The Evolution Of Morphological Diversity In Tropidurine Lizards: the Influence Of Habitat

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Uracentron flaviceps (upper photo) and Microlophus thoracicus (lower photo), two tropidurine lizards adapted to rainforests and deserts, respectively.

I was lucky enough to spend some months working at the Museum of Comparative Zoology of Harvard as part of the Losos lab. There I learned a good deal about anoles and got to meet anole-loving people face to face. Even though this atmosphere tempted me to develop a project related to one of the greatest examples of adaptive radiation, I had other plans in mind involving some of their distant cousins: tropidurine lizards! The results of this study are already published (Toyama, 2017) and I will describe a bit of what I found.

Tropidurinae is a group of lizards whose representatives have diversified across South America. They come in different shapes, colors and sizes, as you would expect from any group of organisms spreading in a diverse territory in terms of habitats, climates and altitudes. Rainforests, deserts, mountains and dry forests are just some examples of the different ecosystems where you can find these lizards. Given this scenario, I wondered if the morphological diversity observed in this clade could be linked to the challenges imposed by the different habitats types found in the continent.

Inspired by similar studies that focused on other lizard radiations, I took measurements of functional morphological traits of several species of lizards coming from 10 out of the 12 genera comprising the Tropidurinae. These traits would allow me to look for a possible correspondence between morphology and habitat.

However, as I was not only interested in the link between morphology and habitat use, but also in the morphological diversity itself, I started looking at purely morphological information. The next figure shows the illustrative results of a Principal Component Analysis (PCA), which tries to separate the species as much as possible based on the morphological measurements. In the figure, we can observe how the dots of each color (representing species of the same genus) occupy a particular zone in the graph. This means that, in general, species of the same genus are, as expected, morphologically more similar between them than to species of other genera (exceptions aside, given the overlaps between some genera).

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Scatter plot showing the morphological space defined by PC1 and PC2. Each dot represents the average values for a species, and species are grouped in genera (colors). Abbreviations are shown for some traits as HL (head length), HW (head width), HH (head height), BW (body width), BH (body height), Dist (distance between limbs), Htoe (longest toe of the hind limb), and Ftoe (longest toe of the forelimb).

Going a bit farther in respect to morphological diversity, Continue reading The Evolution Of Morphological Diversity In Tropidurine Lizards: the Influence Of Habitat

Box Turtle Scavenges Green Anole!

My good friend Trace Hardin, a professional entomologist but also avid herper and snake breeder, just sent me these photos below. Here’s what he had to say about the encounter on Instagram:

hardinherpetologica: Interesting observation while walking through the woods. Found this #BoxTurtle eating a dead #GreenAnole. I’m assuming it was a scavenged find but the entire body was gone by the time I came upon the scene. #Neature

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Has anyone else observed box turtles (or any other chelonian [I guess now testudine?]) interacting with anoles?

What Drives Substrate Use Patterns in Semiaquatic Anoles?

Anolis oxylophus at La Selva Biological Station (left, photo by Christian Perez) and Anolis aquaticus at Las Cruces Biological Station (right, posed).

Anolis oxylophus at La Selva Biological Station (left, photo by Christian Perez) and Anolis aquaticus at Las Cruces Biological Station (right, posed).

Among anoles, West Indian ecomorphs are the best known microhabitat specialists, but they are not the only ones. Semiaquatic anoles, of which there are 11 described species, live exclusively near streams and will sometimes enter water to feed or to escape a threat. The Central American species Anolis aquaticus appears to be specialized for climbing on rocks, particularly relative to other Central American semiaquatic anoles (Muñoz et al. 2015). Recent posts on A. aquaticus have addressed sleep site fidelity, dewlaps and trait scaling, and underwater foraging.

During a field ecology course with the Organization for Tropical Studies last winter, I compared patterns of substrate use between A. aquaticus and another Central American semiaquatic anole, Anolis oxylophus. Unlike A. aquaticus, A. oxylophus perches predominantly on woody and leafy substrates (Table 1). I wondered what was driving the differences in substrate use between these two species that appear broadly similar in morphology and lifestyle. Some Caribbean anoles alter their behavior to use only a narrow subset of available substrates in their habitat, whereas others have a greater breadth of substrate use that more closely reflects habitat-wide availability (Irschick and Losos, 1999; Mattingly and Jayne, 2004; Johnson et al., 2006). To evaluate whether substrate use differences between A. aquaticus and A. oxylophus are driven by substrate availability, species-specific selectivity, or both, I simultaneously quantified lizard substrate use and substrate availability within their streamside habitats.

