Thermoregulation in mice, rats and humans: An insight into the evolution of human hairlessness

Bernard J. Feldman Department of Physics and Astronomy University of Missouri-St. Louis, St. Louis, MO USA

Abstract The thermoregulation system in mammals removes body heat in hot temperatures and retains body heat in cold temperatures. The better the animal removes heat, the worse the animal retains heat and visa versa. It is the balance between these two conflicting goals that determines the mammal’s size, heart rate and amount of hair. The rat’s loss of tail hair and human’s loss of its body hair are responses to these conflicting thermoregulation needs as these animals evolved to larger size over time.

   There have been numerous ideas about why humans lost most of their hair.   They include  

increased dissipation of body heat for living on the open savannah [1] response to a evolving , metabolically active brain [2], improved heat dissipation needed for the transition to
bipedalism, leading to faster running and better success at hunting [3], faster swimming by an
aquatic ape [4], sexual selection, i.e., female preference of males with less hair [5], use of
clothing [6], reduction of parasites (lice, fleas and ticks) that live in the hair [7], discovery and
use of fire [8], neotony—the tendency of humans to retain infant characteristics, e.g., less hair,
for longer periods [9], enhanced social communication [10], parental selection [11], allometry— the study of body size and physiology combined with the observation that larger apes have less
dense hair [12], and pleasure of skin to skin contact between mother, partner and child. [13] This manuscript proposes that a combination of several of these ideas—improved heat
removal, allometry, improved success at hunting and sexual preferences—can explain why
humans became largely hairless. At the heart of this manuscript is a discussion of the regulation
of body temperature. All mammals need to regulate their body temperatures. They need to
remove heat in hot environments and retain heat in cold environments. This temperature
regulation is controlled primarily by the circulatory system, consisting of blood flowing through
arteries, veins, the heart and the lungs. In hot climates, blood carries heat to the skin and the
lungs, where heat escapes to the environment by conduction and evaporation to the air next to the
skin and inside the lungs. In cold climates, warm blood is carried from the inner, warmer parts of
the body to the colder outer parts of the body.
The vast majority of land-based mammals have fur covering almost all of their skin. This
keeps the animal warm in cold temperatures at the expense of cooling in hot temperatures.
This strategy makes sense for two reasons: First, other strategies keep an individual cool in the
heat—take a dip in a lake, sit in the shade or take a nap. Second, because fur retains body heat, it
dramatically reduces the food the individual needs to gather or hunt. In contrast, few strategies
are available to keep warm in cold temperatures—live in a cave or underground. For Homo
habilis, the evidence suggests that hair loss began more than 1.2 million years ago, even though
the last ice age started about 1.6 million years ago.[7] So Homo habilis was losing hair even as
individuals had to deal with ever cooling temperatures and increasingly cold evenings. Evidence
suggests that the use of fire began about 0.5 million years ago and clothing about 0.3 million
years ago, well after major climate cooling.[8]
So why would any land based mammal give up the advantages of having fur? Let us consider
the examples of rats and mice. Rats and mice differ in three important ways that are relevant to
this discussion. Rats are much larger than mice (84 day old rats weigh 183-409 gm; 84 day old
mice, 18 - 32 gm).[14] Rats have a slower heart beat (330 - 480/minute) than mice (310 –
840/minute).[15] Rats have a hairless tail from which they dissipate about 17% of their body
heat while mice tails are covered in fur.[16]
Simply put, the smaller the animal, the higher the ratio of surface area of the skin and
lungs to body volume and the shorter the distance from hot blood vessels to the cold air next to
the skin or inside the lungs. Smaller animals lose heat more rapidly, requiring more fur to
insulate the animal and a higher heart rate to generate heat from the beating heart muscles and to
circulate warm blood more rapidly to the animal’s cold exterior. In contrast, for larger animals,
heat retention is less of a problem and heat removal becomes more of a problem. The evolution
of a hairless tail in the rat is one solution to the problem of heat removal. Now let us consider the largely hairless human. It is estimated that the average adult height
of Homo habilis was no more than