Among marine mammals, dolphins, whales, seals, seal lions, walruses and manatees all practice frequent same-sex genital play. “The lives of male bottlenose dolphins are characterized by extensive bisexuality, combined with periods of exclusive homosexuality,” writes Canadian biologist Bruce Bagemihl in Biological Exuberance: Animal Homosexuality and Natural Diversity. Males of this species form lifelong same-sex pair bonds around the age of 10, and don’t father calves with females until they are 20- to 25-years-old. Male couplings may strengthen alliances between small groups of bottlenose dolphin male and provide practice for later opposite-sex encounters.
Both males and females stimulate same-sex partners using their snouts, flippers, flukes and genitals, and males -- which have genital slits -- have been observed having genital and anal sex with each other. In captivity, male river dolphins sometimes even partake in blowhole penetration, with the older animal, if there is an age difference, on top.