Continue reading What Drives Substrate Use Patterns in Semiaquatic Anoles?

Evolutionary Predictability: Can We Predict the Color of One Lizard Species by Looking at Repeated Patterns of Geographic Variation on Other Islands?

Thanks to the work of Roger Thorpe and colleagues, Lesser Antillean anoles are renowned as an example of adaptive geographic variation. On many islands in the Lesser Antilles, populations in wet areas, where vegetation is lush, are green in color, whereas those in more xeric areas tend to be a drab gray, often with markings on their back. This pattern is repeated on many different islands, the convergent geographic variation thus making a strong case for the adaptive basis of anole coloration.

See Pavitra Muralidhar’s previous post for more information on geographic variation in Lesser Antillean anoles.

In a new paper in PLoS One, Thorpe takes this work a step further, asking whether we can use the parallel patterns seen across Lesser Antillean islands to predict the coloration of an anole species on another island. The focal species is Anolis bonairensis, which occupies the extraordinarily dry island of Bonaire (see our previous posts on this species).

The prediction: A. bonairensis should be grayer and drabber than populations of anoles that occur at the driest sites on Lesser Antillean answers.

The answer: yes! Just as predicted, Anolis bonairensis is one drab lizard. Score one for evolutionary predictability!

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Anolis bonairensis is represented by the red circles. The x-axis goes from aridity on the left to the most mesic on the right. As you can see, A. bonairensis‘s color and patterning is well-predicted by variation in other species.

New Mainland Green Anole Recognized

Anolis biporcatus, one of the prettiest of anoles. Photo by Thomas Marent

Anolis biporcatus is, if I’m not mistaken, the largest mainland beta/Norops anoles, attaining a length of ca. 100 mm snout-vent. In addition, it has an enormous geographic distribution, ranging from southern Mexico to Ecuador. In a new paper in Salamandra, a team of New Mexican and Ecuadorian biologists headed by Janet Armstead have sliced off part of the species, raising the Ecuadorian/Colombian A. biporcatus parvauritus to species status. They make this decision based on a detailed analysis of morphology and molecular data. Their data also find deep genetic subdivisions within A. biporcatus in Costa Rica, suggesting that there may be more cryptic species awaiting recognition.

A key difference between the species is the color of the distal scales on the dewlap of males, white in biporcatus, black in parvauritus.

biporc male

Note, too, that like many mainland anoles, the males and females have very different dewlaps.

biporc females

Here’s the distribution of the two species:

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Factors Restricting Range Expansion for the Invasive Green Anole Anolis carolinensis on Okinawa Island, Japan

 

Photograph was taken in Hahashima, Ogasawara Islands, by Hideaki Mori.

Photograph was taken in Hahashima, Ogasawara Islands, by Hideaki Mori.

We would like to introduce our recent paper on the invasive green anole (Suzuki-Ohno et al. 2017). In Japan, the green anole Anolis carolinensis invaded the Ogasawara Islands in 1960’s and Okinawa Island in 1980’s. In Ogasawara Islands, A. carolinensis expanded its range  and had a significant negative impact on native species and the ecosystem. This becomes a big problem since Ogasawara Islands are designated as a natural heritage.

On Okinawa Island, A. carolinensis was first captured in 1989  and it did not expand its distribution until more than 25 years later, although its density is extremely high in the southern region.  In the northern region of Okinawa Island, Yambaru area, native forests are preserved so that it is important to avoid the invasive effects of A. carolinensis. Thus, It is important to determine whether A. carolinensis has the potential to expand its distribution on Okinawa Island.

Phylogenetic analysis shows that the invader A. carolinensis originated in the western part of the Gulf Coast and inland areas of the United States. Interestingly, all of the invaded A. carolinensis in Ogasawara, Okinawa and Hawaii originated from the Gulf Coast and inland areas of the United States.

