Recently my work on urban lizard locomotor performance was published in Proceedings of the Royal Society B. This manuscript is part of a special issue on urban evolution, “The Evolution of City Life”, which will be coming out in paper form any day now.
Looking for the videos? JUMP straight to them at the bottom or keep reading to learn more about the experiment first.
In short, we wanted to connect the dots between morphological shifts we had previously observed (Winchell et al. 2016) to functional consequences relevant to fitness. Following the classic phenotype–performance– fitness paradigm, we reasoned that if these traits are shaped by natural selection, then we should observe a difference in performance correlated with these traits in the urban environment. So by examining locomotor performance on both anthropogenic (manmade) and natural (vegetation) substrates we gain a better understanding of the mechanism of natural selection shaping urban phenotypes.
How did we do this? Obviously, we had to build a lizard racetrack. We constructed a 9cm wide and 2.5m long track that we could adjust to be at varying angles of inclination (we chose 37° and 60°, which was as flat as we could set the track without lizards jumping out and the steepest we could set the track with enough lizards able to climb on smooth substrates). We also constructed three different surfaces for the lizards to run on: tree bark, painted concrete, and aluminum metal sheeting. We then placed high speed cameras above and alongside the track to capture the lizard sprints for analysis. On each of these 6 tracks we tested 15-19 adult male lizards from paired urban and forest sites in each of four municipalities across the North and West of Puerto Rico: San Juan, Arecibo, Aguadilla, and Mayagüez.
What did we find? Well to get the complete story, check out the manuscript. But the short version:
- Regardless if they were urban or forest lizards, all lizards ran significantly slower on the painted concrete and the metal than they did on the bark! They also tended to slip and slide on these tracks, which didn’t happen at all on the bark. This means that there is a significant cost to using these surfaces in the wild – if they can’t effectively cling to or run on these surfaces then that excludes 2/3 of the urban perch sites and puts them at greater risk of injury (from falls) and predation (if they fail to run away effectively).
- Lizards ran fastest when they had long hindlimbs across all tracks. There were also positive correlations of toepad morphology on some, but not all, tracks and negative correlations of longer forelimbs on steeply inclined tracks. Specific combinations of traits conferred greater performance advantages. Specifically, longer limbs were most advantageous when combined with larger toepads and more lamellae. Sound familiar? It should, that’s the same phenotypic shifts we’ve observed over and over in urban populations (including in this study).
- Urban lizards ran faster than forest lizards across all tracks, a trend that was driven by differences on the gradually inclined tracks regardless of substrate type. This surprised us and suggests that either the same phenotypes that improve performance on smooth anthropogenic substrates also improve performance on bark, or that urban lizards face selective pressures to be able to perform on both anthropogenic and natural surfaces. Regardless, these results combined with the phenotypic differences suggest that selective pressures to run quickly over-ground or on relatively flat surfaces are stronger than those to run quickly up more vertically inclined or smooth surfaces.
- Lastly, we asked if habitat use was associated with performance and found that urban lizards that performed best on the most challenging track (60 degree painted concrete) used smooth vertically inclined perches indiscriminately while lizards that performed poorly on this track discriminately used perches that were the least inclined and roughest. This suggests either that some lizards are constrained in their habitat use by performance abilities or that lizards become more adept at navigating anthropogenic surfaces with experience.
First up: A comparison of average speed on three of the tracks to highlight the reduction in velocity across the tracks.
Here’s a lizard running on the gradual incline wood bark track. This lizard is running at the average speed that lizards ran (regardless of urban or forest context of origin) on this track. This is the track on which lizards ran the fastest on average.
Next, a lizard runs at average velocity on the steeply inclined metal track. This is the track on which lizards ran second slowest on average. Notice the loss of traction when the lizard stops resulting in a slide.
And finally, a lizard runs at average velocity on the steeply inclined painted concrete track. This is the track on which lizards ran slowest on average. Notice how the lizard’s feet slip on the surface at a few points.
In this second set of videos, I want to show you some urban lizards really sprinting well on several tracks.
First up, urban lizards on the 37 degree inclined metal and painted concrete tracks:
Followed by an urban lizard on 60 degree inclined wood bark and painted concrete:
Run little lizard, run!