An urban anole lizard in Puerto Rico (Photo: Kristin Winchell)
For anole lizards living in Puerto Rican cities, the slickness of walls and windows poses a challenge to creatures that evolved on rocks and trees. Yet they are fast adapting to grip well on smooth surfaces. Living on Earth’s Helen Palmer visited anole researcher Kristin Winchell in her lab at the University of Massachusetts Boston to see how these little reptiles are coping with the built environment.
PALMER: Now we think of evolution as a matter of gradual changes over many generations, but that’s not always the case. Indeed, there are creatures that seem to be able to change aspects of their anatomy very speedily, and a case in point is some small lizards that live in Puerto Rico. An Associate Professor of Biology [Correction: PhD candidate] at UMass Boston is documenting these fast-evolving lizards, and I visited her lab on campus.
WINCHELL: My name is Kristin Winchell, and I work on urban ecology and evolution of Anolis lizards. They're a group of neo-tropical lizards. There is around 400 species of them throughout Central, South America and the Caribbean. Of those 400 or so species, there our about 100 that live in the Caribbean Islands, most of those are endemic -- only found on one island or chain of islands. I study a particular species from Puerto Rico, the Crested Anole, Anolis Cristatellus.
PALMER: What do they actually look like?
WINCHELL: So, I actually have some specimens here in this jar.
PALMER: Gosh, there are dozens in here.
WINCHELL: Yeah, these animals were part of a breeding experiment we did. So these are wild-caught individuals. They're all adult males from an urban and a natural population. You can see they're about four inches long from head to their hips. They also have a long tail after that that can be about as long as their body size.
PALMER: They're cute things. I'm interested, they look quite different. There's some with stripes down their backs and some with sort of like a pattern of blobs down their backs.
WINCHELL: Yes, so there's actually a very large amount of variation in patterning and coloration in Anolis and particularly in the species. But the males and females will have chevron patterns and modeled patterns on their backs and some just have a simple stripe, and they also have this dewlap which is a flap of skin on their throat and that dewlap is orange and yellow, and it is different in every species. And it functions to tell other animals, ‘hey I'm of your species’, or ‘hey I'm not of your species.’
PALMER: Which particular populations of Crested Anoles have you been studying and what have you been finding?
WINCHELL: So, I've actually works pretty extensively across the islands. My first studies were up in San Juan one kind of the northeast there.
PALMER: So, that's the capital.
WINCHELL: That's the capital. It's the largest city in Puerto Rico, and one of the largest cities in the Caribbean. It has a metropolitan population of over two million people. So, it's a really really substantially urbanized area. And actually there's very little natural forest there so it makes it a difficult place to do this type of study because you need to have both natural and urban habitats to compare animals. So, in San Juan, there's really only a couple of forests that you can look at. Most of my research the past couple of years has been in the city of Arecibo, which is up on the north coast there. It's a slightly smaller city, but it is an old city and it does have pretty sprawling urbanization there. I've also done some work in Mayaguez on the west coast and then Ponce on the south coast.
PALMER: So, you've done work in all these cities around there, and what have you exactly been looking at and what are you finding?
WINCHELL: I've been looking at how animal lizards are adapting to humans, and we set out with the idea that the Anole lizards that are persisting in these urban habitats, they are subject to natural selection pressures that exist in these habitats. And so, we looked it what we knew about their morphology and how that relates to the habitat, and how that's shifted in natural settings when habitats are modified, and came up with a set of hypotheses of what traits might be changing in this new setting and why. And so, we set out looking specifically at the toepads and the limbs, the limb length, in the urban areas with the hypothesis that in urban areas they tend to be perching on things like walls that are very broad. The habitats tend to be very open and they have to run across very large areas. And they're perching on things that are very different from a tree. If you think about a painted wall, it's much smoother and much harder than a tree trunk, and so we expected that there might be a shift in these functionally relevant traits.
PALMER: So, basically, if you’re running up a wall it's much more slippery than if say you're running up a tree.
WINCHELL: Exactly. Yeah, these lizards, they'll have a toepad that has these unique scales on them that we call lamellae. Basically, they look like folds or flaps of skin, but they are actually specialized scales that have these microscopic hair called cetae on them that interact at a microscopic level with the surface to give them a really strong grip to perfectly smooth services. So, they can climb up these smooth surfaces without the use of claws.
PALMER: Wow. That's clever. Do you have some pictures here?
WINCHELL: I do. So, I pulled up a picture here. This is the rear toepad of an Anolis that I measured, and when we take these photos we use just a high-resolution flatbed scanner. We place the animal on that and just scan it like you make a copy, and you can see here all of the little scales, and we can count the number of scales ... one, two, three, four, five ... all the way down until the toepad ends.
PALMER: It's very clear to see there how these scales sort of overlap each other.
