The Horniest Mammal

Dolphins:

[T]hey really like sex. More than humans. More than bonobos. In fact, dolphins are so sex talented that they have mastered what millions of teenage boys have been unable to achieve. Dolphins can masturbate. With no hands (it mostly involves rubbing against things).

And they even spontaneously ejaculate:

[W]hy does this happen? It actually has to do with how ejaculation is controlled in the male mammal. This hasn’t been tested, but the hypothesis is that if the dolphin is “wired” like some mammals such as rats (and there’s no reason to believe he’s not, in this case), ejaculation is controlled at the spinal level, rather than in the brain, and controlled by the release of various chemicals. During sleepiness (which you can often detect in a dolphin due to one eye being closed, as in this case), this system would be relaxed, and spontaneous ejaculation would be more likely.