Anole lizards are rapidly becoming a model for urban adaptation. Anolis cristatellus, or the Puerto Rican crested anole, is a lizard species that has a great affinity for city-living. Previous work has demonstrated shifts in toepad morphology with consequences for locomotion: urban lizards have larger toepads with more specialized sub-digital scales specialized for clinging to smooth surfaces (known as lamellae) compared to their forest counterparts. Presumably, these larger toepads allow city lizards to cling to smoother surfaces like glass, metal, and painted concrete where the claw is less effective. (Check out a related study on urban claw morphology: The Finer Points of Urban Adaptation.)
When Travis Hagey heard about this, he wondered exactly how urban toepads were changing — Did they simply scale up the same general shape? Did they get longer or wider, perhaps? Were more lamellae fitting onto those toepads because they were smaller or did larger toepads always have more lamellae? Travis had an idea of how to answer this question: using geometric morphometrics. We (Bailey Howell & Kristin Winchell) teamed up with Travis and set off to test this novel application of geometric morphometrics to better understand this important adaptive morphological trait.
Our work focused on understanding how toepad shape changed between city and forest lizards. Shape is complex and sometimes difficult to measure. To measure it for our purposes we developed a novel method using geometric morphometrics. Leveraging a multi-year dataset of over 1000 images from urban and forest populations of lizards across Puerto Rico collected by Kristin, Bailey and Travis developed a landmarking protocol to capture the shape and size of lizard toes, toepads, and lamellae. Bailey carefully placed landmarks at specific locations around the perimeter of the toe and toepad, and marked the edges of several lamellae standardized by distance from the end of the toe.
Using this method, we were able to determine that city lizards have longer and wider toepads, but there is a more dramatic increase in toepad length than width in urban habitats. In other words, urban toepads are getting bigger by getting longer, not wider! This increase in size is accompanied by a larger proportion of the toe covered in adhesive pad, and an increase in the number of specialized lamellae scales as well as increased space between lamellae. In other words, urban toepads do not have more tightly packed lamellae, but rather have both larger toepads and more / larger lamellae as well.
This finding leads us to more questions, such as how does shape impact function? Longer toepads of city lizards could arise as a consequence of functional trade-offs between sprinting and clinging ability, which are important for navigating the novel structural environment of buildings, glass, and pavement in urban habitats. Increased toepad length could also be due to selection on increased urban limb length if there is functional or developmental integration. Longer toepads could optimize functional demands of city lizards by facilitating faster sprinting between perches and improving clinging ability.
Our study is the first attempt, to our knowledge, to study toepad shape (and not simply size and number of lamellae) with the application of geometric morphometrics. Toepads are important adaptive traits in Anolis, and shape may be an important and understudied aspect. Future studies could help us better understand how shape influences function and adaptation in novel habitats.
Read the study:
Featured Image: Anolis cristatellus (all rights reserved, K. Winchell)
This post was co-authored by:
Leave a Reply