Sexual coercion has been observed in many clades of animals, including mammals, birds, insects, and fish.Garner, S. R., Bortoluzzi, R. N., Heath, D. D. & Neff, B. D. "Sexual conflict inhibits female mate choice for major histocompatibility complex dissimilarity in Chinook salmon". Proceedings: Biological Sciences 277, 885–94 (2010). It includes the use of violence, threats, harassment, and other tactics.Smuts, Barbara B. "Male Aggression and Sexual Coercion of Females in Nonhuman Primates and Other Mammals: Evidence and Theoretical Implications". Advances in the Study of Behavior 22 (1993) Such behavior has been compared to sexual assault, including rape, among humans.Stamos, David N., Evolution and the Big Questions: Sex, Race, Religion, and Other Matters, John Wiley & Sons, 2011; Alcock, John, The Triumph of Sociobiology, Oxford University Press, 2003, p.207-9.
In nature, males and females usually differ in reproductive fitness optima.Han, C. S. & Jablonski, P. G. "Female genitalia concealment promotes intimate male courtship in a water strider". PLoS ONE 4, e5793 (2009). 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.Gage, M. J. G., Parker, G. a, Nylin, S. & Wiklund, C. "Sexual selection and speciation in mammals, butterflies and spiders". Proceedings: Biological Sciences 269, 2309–16 (2002). Because of this, there are generally more males available to mate at a given time, making females a limited resource. This leads males to evolve aggressive mating behaviors which can help them acquire mates.Grayson, K. L., De Lisle, S. P., Jackson, J. E., Black, S. J. & Crespi, E. J. "Behavioral and physiological female responses to male sex ratio bias in a pond-breeding amphibian". Frontiers in Zoology 9, 24 (2012).
While sexual coercion does help increase male fitness, it is very often costly to females. Sexual coercion has been observed to have consequences, such as intersexual coevolution, speciation, and sexual dimorphism.Rönn, J., Katvala, M. & Arnqvist, G. "Coevolution between harmful male genitalia and female resistance in seed beetles". Proceedings of the National Academy of Sciences of the United States of America 104, 10921–5 (2007).
Male adaptations
Harassment/aggression
Harassment is a technique used by males of many species to force females to submit to mating.Muller, M. N., Kahlenberg, S. M., Emery Thompson, M. & Wrangham, R. W. "Male coercion and the costs of promiscuous mating for female chimpanzees". Proceedings: Biological Sciences 274, 1009–14 (2007). It has been observed in numerous species, including mammals, birds, insects and fish. Aggression and harassment have been documented in the males of guppies (Poecilia reticulata), bottlenose dolphins (Tursiops aduncus), botos (Inia geoffrensis), dusky dolphins (Lagenorhynchus obscurus), Hector's dolphins (Cephalorhynchus hectori), grizzly bears, polar bears, and ungulates.Fury, C. A., Ruckstuhl, K. E. & Harrison, P. L. "Spatial and social sexual segregation patterns in indo-pacific bottlenose dolphins (Tursiops aduncus)". PLoS ONE 8, e52987 (2013). It is also seen in Chinook salmon (Oncorhynchus tshawytscha), red-spotted newts (Notophthalmus viridescens), and seed-eating true bugs (Neacoryphus spp.).Valero, A., Macías Garcia, C. & Magurran, A. E. "Heterospecific harassment of native endangered fishes by invasive guppies in Mexico". Biology letters 4, 149–52 (2008). Furthermore, it is prevalent in spider monkeys, wild Barbary macaques (Macaca sylvanus) and many other primates.McFarland, R. & Majolo, B. "Grooming coercion and the post-conflict trading of social services in wild Barbary macaques". PLoS ONE 6, e26893 (2011).
In basically all major primate taxa, aggression is used by the dominant males when herding females and keeping them away from other males. In hamadryas baboons, the males often bite the females' necks and threaten them.Nitsch, F., Stueckle, S., Stahl, D. & Zinner, D. "Copulation patterns in captive hamadryas baboons: a quantitative analysis". Primates; journal of primatology 52, 373–83 (2011). 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.Knott, C. D., Emery Thompson, M., Stumpf, R. M. & McIntyre, M. H. "Female reproductive strategies in orangutans, evidence for female choice and counterstrategies to infanticide in a species with frequent sexual coercion". Proceedings: Biological Sciences 277, 105–13 (2010). 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.
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.Han, C. S. & Jablonski, P. G. "Male water striders attract predators to intimidate females into copulation". Nature Communications 1, 52 (2010).
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.Shine, R., Langkilde, T. & Mason, R. T. "Courtship tactics in garter snakes: how do a male's morphology and behaviour influence his mating success"? Animal Behaviour 67, 477–483 (2004)
Grasping and grappling
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 spiky 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. 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.Puniamoorthy, N., Su, K. F.-Y. & Meier, R. "Bending for love: losses and gains of sexual dimorphisms are strictly correlated with changes in the mounting position of sepsid flies (Sepsidae: Diptera)". BMC Evolutionary Biology 8, 155 (2008).
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.Bergsten, J. & Miller, K. B. "Phylogeny of diving beetles reveals a coevolutionary arms race between the sexes". PLoS ONE 2, e522 (2007).
Male waterfowl have developed another modification; while most male birds have no external genitalia, male waterfowl (Aves: Anatidae) have a phallus (length ). 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.Brennan, P. L. R. et al. "Coevolution of male and female genital morphology in waterfowl". PLoS ONE 2, e418 (2007).
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, whose genital organs take much longer to deflate than females do; the locking (most commonly known in canids as a "tie") allows the males to copulate for as long as they need to until they are finished. In dogs, the male has a knot in his penis that gets engorged with blood and ties the female, locking them together during copulation, until the act is complete. Male dogs have evolved this mechanism during mating in order to prevent other males from penetration whilst they are and the use of the tie enables them to be more likely to inseminate the female and produce a healthy litter of pups. Breaking this "tie" can be physically harmful to both females and males.Mazzi, D., Kesäniemi, J., Hoikkala, A. & Klappert, K. "Sexual conflict over the duration of copulation in Drosophila montana: why is longer better?" BMC Evolutionary Biology 9, 132 (2009).