Male dogs possess nipples because this anatomical feature is a biological remnant from the earliest stages of life in the womb. Every canine embryo, regardless of its eventual sex, follows the exact same initial physical blueprint during development. The presence of these small, non-functional structures on a male dog’s abdomen confirms this shared anatomical origin.
The Universal Mammalian Blueprint
The formation of nipples in a dog embryo occurs very early in development, long before any physical differences between the sexes begin to appear. The developing embryo follows a single, shared set of genetic instructions for constructing a basic mammalian body plan. During this period, a specialized thickening of the ectoderm, known as the mammary ridge or milk line, forms along the underside of the embryo.
This mammary ridge extends in two parallel lines from the chest area down toward the groin. Cells within this ridge then differentiate and sink into the underlying tissue, creating the rudimentary structures of the mammary glands and nipples. This process is completed well before the genetic instructions for sexual development are activated.
In canines, sex differentiation—the process where the embryo becomes morphologically male or female—does not begin until roughly day 35 to 40 of the gestation period. This differentiation is triggered by the Y chromosome, which activates the SRY gene and leads to the production of testosterone and the development of male reproductive organs. Since the nipples and mammary ridges have already formed weeks earlier, the subsequent development of male characteristics does not cause these structures to disappear.
Lack of Function and Purpose
While the underlying genetic blueprint ensures the nipples form, the necessary hormonal environment for them to develop into functional mammary glands is absent in male dogs. The structures that do form are considered vestigial, meaning they are non-functional remnants of an organ that was functional in an ancestor or in the other sex.
Functional mammary glands in female dogs require a complex interplay of hormones, primarily prolactin and progesterone, to spur the growth of ducts and secretory lobules. In a male dog, high levels of testosterone and low levels of estrogen and prolactin prevent the necessary proliferation and development of the internal mammary tissue. The male hormones suppress the further development of the glands, leaving behind only the surface-level nipple structure.
The small, raised bumps remain in place because their retention poses no disadvantage to the dog’s survival or reproduction, so there has been no evolutionary pressure to eliminate them entirely. It is genetically simpler to keep the initial developmental step of forming the nipples than to introduce a new, sex-specific genetic signal to remove them in males.
Nipple Anatomy and Owner Concerns
Male dogs typically have between eight and ten nipples, arranged symmetrically in two parallel rows running down the underside of the abdomen. The number can vary slightly between individuals and breeds, but they are consistently positioned along the former embryonic milk line. In males, these nipples are usually small, appearing as tiny, flush or slightly raised bumps. They are often less prominent than a female dog’s nipples, especially those that have been pregnant.
It is important for owners to routinely check their dog’s abdomen to establish a baseline of what is normal for their pet. While these structures are generally harmless, any sudden changes can indicate a health concern requiring veterinary attention. Owners should monitor for signs such as unexpected swelling, redness, discharge, or any noticeable scabbing on or around the nipple.
Though rare, male dogs can develop conditions affecting the mammary tissue, including mastitis, which is inflammation or infection of the glands. More seriously, male dogs can also develop mammary tumors, which, although far less common than in females, tend to be more aggressive when they do occur. Any lump or mass forming near a nipple or any change in the size or texture of the nipples warrants a prompt examination by a veterinarian.