Category: New Research Page 20 of 66

Stephen Jay Gould famously claimed that evolution is “utterly unpredictable and quite unrepeatable,” and we Anolis biologists have relished in proving that statement wrong. In his talk in Austin this week, Alejandro Gonzalez Voyer of UNAM (with coauthors Alvaro Dugo Cota and Carles Vilá) showed that anoles aren’t the only Caribbean herps to exhibit the independent, repeated evolution of ecomorphs across islands – Eleuthrodactylus frogs have joined the club!

Among the remarkably diverse Caribbean Eleuthrodactylus species, nine ecotypes exist, including terrestrial, leaf-litter, aquatic, riparian, bromelicolous, arboreal, fossorial, cavernicolous, and petricolous specialists. Gonzalez and his coauthors first determined that these ecotypes evolved repeatedly, and showed that their distribution resulted from both invasion across islands and intra-island speciation. They also found that eight of the nine ecotypes cluster in morphological space and exhibit significant convergence. (The ninth, the fossorial ecotype, is composed of a monophyletic clade from Hispaniola and so convergence could not be tested.)

In sum, it appears that Eleutrodactylus ecotypes are indeed ecomorphs, and that evolution may be utterly predictable and quite repeatable after all.

Evolution 2016: Genomic Insights into Anolis carolinensis Phylogeography

By Kristin Winchell

On June 19, 2016

In New Research

Anoles, in particular Anolis carolinensis, have long been considered an ideal group for studies investigating thermal physiology, reproductive endocrinology, and even regeneration. With the recent publication of the A. carolinensis genome (see AA posts on this here and here), the possibilities for new genomic studies in this new model species have significantly increased.

Joseph Manthey and co-authors used this new resource to clarify the phylogeographic relationships of A. carolinensis. Previous research on the phylogeography of A. carolinensis using both mitochondrial DNA and nuclear DNA showed that there were 5 clades. However, the relationships between these groups differed between the two approaches. Joseph looked at the genomes of 42 individuals from 26 localities across the native range to determine the true evolutionary relationship between regional groups and to shed light on the demographic histories of the groups. Manthey sequenced 500 loci using an anchored hybrid enrichment approach.

STRUCTURE analysis showed that the clusters had little admixture

Manthey et al. found that the genomic data predicted 5 genetic groups, in agreement with both the nuclear and mitochondrial analyses previously done. Their results also indicated that the 5 genetic clusters were distinct with little admixture. However, the relationships between groups did not agree with either the mitochondrial or nuclear trees, yet all nodes had extremely high support (93-100%)

Finally, Manthey commented on the likely timing of this diversification and associated demographic trends. Their results indicate that the initial split occurred during the late Miocene or early Pliocene and that the remaining diversification occurred during the Pleistocene. They also found that the most Southern population had a significant number of fixed genes while other populations did not. This suggests that this group was likely the oldest and most stable and supports an “out of Florida” hypothesis of diversification.

Evolution 2016: “Lightning” Strikes Three Times on Anoles in Afternoon Session: Character Displacement, Performance Trade-Offs and Opsin Evolution Matching Dewlap Color in Anoles

By mjohnson216

On June 19, 2016

In New Research

In this afternoon’s round of lightning talks, anoles were the focus of three fantastic (but short!) presentations on adaptation. It’s not easy to summarize a whole project in five minutes, but that’s just what these three speakers did, and each left me wanting to know more!

First, James Boyko, a Masters student working with Luke Mahler at the University of Toronto, described his work on morphological evolution in Lesser Antillean anoles. When similar species compete over a shared resource, there are two possible outcomes: extinction or divergence (i.e., character displacement). Lesser Antillean anoles are an excellent system in which to study the role of character displacement, as these islands all have either one medium-sized species, or one large and one small species. Further, the species on these islands represent two colonization events – one from the north, and one from the south. James first confirmed the classic pattern on body size evolution, finding that a three peak Ornstein-Uhlenbeck model (i.e., one that predicted large and small lizards on two-species islands, and medium lizards on single-species islands) best fit the observed data (consistent with Butler and King 2004). But when he analyzed 20 other ecologically-significant morphological traits, this three peak model did not predict trait evolution better than a model based on random chance, although the northern and southern clades significantly differed in these morphologies. In summary, to understand the evolution of Lesser Antillean anoles: body size matters, as evolution in body size is clearly an important factor to reduce inter-species competition, but lineage matters too, as body shape was predicted by ancestry.

