Anole Annals

Two days ago, Ambika Kamath posted an entry in which she observed that the green anoles in her study site in Gainesville are doing just fine, they’re just high up in the trees and harder to spot than the abundant browns. She concluded that, contrary to what many think, brown anoles are not threatening greens in Florida with extinction.

I’d like to add to Ambika’s conclusion by pointing out how browns and greens interact throughout the natural range. Both species evolved in Cuba. There members of the sagrei group coexist widely with carolinensis’s relatives. Where they co-occur, brown anoles are very abundant and are found on the ground and low in vegetation. Greens, primarily A. allisoni and A. porcatus are seemingly less abundant (population estimates are not available) and they occur on tree trunks on up into the canopy.

This mostly peaceful coexistence is repeated in other places the two species co-occur. In the Bahamas, it’s A. smaragdinus and A. sagrei, on Little Cayman, it’s A. maynardi and A. sagrei. In both cases, sagrei is apparently much more abundant, and the two species occupy different parts of the habitat.

Some time ago (possibly several million years, according to genetic data), green anoles colonized Florida from Cuba. In the absence of browns, the greens took the arboreal to increase their habitat use, a phenomenon termed “ecological release.” Then the browns arrived, thanks to us. They have moved into their ancestral niche and the ancient order has been restored. Greens have moved back up in the trees and, yes, their populations are probably now smaller, because some of the resources they were using are now taken by browns. But they’re not going extinct. Greens and browns stably coexist throughout their range. That’s what they’ll do in Florida, too, as long as all the trees aren’t cut down for shopping malls and parking lots.

Tell almost anyone in Florida that you’re doing research on brown anoles (Anolis sagrei), and they’ll express some distaste for your study organism. “I don’t like them,” they’ll say, “they’re invasive. Aren’t they driving the native green anoles extinct?” Everyone—literally everyone who has lived in Florida for a while—will tell you how their backyards used to be full of green anoles (Anolis carolinensis). Today, they report, these green anoles have disappeared and been replaced by the invading browns.

These backyard tales are supported by some scientific evidence for shrinking populations of green anoles . On spoil islands in Mosquito Lagoon, Dr. Todd Campbell documented precipitous declines in green anole densities following the experimental introduction of brown anoles [1]. In southwest Florida, Cassani et al. repeated surveys of reptile and amphibian abundance fifteen years apart, using identical methods in exactly the same locations [2].  They found a drop in green anole numbers and a sharp rise in brown anole numbers between 1995 and 2011. Based on their results, both Campbell and Cassani et al. suggest that the persistence of green anoles in Florida has been jeopardized by the invasion and spread of brown anoles.

But both Campbell and Cassani et al. acknowledge a second possible explanation for the apparent disappearance of the green anoles: the lizards may simply have shifted upwards, out of sight.* As Cassani et al. put it, “the hope remains that these lizards persist in the face of competition and predation from A. sagrei by shifting habitat use.” We already know that green anoles shift upwards at least a bit in the presence of brown anoles, and have evolved morphological features that likely help them survive at these higher perches [3]. Could green anoles have shifted so high as to be nearly invisible to us, from our vantage points near the ground?

When I started studying brown anoles in Gainesville, FL, in 2014, I was convinced that the green anoles were all gone. But as we spent many hours marking individual brown anoles and repeatedly surveying their habitat to re-spot them, we began to spot a few green anoles too. I guessed that these green anoles were the last few holdouts against the invaders, and that we were seeing the same individuals again and again. To prove this, all we needed to do was catch and individually mark these green anoles using permanent bead-tags, in exactly the same way that we were catching and marking the brown anoles. It didn’t seem like too much extra work, so once I realised that my 2015 fieldsite was also home to quite a few green anoles, we began catching and tagging them as well.

In two months of sampling,  we either caught or re-spotted green anoles a mere 52 times. In the same period and location, we caught or re-spotted brown anoles 4369 times, which certainly seems to suggest more brown anoles than green anoles in this site. But to compare the population sizes of brown and green anoles, you need to compare how often you see new, unmarked individuals relative to how often you see already-marked individuals for each species**. In the graphs below, I’ve plotted the total number of observations against the total number of marked individuals for both A. carolinensis and A. sagrei***, and then zoomed in to just the first 52 observations for both.

