Despite being on opposite sides of the world and separated by millions of years of evolution, Draco and Anolis lizards have converged on many common adaptive solutions.

It has long been suspected that the Draco lizards of Southeast Asia were ecological analogues of the Anolis lizards in the Caribbean. But it has only been recently that we’ve started to truly figure out just how similar these two groups actually are. Especially exciting has been the discovery of how lizards in both groups have converged on remarkably similar adaptations: natural selection appears to repeat itself, time and time again.

The classic textbook scenario in anoles is adaptive convergence in ecomorphology, which reflects where lizards tend to hang out in the environment. This is particularly obvious  in the adaptation of Anolis limb morphology to different types of perches. I’m not sure we can say whether Draco exhibit the same ecomorphs as Caribbean anoles just yet, but Draco do exhibit the same adaptive changes in limb morphology as the anoles of the Caribbean.

Then there’s the obvious case of the dewlap, which has evolved separately in both Anolis and Draco as a key component of their territorial and courtship displays.

Most Anolis and Draco lizards also rely heavily on elaborate sequences of head-bobs in these displays as well.

This in itself isn’t unusual. Many lizards head-bob and push-up over territory and mates. Up to now, however, Caribbean Anolis lizards were the only ones known to tailor their display movements to ensure detection. For any visually communicating animal, visual “noise” from windblown vegetation and poor ambient light make it difficult to see visual signals. Anolis lizards were exceptional because they actively monitored environmental conditions and exaggerated their body movements when it was visually noisy (e.g., on windy days) or extended the duration of their displays when light levels diminished (e.g., on cloudy days or deep inside a forest).

There are a countless ways lizards might produce a conspicuous signal. Many Australian and Chinese lizards add arm-waves or elaborate tail-flicks or simply rely on colourful ornamentation that stands out well in the environment. Many North American lizards rely on performing lots of head-bobs or positioning themselves to accentuate a colourful badge on their throat or sides. But anoles seemed unique in both the complexity of their visual displays and their capacity to modify their behaviour to the prevailing conditions in the environment.

But after years of studying Anolis on Jamaica and Puerto Rico, and then even more years studying Draco in the Philippines, Borneo and Malaysia, we have now discovered more astonishing parallels between the two lizard groups that extends beyond just morphology.

Southeast Asian Draco lizards exhibit virtually identical strategies for coping with visually difficult environments as do the Caribbean Anolis lizards. Draco use the dewlap in the same way as the anoles, and change the speed (or exaggeration of movements) and the duration of displays in the same way as anoles, and this capacity to tailor displays to the conditions of the environment has also tended to precede what seems to have been adaptive divergences in display behaviour among species.

To discover all this, we had to study many different species of Draco and Anolis (11 and 12 species respectively), including hundreds and hundreds of lizards (727 to be precise), and then conduct thousands and thousands of hours of video analysis (13,310 hours – !!).

To be perfectly honest, what I was hoping to document from all this work was how differences in evolutionary history between the Anolis and Draco had shaped the trajectory of display evolution. Sure, Draco had evolved a dewlap like Anolis, but how that dewlap has been morphologically constructed was quite different between the two groups. I had become quite interested in these so-called “many-to-one, form-to-function” outcomes in evolution, and I was aiming to show something similar for display behaviour.

To be clear, there were differences in how Anolis and Draco lizards responded to environmental conditions, and how plastic changes in behaviour have contributed to display differentiation among species. In fact, the head-bob component of the territorial display has been entirely lost in some Draco species.

But the similarities were stunning and outweighed the differences by a large margin. Even the loss of headbobs in some Draco have intriguing similarities to how some Anolis species have shifted their display effort to the dewlap, which seems to be a more energetically efficient means of producing a conspicuous, complex visual display, than the more tiresome head-bob and push-up movements.

We have also confirmed experimentally in anoles that the manner in which Anolis lizards tailor their displays does actually improve display detection in visually difficult environments. This took a lot of work in itself and required the development of a robotic playback system, but this is now ancient history.

But to complete the loop, a similar type of playback experiment needed confirm the same adaptive benefit in Draco.

Some years ago we had conducted a lengthy field experiment using robot playbacks that were designed to test the response of Draco lizards to different coloured dewlaps. That experiment showed little effect of dewlap colour on detection, but a tangible effect that once lizards saw the dewlap, they used it to evaluate the species identity of the signaller.

I was in the lab one day looking at these old Draco robots to get some inspiration for designing a new system for some other crazy idea I had. As I was fiddling with the mechanism, I noticed that the robots weren’t exactly the same, with the lever controlling the dewlap of one being slightly longer than another. This meant the display probably differed in speed between the robots. These things happen and I didn’t think much of it at the time. The treatments used in the field experiment were systematically inter-changed across the robots to make sure this type of thing didn’t cause any problems.

Later, however, it occurred to me that perhaps this might offer a serendipitous opportunity to confirm the adaptive benefit of at least one of the key convergences exhibited by Draco lizards. I downloaded the data from the original study from its dryad repository, extracted the response times of lizards to the two robots that differed in dewlap speed, and sure enough, detection times were much quicker to the robot with the faster dewlap display.

The top panel (a) shows the differences in dewlap speed between the two robots, while the bottom panel (b) shows the detection time of free living Draco melanopogon.

If you’re interested in a short video introduction to this work, or want to know more about how these findings relate to our general understanding of adaptation and animal communication, you’ll find some answers in this 5 minute video below.

Terry Ord
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