ND2 phylogeny using Okinawan, Ogasawaran, and Hawaiian populations in addition to haplotypes used by Campbell- Staton et al. (2012) and Hayashi et al. (2009). The map was redrawn from Campbell-Staton et al. (2012)

ND2 phylogeny using Okinawan, Ogasawaran, and Hawaiian populations in addition to haplotypes used by Campbell- Staton et al. (2012) and Hayashi et al. (2009).The major branches with high posterior probabilities of the Bayesian inference method (>0.99) are indicated in bold. The map was redrawn from Campbell-Staton et al. (2012). Cited from Suzuki-Ohno et al. (2017). Figure 2 of Suzuki-Ohno et al. (2017) lacks bold lines in error.

We used a species distribution model (MaxEnt) based on the distribution of native populations in North America to identify ecologically suitable areas on Okinawa Island. The MaxEnt predictions indicate that most areas in Okinawa Island are suitable for A. carolinensis. Therefore, A. carolinensis may have the potential to expand its distribution in Okinawa Island.

MaxEnt prediction of suitable areas for A. carolinensis in Okinawa Island according to the presence data for North America. Lighter and darker areas indicate high or low suitability, respectively. Points indicate the presence distribution of A. carolinensis. (a) prediction using all parameters, (b) prediction omitting mean diurnal range and precipitation of warmest quarter

MaxEnt prediction of suitable areas for A. carolinensis in Okinawa Island according to the presence data for North America. Lighter and darker areas indicate high or low suitability, respectively. Points indicate the presence distribution of A. carolinensis. (a) prediction using all parameters, (b) prediction omitting mean diurnal range and precipitation of warmest quarter. Cited from Suzuki-Ohno et al. 2017.

The predictions indicate that habitat suitability is high in areas of high annual mean temperature and urbanized areas. The values of precipitation in summer in the northern region of Okinawa Island were higher compared with those of North America, which reduced the habitat suitability in Okinawa Island. Adaptation to low temperatures, an increase in the mean temperature through global warming, and an increase in open environments through land development will likely expand the distribution of A. carolinensis in Okinawa Island. We think that invasive anoles (A. calrolinensis and A. sageri) prefer open habitats.

Therefore, we suggest that A. carolinensis should be removed by using traps and/or chemicals. In addition, we must continue to be alert to the possibility that city planning that increases open environments may cause their range to expand.

These results were published as Suzuki-Ohno et al. (2017) Factors restricting the range expansion of the invasive green anole Anolis carolinensis on Okinawa Island, Japan. Ecology and Evolution 

Calcium Storage in Anoles

Enlarged endolymphatic glands in two A. lemurinus museum specimens

Enlarged endolymphatic glands in two A. lemurinus museum specimens

I’ve been looking through a lot of anole museum specimens lately, and I’ve noticed that many of them have pretty pronounced endolymphatic glands, which made me curious about their prevalence and function in anoles generally.

Endolymphatic glands serve as calcium reserves, and are present in many animals, including a number of reptile and amphibian clades. According to Etheridge (1959), these glands are present in anoles and a few of their close relatives (e.g. Polychrus), but not in any other Iguanians. But it looks like most of the research on their function (in reptiles) has focused on geckos. In geckos, the size of the glands has been shown to fluctuate in response to both stress and reproductive activity, supporting the idea that the stored calcium is used in egg production, both for the yolk and the shell (Brown et al. 1996, Lamb et al. 2017). However, in anoles and geckos, these glands are present in both males and females, so their function isn’t limited to providing calcium for eggs (Etheridge 1959, Bauer 1989, Lamb et al. 2017).

But I haven’t found much information on these glands in anoles. I personally haven’t noticed them in the wild, but so far I’ve found very pronounced glands in 13/66 museum specimens, and some of them are really striking (see photos)! So I’m curious to hear, how often do you other anole-ologists see these enlarged glands? Is there any other literature about their prevalence, seasonality, or function in anoles that I’ve overlooked? Seems like we might be lagging behind the gecko crowd on this topic!