Zoomed in, you notice that the curves for the brown and green anoles look quite similar. If anything, we see more new individuals per observation for green anoles than for brown anoles. Neither of these curves has begun to plateau (i.e. we’re still seeing lots of new individuals), so we cannot quantify the difference in total population size of these two species. But these limited data suggest that this population of green anoles is not doing that badly.

But if the population is doing okay, then why weren’t we spotting green anoles all that often? The most logical explanation is that the green anoles have shifted up to very high perches, and only rarely descend to heights at which we can observe and catch them easily. Moving a bit beyond the numbers, we find another piece of evidence that supports the idea of a perch height shift—of the 40 green anoles we caught, only eight were males!

We know that male anoles usually perch higher than female anoles [4], that female anoles will often search for and feed on insects on the ground, and that females must descend to the ground to lay their eggs. Males, on the other hand, often move to higher perches to display, seem to feed more opportunistically than females, and are not necessarily compelled to return to the ground after they hatch. Though sex ratios can deviate quite a bit from 1:1 in natural populations of anoles [5], it seems unlikely that a population of green anoles could be comprised of one male for every four females. Taking the sex differences in perch height into account, it makes sense that for every female green anole we spotted, there’s a male green anole perching really high up whom we simply did not see.

None of this means that green anole densities aren’t declining due to the presence of brown anoles in some habitats. In particular, because brown anoles can perch as high as 4 m off the ground, there may be many places in which green anoles previously thrived but where there is simply no “up” for them to escape to once the brown anoles arrive. I suspect that many backyards are exactly such places, and that some reports of local declines in green anole population sizes may in fact be well-founded.

But it’s also certainly possible that, in habitats with sufficiently tall trees, brown anoles are not driving green anoles to extinction. Instead, brown anoles may simply have precipitated a substantial upward shift in the perch height of green anoles towards their ancestral trunk-crown niche. It’s therefore possible that green anoles are thriving, just out of our sight. If that’s the case, then brown anoles don’t deserve quite so much of our animosity after all!

References:

[1] CAMPBELL, T.S. 2000. Analyses of the effects of an exotic lizard (Anolis sagrei) on a native lizard (Anolis carolinensis) in Florida, using islands as experimental units. Unpublished Ph.D. Thesis. Knoxville, USA, University of Tennessee

[2] CASSANI, J.R., D.A. CROSHAW, J. BOZZO, B. BROOKS, E.M. EVERHAM, D.W. CEILLEY, AND D. HANSON. 2015. Herpetofaunal community change in multiple habitats after fifteen years in a southwest Florida preserve, USA. PLoS One 10(5): e0125845.

[3] STUART, Y.E., T.S. CAMPBELL. P.A. HOHENLOHE, R.G. REYNOLDS, L.J. REVELL, AND J.B. LOSOS. 2014. Rapid evolution of a native species following invasion by a congener. Science 346: 463-466.

[4] SCHOENER, T.W. 1968. The Anolis lizards of Bimini: resource partitioning in a complex fauna. Ecology 49: 704-726

[5] SCHOENER, T.W., AND A. SCHOENER. 1980. Densities, sex ratios, and population structure in four species of Bahamian Anolis lizards. Journal of Animal Ecology 49: 19-53.

*Cassani et al., in particular, trapped reptiles and amphibians in ground-level traps, and very likely missed many anoles. Campbell, however, did sample in arboreal habitats, and did not find this explanation compelling in the context of his study. Trees on the islands he sampled were relatively short (~6 m), “allowing the vertical habitat to be searched thoroughly with small binoculars and some healthy tree climbing.”

**The logic is this: once you’ve marked every individual in a population, you will only re-spot marked individuals and not see new individuals, and the size of your population  will be equal to the number of individuals you’ve marked. In reality, you’ll almost never mark every individual, but the rate at which you spot new individuals relative to the total number of individuals you observe (new and marked) can still be revealing. Say you have two populations, A and B. If population A is much smaller than population B, then you will reach the point of mostly re-spotting marked individuals and not seeing new individuals more quickly in population A than in population B.

