaces. To get the females to submit, males shake the females violently and keep them submerged underwater (diving beetles cannot go long without atmospheric oxygen). Unable to get air, female diving beetles submit to the male’s advances in order to avoid drowning (and they lose the energy to resist). Once the males attach, copulation can occur.[17]
Male waterfowl have developed another modification; while most male birds have no external genitalia, male waterfowl (Aves: Anatidae) have a phallus (length 1.5–4.0 centimetres [0.59–1.57 in]). Most birds mate with the males balancing on top of the females and touching cloacas in a “cloacal kiss”; this makes forceful insemination very difficult. The phallus that male waterfowl have evolved everts out of their bodies (in a clockwise coil) and aids in inseminating females without their cooperation.[18]
Another such technique is having a “lock-like” mechanism, found in Drosophila montana, dogs, wolves, and pigs. Towards the end of copulation, females struggle to try to dislodge the males, who prefer to copulate much longer than females do; the locking allows the males to copulate for as long as they like. In dogs, the male has a knot in his penis that gets engorged with blood and locks into the female. Breaking this “lock” while the males try hard to continue copulation can be physically harmful to females.[19]
Males of many species simply grab the females and force a mating. Coercive mating is very common in water striders (Gerridae) because in most of the species, the female genitalia are often exposed and easily accessible to males.[3] Without any courtship behavior, males initiate by forcefully trying to mount the females. Carrying the males on their backs is energetically costly to females, so they try to resist and throw off the males. The males fight back even harder and use their forelegs to tightly grasp the female’s thorax and keep them from escaping.[20] The males then forcefully insert their genitalia into the female vulvar opening.[3] In the newt species Notophthalmus viridescens, males carry out a courtship behavior called amplexus. It consists of males capturing females that do not want to mate with them and using their hind limbs to grasp the females by their pectoral regions.[5]
Male guppies (Poecilia reticulata) have been observed to forcefully copulate with females by trying to insert their gonopodium (male sex organ) into female’s genital pores, whether or not they are accepting.[6] Sometimes, male guppies also try to forcefully mate with Skiffia bilineata (goodeid) females, which resemble guppy females and tend to share the same habitat, even when guppy females are available. A possible explanation for this is the deeper genital cavity of S. bilineata, which stimulates the males more than when mating with guppy females.[11]
Males of some species are able to immobilize females and force copulation. In pigs and boars, males grab females and maneuver the pelvis to lift the vaginal opening and facilitate copulation. The stimulation following intromission causes the female to be immobilized. The male can then freely continue copulation without worrying about the female escaping.[21] Immobilization of the female also occurs in muscovy ducks.
Grasping and/or grappling mating situations have also been documented in Calopteryx haemorrhoidalis haemorrhoidalis (Odonata),[22] fallow deer (Dama dama),[6] wild orangutans (Smuts 1993), wild chimpanzees,[1] water voles (semi-aquatic rats) Arvicola amphibius,[21] feral fowl,[23] mallard (Anas platyrhynchos),[24] hamadryas baboons [25] and many other primates,[1] coho salmon (Oncorhynchus kisutch),[6] and others.
Infanticide (zoology)
In some mammal species, mostly nonhuman primates,[citation needed] it is common for males to commit infanticide to mate with females. This happens often in species that live in groups, such as Old and New World monkeys, apes, prosimians, and hamadryas baboons.[25] There is usually a single breeding male in a group, and when an outside male aggressively takes over, he kills off all of the young offspring. The males kill infants that are not their own to assert their strength and position, and mate with the females.[1] Sometimes, multiple males will invade a troop and gang up on females, killing their offspring and subsequently mating with them. This occurs in spider monkeys, red-backed squirrel monkeys, chimpanzees, and red howlers.[1]
Coercive faithfulness
Post-copulatory guarding
Another form of coercion is male mate guarding, used to keep females from mating with other males, and often involves aggression.[9] Guarding allows the males to assure their paternity. A classic example occurs in diving beetles, family Dytiscidae. After copulation, males continue to guard females for up to six hours. They hold them underwater, occasionally tilting them up for air.[17] Guarding also occurs in water striders where, once males complete their sperm transfer, they often remain on top of the females. This guarding duration varies, lasting from several minutes to several weeks. The purpose of such long guarding periods is for the males to see the females lay their eggs and be assured that the offspring are theirs.[20] This behavior also occurs in hamadryas baboons (Papio hamadryas), where the leader males practice intensive mate guarding.[25] In Drosophila Montana, studies have shown that following mate guarding, the chances of a female mating with or being inseminated by another male were greatly diminished. This shows that the mate guarding tactic can be very effective.[19]
Acceptance/Submission
Sometimes, females choose not to struggle and simply acquiesce to forceful matings. This can happen when they decide that the cost of resisting would be greater than the cost of mating.[22] They use submission to avoid further harassment or aggression, which could end in death or injury.[26] This is often seen in primate species, such as chimpanzees and hamadryas baboons.
Female benefits to rape or sexual coercion
Some possible benefits of sexual coercion for females have been hypothesized.
Proximate
A possible proximate benefit for females is that sometimes after a male mates with a female, he becomes her mate. Then, he would defend and protect her.[22] This is seen in many primate species.[1]
Ultimate
A possible benefit of sexual coercion that would come out in the long run is the “good genes” hypothesis.[17] If males can overcome a female’s resistance, then they must possess good genes that would increase the survival, mating success, and ultimately the fitness of her offspring. The hypothesis is that females can use the sexual coercion process to assess the quality of a male.[3]