Fish and amphibians fertilize their eggs externally. They spend much or all of their lives in water, and their sperm can swim short distances to fertilize the eggs.

Animals that use external fertilization typically produce many eggs. The developing embryos have far less protection than embryos developing inside the female parent’s body. This male stickleback has made a rounded nest of debris to contain the eggs and he aggressively protects both the eggs and the nest.

This species of toad actually carries the eggs around on its back. When they are laid the male fertilizes them and presses the eggs onto the female’s back. Cysts grow around the eggs, protecting them until they hatch as tiny toads after about 80 days.

Internal fertilization involves the male placing his sperm in or near the female’s reproductive tract during mating.

Mating behaviors have evolved that allow males and females to be attracted to each other in order to mate - even if members of a species are by nature mutually aggressive.

Copyright 2006 The Regents of the University of California and Monterey Institute for Technology and Education