Allen’s Rule is a rule of biology that states that the appendages (ears, limbs, and tails) of animals living in cold geographical regions are consistently shorter than those of closely related animals occupying warmer climates.
Allen’s Rule is almost wholly dependent on environmental temperature; that is, a secondary growth response in mammals to help maintain constant core body temperatures, and is not a result of nutrition or exercise practices of the animals in question.
Allen’s Rule also makes itself apparent very quickly.
When researcher Maria A. Serrat housed mice at cold (7 °C), room temperature (21 °C), or warm (27 °C) ambient temperatures from weaning through maturity (3.5–12 weeks age), it was confirmed that the ears, limbs, and tails of the mice kept in the warmth were significantly longer than those of siblings raised in the cold, but with no change in total body mass.
Differences in core organ size appeared to account for the latter—hearts and kidneys were enlarged in cold-reared mice so they weighed as much as the warm-reared mice.
Most interesting was that the differences in limb length were not explainable by difference in diet and/or activity level, because cold-reared mice consumed substantially more food and were more active than their warm and control counterparts.
Yet the bones of the warm mice were in fact larger than the bones of the cold mice.
This has a few very interesting applications, starting with osteoporosis…
Osteoporosis is the most prevalent metabolic bone disease in the world, with one third of postmenopausal women affected. It is characterized by low bone mass and microarchitectural deterioration, leading to weaker bones and an increased risk of fractures.
In humans, populations under chronic cold exposure generally exhibit an overall lower bone density and cortical thickness as well as accelerated rates of bone loss during aging, when compared to populations in warmer living areas.
Further studies have found cold-dwelling humans have lower cortical thickness and bone mineral density as well as accelerated bone loss with aging, showing that prolonged exposure to low temperature is deleterious to overall skeletal health and to bone mass.
Conversely, warm temperature exposure seems to be able to improve bone mass.
This research is early, and has mostly been completed in animal models, but it does hold promise as a non-invasive way to hold off, or at least lesson, the bone density losses we typically see with aging.
For more information on the health benefits of heat exposure, please check out The Book on Heat. https://thebookonheat.com/