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Urban lizards show physical and genetic adaptations

As more and more of the world’s population moves to cities, the extent of the land occupied by urbanization increases. Currently, approximately 56 percent of people (4.4 billion) on Earth live in cities, and this is set to increase to over six billion by 2050. Urbanization brings radical changes to landscapes and ecosystems, and results in significant losses in biodiversity. However, some species can adapt to life in these areas, and may even thrive. 

A study of Puerto Rican crested anoles in three cities and three adjacent forested regions has now shown that these lizards have adapted morphologically to life in urban environments, and that their adaptations are reflected in their genomes.

“Urbanization impacts roughly two-thirds of the Earth and is expected to continue to intensify, so it’s important to understand how organisms might be adapting to changing environments,” said study first author Professor Kristin Winchell of New York University. “In many ways, cities provide us with natural laboratories for studying adaptive change, as we can compare urban populations with their non-urban counterparts to see how they respond to similar stressors and pressures over short periods of time.”

Crested anoles (Anolis cristatellus) are small-bodied lizards that are commonly found in both urban and forested areas in Puerto Rico. Prior studies by Winchell and her colleagues found that urban Anolis cristatellus have evolved certain traits that are beneficial to them in cities. They have larger toe pads with more specialized scales that allow them to cling to smooth surfaces, like walls and glass, and have longer limbs that help them sprint across open areas. 

In the current study, published in the journal PNAS, the researchers looked at 96 Anolis cristatellus lizards from three regions of Puerto Rico – San Juan, Arecibo, and Mayagüez – and compared lizards living in urban centers with those living in forests surrounding each city. They investigated whether the physical adaptations were similar in lizards from all three cities, and analyzed their genomes to establish genetic differences between urban-dwelling and forest-dwelling lizards in each region.

After confirming that the lizard populations in the three regions were genetically distinct from each other, they measured the toe pads and leg lengths of all 96 lizards that they collected. They found that the urban lizards did indeed show similar morphological changes in all three cities, including having significantly longer limbs and larger toe pads, with more specialized scales on the toes, than did the forest lizards. 

The researchers were able to conclude that these similarities could be attributed to similar environmental pressures on lizards that lived in the cities, and that the evolution of these traits helped urban lizards to survive and thrive. 

Furthermore, analysis of exome DNA (the regions of the genome that code for production of proteins) revealed that there was a genetic basis for the physical changes observed in the urban-living lizards. The researchers identified a set of 33 genes, found in three regions of the lizard genome, that were repeatedly associated with urbanization across populations, including genes related to immune function and metabolism.

“While we need further analysis of these genes to really know what this finding means, we do have evidence that urban lizards get injured more and have more parasites, so changes to immune function and wound healing would make sense. Similarly, urban anoles eat human food, so it is possible that they could be experiencing changes to their metabolism,” said Professor Winchell.

In additional analysis, the researchers found 93 genes in the urban lizards that are important for limb and skin development, offering a possible genetic explanation for the increases in their leg lengths and toe morphology. 

“The physical differences we see in the urban lizards appear to be mirrored at the genomic level,” said Professor Winchell. “If urban populations are evolving with parallel physical and genomic changes, we may even be able to predict how populations will respond to urbanization just by looking at genetic markers.”

Animals that adapt to living in urban environments typically experience a change in diet and probably an increased ambient temperature, since urban areas are often “heat islands” when compared to surrounding rural areas. Urban animals would thus be expected to show changes in behavior and metabolism, as well as increased tolerance of heat. 

“Understanding how animals adapt to urban environments can help us focus our conservation efforts on the species that need it the most, and even build urban environments in ways that maintain all species.”

Do the differences in urban lizards apply to people living in cities? Not necessarily, according to Professor Winchell, as humans aren’t at the whim of predators like lizards are. But humans are subject to some of the same urban factors, including pollution and higher temperatures, which seem to be contributing to adaptation in other species.

By Alison Bosman, Staff Writer

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