Sexual coercion among animals

Sexual coercion has been observed in many clades of animals, including mammals, birds, insects, and fish. It includes the use of violence, threats, harassment, and other tactics. Such behavior has been compared to sexual assault, including rape, among humans.

In nature, males and females usually differ in reproductive fitness optima. 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. 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.

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.

Harassment is a technique used by males of many species to force females to submit to mating. 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. It is also seen in Chinook salmon (Oncorhynchus tshawytscha), red-spotted newts (Notophthalmus viridescens), and seed-eating true bugs (Neacoryphus spp.). Furthermore, it is prevalent in spider monkeys, wild Barbary macaques (Macaca sylvanus) and many other primates.

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. 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. 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.

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.

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.

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.