Grin and bear down

 kw: book reviews, nonfiction, science, teeth, odontology, paleontology, biology

It is easier to get the bite force of an alligator than a human. With a large alligator, it may be more dangerous, but the process is simple: restrain the beast, tap the tip of its nose so it will open its mouth, and stick the measuring tool in between the teeth at the back of the mouth. The measuring tool is typically a steel rod fitted with one or two force plates that can survive a few tons of force. When the gator feels the intruding rod it will bite as hard as it can, even though one or more teeth might be knocked out or broken.

Humans are more cooperative than alligators. However, a human will never bite with full force, particularly when his or her teeth meet the instrument. We have a protective reflex that snaps our jaw back open when we bite on something harder than the average bit of gristle. Very few people can bite hard enough to crack open a hazelnut, for example. We are all strong enough to do so, but the reflex prevents it. Why can an alligator (or a shark) bite hard enough to break teeth, but we cannot?

It isn't for lack of strength. As adults, if we break a tooth, it stays broken. Without the help of a dentist to fit a denture or insert an implant (both quite costly), we have lost that tooth forever. But an alligator's teeth are replaced when any break. Even more so the teeth of sharks. So it makes sense that humans have a reflex that protects our teeth, but sharks and alligator's don't need that reflex.

This is just one subject covered in a very entertaining way in Bite: An Incisive History of Teeth, from Hagfish to Humans by Bill Schutt. The first section of the book consists of eight chapters that deal with various specialized kinds of teeth. The subject of bite force is found in chapter eight, where we find that a 17-foot crocodile can exert 3,700 pounds (~1,700 kg), and that regardless which of the four kinds of crocodilians were studied, bite force was based almost totally on size (the four "crocs" are alligators, crocodiles, caimans, and gharials). For example, even though the mouth of an alligator is twice as wide as the mouth of a crocodile of equal length, the length of their jaws is very nearly the same, and the bite force is the same.

Compared to the peg-shaped teeth of these reptiles, a human molar is rather complex, as this dentist's sign shows. The strong, curved roots of the molar hold it firmly in the jaw. This is why, whenever someone needs to have a molar removed, the dentist schedules a double-length appointment, and these days takes great care to inject enough Novocain or Lidocaine, because the tooth will probably need to be broken in half to get it out, and removing each section requires a great deal of force. I've noticed that dentists tend to be built like weightlifters, and it makes sense!

Human teeth are not particularly sharp. Our pet cat's teeth are much sharper than mine. But perhaps the sharpest teeth belong to vampire bats, the subject of the first chapter. The incisors of a vampire bat are so sharp that the bat can slice off a bit of skin, really a shallow scrape, without disturbing the sleep of the victim. The typical location of the bite is a toe or ear or other easy-to-reach extremity—certainly not the neck! The shallow wound will ooze blood, which the bat laps up. Along with this understanding, we learn how the mythology of vampires and of bats had to be gradually swept aside so scientists could learn how these tiny mammals really live.

A side note on the subject of Chapter 2, the candiru fish of Brazil. Contrary to rumors, these slender catfish don't swim into you while you are peeing in the river. They are about half an inch in diameter, and won't fit. The author was able to track down just one apparently verified account of a man who had this happen, but "apparently verified" turned out to be not really verifiable.

Other kinds of specialized teeth include fangs, both the large canines of baboons and the venom-bearing fangs of certain snakes and lizards such as the Gila monster. They include tusks, which are teeth that project out of the mouth or skull. And we learn why the teeth of horses continue to grow with age: Grass is so abrasive it wears down the crowns of the teeth, so new material has to be grown continually, or no horse would live more than a few years.

Does every animal have teeth? Or at least, all the animals big enough for us to notice? Apparently the earliest vertebrates had no teeth, and no jaws. Hagfish have a weird Y-shaped sort-of-tongue with two patches of toothlike scrapers. They eat from a fallen fish or whale by gripping a bit of flesh with the Y, then tying themselves into a knot that they slip forward until they can pry against the flesh and tear out the piece, which they just swallow. Some frogs and toads are toothless, and turtles have a beak of keratin rather than teeth. Of course birds have a beak also, and the proverb "scarce as hens' teeth" is apt. Certain experiments show that the genes for making teeth are present in birds, but are inactive. The dinosaur ancestors of birds probably all, or nearly all, had teeth.

While the toothed whales—porpoises, orcas, and sperm whales—have many teeth, the other major group of whales instead have baleen or "whalebone", a filtering organ, and no teeth. The echidna, or spiny anteater, and other anteaters, have no teeth, swallowing their prey whole; their main "utensil" is the tongue.

Cephalopod mollusks (octopus, squid, cuttlefish, etc.) have beaks similar to a parrot's beak, made also of keratin. Contrary to this fanciful image of Jack Sparrow about to enter the gullet of a Kraken, or mythical giant squid, squids don't have any teeth. The beak of a giant squid is about the size of a fist; the exposed portion that actually bites is just 2-3 inches across (5-7 cm).

The last section of the book is all about human teeth, and the things people do and have done with them. You may have heard that George Washington had wooden teeth. He lost most of his teeth rather early, probably because of mercury poisoning due to medical practices of the time. His dentures contained teeth of porcelain and of human origin. When dentists of the era extracted teeth, they kept them to be fitted into dentures. Certain cultures drilled holes into the front of incisors to put jewels or other ornaments in. That's a rather painful way to show status! 

As someone who has a bunch of fillings, and now a few crowns, I know the rigors and discomfort, and sometimes the pain, of dental work. The author tells of research that may one day allow us to re-grow lost teeth. It would be a slow process, but a few months of careful chewing while the new tooth grows into place would be no worse than a few months of careful chewing while the titanium root of an implant becomes firmly incorporated into the jaw, until the titanium-and-ceramic crown can be attached. And it may cost a lot less.

Appreciate the teeth you have. Take care of them. Perhaps some folks alive today will live to see the re-growth of lost teeth. Until then, love your dentists; they really try to cause minimal pain while doing maximum good!

In French spider is araignée

 kw: blogs, blogging, spider scanning

I logged in to Blogger to begin a post and saw that there has been a lot of activity. In the recent past, when spiders aren't active, the total number of views has been around 700, with half of that from the US. For the past week, however:

I think that the roughly equal spread of ~14,000 views between France, the US and Singapore reflects VPN usage, probably by several entities. Getting such large numbers, for a blog with less than 3,000 posts, doesn't make sense otherwise. However, at this point I can do no more than marvel at it.