The common wall lizard (Podarcis muralis) offers a great opportunity to study how humans are influencing the course of evolution. These lizards have a large native range, covering much of Southern Europe, but are also successfully established in many places, including Cincinnati, Ohio, USA. The common wall lizard was introduced to Cincinnati in the early 1950s. A young boy took a vacation to Northern Italy and found these lizards there. He grabbed a few (around 10 individuals), brought them back to his home in Cincinnati, and released them. They have done very well for themselves in this new environment and likely number in the hundreds of thousands across urban Cincinnati and the surrounding areas.
We were very interested in how this new urban environment influenced the morphology of these lizards and how that morphology, in turn, affects performance. We investigated some aspects of that relationship in a previous paper (you can read a blog post here), but we wanted to go a bit deeper. We designed a follow-up experiment in efforts to understand two main questions: (1) How does living in a novel environment influence claw morphology, (2) How does morphology and environment influence performance?
Why lizard claws?
In our previous study, we analyzed body morphology and how it changes in the population as a result of living in this new environment. This time around we wanted to test for changes in the claws of these lizards. Claws are very important because they allow lizards to grip and climb on different surfaces. Other groups of lizards, like geckos and anoles, have sticky toepads which allow them to climb up smooth surfaces. However, these wall lizards don’t have them, so their ability to cling and climb is directly dependent on their claws. In urban environments, these lizards are often found climbing on the sides of buildings and decorative stone walls, so we thought that claw shape could change to allow these lizards to use the urban environment better.
We compared the claw shape of museum specimens collected in the 1980s (courtesy of the Cincinnati Museum Center) and contemporary lizards to understand how claw shape changes over time. We collected claws from both groups and utilized scanning electron microscopy (SEM) to analyze microscopic variations in claw shape.
Following methods of a previous paper (read a summary here), and with help from its authors (big thanks to Cleo Falvey and Kristin Winchell), we used geometric morphometrics to analyze claw shape from our SEM images.
No difference in claw shape over time
We found that claw shape hasn’t changed over time. We have two hypotheses as to why we found no differences. First, claws could already be good in the city. In the wild, these lizards live in rocky outcrops. There might not be much structural difference between a rocky outcrop in Northern Italy and a stone wall in Cincinnati, so the claw shape might not have had to change when moving to the city. Our second hypothesis is that selection already occurred and we can’t see it. These lizards were introduced in the 1950s, but we only have museum specimens dating back to the 1980s. In the 30 years between the introduction and our first museum specimen, selection could have already occurred, changing claw shape. Since we have no way of knowing what the claw shape of the original founding population was, we could be blind to this selection.
Clinging and climbing
We were also interested in the relationship between environment, morphology, and performance, so we decided to test for clinging and climbing performance. For clinging performance, we tied a harness around the pelvis of the lizard and pulled it back, recording how much force it generated as it was trying to cling to the patch of substrate. We tested clinging performance on three different substrate types which mimic materials these lizards regularly encounter in the urban environment: cork mimicking wood, sandpaper mimicking stones, and turf mimicking grass.
To test climbing performance, we used a vertical racetrack and tested it on two different substrates, cork and turf (we wanted to also use sandpaper, but the lizards couldn’t climb on it). For both clinging and climbing performance, we ran all trials at two different body temperatures, one cool temperature (24°C) and one warm temperature (34°C), to test the effects of temperature on performance.
Complex relationships determine clinging and climbing performance
Climbing performance was influenced by temperature and claw shape. We found that warm lizards climb faster than cool lizards and that lizards with longer claws climb faster.
When it comes to clinging performance, we found an interesting relationship that suggests that the interaction between body morphology, claw morphology, and environment determines performance. Each substrate has a different combination of morphology that produces maximum clinging performance. For cork, individuals with short claws, small limbs, large feet, shoulders, and pelvises cling the best. For turf, having short claws will produce the best performance, regardless of your body morphology. Individuals that possess short claws, large limbs, small feet, shoulders, and pelvises cling best to turf. Our results highlight the complex interactions that shape performance.
Next steps
After finishing undergrad at Ohio Wesleyan University, I’ve moved away from the wall lizard system (for the most part) and am now focusing on my graduate work working with anole lizards. Working in this system has been great because I can ask a lot of the same questions I did when working with wall lizards, but go about answering them in a different way.
But that’s not to say the wall lizards aren’t being studied! Many undergraduate students continue to work with Dr. Eric Gangloff to answer questions about the ecology and evolution of these successful invaders on an NSF-funded project, which includes studies of their thermal biology, physiological variation among color morphs, and using genomic approaches to better understand their evolutionary history.
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