12 Sex Differences and Evolutionary Adaptation to Heat

Overview

Biological sex plays a central role in thermoregulation. Differences in body composition, hormonal profiles, sweat gland density, and cardiovascular responses all contribute to how people experience and manage heat. Over evolutionary time, these differences likely developed in response to reproductive pressures, climate, and energy trade-offs.

This chapter explores:

• Key physiological sex differences in heat response

• Evolutionary theories behind thermoregulation

• Why females may be more vulnerable to heat during pregnancy

• How these patterns manifest in real-world data

Physiological Sex Differences in Heat Response

Key differences include:

• Body composition:
  Women generally have a higher body fat percentage, which insulates but can impair heat dissipation. Men typically have more lean mass, which increases heat production but also improves heat loss via sweat.

• Sweat rate:
  Men tend to sweat more than women for the same heat load. Women often rely more on cutaneous vasodilation (blood flow to the skin) than on evaporative cooling.

• Surface area to mass ratio:
  Women have a slightly higher surface-area-to-mass ratio, which aids passive heat loss but may also increase environmental heat gain in hot, humid climates.

• Hormonal variation:
  The menstrual cycle causes cyclical changes in core temperature, vasomotor responses, and sweat threshold, as covered in Chapters 5 and 6.

Gagnon & Kenny (2012) found that under equivalent heat loads, women experienced higher cardiovascular strain and reached exhaustion sooner than men in both dry and humid heat.

Evolutionary Perspectives

Thermal tolerance may have evolved differently in males and females due to reproductive trade-offs and energetic constraints.

• Pregnancy requires precise thermal regulation to protect embryonic development

• Invasive placentation in humans increases fetal dependency on maternal thermoregulation

• Selection pressures may have favoured heat-tolerant traits in males who ranged or hunted, while females developed adaptations to buffer internal environments

The 37°C set point in humans may represent a balance between enzymatic efficiency and reproductive safety (Marino, 2008).
Estradiol’s effects on promoting vasodilation and cooling may be part of this evolutionary strategy.

Why Are Female Fetuses More Resilient?

Studies have shown that male fetuses are more vulnerable to preconception stress, heat exposure, and growth restriction.

• Male embryos grow faster but are less buffered against environmental insults

• Female fetuses are more likely to survive adverse uterine conditions, though at the cost of smaller size

Hipwell et al. (2023) found that maternal caregiving stress predicted shorter gestation in male infants but not female infants.

This suggests a sex-specific developmental trade-off, where male fetuses are more growth-focused, while females are more resilient to suboptimal environments.

Global Trends and Climate Risk

• Stillbirth rates rise during heatwaves, disproportionately affecting regions with poor maternal health infrastructure

• Female labourers in agriculture or manufacturing are often exposed to high ambient heat during key reproductive years

• In some regions, low birthweight prevalence increases in hotter months, especially among female-headed households

Understanding sex differences in thermal biology can guide climate adaptation policies, workplace protections, and public health interventions.

Recap

• Biological sex influences how the body responds to heat, with women generally relying more on vasodilation and less on sweat

• Evolution may have shaped thermoregulatory strategies in females to protect reproductive success

• Male fetuses are more sensitive to heat and stress, while females are more resilient under suboptimal conditions

• These differences have real-world implications in a warming world

References

• Gagnon, D., & Kenny, G. P. (2012). Sex differences in thermoeffector responses during exercise at fixed requirements for heat loss. Journal of Applied Physiology, 113(5), 746–757.

• Marino, F. (2008). The evolutionary significance of the thermoregulatory set point in humans. Physiology & Behavior, 95(2), 151–154.

• Hipwell, A. E., et al. (2023). Maternal stress, infant sex, and gestational age: Evidence for sex-specific vulnerability. Development and Psychopathology, 35(1), 295–304.