What extremes can teach us

kw: book reviews, nonfiction, biology, extremes, biggest, smallest, fastest, slowest, most dangerous

If you want to know what is the biggest or smallest mammal, bird, tree, or whatever, it has become incredibly easy to do so. However, sometimes the answer is "it depends". You won't get an "it depends" answer from a search engine. But you will from a knowledgeable person. Then, whatever results, it's helpful to dig a bit deeper, to find out the why or how of it.

I was thinking of such a thing, starting with, "What is the heaviest land mammal?". If you look to historic times, the African elephant, which gets as heavy as seven tons (14,000 lbs, 6,400 kilos) easily tops the list. However, if we look back in time, it was probably one of the Titanosaurs. A species recently found in Patagonia (Argentina) must have weighed 85 tons (12 x the biggest elephant). What keeps a species that large from arising again? I first checked on historic oxygen levels…and found that during the "age of dinosaurs" there was a little less oxygen in the atmosphere than there is now. But that may have actually facilitated huge size, because my next thought was physiology. Elephants are mammals, and as such are exothermic; the muscles of all mammals twitch constantly to keep the body warm. This is called "muscle tone". Although larger dinosaurs were almost certainly warmer than their surroundings, they probably didn't have the level of muscle tone found in birds and mammals. If you model an 85-ton mammal, it will overheat, badly. I suppose that elephants represent the extreme end of the trade-off between large size (efficiency and ability to ward off predators) and heating. With a little less oxygen, and a slower overall metabolism, the titanosaurs were possible.

In Superlative: The Biology of Extremes, by Matthew D. LaPlante, we don't find the above comparison, but there are numerous others. For example, there are two contenders for the title of lightest mammal, the Etruscan Shrew and the Bumblebee Bat (Kitti's hog-nosed bat). Both weigh just under 2g, and stretch the limits of packing all the things a mammal needs to live into a tiny body. More to the point in the author's analysis, a 2-gram shrew has such a small brain that its cerebral cortex is less than 1mm thick, and so its neurology can be studied with certain imaging techniques. Larger critters have a cortex that is too thick to be studied that way.

Other small things reveal other limits. The smallest flying insects, fairyflies, include a species that gets as small as 0.16 mm long. The period in a printed book is typically 0.3 to 0.4 mm in diameter. So, while I don't know how many angels could dance on the head of a pin (assuming you could induce them to do so), three or four or more of that species of fairyfly could crowd onto a printed period (assuming you could induce them to do so). But it's a stretch to call what they do "flying". To a fairyfly, air is a thick fluid, and they really kind of row themselves along, using a motion similar to the sculling we might do with our hands to stay afloat in water.

In addition to dealing with size, weight, and speed (cheetahs and sloths, for example), the book contains a chapter on danger. The title of "most dangerous" is incredibly hard to confer. Dangerous how? A handful of the most venomous snakes produce venom that can kill a human in a half hour or less, with an amount almost too small to see. But like other extreme animals, they reward study with discovery. Some components of snake venom are being studied for medical use, such as relieving pain, or liquefying tissue, or conversely, protecting tissues from being liquefied by other components. But there are other measures of danger. A snake species with very potent venom may be shy, and less likely to bite, so it poses less of a danger than an aggressive species.

We must remember that the most dangerous animal is ourselves. Besides that, then, what animal causes the greatest number of untimely human deaths? The hands-down winner is the genus of mosquito, Anopheles, that carries malaria. A few other insects, such as a different mosquito that carries Zika and another that carries West Nile virus, are somewhat distantly behind, as is the Tsetse fly.

There are other ways to measure danger, as in "Dangerous to whom or what?" If you're a flying insect, you're more likely to find yourself in the gullet of a bat than in any other creature. A bat consumes half its body weight in insects nightly; a nursing female takes in twice that. And there are a lot of bats, each of which must eat hundreds to thousands of insects each night. One cave in New Mexico hosts a colony of 20 million bats, and they consume 50 tons or more of insects every night! Someone else can figure out the overall world total, but the yearly "take" of insects by bats exceeds the total biomass of all mammals.

These examples just scratch the surface of a delightful book, that doesn't just tell what is the biggest, smallest, fastest, or whatever, but describes how the quantity is measured, and in many cases, "What's it good for?" Studying the extremes can help us better understand the "usual".