Male cattle do have nipples, usually four of them in the same location as a cow’s udder, and under rare circumstances the tissue behind those nipples can produce small amounts of fluid. In normal life, though, a bull or steer will never fill a milk pail. The reason males carry nipples at all, and the biological barriers that keep those nipples from doing much, trace back to how mammalian embryos develop before sex hormones enter the picture.
Why Bulls Have Nipples in the First Place
Every calf, regardless of sex, starts out with the same basic body plan. Mammary tissue begins forming early in embryonic development, well before the genes on the sex chromosomes ramp up testosterone or estrogen production. By the time the embryo “knows” whether it is male or female in a hormonal sense, the groundwork for nipples has already been laid down. In species like cattle, guinea pigs, and humans, male embryos simply keep the nipples that were built during that early shared stage of development.
Not every mammal works this way. In mice, for example, androgen receptors are present in the developing mammary bud during sexual differentiation. Testosterone binds to those receptors, triggering programmed cell death in the nipple tissue, so male mice are born without visible nipples at all. Guinea pigs lack that androgen receptor activity in the mammary bud, which allows cell proliferation to continue and nipples to form in both sexes.1PubMed Central. Developmental basis of male nipple loss and retention in mammals Cattle fall into the guinea-pig camp: the androgen-driven destruction pathway is not active in their mammary tissue, so bulls are born with four teats just like heifers.
This is not a design flaw or an evolutionary leftover waiting to disappear. Eliminating male nipples would require active selection pressure against them, and since nipples on a bull do not cost the animal much energy or create a survival disadvantage, there is no strong evolutionary reason to get rid of them. They persist because there is nothing pushing them away.
What Keeps Bulls From Making Milk
Having nipples is only the first step toward producing milk. The mammary gland itself needs extensive development, and that development is driven by a cocktail of hormones that bulls simply do not produce in the right amounts or combinations. Mammary growth depends on growth hormone, prolactin, estrogen, progesterone, and adrenocortical steroids working together.2PubMed. Endocrinology of milk production In a female cow, the hormonal shifts of puberty, pregnancy, and the period just before calving orchestrate this process over months. The mammary ducts branch, the milk-secreting cells multiply, and the gland fills with the nutrient-rich fluid we call milk.
A bull’s endocrine system runs a completely different program. Testosterone dominates, prolactin stays low, and the estrogen and progesterone surges that would tell mammary cells to proliferate and begin secreting never arrive. The underlying tissue is there in a rudimentary form, tucked behind those four teats, but it remains dormant, like wiring in a wall that was never connected to the breaker box.
When Bull Mammary Tissue Does Produce Fluid
Dormant does not always mean silent. There are documented situations in which the mammary glands of male cattle swell and produce expressible fluid, though what comes out is generally not true milk.
The best-studied scenario is mastitis in bulls. At one bull test station, 21 out of 97 bulls were found to have enlarged mammary glands with expressible fluid and purulent material. Bacterial cultures of the discharge grew a range of common environmental pathogens. Treatment with intramammary antiseptics and antibiotics was attempted but did not resolve the infections.3The Bovine Practitioner. Mastitis in beef bulls on a feeding test The fluid in these cases was not milk in any useful sense; it was inflammatory discharge produced by tissue responding to bacterial invasion. But the finding highlights that the mammary tissue behind a bull’s teats is real, living tissue that can swell, become infected, and secrete fluid under the wrong conditions.
Environmental exposure to estrogenic compounds can also trigger abnormal mammary development in males. Young dairy goats accidentally fed hay contaminated with formononetin, a plant estrogen found in certain clovers, developed precocious mammary gland growth and even began lactating, despite being nonpregnant and only six to eight months old.4Veterinary Clinics: Food Animal Practice. Reproductive Toxicoses of Food Animals While that study involved female goats, the same estrogenic pathway exists in males. Plant estrogens and synthetic endocrine disruptors can mimic the hormonal signals that tell mammary tissue to grow, and male animals exposed to high enough levels may show similar glandular changes.
There is also older evidence, referenced in veterinary literature, that castrated male cattle may lactate under certain circumstances. Castration removes the main source of testosterone, and without that hormonal brake, the balance can shift just enough for the rudimentary mammary tissue to respond to whatever small amounts of estrogen and prolactin circulate in the body. This does not produce anything resembling a dairy cow’s output, but it does show that the biological machinery is not entirely absent in the male.
Male Lactation Beyond Cattle
The question of whether males can make milk is not unique to cows. Across mammals, the answer is almost always “no, but the equipment is technically there.” The rare exceptions are fascinating.
Until fairly recently, researchers had no confirmed examples of male lactation in wild mammals. That changed when two species of Old World fruit bats, the Dayak fruit bat and the masked flying fox, were documented producing milk from functional mammary glands.5PubMed. Male lactation: why, why not and is it care? Why these bats and not other species remains an open question. One hypothesis is that the bats’ diet, rich in plants containing phytoestrogens, nudges the hormonal balance enough to activate mammary tissue. Another is that these species evolved male lactation as a form of parental investment, with fathers supplementing the mother’s milk supply to improve offspring survival. Neither explanation has been definitively confirmed.
In humans, male lactation is rare but medically documented. A case study of a 27-year-old man with galactorrhea caused by abnormally high prolactin levels found that his breast secretion contained lactose, alpha-lactalbumin, and lactoferrin, the same key components found in normal human breast milk. The concentrations of sugars, proteins, and electrolytes in his secretion fell within the range seen in colostrum and milk from lactating women.6PubMed. Composition of breast fluid of a man with galactorrhea and hyperprolactinaemia The condition was caused by a hormonal imbalance, not a normal physiological state, but the finding demonstrates that male mammary tissue, when given the right hormonal cues, can produce fluid that is biochemically genuine milk.
