Human breasts might have evolved to keep big-brained infants warm

A study in Finland measured heating capacity and resilience to temperature changes of breasts of nursing women, non-nursing women, and men. Results showed that breastfeeding women had overall higher breast surface temperature compared to the other groups. Their breasts were also more resilient to cooling. The paper was published in Evolutionary Human Sciences.

Human breasts are unusual among primates because they remain relatively prominent throughout adult life rather than enlarging mainly during lactation. Much of their size and shape is produced by subcutaneous fat and fat around the milk-producing glands rather than by the milk-producing glandular tissue itself. Breast size varies considerably between women, and larger breasts can also create physical disadvantages such as increased strain on the back.

One evolutionary possibility is that permanent breasts emerged as a by-product of increased body fat during human evolution and were later shaped by other selective pressures. Some researchers have proposed that enlarged breasts function partly as sexual signals of fertility, health, or reproductive fitness, although this interpretation is not universally applicable across cultures.

Another hypothesis is that breast size may have helped mothers warm infants during close skin-to-skin contact, particularly after humans lost most of their body hair. Under this view, additional breast tissue could increase the warm surface area surrounding an infant and reduce heat loss. This possibility is especially relevant because human infants have unusually high energetic demands and substantial thermoregulatory needs during early development, largely due to encephalization, which is the evolutionary increase in brain size.

Study author Tiina Kuvaja and her colleagues investigated if the female breast tissue would have a distinct response to changing temperature. If this happened, it would indicate that it has a potential to stabilize the temperature of infants and especially newborns. They also noticed that lactation influences breast skin temperature and that relatively large changes in breast metabolic rate and temperature take place in early pregnancy. That is why they decided to include both breastfeeding women and women who do not breastfeed in their study.

Study participants were 8 currently breastfeeding women, 12 non-breastfeeding women, and 7 men. Participants were 20 to 40 years of age. This age category was selected because it represents the typical age of nursing women and avoided individuals undergoing puberty or menopause. The group of non-breastfeeding women included both women who had not given birth and those who had stopped breastfeeding at least one year prior to the study.

In the study, participants were exposed to three different temperatures in a climatic chamber: 32°C, 27°C, and 18°C. The study authors note that 27°C is a thermoneutral temperature for humans, meaning it is the temperature at which they can maintain their normal body temperature with minimal energy expenditure. It is the mean daytime temperature of the African savanna. The other two temperatures were taken to represent higher and lower extremes. The researchers used thermal imaging to record breast surface temperature at the start of the exposure, after 10 minutes, and at the end of the exposure, which lasted 20 minutes in total.

Results showed that breastfeeding women had overall higher breast surface temperatures compared to the other groups. Furthermore, they showed a distinct resilience against cooling during the 18°C exposure. During this exposure, they lost 2.5°C of their breast surface temperature on average, whereas the other study groups lost 4.3°C and 4.7°C.

“This proof-of-concept study clearly indicated the potential of the nursing women’s breasts to support infant’s thermal balance,” the study authors concluded.

The study contributes to the scientific knowledge about the evolutionary roles of human breasts. However, it should be noted that the study was conducted on a very small group of participants, particularly of breastfeeding women. Studies on larger groups might not yield identical results.

The paper, “Infant’s thermal balance and the evolution of the human breast – a proof-of-concept study,” was authored by Tiina Kuvaja, Tiina Väre, Sirkka Rissanen, Hannu Rintamäki, Petri Lehenkari, and Juho-Antti Junno.