Citations:

Bauer A (1989) Extracranial Endolymphatic Sacs in Eurydactylodes ( Reptilia : Gekkonidae), with Comments on Endolymphatic Function in Lizards. J Herpetol 23:172–175.

Brown SG, Jensen K, DeVerse HA (1996) The Relationship Between Calcium Gland Size, Fecunduty and Social Behavior in the Unisexual Gecks Lepidactyluse Lugubris and Hemidactylus Garnotii. Int J Comp Psychol. doi: 10.5811/westjem.2011.5.6700

Etheridge R (1959) The relationships of the anoles (Reptilia: Sauria: Iguanidae) an interpretation based on skeletal morphology.

Lamb AD, Watkins-colwell GJ, Moore JA, et al (2017) Endolymphatic Sac Use and Reproductive Activity in the Lesser Antilles Endemic Gecko Gonatodes antillensis (Gekkota: Sphaerodactylidae). Bull Peabody Museum Nat Hist 58:17–29.

 

Legendary Brazilian Anoles Rediscovered

Several anole species are known from a single remote locality or only a few individuals, sometimes collected long ago. Because sampling these species is hard, we have a limited understanding about their biology and evolution. In a recent paper, we report on the rediscovery of Anolis nasofrontalis and Anolis pseudotigrinus, two mainland species from the Brazilian Atlantic Forest that were not reported for more than 40 years. Based on DNA sequence data, we examine their placement in the Anolis tree of life and estimate divergence times from their closest relatives. Moreover, based on the morphological attributes of newly and previously collected specimens (some of which were overlooked due to misidentification), we provide much needed taxonomic re-descriptions.

Fig. 1. Coloration in life of Anolis nasofrontalis (A, B) and A. pseudotigrinus (C, D). In A, inset shows the black throat lining of A. nasofrontalis, an uncommon trait that may be indicative of close relationships with Andean anoles (such as A. williamsmittermeierorum). Photographed specimens are females.

Coloration in life of Anolis nasofrontalis (A, B) and A. pseudotigrinus (C, D). In A, inset shows the black throat lining of A. nasofrontalis. Photographed specimens are females.

This study starts with efforts by collaborator Dr. Miguel T. Rodrigues (Universidade de São Paulo) to investigate reptiles and amphibians that have been undetected for years – a gap that could indicate human-driven extinctions. On late 2014, Dr. Rodrigues and his students (including co-author Mauro Teixeira Jr.) launched an expedition to the mountains of Santa Teresa (state of Espírito Santo, Brazil), the type locality of both A. nasofrontalis and A. pseudotigrinus. After a few days (and nights) of search, the team spotted the first A. pseudotigrinus in decades. The adult female was found sleeping on a narrow branch, (probably) unaware of its significance for South American biogeography (so were we). No signs, however, of A. nasofrontalis.

Shortly after, PhD students Paulo R. Melo-Sampaio (Museu Nacional) and Leandro O. Drummond (Universidade Federal do Rio de Janeiro) decided to visit Santa Teresa, inspired by conversations with Dr. Rodrigues. At this point, Dr. Rodrigues, my supervisor Dr. Ana C. Carnaval (City University of New York), and I had agreed that a phylogenetic study of A. pseudotigrinus would fit my PhD research well. Then, on early 2016, we got an unexpected email from Paulo and Leandro, with the first picture ever taken of an A. nasofrontalis in life. Both legendary anoles were real!

Back to the lab, we generated DNA sequence data and performed phylogenetic analyses, with completely unexpected results. First, A. nasofrontalis and A. pseudotigrinus are not closely related to the other (confirmed) Atlantic Forest species (A. fuscoauratus, A. ortonii, and A. punctatus); instead, they are close relatives of a species from western Amazonia, the “odd anole” Anolis dissimilis. These three species were found to compose a clade with A. calimae from the western cordillera of the Colombian Andes, A. neblininus from a Guiana Shield tepui on the Brazil-Venezuela border, and two undescribed Andean species (Anolis sp. R and Anolis sp. W from Poe et al. 2015 Copeia). This clade falls outside of the five major clades previously recovered within the Dactyloa radiation of Anolis, which have been referred to as species series (aequatorialis, heterodermus, latifrons, punctatus, roquet). Based on these results, we define the neblininus species series of Anolis.

