Breastfeeding Mothers' Breasts Run Warmer - and That May Have Saved Newborns for 1.8 Million Years

Human female breasts are anatomically unusual among mammals. Other great apes have flat chests with small nipples that swell only during lactation; humans alone have permanent, fatty, prominently shaped breasts regardless of reproductive status. Evolutionary biologists have proposed competing explanations - signaling of reproductive fitness, storage of fat reserves during times of scarcity, byproducts of other selection pressures - but none has been entirely satisfying. A new study from the University of Oulu proposes a function that previous hypotheses overlooked: keeping newborns warm enough to survive.

The study, published in Evolutionary Human Sciences, tested whether breastfeeding women's breasts produce and retain more heat than those of non-lactating women and men - and whether that warmth persists when ambient temperature drops. The researchers used a climate-controlled chamber at the Finnish Institute of Occupational Health's Kastelli Research Centre to expose participants to different temperatures while measuring skin surface temperatures across body regions.

The Vulnerability of Human Newborns

The evolutionary rationale for the hypothesis rests on a distinctive feature of human infants: they are born extraordinarily underdeveloped relative to other primates. Human newborns cannot walk, cannot regulate their own body temperature effectively, and depend completely on caregiver warmth. In the environments where modern humans evolved and where our ancestors lived before the controlled climates of modern housing, a newborn that could not maintain its core temperature faced a rapid and serious threat.

The breast's anatomy supports a thermoregulatory role. A large proportion of breast tissue is adipose - fat - which is a thermal insulator. The shape and size of the breast allow for substantial surface contact between mother and infant during feeding. If the breast surface runs warmer than surrounding body surfaces, and if that warmth is maintained even in cold conditions, the contact during nursing would transfer heat directly to the infant across a broad area - a mechanism that could make a meaningful difference to a neonate struggling to maintain 37 degrees Celsius in a cool environment.

What the Experiment Found

The study compared three groups: breastfeeding mothers, men, and non-breastfeeding women. In the climate chamber, participants were exposed to a range of temperatures while a thermal imaging system measured skin surface temperatures across body regions.

The results showed that breastfeeding women maintained higher breast surface temperatures than either control group. More importantly, that temperature advantage persisted even when the chamber temperature was lowered - suggesting the effect is not simply a consequence of higher baseline metabolism during lactation, but reflects a physiological priority placed on maintaining breast warmth even under thermal challenge.

"This could improve a newborn's chances of survival and provide an evolutionarily grounded explanation for the development of external breasts in humans," said Juho-Antti Junno, adjunct professor and bioarchaeologist at the University of Oulu.

An Honest Assessment: This Is a Proof-of-Concept

The researchers are explicit about the study's scope. The sample was small - the paper describes it as a proof-of-concept study - and the investigators are not claiming definitive evidence for the thermoregulatory hypothesis, only that the physiological observation they predicted actually exists. The next steps would involve quantifying how much heat transfer occurs during actual breastfeeding under realistic temperature conditions, and comparing that transfer to the thermoregulatory needs of neonates.

The study does not address why permanent breast anatomy developed specifically, rather than tissue that enlarges and warms only during lactation. A thermoregulatory function during nursing would not necessarily require permanently shaped breasts outside of that period. The authors propose that permanent breast anatomy may reflect other evolutionary pressures operating in parallel, and that thermoregulation during lactation is one component of a multi-factor explanation rather than a complete account on its own.

Testing the hypothesis further would require both physiological measurement in mother-infant pairs and paleontological inference about when in hominin evolution the permanent breast anatomy appeared - a record that soft tissue does not preserve.

A Convergence of Disciplines

The research group reflects an unusual combination of expertise: archaeologists, medical researchers, and occupational health scientists working together to address an evolutionary question through physiological experiment. Junno noted that he considers the finding exceptionally significant in the context of Finnish evolutionary biology research.

"This is one of the most interesting discoveries concerning early human evolution made in Finland," Junno said.

The research team includes Tiina Kuvaja, Tiina Vare, Sirkka Rissanen, Hannu Rintamaki, Petri Lehenkari, and Juho-Antti Junno from the University of Oulu and the Finnish Institute of Occupational Health.