Anole Annals

My foray into lizard studies (including  anoles) began when I joined Bob Cox’s (no relation!) laboratory at the University of Virginia around three years ago, after years of studying the evolution and physiology of snakes (e.g., Cox and Secor 2008; Cox and Secor 2010; Cox et al. 2012; Cox and Davis Rabosky 2013). One aspect of anoles that fascinates me is that they exhibit great diversity in sexual dimorphism (any difference in any trait between the sexes). Many species are highly dimorphic in traits like head shape, dewlap size (and color), and body size, while other species are sexually monomorphic in some or all of these traits. This evolutionary diversity in whether or not a trait differs between the sexes suggests that dimorphism can evolve relatively rapidly. These traits are likely encoded by genes on the shared autosomal genome and thus shared between the sexes, which should theoretically impede evolution of these traits. One solution to this problem is to link the expression of shared traits (such as dewlaps, head shape, and body) to sex steroids (androgens and estrogens). However, typically “female” steroids such as estrogens have important functions in males (e.g., Sanger et al. 2014), and typically “male” steroids such as testosterone have important functions in females (e.g., Ketterson et al. 2005). Thus, the evolution of sexual dimorphism requires not only linking the expression of dimorphic traits to testosterone in males, but also unlinking testosterone from the expression of these traits in females. Unfortunately, reflecting the recent post by Ambika Kamath pointing out the paucity of research on female anoles, we know relatively little about how testosterone impacts growth and development in female lizards. Our work begins to address this issue by studying how both male and female brown anoles respond to testosterone (Cox et al. 2015).

Our research was focused on brown anole lizards (Anolis sagrei), which have become commonly used in evolution and ecology research. This species was ideal for our studies because they are very sexually dimorphic, with males much larger than females (averaging up to 33% longer, and three times as massive), possessing much larger dewlaps, along with a suite of other morphological divergences (differently shaped heads, longer forelimbs, etc).  Importantly, previous work by Bob Cox and Ryan Calsbeek (and their collaborators) has shown that the expression of many of these traits is altered by testosterone, which naturally circulates at much higher levels in males (Cox et al. 2009a; Cox et al. 2009b). We followed up that previous research and used hormone manipulations to test how testosterone impacts the development of a suite of sexually dimorphic characters in both males and females. We were specifically interested in testing whether females could respond to testosterone and whether or not they responded to testosterone in the same way as males. If females respond to testosterone as males do, then this implies that the decoupling of dimorphic trait expression between the sexes is accomplished merely by circulating testosterone at lower levels in females.

We conducted this experiment on juveniles (offspring of brown anoles captured from Great Exuma in The Bahamas) that were 5-6 months of age at the beginning of the experiment. This is a point in development when sexual dimorphism is just starting to develop, but the sexes are still pretty similar. We used small Silastic implants (about 5 mm in length) that were filled with either crystalline testosterone or a control. These implants were then surgically implanted into the coelomic cavity of both male and female anoles.  Most of my limited surgery experience has been with snakes (mostly Burmese pythons as part of my Master’s work with Stephen M. Secor at the University of Alabama), so it was interesting to learn the techniques used for small (about 2 g) lizards. We simply opened a small abdominal incision, inserted the implant, and the closed the opening with skin glue. We then allowed the lizards to develop naturally in the brown anole colony at the University of Virginia for about two months. At the end of that period, we measured a suite of traits in both sexes, including size (mass and length), metabolic rate, fat storage (by weighing the visceral fat bodies), growth of various skeletal elements (via X-rays), and dewlap size and color.

We found that males and females brown anoles responded similarly to testosterone for most traits. Both sexes with testosterone grew faster than the control animals, and females given testosterone grew at similar rates to males with testosterone. Testosterone also increased metabolic rate, and a decreased the amount of fat stored in both sexes. Beyond growth and energetics, we also found that testosterone stimulated the size and darkened the color (decreased brightness and saturation) of the dewlap. Interestingly, we did not find an impact of testosterone treatment on head shape (jaw length and head width) in either sex, which is perhaps not surprising in the light of Thom Sanger’s (and colleagues) work highlighting the role of estrogens in skull development (Sanger et al. 2014).