Next came Ann Cespedes, a Ph.D. student with Simon Lailvaux at the University of New Orleans. Ann is studying functional trade-offs in green anoles (Anolis carolinensis), focusing on relationships between fitness and performance. Many studies have searched for these trade-offs in the past, and some have found them, but others haven’t. Why the discrepancies? Ann proposed that previous studies haven’t always considered sex differences in functional trade-offs, that measuring only two traits (one associated with fitness and one with performance) may not reveal real trade-offs, and that differences in individual quality are often ignored. To consider all of these factors, she measured a suite of performance and morphological traits in 60 male and 60 female green anoles. Illustrating the limitations of examining raw data on sprint speed and endurance, Ann found no suggestion of the predicted trade-off between these traits. But when using a composite measure of all performance measures (sprint speed, bite force, clinging ability, exertion, endurance, jumping ability, and climbing ability) as a control for individual quality, the trade-off between speed and endurance became clear. Males and females also differed in their speed-endurance trade-offs, as body size predicted performance in different traits between the sexes, and body shape predicted male but not female performance. So performance trade-offs do exist, but you have to know how to look for them!

To conclude the session, Alexander Stubbs, a graduate student in Jimmy McGuire’s lab at the University of California, Berkeley, described the differences between opsin gene expression in two Cayman Island anoles: Anolis sagrei (a species with a red dewlap that reflects long wavelength radiation) and Anolis conspersus (a species with a blue dewlap that reflects short wavelength radiation). Alexander proposed that these different dewlap colors might provide different selective pressures on opsins in the two species to allow better color discrimination and angular resolution. Using RNAseq to measure mRNA in the eyes of six males of each species collected at solar noon or at sunset, the results were exciting. As predicted, Anolis conspersus had higher expression of opsins that increase visual sensitivity to UV, blue, and green wavelengths, and Anolis sagrei had higher expression of opsins that increase long wavelength sensitivity. Alexander also found that gene expression different substantially between noon and sunset, and further, there was surprisingly little variation in opsin expression between lizards, in stark contrast to the wildly varying opsin expression observed in humans.

Evolution 2016: It’s Getting Cold in Here!

By Kristin Winchell

On June 19, 2016

In New Research

Tamara Fetters with her poster at Evolution 2016

Tamara Fetters, from the McGlothlin lab at Virginia Tech, reported on her ongoing work on thermal physiology in Anolis sagrei during the first poster session here at Evolution 2016 in Austin, Texas. Tamara looked at thermal tolerance and sprinting abilities at different temperatures and how that related to the latitude of the population. Specifically, she asked if lower temperatures regularly experienced by the Northern populations influence cold tolerance and performance at those temperatures. She expected that Anolis sagrei, native to Cuba and the Bahamas and introduced into the Southern U.S., would show signs of adaptation to its new, colder home in the more Northern mainland populations compared to the native range island populations in the South.

Tamara’s poster focused on two main experiments. In the first she calculated thermal tolerance to cold temperatures using a classic critical thermal minimum (CTmin) setup: with an ice bath she slowly lowered the body temperature of each animal until it was unable to right itself. This method approximates the minimum temperatures that the animals can handle in the wild. She found a clear trend showing a decrease in the minimum temperature tolerated as latitude increased. In short, Northern populations could handle the cold and Southern populations could not.