In rats, prolactin-secreting pituitary tumors can flood the body with prolactin while simultaneously suppressing testosterone, effectively creating a hormonal environment that mimics some features of the female lactation cycle.7Karger Publishers (Neuroendocrinology). Effects of a prolactin-secreting pituitary tumor on hypothalamic, gonadotropic and testicular function in male rats The pattern across species is consistent: male mammals have the cellular raw material for milk production, but the endocrine environment actively suppresses it. When disease, tumors, castration, or chemical exposure disrupts that suppression, the tissue can wake up.
Can Hormones Force the Issue
Researchers have tested whether exogenous hormones can deliberately induce lactation in animals that would not normally be producing milk. Most of this work has been done in females, not males, but it reveals how close the hormonal trigger sits to the surface.
In a study on non-pregnant sows, researchers used estradiol and prostaglandin treatments to mimic the hormonal cascade of late pregnancy. Lactation was successfully induced in just over half the animals treated. The milk collected from those sows was not only real milk but contained high levels of immunoglobulins, the antibodies that protect newborns from infection.8PubMed Central. Artificial lactation by exogenous hormone treatment in non-pregnant sows The study was a proof of concept that the mammary gland can be activated pharmacologically, without pregnancy ever occurring.
Applying the same approach to a male animal would face steeper hurdles. The rudimentary mammary tissue in a bull has not undergone the ductal branching and lobular development that pregnancy hormones drive in females. You would need not just a prolactin surge to initiate secretion, but a prolonged estrogen-progesterone treatment to build the gland architecture first, and even then the yield would be minuscule. No one has published a protocol for inducing commercially meaningful milk production in a bull, and there would be no practical reason to try when female dairy cows are already optimized for the job through thousands of years of selective breeding.
Bulls That Shape the Milk Supply Without Producing a Drop
Here is where the story takes an ironic turn. Bulls never produce milk themselves, yet individual bulls have a greater influence on the dairy industry’s total milk output than any single cow ever could. The reason is genetics and scale.
A dairy cow produces milk for her own lifetime. A top-performing bull, through artificial insemination, can sire tens of thousands of daughters across an entire country. If those daughters inherit genes for higher milk yield, longer lactation, or richer fat content, the bull’s genetic contribution ripples across the industry for generations. Evaluating the “breeding value” of bulls for milk traits they personally never express is central to dairy genetics. Researchers use statistical models that compare the performance of a bull’s daughters against the herd average, ranking bulls based on how much their offspring exceed or fall short of expected production for traits like milk volume, fat yield, and fat percentage.9Biotechnology in Animal Husbandry. The evaluation of breeding value of simmental bulls for milk performance in Serbia
This means that the most important “milk animal” on a modern dairy farm may be a bull whose semen is stored in a tank of liquid nitrogen. He will never lactate, but his DNA determines whether the next generation of cows fills the tank faster, produces creamier milk, or stays healthier while doing it. The disconnect between a bull’s own body and his genetic value for milk production is one of the more counterintuitive features of livestock breeding.
Transgenic Animals and the Mammary Gland as a Factory
The mammary gland’s ability to produce complex proteins in large quantities has attracted interest far beyond the dairy aisle. Transgenic livestock species have been explored as living factories for recombinant proteins, where the mammary gland secretes pharmaceutical compounds directly into the milk.10PubMed Central. Use of Genome Editing Techniques to Produce Transgenic Farm Animals The idea is straightforward: insert a gene for a medically valuable protein (an anticoagulant, an antibody, a clotting factor) into an animal’s genome, linked to a promoter that activates only in mammary tissue. When the animal lactates, the target protein appears in the milk, where it can be purified on an industrial scale.
This technology has been used primarily in female goats, sheep, and cattle, since they are the ones that lactate routinely. But the underlying concept highlights an interesting point about male nipples. The tissue is there, the cellular machinery for secretion exists in a primitive form, and the genome of a male animal carries the same milk-protein genes as a female. The bottleneck is hormonal, not genetic. In theory, a transgenic bull given the right hormonal treatment could express a pharmaceutical protein through its mammary tissue, though no one has demonstrated this at a practical scale. The female mammary gland remains a far more developed and efficient production system.
Common Misunderstandings About Male Cattle Anatomy
Several misconceptions float around this topic that are worth clearing up. One is the idea that the bumps visible between a bull’s hind legs near the sheath are nipples. They usually are nipples, but people often mistake the prepuce (the sheath covering the penis) for an udder and the actual teats for something else entirely. Bulls carry their small, vestigial teats in roughly the same inguinal region as a cow’s udder, just much less prominent. They can be easy to miss on a heavily muscled animal.
Another common confusion involves the term “boy cow” itself. In cattle terminology, a bull is an intact adult male, a steer is a castrated male, and a cow is technically an adult female that has calved. A heifer is a young female that has not yet had a calf. All of these animals have nipples. The distinction between bulls and steers matters for the lactation question because, as noted earlier, castration removes testosterone and can shift the hormonal balance enough to occasionally coax small amounts of secretion from rudimentary mammary tissue. A steer is marginally more likely than a bull to show any mammary activity, though even then it remains extremely uncommon.
Perhaps the most persistent myth is that male nipples in mammals are “vestigial” in the evolutionary sense, implying they are on their way out. They are not. Vestigial structures are ones that had a function in an ancestor and have lost it over time, like the human appendix or the pelvic bones in whales. Male nipples were never functional in ancestral males; they exist because they develop before sex differentiation, and evolution has not removed them because they impose no meaningful cost. They are a byproduct of shared developmental programming, not an organ in decline. The distinction matters because calling them vestigial implies evolutionary pressure to eliminate them, when in reality there is none.