(Picture from here.)
I went into graduate school to study environmental physiology, got side tracked, and ended up in neurophysiology. Then, of course, went into software engineering as one does.
Temperature is a problem for any organism. The enzymes, proteins, nucleic acids—essentially, the chemistry of life—operate in a relatively narrow range of temperature. This range is shifted up or down depending on the animal heritage and evolutionary history—some organisms live close to the boiling point of water. Others operate a hair’s breadth from water’s freezing point. But there is an optimal chemical temperature range for the cellular chemistry as a whole. Outside of that optimum, and cellular operations lose efficiency. Further outside, they fail.
A good visual example is a chicken egg. It has a temperature range in which the egg remains nice and liquid—a fertile ground for another chicken. Cook it and the egg proteins congeal into a rubbery substance suitable for consumption.
Thus, organisms work to maintain that optimum range if they can. Or they change their internal conditions to survive as spores or other dormant conditions. Or they trust to numbers that only a portion of the population dies. Or they go extinct.
Some complex animals—fish, clams, lizards, humans—have mobility and can modify the conditions with their behavior such as ascending or descending a water column, migrating to a different area, or, as I saw one heat-stressed sea lion do, piss themselves to cause evaporative cooling.
Such complex animals have a complex problem with thermoregulation. Remember, I said the cell had an optimum temperature range for its chemistry as a whole. A complex organism has a larger problem in that it has an optimum temperature range for the chemistry of the entire organism. These optimum temperatures might be different for different tissues. Muscles have one temperature range. Brains have a different one.
Animals fall, roughly, into two categories of thermoregulation: endothermic, where there are important mechanisms that keep the internal temperature constant in a changing world, and ectothermic, where temperature is maintained behaviorally or endured through physiological means.
This is very rough. Placental mammals are generally endothermic, though there are significant exceptions for animals that hibernate. Non-placental mammals —marsupials and monotremes—are endothermic but the maintained temperature range is looser. Birds are endothermic. Other vertebrates such as amphibians, fish, and reptiles, tend towards ectothermic. This isn’t universally true since tuna and some kinds of sharks have developed the capacity of keeping some tissue significantly warmer than the surrounding water.
Whether dinosaurs were endothermic or not has been an ongoing debate for over a century. It seems that there is a sort of consensus that they were, sort of. Certainly, some dinosaurs might have been as endothermic as birds. But others—especially the large sauropods—might have just relied on unregulated activity to keep them warm enough. True endothermy is expensive and problematic.
Every chemical reaction creates waste heat. This means every heartbeat, thought, muscle movement, alcohol metabolism in the liver, produces heat. This is in addition to any metabolism that is used to maintain body temperature. Animals can handle cold. Penguins regularly handle -40C temperatures. They have biochemical and physiological mechanisms that enable them to do this. But they also have a raging endothermic furnace that keeps their insides warm. The problem there is to prevent heat loss to an unforgiving outside world.
Heat gain is a different problem. There are two main sources of heat in the endothermic body: inside and outside. The problem is to cool the inside from both sources. But in this case, the mechanism of getting rid of the heat produces heat.
This is one of the sources of runaway heat exhaustion leading to heat stroke. Consider the problem if heat intake from the environment: air temperature, solar or other infra-red radiation. The problem is how to lose heat that is generated internally in an environment where the outside temperature is higher than the inside temperature.
Remember, the whole point of thermoregulation in endotherms is to keep a constant temperature. When sweat fails—as in a high humidity environment—the system can’t keep the outside temperature from driving up the inside temperature. Worse, all of those lovely thermoregulatory mechanisms such as sweating, increased heart beat and circulation, panting, generate heat, too. This all adds up. As long as there is a net heat loss, the system works. If the outside conditions change that to a heat gain—or the system fails as in sweat exhaustion where the glands give up—things get difficult.
In furred animals, fur serves as a barrier to heat intake—certainly it interferes with heat output, but if the net gain from that desert sun is prevented by the fur, it’s a win. It’s part of the reason for the clothing style in some desert cultures. The bet is that the heat not rejected by the fur or hair insulation is more than that gained by the retention of internal heat.
We’re not furred animals. When we’re out in the hot sun it transmits that heat right onto us. Unless we’re wearing special clothes, we take that heat in.
Humans do this in two ways: sweating and behavior.
Sweating is astonishingly effective when it works. People talk about a “dry heat.” What they mean is it’s a heat state where sweat works. If the humidity is 5% in the desert, that sweat evaporation is often sufficient to keep much hotter temperatures at bay. Picking a number at random, let’s say a 98.6F human needs to keep the skin temperature at about 70F for a net heat loss maintaining that internal temperature. As the temperature climbs—maintaining low humidity—the human can keep ahead of heat stroke as long as the sweat production can evaporate that skin to 70F.
The curve for that is fairly long in humans. In an effective 0% humidity, Army experiments were able to keep humans alive in an oven well in excess of any earthly temperature. (See here. I looked for the original paper and couldn’t find it but this high temp (~248F) is close to what I recall from Knut Schmidt-Nielsen’s excellent book, How Animals Work. The studies were done during and after WW II by the Army.) However, survivability drops quickly with increasing humidity to the point that a 95F temperature at 100% humidity is likely fatal.
This is what makes the current heat wave down south so incredibly dangerous. It’s hot and humid. In addition, many of the more dangerous places are heat islands, areas that by virtue of buildings, human activities, or reduced natural landscapes, magnify the problem. Buckminster Fuller once pointed out that the skyscraper structure of New York City resembled a heat sink—a attachment to a heat producing device that increases the surface area in order to radiate waste heat. In the summer, each building is not only reflecting solar heat on those outside, but also radiating its own heat from lights or air conditioning. Thus, the air temperature outside the city might be many degrees cooler than the city itself. Take New York City and transplant it to southern Texas and you have Houston.
Which brings us to the second mechanism: behavior. Humans have a huge range of possible behaviors from summering up in Maine, going inside to the air conditioner or under the shade of a tree, jumping in the pool, to drinking ice water. Other animals do the same, hiding in the shade during the heat of the day.
Just like humans can engage in behavior to protect themselves from the heat, they’re also smart enough to engage in behavior that exacerbates the problem. Like going to a baseball game, drinking alcohol, working outside when it’s dangerously hot.
So be smart and take this stuff seriously.