CHOMP

 kw: book reviews, nonfiction, teeth, history

I asked Whisk for a picture of someone with perfect teeth. It delivered. The book of the week is Bite, An Incisive History of Teeth, From Hagfish to Humans by Bill Schutt.

The first three parts of this four-part book are titled

• Toothy Adaptations in Nature: The Specialists
• Who, What, Where, and How Many?
• Teeth, Who Needs 'Em?

The 13 chapters in these parts cover a number of nonhuman animals with interesting teeth, or interesting things teeth do. Naturally, the scariest ones come first, Vampire Bats. Guess how big a vampire bat is. To use Charades categories, are they Bigger than a breadbox, Smaller than a cigar, or somewhere in between? For simplicity, in the list below, keep in mind that the wingspan is about twice the body length. There are three species:

1. The Common vampire bat, Desmodus rotundus, has a body length of about 9 cm (3.5 inch), and weighs 25-40 grams with an empty stomach; about an ounce.
2. The Hairy-legged vampire bat, Diphylla ecaudata, is the same size, with a little more variation, but the largest ones don't exceed 9.5 cm or 3.75 inch.
3. The White-winged vampire bat, Diaemus youngi, is a little smaller, around 8.5 cm (3.3 inch) and usually weighs less than 35 grams.

Did you say, Smaller than a cigar? ding-ding-ding!

The body length of all three vampire bat species is very nearly the same as the Little Brown Bat, Myotis lucifugus, though vampire bats weigh about twice as much as a Little Brown Bat, and have shorter wings. I see Little Brown Bats frequently, flitting about in the warm season here. They are found throughout North America. Vampire bats, by contrast, are found in South America. An elderly Little Brown Bat fell dead right in front of me while I was walking in a park near home. Naturally, I collected it and took it to the nearby Museum of Natural History for their collection. I was surprised to see that its body was no bigger than my thumb.

Now, as to vampire bat teeth…they don't puncture the way Dracula would. They shave off a small bit of skin with teeth so sharp their victim may feel nothing. Then they lap the blood as it seeps out (no gushing), and their saliva, like the saliva of a mosquito, has an anticoagulant, keeping the blood flowing longer.

So vampire bats don't have Fangs. Snakes do. There is much discussion about the varieties of fangs. Vipers such as rattlesnakes in the New World and the Gaboon viper, Bitis gabonica, of Africa (with the longest fangs, at 5+ cm, or about 2 inches) have hypodermic-style fangs attached at the front of the mouth. The mouth opens almost flat when they strike, so the fangs drive straight in. Cobras and their relatives, including the Coral snake of North America, have fangs set a little farther back, and tend to strike and then chew, to give venom more time to emerge. Snakes such as the Boomslang, Dispholidus typus, also of Africa, have their fangs in the back of the mouth, where they really need to chomp down to inject venom. Also, the fangs are not hollow like a viper's, but grooved. They make up for such disadvantages by having extra-toxic venom.

The largest teeth are the various tusks, with African elephants, Loxodonta africana, holding the prize for the largest ones among living animals. In the 1890's a few elephants were killed that had tusks exceeding 3 m in length (~10 ft), weighing as much as 100 kg (220 lbs) each. The largest fossil tusks, of a Columbian mammoth, are almost 5 m long (~16 ft) and weigh considerably more than elephant tusks. Another prize-winner would be narwhal, Monodon monoceros; each male has a single spiral tusk that looks like the horn of a Unicorn (which led to many myths), 2-3 m long (7-10 ft), but it is slender and hollow, seldom weighing more than 7.5 kg (16.5 lbs).

There are a small number of toothless mammals, primarily the anteaters and echidnas, but larger numbers are found in other groups such as birds and many frogs. The largest toothless animals are the great baleen whales. 