Our work demonstrates that despite differing greatly in appearance from males, female brown anoles retain the physiological capacity to respond to T similarly to males, at least as juveniles. Thus, the evolution of sexual dimorphism in brown anoles can be explained simply by higher circulating testosterone in males. More broadly, our work suggests that sexual dimorphism can evolve simply by coupling trait expression to differences between the sexes in circulating testosterone, without unlinking trait expression from testosterone in females. Some of our recent and ongoing work continues to explore these themes by examining how testosterone impacts tissue-level transcriptomes and gene expression of growth-regulatory networks in brown anoles of both sexes, and how the gene expression response to testosterone might differ between species with different patterns of sexual dimorphism. Our ultimate goal with this research is to understand how the evolutionary modulation of interactions between developmental networks mediates the evolution of sexual dimorphism.

 

References

 

Cox, C. L., and A. R. Davis Rabosky. 2013. Spatial and temporal drivers of phenotypic diversity in polymorphic snakes. American Naturalist 182:E40-E57.

Cox, C. L., A. F. Hanninen, A. M. Reedy, and R. M. Cox. 2015. Females retain responsiveness to testosterone despite the evolution of androgen-mediated sexual dimorphism. Functional Ecology 29:758-767.

Cox, C. L., A. R. D. Rabosky, J. Reyes-Velasco, P. Ponce-Campos, E. N. Smith, O. Flores-Villela, and J. A. Campbell. 2012. Molecular systematics of the genus Sonora (Squamata: Colubridae) in central and western Mexico. Systematics and Biodiversity 10:93-108.

Cox, C. L., and S. M. Secor. 2008. Matched regulation of gastrointestinal performance in the Burmese python, Python molurus. Journal of Experimental Biology 211:1131-1140.

—. 2010. Integrated postprandial responses of the diamondback water snake, Nerodia rhombifer. Physiological and Biochemical Zoology 83:618-631.

Cox, R. M., D. S. Stenquist, and R. Calsbeek. 2009a. Testosterone, growth and the evolution of sexual size dimorphism. Journal of Evolutionary Biology 22:1585-1598.

Cox, R. M., D. S. Stenquist, J. P. Henningsen, and R. Calsbeek. 2009b. Manipulating testosterone to assess links between behavior, morphology, and performance in the brown anole Anolis sagrei. Physiological and Biochemical Zoology 82:686-698.

Ketterson, E. D., V. Nolan Jr., and M. Sandell. 2005. Testosterone in females: mediator of adaptive traits, constraint on sexual dimorphism, or both? The American Naturalist 166:S85-S98.

Sanger, T. J., S. M. Seav, M. Tokita, R. B. Langerhans, L. M. Ross, J. B. Losos, and A. Abzhanov. 2014. The oestrogen pathway underlies the evolution of exaggerated male cranial shapes in Anolis lizards. Proceedings of the Royal Society B 281:2014039, online early.

 

 

This year’s Evolution meeting just kicked off at the Casa Grande Hotel in the lovely coastal town of Guarujá, Brazil. There are some great anole talks lined up for this year’s conference. Kristin Winchell, a Ph.D. candidate in Liam Revell’s lab at the University of Massachusetts, Boston, will be giving a talk titled, “Urban Evolution: Natural selection and genetic basis of phenotypic shifts in urban Anolis cristatellus.”

The talks on Puerto Rican anoles don’t end there. Matthew McElroy, a Ph.D. candidate in Adam Leache’s lab at the University of Washington, will present a talk titled “Comparative phylogeography of co‐distributed Anolis lizards from Puerto Rico.”

