Like us, chameleons have five digits on each of their hands and feet, but they’re bundled up with connective tissue, making the lizards look two-toed – or like they’re trying to do a Vulcan salute. This helps them grip branches, and according to work published in BMC Evolutionary Biology last month, their expert tree-climbing abilities are thanks to previously understudied skeletal elements that also form ball-and-socket joints in their wrists and ankles.
Among living reptiles, chameleons are the best adapted for a tree-climbing lifestyle. To find out more about how their adaptations developed, La Sierra University’s Raul Diaz and Paul Trainor from the Stowers Institute for Medical Research studied embryos of the veiled chameleon (Chamaeleo calyptratus). First, the duo wanted to know if the same mutations that give rise to split hand (or split foot) syndrome in humans are also behind the chameleon’s two-toed appearance. In people, mice, and birds alike, these mutations occur in genes involved in maintaining limb outgrowth, such as Fgf8.
Surprisingly, the team discovered that Fgf8 is expressed without problems in chameleons. "Most of what we know about vertebrate development comes from zebrafish, frogs, chickens, mice and humans," Diaz said in a statement. "Looking at atypical species, such as the veiled chameleon, forces us to begin to think within an evolutionary framework to try and figure out how a unique chameleon body was made."
Here, you can see a veiled chameleon's left hand (left) and left foot (right). Digits 1, 2, 3 and digits 4, 5 are bundled on the hand, while digits 1, 2 and digits 3, 4, 5 are bundled on the foot.
The researchers then turned to the chameleons' wrists and ankles. Perhaps what’s happening in those joints are affecting the fingers and toes? By clearing and staining chameleons in various stages of development, the team found never-before-seen skeletal elements: nine in the wrist and seven in the ankle. Until now, we thought they had half as many. Some of these were transitional elements that condensed as cells, others differentiated into cartilage or fused with nearby skeletal elements, and only half actually went on to form the bones we see in adults.
This higher number of independent skeletal elements, along with the ball-and-socket joints in their wrists and ankles, offer greater flexion. "They’ll hold onto a branch and rotate their entire body around a particular gripping point," Diaz told IFLScience. "If we tried to do that, we’d tear a ligament." When they rotate, they have two swivel points.
Big, colorful, specialized chameleons like the veiled chameleon evolved more recently. Earlier members of the lineage were likely small, drab, and lived on the ground. There are small, ground-dwelling (maybe bush-climbing) chameleons around today; they have fewer wrist and ankle components than veiled chameleons, and they also walk with a stiff footing. Turns out, the angle between their two bundles of digits is about 180 degrees (that means fingers on the same hand can point in two opposite directions).
In veiled chameleons, that angle is reduced: Having more wrist and ankle parts actually pushes the two bundles of digits closer together. "Their hands and feet are able to move faster and hold onto more of a diversity of branch sizes as they climb," Diaz explained to IFLScience. "The more small parts you have, the more complex movements you can do."
The veiled chameleon has a laterally compressed body, prehensile tail, turreted and independent eyes, and a projectile tongue. Chameleon hand (B) and feet (C). Diaz & Trainor BMC Evolutionary Biology 2015.
Image in text: Raul Diaz/La Sierra