Domestic sheep
Domesticated sheep are one species known to show exclusive homosexuality, like we see in humans. The biologist Charles Roselli has found that 6 to 8% of rams (male sheep) only show interest in other rams over a long period of time, instead of ewes. He has cut open the brains of homosexual rams and found that a region of the brain in the hypothalamus called the SDN was the same size of the SDN in a ewe (female sheep). The size of this brain region appears to determine whether a ram likes females or males, and is determined by sex hormones while it was growing during its mothers pregnancy.