Many non-human animal species also exhibit bisexual behavior. This is, of course, common in hermaphroditic animals, but is also known in many other species. Examples of mammals include the bonobo (formerly known as the pygmy chimpanzee), orca, and bottlenose dolphin. Examples of avians include some species of gulls and Humboldt Penguins. Other examples occur among fish, flatworms, and crustaceans.
Many species of animals are involved in the act of forming sexual and relationship bonds between the same sex; even when offered the opportunity to breed with members of the opposite sex, they picked the same sex. Some of these species are gazelles, antelope, bison, and sage grouse.
In some cases animals will choose intercourse with different sexes at different times in their life, and sometimes will perform intercourse with different sexes at random. Homosexual intercourse can also be seasonal in some animals like male walruses, who often engage in homosexual intercourse with each other outside of the breeding season and will revert to heterosexual intercourse during breeding season.
In some cases bisexuality is actually a form of fitness favored by evolution. For example, in the absence of male whiptail lizards (Cnemidophorus), females reproduce by pairing up with each other. During the breeding season females will take turns switching between "male" and "female" roles as their hormones fluctuate. Estrogen levels are high during ovulation ("female" role) and much lower after laying eggs ("male" role). While in the "male" role, a female lizard will mount another in the "female" role and go through the motions of sex to stimulate egg-laying. The hatchlings produced are all female. This all-female species has evolved from lizards with two sexes, but their eggs develop without fertilization (parthenogenesis). Female whiptail lizards can lay eggs without sex, but they lay far fewer eggs than if they engage in sexual stimulation by another female.
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