American psychologist Dr. John Gray famously wrote a book that Men are from Mars, Women from Venus to illustrate the many emotional and behavioral differences between the sexes. One of the differences is their reaction to temperatures. 

Israel’s new transportation minister and feminist, Merav Michaeli, made headlines recently when she said she would investigate complaints from women that the country’s trains are “too cold,” with air conditioning set too low. The scenario in which women bring a sweater into work, while their male counterparts feel comfortable wearing short sleeves in an air-conditioned office is also common. 

Now, researchers at Tel Aviv University (TAU) have confirmed that this is a fact not only in people but also among numerous types of other mammals and of birds. The study was led by Dr. Eran Levin and Dr. Tali Magory Cohen from TAU’s School of Zoology and Steinhardt Museum of Natural History; Yosef Kiat from the University of Haifa; and Dr. Haggai Sharon, a pain specialist from TAU’s Sackler Faculty of Medicine and Tel Aviv Sourasky Medical Center. The article was published in the journal Global Ecology and Biogeography under the title “An alternative hypothesis for the evolution of sexual segregation in endotherms.” 

The researchers explained that the phenomenon “is not unique to humans. In many species of mammals and birds, the females prefer warm places whereas the males prefer cooler temperatures.” The researchers suggest that this is a built-in difference between the temperature-sensory systems of the two genders that developed over the course of evolution. This difference is adaptive as it leads to spatial separation between males and females, which lowers competition and aggression between the sexes.

The researchers concluded that this phenomenon is not unique to humans, with many male species of endotherms (birds and mammals) preferring a cooler temperature than the females.

The researchers: “We propose that males and females feel temperature differently. This is a built-in evolutionary difference between the heat-sensing systems of the two sexes, which is related, among other things, to the reproduction process and caring for offspring.”

The new study included an in-depth statistical and spatial analysis of the distribution of dozens of bird and bat species living in Israel, along with a comprehensive review of the international research literature on the subject. Levin, who among other things studies the physiology and behavior of bats, noted in his previous studies that during the breeding season males and females tend to segregate, with the males inhabiting cooler areas. 

For example, entire colonies in caves on the slopes of Mount Hermon are composed of only males during the breeding season, while in the warmer area of the Sea of Galilee (Kinneret), there are mainly females that give birth and raise their pups there. It was this phenomenon that aroused his curiosity.

A study of the research literature reveals several examples of a similar phenomenon being observed in many species of birds and mammals. In migratory bird species, males spend the winter in colder areas than females (it should be noted that in birds, the segregation between the sexes takes place outside of the breeding season, since the males participate in the raising of the chicks). Among many mammals, even in species that live in pairs or in mixed groups all their lives, the males prefer shade whereas the females prefer sunlight or the males ascend to the peaks of mountains while the females remain in the valleys.

Following their study of the scientific literature, the researchers conducted their own study. They sampled information collected in Israel over the course of nearly 40 years (1981 to 2018) on thousands of birds from 13 migratory bird species from 76 sites (data from Birdlife Israel and the Steinhardt Museum of Natural History) and 18 species of bats from 53 sites (data from the researchers and the Society for the Protection of Nature.) In total, the study included more than 11,000 individual birds and bats, from Mt. Hermon in the north to Eilat in the south.

The reasoning behind the choice of birds and bats for the study is the fact that they fly and are therefore highly mobile, and the researchers suggested that the spatial separation between the sexes – sometimes extending to different climatic zones – could be particularly clear in these groups. In addition, the diversity of Israel’s climate allowed them to study individual animals of the same species that live in very different climatic conditions.

The findings of the study clearly showed that males prefer a lower temperature than females, and that this preference leads to a separation between the sexes at certain periods during the breeding cycles, when the males and females do not need, and may even interfere, with each other.

“In light of the findings, and the fact that this is a widespread phenomenon,” said Levin, “we have hypothesized that what we are dealing with is a difference between the females and males’ heat-sensing mechanisms that developed over the course of evolution. This difference is similar in its essence to the known differences between the pain sensations experienced by the two sexes and is impacted by differences in the neural mechanisms responsible for the sensation and also by hormonal differences between males and females.”

Magory Cohen noted that this difference has a number of evolutionary explanations. First, the separation between males and females reduces competition over resources in the environment and keeps away males that may be aggressive and endanger the young offspring. In addition, many female mammals must protect their offspring at a stage when they are not yet able to regulate their body temperature on their own, so they developed a preference for a relatively warm climate.

The team concluded that “bottom line is that this difference in thermal sensation did not come about so that we could argue with our partners over the air conditioning, but rather the opposite. It is meant to make the couple take some distance from each other so that each individual can enjoy some peace and quiet. The phenomenon can also be linked to sociological phenomena observed in many animals and even in humans, in a mixed environment of females and males: females tend to have much more physical contact between themselves, whereas males maintain more distance and shy away from contact with each other.”