Breast development extension adrenocorticotropic hormone (ACTH)), and thyroid hormones such as thyroxine (and by extension thyroid-stimulating hormone (TSH) and thyrotropin-releasing hormone (TRH)) also play permissive but less well-understood/poorly-characterized roles in breast development during both puberty and pregnancy, and are required for full functional development. Leptin has also been found to be an important factor in mammary gland development, and has been found to promote mammary epithelial cell proliferation. In contrast to the female-associated sex hormones, estrogen and progesterone, the male-associated sex hormones, the androgens, such as testosterone and dihydrotestosterone (DHT), powerfully suppress the action of estrogen in the breasts. At least one
Sexual selection in mammals neck length is not correlated to increases in other parts of the body, which would be expected from foraging selection, so sexual selection may be a more satisfactory explanation. Studies have failed to resolve the causes involved: perhaps the neck was a result of both or other forces. Precopulatory mechanisms determine who father an offspring prior to sex. Male-male competition is the biggest precopulatory mechanism in mammals. Sexual dimorphism is a result of male-male competition that is easily seen in species. Male-male competition to copulate with the opposite sex is often seen in mammals. African elephants strongly promote male-male competition.