Fig. 2. Phylogenetic relationships and divergence times between species in the Dactyloa clade of Anolis inferred using BEAST. Asterisks denote posterior probabilities > 0.95.

Phylogenetic relationships and divergence times between species in the Dactyloa clade of Anolis inferred using BEAST. Asterisks denote posterior probabilities > 0.95.

The neblininus series is composed of narrowly-distributed species that occur in mid-elevation sites (or adjacent habitats in the case of A. dissimilis) separated by large geographic distances. This pattern suggests a complex biogeographic history involving former patches of suitable habitat between regions, followed by habitat retraction and extinction in the intervening areas. In the case of A. nasofrontalis and A. pseudotigrinus, for instance, past forest corridors may explain a close relationship with the western Amazonian A. dissimilis. Atlantic and Amazonian rainforests are presently separated by open savannas and shrublands, yet geochemical records suggest that former pulses of increased precipitation and wet forest expansion have favored intermittent connections between them. These connections may have also been favored by major landscape shifts as a result of Andean orogeny, such as the establishment of the Chapare buttress, a land bridge that connected the central Andes to the western edge of the Brazilian Shield during the Miocene.

Fig. 3. Geographic distribution of confirmed and purported members of the neblininus species series. The inset presents a schematic map of South America around 10-12 mya, when the ancestor of A. nasofrontalis and A. pseudotigrinus diverged from its sister, the western Amazonian A. dissimilis. The approximate locality of the Chapare buttress, a land bridge that connected the central Andes to the western edge of the Brazilian Shield, is indicated.

Geographic distribution of confirmed and purported members of the neblininus species series. The inset presents a schematic map of South America around 10-12 mya, when the ancestor of A. nasofrontalis and A. pseudotigrinus diverged from its sister, the western Amazonian A. dissimilis. The approximate locality of the Chapare buttress, a land bridge that connected the central Andes to the western edge of the Brazilian Shield, is indicated.

During our morphological examinations of A. nasofrontalis and A. pseudotigrinus, it became apparent that these two species are not very different from Caribbean twig anoles, with whom they share short limbs and cryptic coloration. We learned that these features are also present in other, distantly-related mainland anoles, such as A. euskalerriari, A. orcesi, A. proboscis, and A. tigrinus. Phylogenetic relationships support that a twig anole-like phenotype was acquired (or lost) independently within Dactyloa, perhaps as a result of adaptive convergence. Alternatively, this pattern may reflect the conservation of an ancestral phenotype. In the former case, an apparent association with South American mountains is intriguing.

Unfortunately, natural history data from A. nasofrontalis and A. pseudotigrinus are lacking. It is currently unclear whether they  exhibit the typical ecological and behavioral traits that characterize the Caribbean twig anole ecomorph, such as active foraging, slow movements, infrequent running or jumping, and preference for narrow perching surfaces.

Fig. 4. Anolis dissimilis, the 'odd anole'.

Anolis dissimilis, the ‘odd anole’.

It has become increasingly clear that broader sampling of genetic variation is key to advance studies of mainland anole taxonomy and evolution. This significant challenge also provides exciting opportunities for complementary sampling efforts, exchange of information, and new collaborations between research groups working in different South American countries.

To learn more:

Prates I, Melo-Sampaio PR, Drummond LO, Teixeira Jr M, Rodrigues MT, Carnaval AC. 2017. Biogeographic links between southern Atlantic Forest and western South America: rediscovery, re-description, and phylogenetic relationships of two rare montane anole lizards from Brazil. Molecular Phylogenetics and Evolution, available online 11 May 2017.

More On Blue-Eyed Anoles

Anolis etheridgei. Photo by Rick Stanley.

Anolis etheridgei. Photo by Rick Stanley.

Three-and-a-half years ago, I wrote a post on the phylogenetic distribution of blue eyes in anoles. They pop up all over anole phylogeny and in species with diverse habitats and geography. The post attracted 32 comments.

At the time, I asked if anyone had a photo of the blue-eyed Anolis etheridgei. Photographer par excellence Rick Stanley quick obliged, but I never got around to posting his photo, so here it is.

