Hmmm so it seems that we have to abandon some old cherished beliefs:
When it comes to wrapping up on a cold winter's day, a cosy hat is obligatory. After all, most of our body heat is lost through our heads - or so we are led to believe.
Closer inspection of heat loss in the hatless, however, reveals the claim to be nonsense, say scientists who have dispelled this and five other modern myths.
They traced the origins of the hat-wearing advice back to a US army survival manual from 1970 which strongly recommended covering the head when it is cold, since "40 to 45 percent of body heat" is lost from the head.
Rachel Vreeman and Aaron Carroll, at the centre for health policy at Indiana University in Indianapolis, rubbish the claim in the British Medical Journal this week. If this were true, they say, humans would be just as cold if they went without a hat as if they went without trousers. "Patently, this is just not the case," they write.
The myth is thought to have arisen through a flawed interpretation of a vaguely scientific experiment by the US military in the 1950s. In those studies, volunteers were dressed in Arctic survival suits and exposed to bitterly cold conditions. Because it was the only part of their bodies left uncovered, most of their heat was lost through their heads.
The face, head and chest are more sensitive to changes in temperature than the rest of the body, making it feel as if covering them up does more to prevent heat loss. In fact, covering one part of the body has as much effect as covering any other. If the experiment had been performed with people wearing only swimming trunks, they would have lost no more than 10% of their body heat through their heads, the scientists add.
Even though yawning is very common, the physiological and evolutionary reasons for yawning behavior are poorly understood.
Yawning is a familiar behavior characterized by a large gaping of the mouth, accompanied by a deep inhalation of air, followed a shorter expiration. This behavior is under involuntary control, so it cannot be consciously controlled or suppressed. Further, yawning is a stereotyped behavior expressed by all classes of animals, and is correlated by a variety of neurochemical changes in the brain. Previous research suggests that yawning is a biological mechanism in humans and non-human apes, such as chimpanzees, to keep the brain from overheating.
"Brains are like computers," reports Andrew Gallup, a researcher in the Department of Biology at Binghamton University who led the study. "They operate most efficiently when cool, and physical adaptations have evolved to allow maximum cooling of the brain."
This is known as the "radiator hypothesis" or the "brain-cooling hypothesis," which can be tested.
"Based on the brain-cooling hypothesis, we suggest that there should be a thermal window in which yawning should occur," Gallup proposed. "For instance, yawning should not occur when ambient temperatures exceed body temperature, as taking a deep inhalation of warm air would be counterproductive. In addition, yawning when it is extremely cold may be maladaptive, as this may send unusually cold air to the brain, which may produce a thermal shock."
To test this hypothesis, Gallup and his colleagues, Michael Miller and Anne Clark, studied yawning in wild budgerigars, Melopsittacus undulatus. They chose budgerigars because they have relatively large brains for their body size, they live wild in arid regions of Australia that are subject to frequent and dramatic temperature changes, and -- most important for their experimental design, where they tested the birds in small groups -- budgerigars do not experience contagious or sympathetic yawning, as humans and some other animals do....
"In fact, covering one part of the body has as much effect as covering any other."
False. Heat loss is at its worst where blood circulates close to the skin, e.g. armpits and head. On the other hand, subcutaneous fat is a thermal insulator.
"Brains are like computers"
I HATE that expression, and I'm a computer engineer, so I know quite a lot about them. Transistors have nothing in common with neurons. Failure mechanisms due to heat are very different. For example, computers recover when cooled, but brains are damaged permanently when the proteins coagulate.
Transistors have nothing in common with neurons. Failure mechanisms due to heat are very different. For example, computers recover when cooled, but brains are damaged permanently when the proteins coagulate.
Normal temperatures for neural systems run 33-40C, coagluation doesn't really set in until 60C or more -- something like 3X the normal temperature range above operating maximum.
Normal temperatures for transistor circuits run -40 to 125C, and the same adjustment produces 620C -- at which point I promise you that the physical structures of silicon transistors will be irreparably damaged from carrier diffusion if nothing else.
Now, comparing brains to testes works ...
OK, let's start nitpicking...
Celsius is an arbitrary scale. In physics, Kelvins make more sense. 33-40C is 306-313K and 60C is 333K. Pretty narrow margin.
-40 to 125C is the filthy expensive mil temp spec. Computers are usually built using commercial transistors that live in the 0 to 70 C range (273 to 343K). Besides, defining a comparable operating range is not easy, because the lowest temp is set by packaging technology, not the transistor chip. Even commercial chips will work in cold, if you make sure there is no water to freeze inside the plastic pack.
You can compare also computers to testes. About the same amount of intelligence.
I wonder how many people yawned while reading this article. I certainly did.
More interesting than how much heat you loose by exposing body parts is what effect cooling those body parts can have. For example, having your head uncovered could rapidly cool your brain, which could cause all sorts of problems, even if the rest of the body does not have it's temperature reduced too much (in my cause, it caused fatigue and depression, of whihc I was swiftly cured by wearing a cap someone lent me).
I think this question becomes relevant once the previous one about percentage of body heat lost has been settled.