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By Daniella Gray
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For decades, scientists have puzzled over a simple question: why do human women have permanently enlarged breasts, even long before pregnancy or breastfeeding?
A new study suggests the answer may have far less to do with appearance—and far more to do with keeping babies alive.
Researchers from the University of Oulu, Finland have proposed that human breasts may have evolved to help regulate a newborn’s body temperature, offering warmth during close, skin-to-skin contact at a time when infants are especially vulnerable to the cold.
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Unlike the females of most other mammal species, human women develop and retain prominent breasts from puberty onward. This size and shape is not required for producing milk—and such come with downsides, including physical strain and health issues.
Previous theories suggested that breasts evolved as fat reserves or as signals shaped by sexual selection, but none fully explained why they remain enlarged year-round.
The new research puts forward a different idea: breasts may act as a built-in warming surface for infants.
Newborns struggle to regulate their own body temperature because their thermoregulatory systems are still developing. Even small drops in temperature can be dangerous, particularly in early human environments before modern clothing and heating.
To test their hypothesis, the researchers conducted a proof-of-concept experiment in a controlled climate chamber.
They measured skin surface temperatures using thermal imaging in three groups: breastfeeding women, non-breastfeeding women and men. Participants were exposed to three different ambient temperatures—warm, mild and cool—while researchers tracked how their skin temperature changed over time.
The team found that breastfeeding women consistently showed higher breast surface temperatures than the other groups. Even more notably, their breast temperature dropped much less when the surrounding environment cooled.
While non-breastfeeding women and men experienced larger decreases in skin temperature, nursing women’s breasts retained warmth far more effectively.
That combination—higher baseline warmth and resistance to cooling—suggests breasts could help transfer heat to a baby held against the chest. The broad surface area created by breast tissue may enhance heat exchange during feeding or contact, reducing the risk of hypothermia.
“This could improve a newborn’s chances of survival and provide an evolutionarily grounded explanation for the development of external breasts in humans,” said study author Juho-Antti Junno. Importantly, the researchers stressed that this is an early, exploratory study rather than a definitive answer.
While the results support the idea that breasts play a role in infant thermal balance, further research is needed to understand how this function may have shaped human evolution over thousands of years.
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Reference
Kuvaja, T., Väre, T., Rissanen, S., Rintamäki, H., Lehenkari, P., & Junno, J.-A. (2026). Infant’s thermal balance and the evolution of the human breast – a proof-of-concept study. Evolutionary Human Sciences, 8. https://doi.org/10.1017/ehs.2025.10024
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