Teeth are hard, calcified structures found in the jaws or mouths of many vertebrates. They are primarily used to help break down food, in a process called mastication, which is the first step of digestion.
Many animals also use their teeth to catch or wound prey, to carry things including their young, to tear food into smaller pieces before eating it, and for defence.
Most mammals, fish, amphibians, and reptiles have teeth which vary in size, structure, and arrangement in the mouth, and which have evolved to have different uses. For example, some snakes have hollow teeth which function as needles to inject venom.
Birds, however, are the exception, as the only class of vertebrates not to have teeth.
About 300 million years ago, a group of reptiles called the archosaurs appeared, descended from an earlier group of diapsid reptiles which had two openings in the skull behind each eye. Archosaurs included flying and non-flying dinosaurs and differed from the diapsid reptiles by the presence of just one opening in the skull in front of each eye. Another feature of the archosaurs was serrated teeth set in sockets, rather than a shallow groove, unlike other reptiles.
Fossil records show that modern birds appeared about 60 million years ago, descended from those early archosaurs. Crocodiles and alligators are the only other extant archosaurs and the closest living relatives to birds.
Like the early archosaurs, crocodiles still retain their teeth, which means that somewhere during their evolution birds lost their teeth, rather than lacking them in the first place. And science has shown that the trigger to enable the genes to produce teeth in birds was switched off about 100 million years ago.
For many years it was thought that modern birds didn’t have teeth because they are relatively heavy and would impede flight. But this hypothesis doesn’t explain why many Mesozoic flightless dinosaurs didn’t have teeth either. Conversely, the Archaeopteryx, long thought to be the earliest bird, flew, and it had teeth that aided its carnivorous diet. And flying mammals such as bats can fly well despite having teeth.
A further suggestion was that a beak is a better adaptation for eating certain types of food. Most species of birds do not hunt prey, for example, and those that do have evolved sharper and more powerful beaks and claws to help them catch and tear up food. For birds who eat smaller animals, insects, and seeds, teeth would make it harder to use their beaks as efficiently.
The theory went that because birds have a gizzard, a muscular organ in their stomach that grinds up food, chewing with teeth would be a waste of energy so over time birds lost their teeth because they didn’t need them to eat.
But again, toothless is not unique to birds. Turtles and tortoises as well as some extinct reptiles don’t have teeth either.
In 2018 a new study by scientists at the University of Bonn came up with a new theory. They found that the eggs of flightless dinosaurs took between 3 and 6 months to hatch which was much longer than previously thought.
By studying the fossilised teeth of these dinosaur embryos and examining the growth rings, they found that the long incubation period was due to the development of the teeth, a complex process that can take up to 60% of the total egg incubation time.
Eggs are particularly vulnerable to predators or damage so dinosaurs and many living reptiles typically bury their eggs in the ground to keep them safe during this long incubation period.
Faster hatching boosts the chances of survival so by not developing teeth chicks were able to escape from their shells earlier and reduce the risk of either the eggs or the incubating parent being lost. A long incubation period is also very stressful on adult birds resulting in a dramatic loss of body weight, putting them at risk of malnutrition and disease.
Although there are some exceptions, modem birds’ eggs typically hatch after just a couple of weeks of incubation. The loss of teeth was a trade-off for fast embryo growth and a shorter incubation, and allowed birds to lay their eggs in the open rather than expend energy burying them in the ground.
In the middle of the last century, a mutant chicken, which had severe limb defects, died before hatching. For 50 years, the embryo lay untouched in a lab until a researcher at the University of Wisconsin re-examined it and noticed it had tiny bumps along the edge of its beak that looked like teeth.
Much like an alligator or crocodile, the chick, nicknamed Talpid, had conical teeth, which closely resembled those of the archosaurs, providing further evidence that crocodiles and birds have a common ancestor.
This was not the first time a bird had been discovered that had teeth. In the early 19th century, the French naturalist Étienne Geoffroy Saint-Hilaire, and one of the founders of comparative anatomy, observed some embryonic parrots had with tiny teeth, which supported his theory that modern animals descended from more primitive species.
The discovery of Talpid’s teeth, inspired a team of scientists from the Universities of Manchester and Wisconsin to find out whether chickens still possessed the gene responsible for growing teeth. Previous experiments that had introduced genetic material from mice resulted in chickens growing round teeth similar to mammals, but had not proved that birds still retained their ancestors’ molar genes.
By making some changes to the expression of particular molecules, the scientists activated the tooth gene that had lain dormant for millions of years and induced teeth growth in healthy chickens.
Mutant chickens that naturally bear this recessive trait are as rare as the proverbial hens’ teeth, and never survive long enough to hatch, so are never found in the wild.
The cutting edges of the two mandibles of a bird’s beak are called tomia and in some species these have evolved to look and act like teeth to help them handle food more easily. Fish-eating birds, for example have saw-like serrations that help them grip slippery fish more easily, while seed-eating birds have ridges in their tomia which help them cut through a seed’s outer husk.
Some shrikes and birds of prey have a ‘tomial tooth’ on the upper mandible which they use a little like a can opener to sever their prey’s spinal cord. The double-toothed kite is so named because it has two pointed tooth-like notches on its upper mandible, but these are not teeth in the true sense of the word, as they are not coated in enamel.
Baby birds have what is known as an ‘egg tooth’ a small, sharp structure on the end of their beaks that they use to help them hatch. First, they use their egg tooth to pierce the air sac between the membrane and the eggshell giving them a few hours of air during which time they repeatedly force the egg tooth through the shell until they have made a hole big enough for them to hatch through.
Some species of birds, such as woodpeckers, have two egg teeth on both the top and bottom mandible, while some precocial species are strong enough not to need an egg tooth and use their legs and feet to crack the shell instead.
The egg tooth will fall off a few days after hatching or will be absorbed into the chick’s bill.
