Why Don’t Birds Have Teeth?

Modern birds have very short incubation times compared to dinosaurs, and this evolutionary innovation gave them an advantage over dinosaurs — but it came at the expense of teeth

by GrrlScientist for Forbes | @GrrlScientist

Hungry robin nestlings.
(Credit: Bryancalabro / CC-BY-SA 3.0)

Evolution is complicated, and thus, there often are multiple overlapping reasons that particular traits are adaptive. One such evolutionary puzzle is the reason that birds have beaks instead of teeth. Modern birds evolved from theropod dinosaurs, which include some of those ferociously carnivorous beasts with mouths full of pointy teeth, like velociraptors and Tyrannosaurus rex, that were featured in the film, Jurassic Park.

Traditionally, toothlessness in modern birds was thought to be an adaptation for flight (ref), because teeth are heavy. This hypothesis sounds reasonable, but doesn’t explain why toothlessness arose repeatedly in a variety of Mesozoic carnivorous dinosaurs that were flightless (Figure 1). An alternative hypothesis, based on the observation that primitive birds with teeth had disappeared by the end of the Mesozoic, is that a beak is better suited for eating particular types of foods. But this idea doesn’t really make much sense when you think about it, since teeth really are remarkably well-adapted to diet.

“Several recent studies have suggested that changes in gene regulation are responsible for lost teeth rather than diet changes,” said lead author of a new study, Tzu-Ruei Yang, a doctoral candidate, at the University of Bonn. “So there may be an unknown mechanism that contributes to tooth loss.”

Figure 1. Toothlessness in dinosaurs. Consensus cladogram of Archosauria shows that edentulism frequently evolved independently in several lineages. All Ornithischia, Therizinosauroidea, Incisivosaurus and toothed Enantiornithes exhibit partial tooth loss. Ontogenetic edentulism has been reported for the theropod Limusaurus from China. Complete edentulism is observed in some pterosaurs, Ornithomimosauria, Caenagnathoidea (including Oviraptoridae), some Mesozoic birds such as Confuciusornis and Gobipteryx, and all extant birds (Neornithes). Silhouettes were taken from public domain images on phylopic.org.
(doi:10.1098/rsbl.2018.0090)

Evolution of beaks are thought to be ecological adaptations (ref) that triggered the explosive diversification of birds after the mass extinction at end of the Cretaceous, 66 million years ago. But a quick glance through the animal kingdom reveals that toothlessness isn’t unique to birds: other modern vertebrates, such as baleen whales, anteaters, and turtles, as well as some extinct reptile lineages, also lack teeth. So this trait suggests that these groups all share something — ecology, behavior or something — that specifically predisposes them to toothlessness. But what?

A recent study where the authors counted growth rings of embryonic dinosaur teeth discovered that their incubation period was roughly three to six months months long (ref). This long incubation period is due to tooth development — a complex process that takes up to 60 percent of egg incubation time.

“The embryo basically has to ‘wait’ in the egg until its teeth are done, only then can it hatch,” Mr. Yang said.

Oviraptosaurs were omnivores but had a toothless beak. This artist’s impression was released by the University of Nagoya, Japan.

This research inspired Mr. Yang and his doctoral supervisor, palaeontologist Martin Sander, a professor at the University of Bonn, to rethink the evolution of avian toothlessness. They proposed that a key selection pressure for the evolution of avian toothlessness may be incubation period. Basically, they argue that birds are under evolutionary pressure to speed up embryonic development so vulnerable incubation time periods are shortened. This then, reduces the likelihood that either the eggs or the incubating parent will be lost to predators, disease or natural disasters.

“The omission of time-consuming tooth formation could have been an effective way to shorten the dangerous incubation period,” Mr. Yang explained. Modern bird eggs typically hatch after a couple weeks incubation.

“We suggest that (evolutionary) selection for tooth loss (in birds) was a side effect of selection for fast embryo growth and thus shorter incubation,” Mr. Yang and Professor Sander write in their paper (ref) (Figure 2).

Figure 2. Schematic illustration of the hypothetical four stages of the evolution of edentulism. Stage 1, neither edentulism nor heterochronic truncation of tooth formation. Stage 2, ontogenetic edentulism, heterochronic truncation of tooth replacement occurs after the eruption of functional generations of teeth. Stage 3, complete edentulism, heterochronic truncation occurs at the cessation of embryonic tooth formation. Stage 4, complete edentulism, heterochronic truncation occurs at the onset of embryonic tooth formation, as indicated by the existence of tooth primordia in modern birds.
(doi:10.1098/rsbl.2018.0090)

Although this paradigm shift is a intriguing reevaluation, Mr. Yang’s and Professor Sander’s hypothesis does not explain toothlessness in all animals. For example, turtles have beaks instead of teeth, but they still have a long incubation period that lasts between 42–112 days. Of course, turtles live vastly different lives than birds do, so turtle toothlessness could potentially result from different evolutionary pressures than those faced by birds.

Mr. Yang and Professor Sander are optimistic that they are on to something.

“The combination of developmental biology and palaeontology could enlighten more hidden mechanisms and help us to understand how a particular trait developed,” Mr. Yang said.

Source:

Tzu-Ruei Yang, and P. Martin Sander (2018). The origin of the bird’s beak: new insights from dinosaur incubation periods, Biology Letters, published online on 23 May 2018 ahead of print | doi:10.1098/rsbl.2018.0090

Also cited:

Gregory M. Erickson, Darla K. Zelenitsky, David Ian Kay, and Mark A. Norell (2017). Dinosaur incubation periods directly determined from growth-line counts in embryonic teeth show reptilian-grade development, Proceedings of the National Academy of Sciences of the United States of America 114(3):540–545 | doi:10.1073/pnas.1613716114

Originally published at Forbes on 30 June 2018.