But the bigger question is: what about those blue eyes? Why hasn’t anyone studied the phenomenon? If you’ve got a good photo of a blue-eyed anole, send it here!

Noose Pole Poll

We anolologists (and herpetologists generally) are a devoted bunch, particularly when it comes to our field equipment. It is therefore very troubling to learn that an essential component of our field kit is being discontinued. Perhaps most chilling is the thought losing access to our beloved [1] [2Cabela’s Panfish Poles. A recent series of tweets between AA stalwart James Stroud and Cabela’s customer service revealed noose poles are currently out of stock and may not return:

We have experienced the disappearance and return [1] [2] [3] of these poles before and, despite our best efforts, have not found a good alternative. With this essential tool at risk, I am taking up the effort to convince Cabela’s it is worthwhile to continue producing panfish poles. I would like to present them with the economic argument that many herpetologists use, and will continue to buy, this product.  I created a Twitter poll below and will present the results to Cabela’s customer service in making our case. Please take a moment to share your thoughts using the poll and in the comments. Thanks!

Vanzolini’s Anole Video

I stumbled onto an old video from a past trip that might interest some of you.  Anolis vanzolinii, named after herpetology and samba master Paulo Vanzolini, is a poorly-known species from northern Ecuador.  While this video is not the most exciting–it is only a video of one crawling on a bed–it does demonstrate almost chameleon-like qualities in its movement.  On a trip where we caught quite a few Anolis proboscis, this species still stood out to me as the most interesting.  Hope to see them again sometime!

Identification Request for Panamanian Anole

 

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Last month I spent a week in Bocas del Toro for a marine invertebrate biology course. However, I made some obligatory terrestrial excursions in search of our favorite vertebrate, the anoles! The habitat surrounding the STRI facility was secondary forest, and anoles were most commonly seen at forest edges. On one tree I found two A. limifrons scurrying about. They both promptly flattened their bodies against the thin branches when they detected my presence. A few seconds later, I noticed that a slightly larger anole was staring right at me from several inches away. I haven’t been able to get a solid ID on this female yet, and I would appreciate any input!

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Sex Ratios and Sexual Selection in Anolis lizards

The adult sex ratio is an important characteristic of a population, influencing the number of available mates in an area, the strength of sexual selection, and the evolution of mating systems. In our new paper in the Journal of Zoology, Michele Johnson and I use anoles to look at variation in sex ratios within and across species within a clade.

Photo by Michele A. Johnson

Photo by Michele A. Johnson

This paper had its roots when Jonathan Losos put me in touch with Michele in my first semester of grad school. Michele had compiled a massive database of detailed behavioral observations for Anolis populations and species across the Greater Antilles during her PhD on territoriality and habitat use (see Johnson et al. 2010 for more details!). While still trying to familiarize myself with the data set, I came across papers by Bob Trivers on sexual selection in anoles and his publication on the name-sake Trivers-Willard hypothesis; the combination of these topics made me curious about sex ratios and their role in sexual selection. I decided to quickly calculate the sex ratios of our localities, and given their distribution, realized that we should definitely look into this more.

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Sex ratios are generally very hard to measure in the field. You need to be certain that you haven’t had any biased sampling, or in other words, that you’ve made a fair attempt at censusing the population. This is quite difficult during short sampling periods! However, Michele conducted extended behavioral observations, and carefully tagged and monitored every individual in large habitat areas for ~3 weeks in each locality. This meant that we could be fairly confident that she had captured every individual in the population during her sampling periods, and her total counts of male and females in the population would be accurate. Even more, she had these adult sex ratios for 14 species, with some of those species being sampled at multiple localities. Given these data, we could actually both look at sex ratios across the Anolis clade, and within multiple anole species, for the first time.

We had two main questions: 1) were the sex ratios of these anole populations significantly skewed (i.e., were they very far off  from a 50:50 male-to-female ratio?) and 2) did the adult sex ratio of a population correlate with the strength of sexual selection in that population? For question 2, we used two measurements of sexual size dimorphism as a proxy for the strength of sexual selection. Sexual selection generally drives an increase in sexual size dimorphism (i.e., the difference between males and females in body size), but is also thought to be related to sex ratio skew (as the more skewed a population sex ratio, the more competition for mates or mating opportunities). We predicted that species with more skewed sex ratios would show an increase in sexual size dimorphism. Given that ecomorphs are an important component of evolution in anoles, and are commonly associated with varying levels of sexual size dimorphism, we also decided to test for a correlation between sex ratio skew and ecomorph type.

