My, What Strange Teeth You Have
If you caught a glimpse of New Zealand’s iconic reptile, the tuatara, you’d be apt to call it a lizard. With its long tail, spiny back, and gray-green coloring, and stretching to less than the length of your arm, all told, it looks like a pretty ordinary member of the iguana family. But two surviving tuatara species are in fact the only living members of the order Rhynchocephalia, which over time has included more than 50 species of now-extinct reptiles, and which separated from the modern snake and lizard lineage during the Mesozoic. As a result of that schism, the tuatara has had more than 225 million years to differentiate itself from all other existing reptiles. "You and I are actually more closely related to a kangaroo or a duckbill platypus than they are to lizards," says Marc E. H. Jones, an evolutionary biologist at University College London who studies the species. Though their ancestors ranged all over the globe, tuatara now live only on 35 small near-shore islands and in a single colony outside Wellington. They’ve also evolved some highly specific traits -- chief among them, a chewing method that a new study by Jones and others demonstrates is one-of-a-kind.
Researchers have long known that tuatara have two rows of jagged teeth at the top of their mouths and a row at the bottom that fits in between them. But figuring out exactly how they chomp has proved tricky: Tuatara are a protected species, so they can’t be probed in the lab; and even if they could be rigged with wires to the sorts of devices that typically measure jaw pressure, it’d be hard to be sure they were chewing as they do in the wild. To get around those problems, Jones and colleagues at the University of Hull and Hull York Medical School used special engineering software to model the tuatara’s bite. They found that the bottom row of teeth works much like a saw, punching through food as through a piece of wood resting on two hobby horses, then sliding forward slightly to serrate. The strategy allows tuatara to ingest a wide range of prey -- insects, snails, lizards, even other tuatara -- that would otherwise be too big to fit in their mouths. They also attack the nests of seabirds, munching on eggs and chicks that they decapitate with their steak-knife dentition.
Chewing, as opposed to swallowing prey whole, has a number of evolutionary advantages. In allows animals to eat a wider variety of foods -- including morsels that are bigger than a mouthful -- and to expend less energy digesting nutrients, which are already partially broken down. Chewers can also gobble bigger meals, since masticated food can be packed more tightly into the gut. Mammals, including humans, are particularly adept at munching a broad-rage of edibles, and researchers have theorized that our complex chewing abilities may have given us the extra fuel we needed to grow bigger brains than our reptilian counterparts, maintain a constant body temperature, and move around a lot. But tuatara appear to muddy that theory. Despite their odd chewing techniques, they still depend on external temperatures to regulate their metabolic rate, unlike mammals, and they aren’t any more active or intelligent than reptiles that consume their meals in a simpler fashion.
Beyond its bite, the tuatara possesses other unusual features. Special hemoglobin and enzymes allow them to be lively at night and to survive in much cooler temperatures than most of their fellow ectotherms. They also have a third eye at the top of their head that can sense light and may even help regulate their body clock and breeding cycles. Tuatara can live to be at least a hundred years old -- and possibly much older. They take around 15 to 20 years to reach reproductive age, and females require about four years to prepare a clutch of roughly a dozen eggs, which they then incubate for a full year. Tuatara remain fertile for decades: In 2009, a captive 111-year-old tuatara fathered offspring with his first-known mate, herself 80 years old. And despite their longevity, they never grow new teeth the way alligators, sharks, and many other toothsome predators do. Depending on the consistency of their local diet, their chewing equipment can wear down quickly, forcing them to make do with smaller or softer fare.
Fossil evidence shows that tuatara once flourished on New Zealand’s main North and South Island. But when Polynesian settlers arrived 750 years ago, they brought rats with them that devoured or outcompeted many native species. With the arrival of European settlers -- and their rats, pigs, dogs, and cats -- tuatara became extinct on the main islands. But on New Zealand’s smaller islands, Sphenodon punctatus and a second species that exists only on the football-field-sized North Brother Island, Sphenodon guntheri, survived long enough to benefit from successful conservation efforts to exterminate exotic invaders. Recently, a wild tuatara population was reintroduced on the North Island, near Wellington.
The main threat facing tuatara now, Jones says, is climate change. The temperature at which tuatara eggs incubate determines what sex hatchlings will be; global warming could cause their population to skew male. To improve their odds of holding onto some suitable habitat, wildlife officials are now working to establish tuatara populations across a greater geographic area. On a hopeful note, Jones points out that tuatara have already "survived a lot of climate change -- they’ve had ancestors on New Zealand since it broke off from Antarctica 85 million years ago." Granted, the planet is heating up faster now than it has in the past. But if the evolutionary innovations tuatara have rolled out so far are any indication, they may yet find a way to endure.