Finally, Fernanda de Pinho Werneck, who is based out of the Instituto Nacional de Pesquisas da Amazonia in Manaus, Brazil will present a talk titled, “Cryptic lineages and diversification of an endemic anole lizard (Squamata, Dactyloidae) of the Cerrado hotspot.”

Stay tuned to Anole Annals for upcoming posts on all the anole talks! You can also follow the happenings at Evolution 2015 on Twitter using #Evol2015.

Heidi Fagerberg, a children’s book writer, is in the middle of writing a book featuring the green anole of St. Kitts. Photos below. Can anyone confirm that these are Anolis bimaculatus? More importantly, does anyone know about their color-changing abilities and proclivities? Under what conditions does color change occur?

Some months ago, I posted a quick analysis of a dataset from 2010 on the movement rates of green anoles (Anolis carolinensis) in the presence and absence of brown anoles (Anolis sagrei) on spoil islands in Mosquito Lagoon, FL. At Jonathan Losos’s urging, Yoel Stuart and I turned this blog post into a paper, which was recently published in Breviora.

The story has not changed since the blogpost (though the analyses are now slightly more sophisticated): male and female green anoles seem to respond differently in their movement behaviour to the presence of brown anoles. There are many possible reasons for these differences, discussed in the paper, that can be summarized as “variation in the motivation for movement between the sexes.” Do read the paper if you’re interested in further details, but be forewarned that we engage in quite a bit of speculation because of one simple fact: compared to what is known about movement rates of male anoles, we know much less about how movement rates in female anoles vary with microhabitat.* Similar disparities between what we know about males and females exist in other aspects of anole biology too.

Though the subject matter of our paper is rather niche (anole ecology pun intended), it contains one paragraph that I think is more broadly pertinent:

 Much more attention has been paid to the behavioral ecology of male anoles than to that of female anoles (Butler et al., 2007; Losos, 2009). Our results suggest that male and female anoles can differ in their behavioral responses to ecological pressures. Understanding the mechanisms leading to behavioral and ecological variation within a species will therefore depend upon documenting this variation in both males and females, a conclusion that is hardly surprising. It is disappointing that research on fundamental aspects of the biology of even organisms as well-studied as Anolis lizards remains largely focused on males

There are reasonable reasons for focussing research on males. Male anoles are indeed often easier to spot in the field, and are certainly easier to catch. And incorporating an effect of sex into our statisical models will require us to double our sample sizes. But in many species, observing and catching females isn’t so difficult as to excuse not studying them. For example, in the last month or so, my undergraduate collaborator Rachel Moon and field assistant Barbara Da Silva have measured the ecology and morphology of over 300 Anolis sagrei females—an enviable sample size in any circumstances.

Females have been ignored in all sorts of studies of all sorts of organisms. The absence of female subjects in biomedical trials, for instance, has far more serious consequences than the gaps in our knowledge of the biology of female anoles. Nonetheless, given that many of us are dedicating some portion of our lives to understanding these animals, making sure that we don’t ignore half of them seems like a worthwhile goal. 

*This gap exists for two reasons: some previous studies don’t sample females, others sample both sexes but don’t distinguish between them.

I spent most of last week battling a cold. One of those awful, end of the semester, got-a-taste-of-freedom-but-now-I’m-bedridden colds. If you’re anything like me, the first sign of illness sends you scrambling trying to recall what your mother said about coping with illness. I always seem to remember my mother explaining fevers to me.

I was shivering under a pile of blankets, influenza-ridden and miserable, suffering from the worst fever I experienced as a child. My mother described a war being waged inside me, self versus virus, and tried to convince me that this crummy feeling might actually be a good thing. Impossible! I spent that day wrapped up a little too tightly, flashing from hot to cold, cold to hot. The next morning, I was achy and exhausted—a war had taken place in there, after all—but miraculously, I felt better. I was amazed that my body’s response to the flu-y invader had contributed to its eradication!

