Studying animals can help in understanding things about people. In mammals, testicles can be very big or very small in relation to the size of the animal itself. The size has to do with the amount of sperm that the male animal needs to make. Some types of male animal generally only have one sexual partner at a time and are called monogamous. Some types of animals generally live in a herd or flock where one male will have many female sexual partners. These males are called polygamous. More sperm is needed by "polygamous" males than by "monogamous" males. "Polygamous" males generally have larger testicles than "monogamous" males. The testicles grow larger to make more sperm.