Some animals may be genetically infertile. Genetic mutations can occur which can produce infertility. They can also give rise to abnormal development in embryos so much so that the young fail to develop properly and cause foetal death or atrophy. These are called lethal factors.
Poultry seem to be more affected with lethal factors than other animals.
A peculiar form of sterility occurs in heifers born twin to a bull. The male then remains normal but the female becomes a freemartin . To some extent, the female becomes masculinised by androgen produced by its twin. Generally the heavier the animal the more fertile, but overfat animals may have difficulty mating, conceiving, and birthing. Puberty is determined by body weight not by age of the animal.