Male monarch butterflies have a unique sexual adaptation: Their penises double as dipsticks to detect how much sperm has already been deposited in a female.

If her tank is running on empty, the male will inject a diluted sperm mixture to fill her up and trick other males into thinking she's full of active sperm. However, if a male determines that she's carrying watered down sperm, he can inject his own more potent mix in an attempt to overpower the sperm already present.