Ecomorphology describes how an animal’s physical traits (called morphology) match how it uses its habitat. It is assumed that ecomorphology results from performance-morphology relationships across different habitats. For instance, running speed (a measure of performance) could be affected by limb length (a measure of morphology), but this relationship varies based on habitat (e.g. type of surface on which the running occurs). We see this pattern in humans—a person moving through water benefits from having broad, paddle-like hands whereas this morphology might not be as important for someone moving over land.
Enter urbanization! When an area becomes urbanized, there is a shift in habitat and this probably alters morphology-performance relationships in animals. First, animals that commonly perch on native vegetation in natural areas may no longer have this option in highly urban areas. Instead, they might switch to using human-made substrates such as concrete walls. Second, animals may experience higher temperatures in urban areas because concrete reflects solar radiation and fewer trees mean less shade. This is known as the urban heat island effect and animals may experience morphological shifts to deal with this heat.
Lizards Across an Urbanization Gradient
In a recent study published in Urban Ecosystems, my colleagues and I tested whether western fence lizards that live in more-urban habitats have different morphological traits than those in more-natural habitats. Western fence lizards (scientific name: Sceloporus occidentalis) are habitat generalists and are wide-ranging, occurring from Washington all the way down to Baja California. We focused our work in Los Angeles County, California because Southern California is one of the most urbanized places in North America. We examined lizards at four sites that formed an urbanization gradient in terms of housing density, percent impervious surface cover (basically the amount of concrete in an area) and percent tree cover. The least urban site had zero houses, nearly 0% impervious surface, and moderate tree cover while the most urban site had medium-density housing, over 50% impervious surface, and less than 5% tree cover. We hypothesized that the extreme habitat changes that occur with urbanization would affect the morphology of the lizards living there.
Habitat Use and Morphology
At each site, we measured how western fence lizards used the habitat and then related their behaviors to morphological traits, such as limb length. We found that urban lizard populations used more human-made substrates (e.g. buildings and walls instead of trees), were found on a broader range of substrate types (wood, rock, ground, other materials), and tended to be more ground-dwelling than natural lizard populations. These differences in habitat use were strongly associated with limb and toe lengths—urban lizards had shorter limbs and toes than natural lizards! Although we did not test whether shortened limbs and toes affect the performance of lizards at urban sites, our results are consistent with previous research which showed that lizard populations found in rocky habitats (like the concrete urban environment) tend to have shorter limbs than lizard populations found in woodland habitats.
Temperature and Scale Counts
Lizards at the most urban site also experienced warmer temperatures throughout the year, confirming that there is likely an urban heat island effect. How can these lizards deal with the heat? They might respond behaviorally by seeking cool, shady spots. Additionally, they could respond morphologically by changing a physical trait that affects evaporative water loss (the process that happens when you sweat). Lizards are in the clade Lepidosauria along with the tuatara and snakes (snakes are actually a group of lizards making the term “lizard” incorrect, but that discussion is for another day). Lepidosauria means “scaled lizard” and lizards indeed have hard keratinized scales covering their skin. Lizards’ skin is exposed between the scales and the more skin exposed, the more evaporative water loss occurs. Lizards with many small scales have more skin exposed than lizards with fewer large scales. Therefore we might expect the urban heat island effect to influence the number of scales on an urban lizard. This is exactly what we found! Urban lizards had fewer scales than lizards living in more natural areas, consistent with living in a warmer habitat (because fewer large scales will reduce skin exposed and evaporative water loss).
What does it all mean?
In sum, we found that western fence lizards exhibit population-level differences in morphology consistent with differences in habitat use. Most importantly, our results show that western fence lizards respond to urbanization differently than other lizard species, which often have longer limbs in urban habitats (see here, here, and here). This demonstrates that studies on various species are needed to form a larger picture of how animals adapt to urban life.
Article citation: Putman, B.J., Gasca, M., Blumstein, D.T., Pauly, G.P. (2019) Downsizing for downtown: limb lengths, toe lengths, and scale counts decrease with urbanization in western fence lizards (Sceloporus occidentalis). Urban Ecosystems. https://doi.org/10.1007/s11252-019-00889-z
Featured photo: “LA Skyline Mountains2” by Nserrano is licensed under CC BY-SA 3.0
website: www.breeputman.com
Twitter: @breeput
Leave a Reply