In the second experiment, Tamara acclimated the lizards to 6 temperatures ranging from 12-41 °C before running them up a track to calculate sprint speed. Tamara used an impressive 25-50 animals from each of 5 populations! She calculated sprint speed from the high-speed video she took using the program Kinovea. Tamara found that across all temperatures the most Southern population ran the slowest while the most Northern population ran the fastest, with the differences remaining fairly constant.

So what’s next for Tamara? She is planning on rearing animals in a common garden setup with some animals in hot temperatures with low variability between day and night (mimicking the native range, Southern habitats) and some animals in cool temperatures with high variability between day and night (as is experienced in the Northern habitats). She hopes that these studies will help her understand the genetic basis of this thermal tolerance and the extent of plasticity in thermal adaptation.

One last note – Tamara wanted to thank Anole Annals for helping her determine her study locations. She was able to determine which areas were likely to have Anolis sagrei and how far North they have spread because of Anole Annals posts (like this one) and comments.

Click to view a bigger version of Tamara’s poster

Predation, Food Abundance and Male-male Fighting Drives Natural Variation in Lizard Tail Autotomy

By Chi-Yun Kuo

On June 14, 2016

In New Research

If you have a truly outstanding memory or if you enjoy re-reading old AA articles, you might remember this post on how bolder lizards autotomized their tails more readily to compensate for risky behavior. As unlikely as it is, you might also remember me saying that this study solved one piece of the puzzle by explaining why the propensity of tail autotomy would vary within a population. Two years have since gone by, and I am glad to present to you, my fellow AA readers, another piece of the puzzle: how ecology might shape the variation in tail autotomy among populations.

A side-blotched lizard couple snuggling (or more accurately, avoiding human nuisance that was the researcher)

Which aspects of ecology should we be looking at? Fortunately, the rich literature in tail autotomy helped us identify three main players: predation, food abundance and male-male fighting. Among these three ecological factors, the relevance of predation is the most straightforward: lizards will benefit from autotomizing more readily if predation pressure is high. On the other hand, the importance of food abundance lies in the fact that lizards need resources to grow the tails back, and the rate of regeneration depends on food abundance. Therefore, high food abundance will allow for faster regeneration and likely favors higher propensities for tail autotomy. The inclusion of male-male fighting as a key factor stems from the common observation that the tail is a common target for attack when males engage in territorial combats. In fact, quite a few studies have reported tail autotomy as a consequence of male-male fightings. As fights between males are rarely life-threatening (i.e. autotomy-worthy), tail autotomy under those circumstances would be undesirable. Consequently, environments in which male–male combat is common should favor lower autotomy propensities, with other things being equal.

To test these hypotheses, we first built a theoretical model in which we simultaneously varied predation, food abundance and the degree of male-male fighting and examined the propensity for tail autotomy that conferred the highest survival. Results from the model supported our hypotheses: higher predation and higher food abundance favored higher autotomy propensities, whereas higher intensities of male-male fighting favored the opposite. We then took one further step: we collected data on these three factors from five populations of side-blotched lizards (Uta stansburiana) in the western U.S. and used our model to explain the variation in the propensity for tail autotomy among those populations. It turned out that our model did a pretty good job, and we are confident that the variation in tail autotomy at the population level represents the outcome of ecological adaptations to predation, food abundance and male-male competition.

Are you wondering which of the three factors played the most important role in determining the propensity for tail autotomy (hint: it was NOT predation!)? Are you interested in more details about how we actually constructed the model? If so, you might want to give our recent paper a read:

CHI-YUN KUO and DUNCAN J. IRSCHICK. (2016). Ecology drives natural variation in an extreme antipredator trait: a cost-benefit analysis integrating modelling and field data. Functional Ecology 30: 953-963. doi: 10.1111/1365-2435.12593

Age- and Sex-Specific Variation in Habitat Use by Brown Anoles

By David Delaney

On April 23, 2016

In New Research

Little guys like it narrow. Photo from Daffodil’s Photo Blog.

The influence of habitat use on ecological and evolutionary patterns in Anolis lizards is well documented. Despite extensive work on interspecific variation, how habitat use varies within a species is relatively understudied.