We obviously could not catch every lizard, and we were better at catching brown anoles than green anoles, so don’t use these data for any serious estimates of population size. But, if anything, our relative inability to catch green anoles means that there are more green anoles in this site than we document.

***Sampling for A. sagrei began about a month before sampling for A. carolinensis, explaining the mismatch in numbers between graph and text.

Bonnie Kircher found this image inside the cover of Florida’s Fabulous Reptiles and Amphibians. Pay close attention to the last lines of the quotation and you will realize that this is a most well-placed anole photograph. 

Over on Daffodil’s Photo Blog, Karen Cusick has documented a knock-down, drag out fight between two brown anoles (which we have elsewhere suggested should be re-branded as “festive” anoles). Check out the dorsal and nuchal crests!

Game: find the snake (Elaphe sp) and the lizard (male Anolis sagrei)

Last April and May, I was in Florida… not for holiday but in order to discover the “lizard word” during a field session with Ambika Kamath. For her Ph.D. project in the Losos lab, we collected data on the spatial use of male and female Anolis sagrei to determine the reproductive system of this species. I was surprised when I realized how easy it is to observe these little guys directly in the field! I was really excited to see them mate, fight, display and eat: it was incredible! But the more impressive memory is when I saw a male A. sagrei being eaten by a baby rat snake. I wondered whether this was a really common sight… but I was told I was lucky that day!

The scene happened during a survey, when I saw this unmarked male displaying on a branch… but not on any tree: a tree occupied by a baby rat snake! It was thrilling to see the snake stalking the lizard in a sit-and-wait foraging strategy (as far as a snake can sit…). Even though five long minutes passed where the lizard had time to look around (photo 1), it still jumped on the very branch where the snake was waiting. A great occasion for the predator, who directly started to bite and wrap his body around the lizard whose helpless bites and dewlappings had no effect on the outcome of the fight (photo 2 & video).

After this, I wondered what the mechanisms of predatory detection and avoidance in lizards (including the tail loss ability) could be. Indeed, during predator-prey encounters, there is a transfer of information between them in the form of signals (e.g. auditory, chemical, visual) which may directly modify the outcome of the encounter. Anolis lizards possess a large and complex behavioral repertoire which consists mainly of visual signals (e.g. dewlapping, push-up, head-bobbing) that are used during social interaction, but also in encounters with a predator.

Leal and Rodriguez-Robles in 1997 showed that Anolis cristatellus may rely more on behavioral signals than on flight to avoid predation. It is indeed what I saw: the male lizard displayed to the predator… But I still wonder why this little guy suddenly jumped directly on the snake! Is it possible that he displayed to me and that in front of two potential “predators,” the lizard focused only on the first (or biggest) one that he saw? Apparently, this lizard was more afraid of me than of the snake… wrong choice, I was not going to eat him!

Then, the last day of my field work, I also saw a female cardinal with a big A. sagrei male in her beak… really strange… but I think that the “lizard world” has not finished to surprise me!

Leal, M. & Rodriguez-Robles, J. (1997). Signalling displays during predator-prey interactions in a Puerto Rican anole, Anolis cristatellus. Animal Behaviour 54(5), 1147–54. doi:10.1006/anbe.1997.0572

And you’ll never guess what happened next! Something that I’ve never experienced in all my years. But I don’t know how to paste a video from Twitter into WordPress, so you’ll have to go to casa martin’s Twitter page to find out.

People pay a lot of attention to the color of a anole’s dewlap, but it’s often forgotten that the perceived color of the dewlap is not just a function of the light that reflects off of it, but also the light that at least sometimes shines through it!

These are two views of the brown anole taken minutes apart from opposite sides of the tree (the lizard was in the same spot, the photographer (me) moved.

For more on this topic, see what Manuel Leal had to say a while back on Chipojolab.

Jill Davidson-Guillen of Boca Raton, Florida, had an unexpected guest when she went swimming in her backyard pool last week. She said that she routinely finds curly-tailed lizards in the pool, but this was the first knight anole.