via olfaction. MHC genes make molecules that enable the immune system to recognize invaders; generally, the more diverse the MHC genes of the parents, the stronger the immune system of the offspring. It would therefore be beneficial to have evolved systems of recognizing individuals with different MHC genes and preferentially selecting them to breed with.

Yamazaki et al. (1976) showed this to be the case for male mice, which show a preference for females of different