As part of my master’s work in Dan Warner’s lab, we caught and recorded the perch height, width, and substrate (i.e., ground vs. vegetation) of 717 brown anoles (A. sagrei) on a small island in the Halifax River, near Ormond Beach, Florida. The island consisted of two main habitat types (open-canopy and forest) with an intermediate between the two.

This Anole’s Signal is…Multimodal?

By Joshua Hall

On April 8, 2016

In New Research

Female Anolis sagrei

In recent months, there has been a lot of talk on the Auburn campus about multimodal signals. Diana Hews gave a phenomenal seminar to the Biology Department last November about complex signaling in Sceloporus lizards, and just last week Eileen Hebets told a similar story about signaling behavior in a group of invertebrates, amblypygids. The latter lecture prompted a momentary side conversation between a Warner Lab postdoc, Tim Mitchell, and me concerning the apparent lack of multimodality in Anolis signaling. Ironically, I just ran across a 2016 publication by Baeckens et al that forced me to eat crow, albeit only a tiny bit of crow.

Anoles, like most iguanians, have been labeled as “visual only” signalers and for good reason. Anoles lack the epidermal glands that secrete the typical chemicals used in lizard chemosensing. Rather, anoles are known widely as models for communication for their reliance on visual signals (which have been demonstrated to be quite complex despite being unimodal) and are also characterized by a low baseline rate of tongue-flicking, even when considered against the backdrop of other visually oriented iguanians. Additionally, previous experiments conducted with A. carolinensis found no significant evidence of chemosensory function in prey selection, assessment of opponents, or in mate choice (Jaslow & Pallera, 1990; Forster et al., 2005; Orrel & Jenssen, 2002). The question of whether or not anoles utilize chemical signals seems to be one answered; however, Baeckens et al have conducted a simple but convincing study that might demonstrate the converse.

How Different Do Two Different Colors Look? Ask an Anole

By Leo Fleishman

On April 2, 2016

In New Research

In trying to understand the evolution of visual signal diversity we are often faced with problem of asking which of two possible signals is most visible against a particular natural background. Alternatively we may want to know how easily different signals (for example signals of sympatric species) can be told apart.

When animals use colorful signals to communicate with conspecifics, selection will tend to favor colors that are highly visible against the natural background and colors that can easily and unambiguously be distinguished from signals utilized by other similar-looking species. Thus, in order to understand the evolutionary pressures on signal properties, we often want to know how distinct two different colors appear to an animal viewer. Perception of color by most animals differs from human perception because of differences in visual-system anatomy and physiology. How, then, can we quantify the difference in appearance of colors to an animal viewer?

A useful approach is to quantify the perceived difference between color stimuli as a distance in the animal’s “perceptual color space.” This refers to some measure of the difference in sensory stimulation elicited by different colors. At present most studies of visual ecology and communication use one of two methods to measure perceptual color space distance between pairs of colors. The first involves determining the relative stimulation of the different classes of cone photoreceptors by each color stimulus in units of relative photon capture, plotting colors in n-dimensional space (where n is the number of photoreceptor classes), and then measuring a simple Euclidian distance between the points. The second approach involves estimating the noise within each photoreceptor channel, determining the detection threshold – defined as the just-noticeable difference, or JND, between the two colors – determined by photoreceptor noise, and then estimating the distance between two colors in units of multiples of this threshold, or in units of JNDs. The photoreceptor noise method has been shown to be effective for estimating color discrimination thresholds, but the idea that differences between supra-threshold colors can be measured in multiples of the JND is more controversial. Each of these methods has some theoretical support, and each has advantages and disadvantages in terms of assumptions required and the information needed to do the calculations. However, empirical tests of which method works best are lacking, and there is not much agreement on the best way to measure perceived differences between colors that are well beyond the threshold for discrimination.