“Fever” occurs when body temperature is elevated above the normal range, and we have long been intrigued by its function in staving off infection. In our own bodies, the brain can trigger the body to heat up when it detects infection. This temperature elevation seems to facilitate the immune system, and the warm temperatures make the internal environment less favorable for infectious agents such as bacteria or parasites attempting to live there (feverish to know more?). Endothermic creatures such as us, which produce and maintain body heat internally via metabolic activity, frequently battle infection with fever. But what about ectotherms?

Read More

One of our favorite topics here at Anole Annals is adaptive radiation. Don’t believe me? Just type adaptive radiation into the search bar on the right and see all the interesting posts that come up. And why shouldn’t we be interested in AR? After all, anoles are one the great examples of the phenomenon.

So, it seems relevant to notice that two new review papers just appeared on the topic, both of which mention anoles at least in passing. Tom Givnish, in a paper in New Phytologist stemming from a conference on plant radiations last year, provides the most convincing analysis to date about why the term “adaptive radiation” should be reserved for clades that have diversified to occupy a wide range of ecological niches, regardless of how fast they have done so and how many species the clade contains (title of paper: Adaptive radiation versus ‘radiation’ and ‘explosive diversification’: why conceptual distinctions are fundamental to understanding evolution).

Some pithy quotes encapsulate his points:

“Of the early writers on adaptive radiation (Osborn, 1902; Huxley, 1942; Lack, 1947; Simpson, 1953; Carlquist, 1965; Mayr, 1970; Stebbins, 1974), only Simpson included what we might term explosive speciation in his concept of the process.”

“Definitions of adaptive radiation that require accelerations of species diversification relative to sister groups will thus fail to identify Darwin’s finches and Brocchinia  as adaptive radiations; excluding such iconic examples of adaptive radiation makes such diversification-based definitions untenable.”

“Why should we care about this distinction? Nothing could be more pointless than a pedantic debate about definitions that goes nowhere. I would argue, however, that making a distinction between adaptive radiation and explosive diversification is fundamental to understanding evolution, and that failure to make such distinctions can blur such understanding and hinder progress.”

Givnish goes on to suggest that “we might consider re-defining adaptive radiation as ‘the rise of a diversity of ecological roles and associated adaptations within a lineage, accompanied by an unusually high level or rate of accumulation of morphological/physiological/behavioral disparity and ecological divergence compared with sister taxa or groups with similar body plans and life histories.’ Such a definition would retain traditional components of adaptive radiation, while suggesting a way forward that includes tempo, not in species diversification, but in the rate of accumulation of disparity.”

Meanwhile, Soulebeau et al., in a new “Forum Paper” in Organisms Diversity & Evolution, conduct a review of the use of the term “adaptive radiation” in the period of 2003-2012 (title: The hypothesis of adaptive radiation in evolutionary biology: hard facts about a hazy concept). Givnish would say that it is hard to draw conclusions from a meta-analysis that lumps different concepts all under one name, but the review does show some patterns in how research is trending. If nothing else, the number of papers purporting to study adaptive radiation doubled over the time period. Moreover, the paper makes an important point that the number of studies that investigate whether adaptive evolution has occurred in a putative adaptive radiation is very low.

The California Academy of Sciences in San Francisco has just opened a new exhibit, The Color of Life. You can read more about it at their website. Naturally, anoles played a prominent role in the exhibit, as the panel above attests. The small print says: “Anole lizards regularly advertise their ownership of their territories. They bob their heads and extend a colorful flap of skin called a dewlap, just in case another male is watching.”

Notably, the anole gets much more prominent billing than frogs, which are relegated to a panel further back in the exhibit, as the photo to the right illustrates. Perhaps it’s not surprising, then, that Cal Acad Herpetology Curator Dave Blackburn has just inked a deal to move to the University of Florida where, sources say, he will not only become the Curator of Herpetology at the UF Museum of Natural History, but will also switch his research program from frogs to the local anoles. A wise move, indeed, and we look forward to his further explanation in the Comments section.

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.