I’m partial to the creepy, crawly, scaly, slithering critters. I spend most of my time thinking about urban lizards, but recently started wondering about urban snakes. To my great disappointment, I wasn’t able to find many recent publications on urban snakes, and most of what I did find looked at urban ecology and biodiversity, rather than urban evolutionary questions. Snakes get a pretty bad rap and perhaps you don’t think of them as common urban animals, but I suspect they are a lot more common than we appreciate, and perhaps they play important roles in urban ecosystems.
Growing up in rural northern California, it wasn’t uncommon to find Western Rattlesnakes (Crotalus oreganus) around houses and shopping centers. And while conducting research on urban anoles in Puerto Rico, I’ve encountered the Puerto Rican Racer (Borikenophis portoricensis, featured photo above) slithering around urban habitats on multiple occasions, and even the Puerto Rican Boa (Chilabothrus inornatus) climbing up fences in rural neighborhoods. And all across the United States, the sight of a garter snake (Thamnophis sp.) racing across a garden or walkway is common. A quick search on Google Scholar will turn up a handful of papers on various species of snakes commonly encountered in urban habitats worldwide. For example, a study in northeastern Brazil by França & França over a 3 year period recorded 291 individuals from 28 species of snakes in urban habitats in “streets, homes, churches, university campus, streams, and even in local supermarkets”. The authors point out that although most of the snake species in urban habitats were non-venomous, some of the venomous species do attain high densities.
But is the presence of venomous snakes in cities cause for concern? In the Brazilian study, the authors emphasize that many snakes were found in peri-urban riparian habitat, although some species do make their way deeper into the urban matrix including some highly venomous species (perhaps to take advantage of rodent populations). A similar study in Windhoek, Namibia by Hauptfleisch et al. found 509 snakes of 17 species in urban areas over a 3 year period of reported snake incidents (e.g., requested removals). Included in their survey were some species that certainly strike fear into the hearts of many like the Puff adder (Bitis arietans) and the spitting cobra (Naja nigricincta) (pictures below). Interestingly, snake occurrences were greater in more affluent areas of Windhoek and in rainier months, suggesting irrigation practices of urban environments might attract some species. Importantly, even though 76% of the snakes they encountered were venomous, not a single snake bite was reported.
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Life in the city might have evolutionary consequences for snakes. For example, Gangloff et al. found that two syntopic species of gartersnake (Thamnophis elegans and T. radix) thrived in the urban habitats of Denver, Colorado, with little gene flow and signatures of genetic differentiation between urban populations. In another study, Bury & ZajĄc found that urban populations of the European grass snake (Natrix natrix) in Kraków, Poland experience loss of sexual size dimorphism. This loss of dimorphism was driven by smaller body sizes in females, perhaps because of resource scarcity or persecution of larger snakes.
The reduction in gene flow and the presence of novel selection pressures documented in these studies brings up the intriguing possibility that conditions might exist for urban snakes to become phenotypically differentiated, and perhaps even adapted to urban environments. For example, how common are the results of Ganfloff et al. — is gene flow commonly restricted among populations of urban snakes? If so, what are the barriers to their movement? Could signatures of genetic differentiation indicate local adaptation? What about phenotypic differentiation? Bury and ZajĄc found shifts in body size, but maybe head shape might be influenced by diet, or reproductive patterns altered by resource availability or thermal physiology by the urban heat island effect! And what about behavioral modifications? Is it possible that the persecution of snakes in urban habitats might select for less aggressive snakes? Given the potential for human-wildlife conflict in this group, understanding how snakes are using urban habitats and how they might be adapting to them could impact conservation and management.
We know relatively little about urban snakes and this group faces additional persecution compared to other urban animals. Nevertheless, there are many fascinating urban evolutionary studies just waiting to be done in urban snakes, and many already being done that I just haven’t heard about yet! Do you know of any cool urban snake research? Let us know in the comments so we can feature it in future posts!
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Featured Image at top: Puerto Rican Racer, Borikenophis portoricensis, image by K.M. Winchell
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