Female Mice Prefer Surfaces Roughly 7 Degrees F Warmer, and Testosterone May Be Why

Female mice settle on surfaces roughly 7 degrees Fahrenheit (4 degrees Celsius) warmer than males do, and testosterone appears to be part of the reason, according to a study published Oct. 28 in the Journal of Physiology. Live Science reported on the findings.

The work was done only in mice, so it does not show why women often feel cold more easily than men. But it points to a biological explanation in the skin's temperature sensors rather than in body temperature itself.

The key numbers behind the mouse experiment

• 30 male and 27 female mice: the animals tested. Each was placed on two adjacent plates while a camera tracked how long it stayed on each.
• 91.4 F (33 C): the temperature of the fixed plate. The second plate varied between cooler and warmer settings. Both sexes spent more time on the 91.4 F plate when the other one was cooler.
• 100.4 F (38 C): the warmer comparison plate. Against it, females spent significantly more time on the warm side than males did.
• 89.6 F (32 C) versus 96.8 F (36 C): the average preferred surface temperature for male and female mice, respectively.
• About 95 F (35 C): the preferred temperature of male mice after researchers genetically deleted a warmth-detecting protein, bringing them closer to the females.

How the team traced the difference to skin sensors

Skin nerve endings contain proteins that respond to warmth or cold. Dr. Michael Caterina, a Johns Hopkins University neuroscientist who was not involved in the study, compared them to small electrical generators. When they switch on, charged ions enter sensory nerve cells, which send signals to the brain.

The top of the cage was open, so air temperature could not explain the choices, said study co-author Peter McNaughton, a pharmacologist at King's College London. The team also found no significant sex differences in the mice's internal or skin temperatures. That led them to conclude the preference came from the sensors themselves.

Deleting the warmth-detecting protein made male mice less likely to avoid the warmer plate. That suggests the protein normally signals them to move off hot surfaces. In females, deleting or blocking it made no significant difference.

When the researchers removed or blocked a cold-detecting protein, both sexes avoided cool plates less strongly. Females also spent less time on the warmer plates, while males' preference did not change. Caterina said females appear somewhat more responsive to the cold receptor and somewhat less responsive to the warm one. The study found the temperature-sensing neurons themselves worked identically in both sexes.

What the hormone tests showed

To test sex hormones, the team gave male mice a drug that blocks testosterone production. Weeks later, these males spent more time on the 38 C plate than untreated males. Female mice given testosterone lost their preference for warmer temperatures.

Giving males forms of estrogen and progesterone did not change their preferences, and blocking those hormones in females had no effect. Those hormones are known to affect other parts of temperature regulation, such as heat loss through the skin, but did not appear to alter temperature perception here. McNaughton said testosterone looks like the most important factor.

The effect was only partial, however. Testosterone did not fully erase the differences between the sexes, and McNaughton said something else is going on that the team could not identify.

Why it matters for people

The researchers did not test humans. McNaughton noted, though, that people have the same sensor proteins and that human studies suggest they behave in a similar way. He said the finding could have practical value in designing buildings and spaces that suit the people using them.

Caterina said the work gives valuable information on how temperature-sensing proteins operate. Understanding that could help research into conditions such as neuropathic pain, in which sensitivity to heat and cold is heightened.

What is still unknown

How testosterone acts on the sensors is unclear. Because the effect took weeks to appear, the researchers suggested it does not act directly on the temperature-sensing proteins. It may work indirectly by changing gene activity, but they said further studies would be needed to confirm that.

Read next
Rats Exposed to Mars-Like Radiation Show Heart Scarring; Most Readers Say No
Women With Longer Reproductive Years Had Healthier Brain Vessels at 70
Sleep Apnea Linked to Weaker Sperm Movement, and CPAP Use Brought Gains

Around Prague, based on reporting by Live Science