What we need here is a model system – and that usually means anoles! Anoles have an excellent, well-understood color vision system. Moreover a method has been developed that uses visual attention as an assay of signal visibility. Fleishman et al. (2016) used this method to quantify the visibility of different colors viewed against a gray background by Anolis sagrei. They then used these behavioral results to test the accuracy of the two different popular methods of measuring distance in visual color space. In these experiments anoles viewed a checkerboard of gray squares in their visual periphery (Fig. 1). One square was then abruptly shifted to some other color. The distance in color space from the initial gray to the newly appearing color was determined using the two different metrics described above. The probability of a gaze shift towards the novel colored stimulus was quantified, and colors throughout the anoline color space were tested. The probability of detection was then plotted against the different measures of perceptual color distance.

anole_annal_fig1

Figure 1. (a) Lizard sits on the perch in a small cage. The experiment waits until its gaze is directed straight outward towards the video camera (t1). At t2 a small colored stimulus square is presented in the middle of the checkerboard visual field. The camera records whether or not the lizard notices the newly introduced color-square and shifts its gaze towards it. See the video above. The video at the top of the page illustrates the appearance of the stimulus to a lizard and shows a positive response.

Which method best predicted the results? It turns out both methods gave excellent predictions of the behavioral data! In this case anoles are the peacemakers. Basically, the information available to the color vision system is how much difference there is in the photon capture by different cone classes exposed to different colors. These two different ways of quantifying this change turn out to give rather similar predictions, and both sets of predictions accurately predict detection probability. Thus the anoles give a firm go ahead for both methods of modeling distance in color space. The ability to easily model distances between colors in anoline perceptual color space should greatly facilitate studies of the evolution of dewlap color diversity because we can easily figure out which color is most visible under each set of natural habitat light conditions.

Now we just have to figure out why Anolis dewlaps come in so many different colors!

City Slickers: Performance and Substrate Properties in Urban Anoles

By Kristin Winchell

On March 29, 2016

In New Research

Anolis cristatellus on a smooth, vertical substrate in Puerto Rico (photo by K. Winchell)

In urban areas, the number of natural substrates (e.g. trees) is reduced. In their place are novel manmade substrates (e.g. walls, metal gates). These surfaces undoubtedly have different properties relevant to anole locomotion: they are smoother, harder, and (in the case of walls) much broader and flatter compared to natural surfaces in a forest. In urban areas lizards still use these substrates at high frequency, but do they do so effectively? Kolbe and co-authors began to dive into this complex topic in their recent publication, “City slickers: poor performance does not deter Anolis lizards from using artificial substrates in human-modified habitats” (Kolbe et al. 2015).

The relationship between habitat use, morphology, and performance for anoles has been extensively studied in natural environments (reviewed in Losos 2009). Urban environments add new dimensions to this area of research. Resource distribution and abundances differ drastically compared to natural areas. For example, the distribution of available perches and what they are made of in urban habitats is very different from a forest. Moreover, the properties of these resources differ drastically as well: urban substrates are smoother, broader, and have different thermal properties, to start. Understanding these differences in habitat use and how they influence performance and, ultimately, adaptive responses in anoles is the topic of ongoing collaborative research that I (K. Winchell) and the Kolbe lab have been conducting.

Lizard Populations Offer Fresh Look at Island Biogeography

By Joshua Hall

On February 22, 2016

In Introduced Anoles, New Research

Female Anolis sagrei, Palm Coast, Florida

Any observant individual has noticed and possibly even been astonished by the incredible densities that some insular anole populations (i.e. A. sagrei) can achieve. Islands necessarily create a unique combination of environmental factors, several of which have traditionally been suggested as reasons that insular species are capable of attaining such densities. Species richness tends to be quite low on islands and so the diversity of predators remains low and there are fewer other species with which to compete for resources. A lack of predation pressure and competition can allow a species to more broadly utilize a traditionally occupied niche or even evolve to fill new regions of adaptive space, further utilizing resources in ways that increase population growth. A newly published meta-analysis of lizard densities across the globe confirms some of what we already knew about island biogeography, but also challenges some traditional thinking on the subject.