Male pigs (boars) have nipples, which often causes curiosity since these structures serve no obvious purpose in males. Pigs possess a row of small external projections along their underside, properly referred to as teats or mammary papillae. This feature is not a random evolutionary leftover but a predictable outcome of the sequence of events that occur very early in the embryonic stages of life. Understanding why a male possesses a characteristic primarily associated with female reproduction requires examining the fundamental blueprint for mammalian development.
Embryological Basis for Nipple Formation
Nipple formation begins in the developing pig fetus long before the biological sex is determined. The initial structure is the mammary ridge, often called the “milk line,” a pair of thickened ectodermal bands that run along the ventral side of the embryo. This formation of the external nipple structure is essentially the default setting in the mammalian developmental timeline. The primordia of the mammary glands are observable around 23 days of gestation, with the external teats established shortly thereafter.
Sexual differentiation, which determines whether the fetus develops male or female reproductive organs, occurs later. In male pigs, the testes produce androgens, such as testosterone, which trigger male characteristics and arrest the internal development of the mammary gland tissue. This hormonal signal prevents the underlying epithelial tissue from forming the complex ductal and lobuloalveolar network required for milk production. Since the external nipple structure has already formed before the androgen surge, and male hormones do not cause its regression in pigs, the physical structure remains visible into adulthood.
Nipple Presence Across the Mammalian Kingdom
The presence of nipples in male pigs is a universal feature across most of the Class Mammalia, including humans, dogs, horses, and cattle. This reinforces that initial embryonic development is conserved across species, but the key difference is whether the later hormonal environment causes the regression of the nipple. For example, male mice and rats are among the few mammals whose developing mammary tissue is highly sensitive to fetal androgen exposure, causing the complete atrophy of the external nipple. Pigs, like humans, are mostly insensitive to this hormonal signal, and the full milk line means they typically have between 10 to 18 teats, present on both males and females.
Do Male Pig Nipples Serve Any Function?
The nipples on a male pig are considered vestigial structures, having lost their original biological function through evolution. Since the underlying glandular tissue is rudimentary and lacks the full apparatus of ducts, lobules, and alveoli, male pigs cannot lactate. They also lack the necessary hormonal triggers, such as prolactin and oxytocin, released during pregnancy and nursing in females.
While the individual male pig’s teats have no biological function for him, the number of teats on a boar is genetically significant in swine breeding. Because the number of teats is a highly heritable trait, a boar with a high, well-spaced teat count is often selected to pass this trait on to his female offspring (gilts). This ensures the gilts will have enough functional teats to successfully nurse a large litter, making the male’s non-functional teats an important consideration for herd productivity.