WINCHELL: Yeah, so these lamellae, these scales function by increasing the surface area contact with the surface. And so each one of these little flaps because there's so many of them allow the toepad to kind of mold around the surface and really make as much contact as possible so those cetae can engage and increase its adherence.
PALMER: The cetae?? are the...
WINCHELL: The microscopic hairs. Yes.
PALMER: So you've been looking at these things and measuring these things. So, what have you found?
WINCHELL: So we found that in the urban habitats the animals actually do you have more of these scales on their toes and they'll have larger toe pads, and so this was in line with what we predicted because these things have to do with increased adherance. We believe we're seeing selection towards larger toe pads and more lamellae.
PALMER: So, how fast is this change happening?
WINCHELL: So, Anoles can reproduce year-round, and they can reproduce when they're about eight months old I'd say, based off of my studies. So you get really quick generation times, and this is a key factor in any rapid adaptation study because you want an animal that's producing offspring quickly. And so, some of the urban areas I've looked at are only 30, 40 years old, so it seems like not that long of a time, but if you think about that in lizard generations that can be a hundred or more lizard for generations.
PALMER: So you can't actually say exactly how fast this is happening. You can only say that it can't be slower than about 30 years.
WINCHELL: Yes, that's correct. The youngest populations we've studied have not been exposed to urbanization for more than 30, 40 years. So, it definitely is happening on that rapid of a time scale at least.
PALMER: And what's the mechanism for this happening, do you think?
WINCHELL: Well, so this is something that we've tried to address from a number of different perspectives. It makes sense that natural selection is shaping these phenotypes, these characteristics, but natural selection is inherently difficult to measure. What we do know is that there are these differences in urban areas with the animals having the larger toepads and longer limbs, and we do know that those have fitness consequences. We're working on some performance studies now in which we are looking at whether or not these phenotypes are associated with improved sprint performance on smooth urban substrates, and we're hypothesizing that these animals that have the larger toepads and longer limbs are going to be better able to perform in these types of habitats.
PALMER: So, basically, you're going to have Anolis races up the glass?
WINCHELL: [LAUGHS] Yes, actually we did have our own little Anolis Grand Prix during the summer where we raced animals up various tracks: metal, painted concrete, bark. We have over 1,000 videos of Anolis running in slow motion up racetracks.
PALMER: [LAUGHS] So, longer limbs, more scales on feet. Any other changes that you're seeing?
WINCHELL: Yeah, so we have seen as well shifts in their thermal tolerance. The urban animals - and this is very much a work in progress -- but the urban animals do seem to be able to tolerate higher temperatures. And so, that's really interesting, and we're also looking at the gene expression patterns behind that as well.
PALMER: So, why did you focus on this particular species?
WINCHELL: So, Anolis cristatellus is the most common species of Anole in Puerto Rico, and so it made sense to focus on the most abundant animal and they're also the one that is consistently found in urban areas. There's a couple of other species that you'll occasionally see in urban areas, but this is the one that is using the buildings, that's perching on houses, that's going inside and eating food scraps. This is the urban synanthrope, if you will. It's the one that is really thriving in the urban areas.
PALMER: So you've noticed differences in the urban species of this particular lizard. Is this common that there are changes in other urban spaces?
WINCHELL: So, for Anoles, we don't know yet, and this is a topic of future research I'm hoping to address. There are many Anoles that use urban habitats, and we're currently working on a project to try and understand why some species thrive in urban areas while others are merely tolerant and some stay out of it entirely. Outside of the world of Anoles, there are some people who have been looking at adoptive shifts in animals in urban areas and yeah people have found that there are shifts in thermal physiology in ants. There's some researchers in New York who are doing some really great work on the birds and the rats living in New York City, and they're finding some very interesting things there looking at the genomic aspect of urban tolerance. If you were to look at all animals in urban areas, you'd probably start to see some trends, especially at the genomic level of shifts in the ability to tolerate stress and tolerate heat and tolerate noise perhaps.
PALMER: Tolerate people.
WINCHELL: Tolerate people. Anything that you look at with an animal. With frogs they've found shifts behaviorally adjusting the pitch and cadence of their calls because when they sing regularly you can't hear them over the traffic noise and traffic embankments turn out to be pretty decent frog habitat. Behavioral adaptation shouldn't be discounted just because it's not genetically based. Behavioral adaptation is a really interesting part of the story because if the first animal that gets to an urban area can't tolerate heat, well, if it stays in the sun all day, it's definitely not going to pass on its genes. But you know maybe after a few generations, you'll get some animals that can tolerate hotter and hotter, and maybe they don't have to stay in the shadows so much. And so this behavioral adaptation of using a habitat differently or adjusting their communication can set the stage for future genetic change.
PALMER: Well, Kristin Winchell. Thank you very much for welcoming me into your lab and showing me all these interesting things.
WINCHELL: Thank you very much.
PALMER: You’ll find pictures and links at our website, LOE.org.
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