# Infant’s thermal balance and the evolution of the human breast – a proof-of-concept study

## TL;DR

This study suggests that the enlarged human breasts may help keep infants warm through skin-to-skin contact, based on temperature measurements in different groups.

## Contribution

The study introduces a novel hypothesis that human breasts evolved to support infant thermal balance through skin-to-skin contact.

## Key findings

- Nursing women had higher mammary surface temperatures than non-nursing women and men.
- Nursing women's breasts showed greater resilience to cooling environments.
- The study supports the idea that breasts may help maintain infant thermal balance.

## Abstract

The distinct size and shape of breasts in women is a uniquely human trait. This trait has no conclusive explanation as it is not a requirement for milk production. Additionally, breasts are already enlarged at puberty, which is usually long before the first pregnancy. We hypothesized that the perennially enlarged human breasts were potentially developed to support infant’s thermal balance by providing increased warming surface in skin-to-skin contact.

To test the hypothesis, we measured breast surface temperature to explore their heating capacity and resilience to temperature changes in an environmental conditions laboratory. Volunteers, divided in groups of nursing women, non-nursing women, and men, were exposed to three temperatures: 32°C, 27°C, and 18°C. The exposure time in each temperature was 20 min. The changes in breast surface temperature were recorded by thermal imaging camera. Data were analysed using Kruskal–Wallis tests. Breastfeeding women had overall higher mammary surface temperature compared to other groups. Furthermore, nursing women had distinct resilience against cooling environment: they lost the average of 2.5°C of their mammary surface temperature, whereas other study groups lost 4.3°C and 4.7°C of surface temperature, respectively. This proof-of -concept study clearly indicated the potential of the nursing women’s breasts to support infant’s thermal balance.