The fourth part embarks on human dentition, "Human Teeth: The Bad Old Days and Beyond". The author starts with a discussion of Washington's teeth…or the lack thereof. By the time he was a public figure George Washington had only one natural tooth. It was used to anchor a denture until it also was lost. Thereafter, he had various full sets made, none of which worked well. This spring-loaded contraption had to be held in the mouth by constant biting pressure. I suppose the dentists never got springs of the right strength to open easily without causing the wearer to produce huge cheek muscles (look at portraits of Washington, grimly holding his mouth shut). The bases were pieces of lead, quite heavy (and toxic).

It is said that at public dinners George Washington ate nearly nothing and drank little. However, his teeth were not wooden as some legends state. They were a mix of carved ivory, animal teeth such as pig, and human teeth. The rumor that some of those human teeth came from his slaves is discussed and dismissed. Washington's dentist did purchase teeth from slaves, but he is not known to have used any in the dentures he made for Washington.

There is a chapter about the various torture devices used for extracting teeth, which like all surgeries, was done without anesthesia until the 1840's. Of course, even in ancient times alcohol's stupefying effects were sometimes used for that purpose, but it was risky. To really knock someone out with wine you need to almost kill them.

I was surprised to read that more than 80% of humans have problems with wisdom teeth, and at least half have one or more removed by middle age. I wonder if those statistics relate primarily to the West, or are worldwide. I haven't been able to verify. I had all 32 of my teeth until I was 65, when one of them got a cavity that was too big for ordinary "drill-n-fill", and there was already bone recession. It was pulled, so now I have 31. But the last chapter in Part IV introduces technologies being developed that may permit new teeth to be grown in place, and other possible marvels that will make trips to the dentist less of an existential dread.

We think we are sort of average. In the tooth department, at least among mammals, that's close. The mammal with the fewest teeth is the walrus (18), while the giant armadillo has 70…but at sea, dolphins have 250 teeth. So our mouth full of 28 to 32 pearly whites is about in the middle, at least for land mammals. But non mammals can have many more than 70 teeth: 3,000 for some sharks, 9,000 for some catfish (including teeth on their tongue!), and for teeth not in the jaws at all, the common garden snail has a tongue with 15,000-25,000 tiny teeth. 

The Epilogue, "How Science Works", tells the story of a shovel-tusk animal called Platybelodon, one of the Gomphotheres, a relative of elephants. This genus had a very extended lower jaw, more than a meter, with two enormous incisors at the end that resemble a scoop shovel half a meter wide. The author regales us with the tale of the various hypotheses by different scientists, of the way(s) this unique appendage was used. The message: scientific conclusions are always provisional. New ways of studying evidence, new instruments and techniques for gathering evidence, and new discoveries in the field, all redirect the thinking of scientific workers. One hopes that each new theory comes closer to the truth. 

This just scratches the surface. There is much to learn here.

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I must touch upon a few errata:

• On page 42 and a couple of other places the author discusses C3 and C4 photosynthesis, and how the evolution of grasses changed the tooth structure of horses. Grasses and a few other plant types are tougher and contain more silica, so a grazing animal needs teeth that keep growing as they wear down. But he calls C3 and C4 "isotopes". They are not. They refer to a chemical with three carbon atoms in the photosynthesis mechanism of most plants, and a more efficient mechanism using a chemical with four carbon atoms in grasses and their relatives. They are different molecules with closely related functions. One could call them "functional analogs". The word "isotope" refers to an atomic nucleus with a specific number of neutrons and protons, as compared to a nucleus with the same number of protons but a different number of neutrons. For carbon, the common isotope has six protons and six neutrons and is called C-12. The other stable isotope of carbon has seven neutrons and is called C-13.
• On page 79, discussing venoms, we find some components listed, including "proteins (especially enzymes), chains of proteins (called peptides), …". Oops. Peptides are mini-proteins, short chains of amino acids. NOT "chains of proteins".
• On page 242, discussing vestigial anatomy, we read that the tailbone "whose only function seems to be to break if we land on it or even sit down too hard". The tailbone, the coccyx, holds our pelvic floor together. Many muscles and tendons attach to it. Ask your doctor what would result if your coccyx had to be removed.

Nobody's an expert on everything. Get this book anyway!

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!