Women’s Breasts May Have Evolved To Keep Newborns Warm, Researchers Suggest

Upon arrival, newborn babies struggle to regulate their own body heat. And, unlike a newborn pup or monkey, human babies don’t have the luxury of fur for warmth. For most of human history, that combination was potentially fatal. Now, research raises the possibility that the human body developed a response to that vulnerability over millions of years, and it may be hiding in one of the most familiar features of human anatomy.

Researchers at the University of Oulu found that breastfeeding women’s breast tissue resisted cooling at roughly twice the rate of non-breastfeeding women and men. That finding, they argue, points to a function that has largely gone unexamined: human breasts may have developed in part as a thermal support system for newborns during skin-to-skin contact. If the hypothesis holds, it gives a well-known aspect of human anatomy an entirely new purpose.

Why Human Breasts Develop Long Before They’re Needed

No other primate (yes, humans are technically primates) has permanently prominent breasts. Chimpanzees, gorillas, and most other mammals develop noticeable breast tissue only during active nursing, when the milk-producing glands are physically engaged. Human women develop breast tissue at puberty, often years before a first pregnancy, and retain it throughout adulthood. The size and shape are largely due to subcutaneous fat rather than the milk-producing glands themselves.

Scientists have long puzzled over why this trait exists at all. One idea holds that it functions as a fertility signal to attract mates. Another treats it as a byproduct of the broader increase in body fat during human evolution, possibly reinforced later by sexual selection. Some researchers argue these explanations do not fully resolve the question of why the trait appears long before any reproductive need for it exists.

The Oulu team argues that infant survival offers a more grounded explanation. When human ancestors lost their body fur roughly two million years ago, alongside the emergence of the genus Homo, newborns were suddenly exposed to cold in ways that no primate infant had faced before. Human babies also began arriving at an earlier developmental stage than other primates, a consequence of the expanding brain requiring delivery before the skull grew too large to pass through the birth canal. The result was an infant arriving unable to adequately produce body heat, with no fur and no defenses against the cold. A mother’s body was the most immediate and reliable source of warmth available, and anything that made that warmth more effective would have given infants a better chance at survival.

Nursing Mothers Held Their Heat. The Others Didn’t.

The research team recruited 27 volunteers between the ages of 20 and 40, split into three groups: 12 non-breastfeeding women, eight currently breastfeeding women, and seven men. That age range was chosen to represent typical nursing years while avoiding the hormonal shifts of puberty and menopause.

Participants were brought into a climate-controlled laboratory at the Finnish Institute of Occupational Health, dressed in lightweight garments from the waist down, and exposed to progressively cooler temperatures over the course of the experiment. A thermal imaging camera tracked breast surface temperatures throughout each exposure.

Breastfeeding women retained significantly more warmth than either of the other groups. Nursing mothers lost an average of just 2.5 degrees Celsius of breast surface temperature in the coolest setting, while non-breastfeeding women lost 4.7 degrees and men lost 4.3 degrees. Men fared slightly better than non-breastfeeding women, a result the researchers attribute to greater muscle mass providing an additional heat source. Both groups, however, lost nearly twice as much warmth as the nursing mothers.

That divide matters. The heat retention seen in nursing mothers does not appear to be simply a function of having more tissue in the area. It seems tied instead to the active biological processes of lactation, particularly the mammary glands and the blood vessel network in the chest that ramp up during pregnancy and nursing.

How Breast Anatomy Amplifies the Warming Effect

The physical form of the breast may be part of the story as well. The authors suggest that breast shape may increase the surface area available for contact with a baby’s skin compared to a flat surface, while lactation-related blood flow may help sustain warmth during that contact. More surface area means more heat transferred, and the two factors together could make a meaningful difference for a newborn with limited ability to stay warm on its own.

This fits with existing research on newborn care. Studies on mother-infant skin-to-skin contact have consistently found that maternal skin temperature is a key factor in keeping a newborn’s body temperature stable. A 2024 study on wild vervet monkeys found that lactating females maintain higher body temperatures than non-lactating ones, a pattern researchers have linked to infant thermoregulation across primate species.

The researchers do not dismiss sexual selection as a contributing factor in how the trait developed over time. Their argument is narrower: thermoregulation may have been the original pressure that kept breast tissue present between pregnancies, with sexual selection potentially reinforcing the trait later. The appearance of breast tissue at puberty, years before any infant arrives, fits a pattern of the body building resources in advance.

The study, published in Evolutionary Human Sciences, is preliminary and the authors call for follow-up research with larger groups and comparisons across primate species. The findings raise the possibility that thermoregulation played a role in the evolution of permanently enlarged breasts, but the study measures physiological differences in modern women under lab conditions and does not demonstrate evolutionary causation directly. Newborn hypothermia remains a documented clinical risk even in modern hospital settings. That this basic biological reality may have quietly shaped human anatomy over two million years is a question worth pursuing.

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