How the power cascade shapes bird and dinosaur beaks

Bird beaks take an extraordinary variety of forms and sizes, from a hummingbird’s straw-like beak to an eagle’s sharp, slicing beak.

Our research has revealed that a hidden mathematical rule underlies this remarkable diversity, governing the growth and form of beaks in almost all living birds.

The same rule also describes the beaks of birds’ long-extinct ancestors, the dinosaurs. We are delighted to share these findings, which have now been published in the journal iScience.

Examining beaks through this mathematical rule helps us understand how the faces of birds and other dinosaurs evolved over 200 million years. It also allows us to explore why the rules are occasionally broken.

When nature follows the rules

Universal rules are rare and hard to establish in biology: physical laws appear to apply so widely across organisms in only a handful of cases.

When such a rule is found, however, it offers a powerful explanation for the patterns observed in nature. Our team previously identified a new biological rule that accounts for the growth and shape of many pointed structures, including teeth, horns, hooves, shells and beaks.

The simple mathematical rule describes how a pointed structure, such as a beak, widens from its tip towards its base. We named it the “power cascade”.

Following that discovery, we were keen to explore whether the power cascade could explain the shape of beaks in birds and other dinosaurs.

Dinosaur beaks evolved more than once

Most dinosaurs, such as Tyrannosaurus rex, have a sturdy snout containing pointed teeth. Others, including the emu-like dinosaur Ornithomimus edmontonicus, had no teeth and possessed beaks instead.

Beaks evolved at least six times among theropods, the dinosaur group to which T. rex belonged. On each occasion, teeth disappeared and the snout gradually lengthened into a beak over millions of years.

Only one of these impeccable dinosaur groups, however, survived the mass extinction event 66 million years ago. Those survivors eventually gave rise to today’s birds.

Testing the power cascade in bird and dinosaur beaks

We investigated the power cascade growth rule by studying 127 theropod species. Our results showed that 95% of theropod beaks and snouts conform to it.

Using state-of-the-art evolutionary analysis and computer modelling, we showed that the ancestral theropod most likely had a toothed snout that followed the power cascade rule.

This raises an exciting possibility: the power cascade may describe the growth not only of theropod beaks and snouts, but also of the snouts of all vertebrates, including mammals, reptiles and fish.

The rule followers and breakers

After the mass extinction, birds went through a period of remarkable change. They now inhabit places across the globe, with beaks adapted in highly specialised ways to their surroundings.

Their beaks take forms suited to eating fruit, catching insects, piercing and tearing meat, and even sipping nectar. Most conform to the power cascade growth rule.

A few of the birds we studied, however, were rare exceptions. One is the Eurasian spoonbill, whose highly specialised beak allows it to sift through mud for aquatic life. Its distinctive feeding style may be the reason it breaks the common rule.

Rule-breakers such as the spoonbill do not disappoint us. Instead, they further demonstrate just how informative the power cascade is. Most bird beaks grow according to the rule, and those beaks can accommodate most feeding styles.

Occasionally, though, unusual birds such as the spoonbill depart from the power cascade growth rule to catch their special “worms”.

Knowing that most bird and dinosaur beaks follow the power cascade, our next major research step is to examine how bird beaks grow from chick to adult.

If the power cascade is indeed a foundational rule of bird beak growth, we might expect to find it hidden in many other forms throughout the tree of life.

Kathleen Garland, PhD Candidate, School of Biological Sciences, Monash University, and Alistair Evans, Professor, School of Biological Sciences, Monash University

This article is republished from The Conversation under a Creative Commons licence. Read the original article.