Cloaca

Also called: Vent

The cloaca (or vent) is the single opening on the underside near the tail that an amphibian uses for waste, mating and egg-laying — and its shape is often how you sex adults.

In axolotls, for example, a mature male has a puffy cloaca that bulges to the sides, while a female's is a smaller bump — one of the few reliable sexing tells.