The evolution of sticky toes opened up the lizards' long-term tree life

Many lizards are terrific climbers. Their sharp, curved claws are ideal for attaching to tree trunks, rocks and other rough surfaces. However, in the dangerous treetop world is full of slippery leaves and unstable branches, three groups of strange lizards have extraordinary evolved organs: they have sticky toe pads on their fingers and toes.

Sticky toe pads evolved independently in geckos, celestial bulls, and Azoris lizards, producing these tree acrobats who specialized in adapting to forest canopy life. Scientists have long considered sticky toe pads to be a "key innovation in evolution" that has enabled arboreal lizards to adapt to their environment in ways that many padless lizards cannot.

However, some lizards without toe pads have adopted a canopy lifestyle, an observation that has puzzled scientists for decades. Biologists Aryeh Miller and James Stroud of Washington University in St. Louis set out to find out whether lizards with paddeds had an evolutionary advantage over lizards without mats living in trees.

They analyzed data from 2,600 lizard species around the world and found that although hundreds of different types of lizards have independently evolved arboreal lifestyles, species with sticky toe pads have the upper hand.

"Lizards with toe pads have greater ecological advantages in arboreal environments," said Miller, a graduate student in the University of Washington's Program in Evolution, Ecology and Population Biology and lead author of the study. "The toe pad is basically a biological superpower of the lizard to obtain new resources, while the lizard without the toe pad cannot."

"We found that lizards with sticky feet predominate in arboreal environments. Once they've adapted to life on the trees, they rarely leave," said Stroud, a postdoctoral research assistant in the arts and sciences and senior author of the paper. "In contrast, lizards without sticky toe pads often transition from living in trees to living on the ground."

The study was published in Systems Biology.

The evolution of the toe pads shaped the diversity of lizards

Scientists have long wondered what the origins of key innovations played in the subsequent evolutionary diversification. Jonathan Losos, professor at william H. Danforth Distinguished University at the University of Washington, professor of biology in the College of Arts and Sciences and director of the Living Earth Collaborative, said: "Lizards are an excellent type of organism to conduct this type of research because of their extremely rich species, incredible degree of anatomical variation and habitat utilization. "

Using the recently published habitat use database of almost all lizard species worldwide, the researchers were able to perform a comprehensive analysis of toe pad evolution in the context of lizard habitat use, and the evolutionary relationships between which lizards live on trees and which do not become clear for the first time.

Miller and Stroud have developed an elegant new way to understand this diversity and the role that anatomical evolution has played in shaping the vast diversity of lizard species. This work will serve as a model for researchers who study many types of plants, animals and microbes," Losos added.

Miller, who led the analysis, first found that among thousands of lizards, species evolved at least 100 times in trees. In other words, it was evolutionaryly easy for a lizard to become a lizard in a tree.

The difficulty is to "stick with it", and the toe pad evolved after the lizard entered the tree, not before. And the lizard without mats will leave the trees with a high frequency, much higher than the lizards with mats.

"There are hundreds of species of lizards that live in trees, but over time of evolution, many of these species end up leaving life on the ground because, presumably, they interact with these cushioned lizards and have a greater advantage," Stroud said.

The next step in the study is to find out what lizards with mats can do that their matless relatives can't. Scientists can learn about this by looking at how these animals are doing in their natural habitat.

"Analyzing evolutionary relationships can tell us a lot, but then we need to go into nature — to see what parts of the environment lizards use and why those evolutionary relationships exist," Miller said. "