We found that sex ratios varied widely across and within anoles, ranging from a very female biased 0.32 in Anolis krugi to a male biased 0.61 in Anolis smaragdinus (sex ratios are expressed as the total number of adult males divided by the total number of both adult males and females in the population). Adult sex ratios also varied between different localities within a species (we had six species with multiple localities). We found two populations with significantly skewed sex ratios (Anolis krugi and Anolis valencienni) but based on Fisher’s test of combined probabilities, the sex ratios of anoles overall are not skewed away from 50:50.

I should note, however, that it is intrinsically extremely difficult to detect a skewed sex ratio in a natural population. We’re trying to measure deviations from a 50:50 sex ratio, and this requires surprisingly high population sizes since the binomial distribution has a broad center. For instance, to detect a true underlying sex ratio of 0.4 or 0.6 (away from our null of 0.5), we would need population sizes of >780 lizards to detect a significant skew 80% of the time. This is just an illustration, but the main point is that these population sizes might not exist for a given species – and so detecting significantly skewed sex ratios might not be possible at all. This is especially difficult when looking at small or endangered populations – there sex ratio skew might be a big problem, but impossible to demonstrate statistically. The general takeaway here is that sex ratio skew in a population can be biologically important, but not statistically significant.

We then used both the categorization of the anole species by sexual size dimorphism (low or high SSD) and the measured sexual size dimorphism of each population (calculated by average male SVL divided by average female SVL, minus 1). We used both of these estimates of SSD to test whether the sex ratio of a population correlated with the sexual size dimorphism of that population, as predicted by sexual selection theory. Turns out we were completely off – there was really no correlation between sex ratio skew and measured SSD, categorical SSD, or ecomorph (see figure 1, posted below,  for a visual of this lack of correlation!).

Figure 1 (from the paper) : Sex ratio versus sexual size dimorphism. Sex ratio is represented as the proportion of males among adults in the population, while sexual size dimorphism was calculated dividing the average SVL of the larger sex by the average SVL of the smaller sex, and subtracting 1 for each population. Each circle represents 1 of the 21 localities sampled in this study. The dashed line represents an equal sex ratio of 0.5. We found no relationship between sexual size dimorphism and sex ratio across the 21 localities (PGLS: adjusted R2 = −0.08, P = 0.86).

Figure 1 (from the paper) : Sex ratio versus sexual size dimorphism. Sex ratio is represented as the proportion of males among adults in the population, while sexual size dimorphism was calculated dividing the average SVL of the larger sex by the average SVL of the smaller sex, and subtracting 1 for each population. Each circle represents 1 of the 21 localities sampled in this study. The dashed line represents an equal sex ratio of 0.5. We found no relationship between sexual size dimorphism and sex ratio across the 21 localities (PGLS: adjusted R2 = −0.08, P = 0.86).

So what’s the general message here? Sexual size dimorphism does not correlate with adult sex ratios across anole species, and so the relationship between strength of sexual selection, sex ratio bias, and sexual size dimorphism may be more complicated than we initially assumed. However, anole sex ratios can range widely between species, and within populations. Given the variance within anole species, the adult sex ratio is probably a better description of a locality, or population, than an intrinsic quality of an entire species. We also think that the influence of various localized environmental factors may impact sex-specific mortality or dispersal, which in turn which cause differences between localities in adult sex ratio skew.

This is my first anole paper, and it’s really nice to see all the brainstorming and discussions put into print. It was also great to get to know and work with Michele, and learn more about her research and behavioral work in anoles (we even got to meet in person at the Evolution conference last year!). This paper was also my first small step into the world of sex ratio and sex determination theory which now forms a large part of my PhD work, so I’m very grateful for the introduction to the subject. Anyway, feel free to email us with any questions and we hope you enjoy the paper!

Paper here: Sexual selection and sex ratios in Anolis lizards