WHOA. Did you know dolphins have wet dreams? Be honest – you probably didn't. Hardly surprising, though, given that this is the very first footage of a wild dolphin ejaculating completely spontaneously. No overt socio-sexual activity. No direct genital stimulation. Nada. Dude was swimming lazily along when he suddenly became aroused and... well... see for yourself.

The footage was captured by researcher Tadamichi Morisaka and his colleagues at Kyoto University when the dolphin was likely in a drowsy/sleepy state. Close observation revealed the Indo-Pacific bottlenose dolphin's left eye was closed when it became aroused, "an indication of unihemispheric (or bilateral) sleep," write the researchers in PLOS ONE. As dolphin researcher Justin Gregg points out, the state is not unlike the one that commonly precedes spontaneous ejaculation in people and other species:

This is exactly the same kind of phenomenon we observe in human wet dreams: spontaneous ejaculation in the absence of physical stimulation. It has been observed in other mammals (mice, guinea pigs, cats, etc.), but never before in a dolphin [Ed. Note: Or, according to the researchers, in any other aquatic mammal]. What’s the function of spontaneous ejaculation? There are three hypotheses: 1) it’s needed to remove excess or abnormal spermatozoa, 2) it’s a kind of sexual display, 3) it has no function, and happens by accident when a sleepy/drowsy brain stops being able to regulate the inhibitory neural control system.

The researchers recount (at a level of detail that I find amusing mainly because I can't determine whether the specifics provided are totally superficial or not – though they certainly seem that way) the conditions under which the emission was observed (emphasis added):

The video was taken underwater at about 10 m depth off Mikura Island, Japan on July 2, 2012. Indo-Pacific bottlenose dolphins around Mikura Island have almost all been identified using natural marks on the body by underwater video-identification research since 1994 [Ed. Note: over 200 hours' worth of video collected in that time, according to the researchers]. Four researchers and a few other sightseers were involved in a dolphin-swimming program observed dolphins underwater, and one researcher observed dolphin behavior on the boat at the time. It was a cloudy day without rain, and the water temperature was approximately 25°C.

Check out more details on the recording over at PLoS.