The identification of individual bones is confounded by the non-random nature of specimens left by hunters, and by technological practices such as castration and the development of herds of cows. Selection may have encouraged the development of paedomorphic forms with persistent juvenile characteristics, such as pliable behavior, as well as neotenic forms with precocious reproductive development. Superimposed on these genetic changes there may have been changes in body structure associated with the new environment created by animal husbandry. The diminution of brain size relative to body weight was accompanied by allometric changes between parts of the brain. In sheep, the cortical reduction of white matter exceeded that of the gray matter, and the parts of the brain that were involved with the senses of smell, sight and hearing showed the greatest reduction, perhaps in relation to a safer environment and controlled feeding.
Domestication is accompanied by allometric changes in the endocrine glands.
In wild and domesticated ducks, appropriate differences occur in the pituitary-adrenal system, but the nature of the relationship between domestication and changes in behavior and adrenocortical activity is still open to question. Sheep provide a striking example of phenotypic changes that may be produced by what Charles Darwin called unconscious selection. Another factor that may be involved in the interaction of animal breeding with growth is the general trends towards an inverse relationship between functional maturity and growth rate in muscle. Thus, muscles that must be able to function effectively for locomotion at birth may not be able to grow rapidly, and vice versa. This has been demonstrated in different species of birds and could also be operative in domestic animals.