At the time I write this sentence it’s 10:13 pm. The sun has been under the horizon for almost two hours. It’s 93 degrees Fahrenheit outside here in College Station. I believe it peaked out right at 100 during the day, and it feels hotter in the sun. Even the animals are clearly not pleased.
Some of them are the cutest thermometers I’ve ever seen:
I wish I had taken a picture so I could show you exactly how the thermometer works. Fortunately there’s nothing that’s not on the internet, and other people have documented this particular phenomenon on film. The thermometer is a binary state instrument. The above picture is in the “not so hot” state. The below picture is in the “really really hot” state.
They find a shaded area of concrete and just splat themselves right on the ground. They’re so hot they don’t even run when you walk within three feet of them.
Which is interesting from a physics standpoint, because a priori they should have a much easier time staying cool than us humans. An average human being might burn something like 100 watts of basal metabolism when going about everyday business that’s not particularly strenuous. That same person might have a total surface area of maybe 2 square meters. All total that means every square centimeter of the body will on average radiate 5 milliwatts or so.
Now all other things being equal surface area scales as the square of linear dimension. Volume scales as the cube of linear dimension. Assuming that a squirrel has the same rough shape and metabolism per volume as a human, a 6-inch squirrel ought to have a surface area of about 138 square centimeters or so. His body ought to generate about (6 inches / 6 feet)^3 times as much energy, or around 57 milliwatts. That’s about 0.5 milliwatts per square centimeter, much less than a person has to deal with.
At best these are extremely sketchy Fermi-problem estimates, but the general principle is a very firmly established one: ratios of volume to surface area are hugely important in biological processes. For heat dissipation it’s not really the summer most critters have to worry about. Assuming there’s not too many predators about, humans and squirrels can just find a patch of shade and lay around reducing their metabolic heat output. The winter is another story – the squirrels and other small animals have so much surface area per volume that their bodies would rapidly become hypothermic without a way to combat the heat loss. The normal adaptations animals use are things like high heat-generating metabolism, insulating fur, fat stores, insulating nests, that sort of thing. The disadvantage of these adaptations is that not all of them can just be switched off (though many can), leaving them just as uncomfortably hot as the rest of us despite what would otherwise be a much easier job of cooling. In particular due to their metabolic adaptations I’d be pretty surprised if the geometric 57 milliwatt was anywhere near as as high as the real figure probably is.
Here’s hoping the squirrel thermometer switches states soon. It’s mid-July in Texas though, so I don’t we can reasonably expect it to. Sigh.
(I will be mostly out of pocket this weekend. Sunday Function will probably be delayed till Monday. See you then!)