copulation

Play has been reported in reptiles, fish, and even spiders, where sexually immature animals use it to practice copulation.

In duck ponds, Prum said, a lot of forced copulation occurs.

"The females are enormously, amazingly successful at preventing fertilization by forced copulation," he said.

This posture, to which they are easily accustomed, becomes natural to them, since they assume it at the time of their copulation.