![]() We did not aggressively follow dolphins and finished our observations when they went away. We did not use scuba diving equipment, but only fins, snorkels, and masks. ![]() When we spotted dolphins from the boat (about 7 m length), we slowly approached the group with the boat and placed the video system underwater. Video codec was AVCHD of 1920 pixels in width and 1080 pixels in height with 30 frames/s. Video recording was made with an HDR-XR550V (Sony, Japan) with an attached wide conversion lens in an underwater housing system (NTF Corp, Japan). At Mikura Island, we had also frequently observed masturbation and socio-sexual behaviors, but not associated ejaculation (Morisaka, Sakai, Kogi, personal communications). Masturbation, such as rubbing genitals on tank objects or the floor, is frequently observed in several odontocetes, including the bottlenose dolphin, spinner dolphin, killer whale, baiji, and boto –, but never accompanied by ejaculation. Ejaculation and actual copulation are difficult to observe even in captive dolphins, (but see ). Despite many observations of socio-sexual behavior in both sexes in this species, a successful copulation or intromission has never been observed in Shark Bay, the oldest study site for this species, which was implemented in 1988. Male bottlenose dolphins engage in much higher rates of socio-sexual behavior than wild, “hypersexual” bonobos. The dolphins live in fission-fusion societies, which are characterized by sex-segregation and frequent changes in group membership (reviewed in ). Around Mikura Island, the breeding season was estimated to be between April and October with a peak in July and August. Sexual maturity in male Indo-Pacific bottlenose dolphins occurs at approximately 7–8 years (reviewed in ). The Indo-Pacific bottlenose dolphin is a small odontocete that reaches 2.7 m in length and 230 kg in weight, and appears in coastal waters from around South Africa, through the Indian Ocean, to southeast Asia and Australia. This is the first report of spontaneous ejaculation in an aquatic mammal, and the first video of spontaneous ejaculation in animals to be published in a scientific journal. Here, we report spontaneous ejaculation in a wild Indo-Pacific bottlenose dolphin ( Tursiops aduncus), and present an accompanying video. Three (not mutually exclusive) possible “functions” of animal spontaneous ejaculation have been discussed in previous publications: (1) a type of “masturbation” as a sexual outlet and/or for the removal of surplus (or abnormal) spermatozoa, , – (2) an element of sexual display (3) no clear function, or misuse of inhibitory neural control system during drowsiness and sleep, , –. The function of spontaneous ejaculation is unknown. Spontaneous ejaculation could possibly be widespread in various animals, including humans, but has passed unrecognized because it is an unpredictable and rare behavior that lasts only a few seconds, making it difficult to observe. Spontaneous ejaculation, which is defined as the release of seminal fluids in the absence of apparent sexual stimulation, has been reported in several male land mammals, including Rodentia (rats, hamsters, guinea pigs, mice ), Cetartiodactyla (mountain sheep, warthogs (reviewed in ), tsessebes ), Carnivora (domestic cats, spotted hyenas (reviewed in )), Perissodactyla (horses ), and Primates (chimpanzees, (reviewed in ), humans (reviewed in )).
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