Category: New Research Page 33 of 67

A figure from Eric Mueller’s poster showing the conserved pathway of how growth hormone may affect body size.

Anyone familiar with Anolis lizards is aware of the dramatic variation in body size. Think dwarf twig anole and crown giant. Although the ecological and evolutionary processes that can lead to such variation in body size have been studied, it is still unknown what physiological mechanism explains the variation we see today. Eric Mueller, a graduate student at Southern Illinois University – Edwardsville, presented a poster asking just that question. Specifically, do differences in circulating levels of plasma growth hormone regulate evolutionary changes in body size among anole species of differing size and morphology?

Anolis carolinensis (L) and A. equestris (R) have dramatic differences in body size but not in growth hormone levels. (photo 1, 2) Species not to scale.

Growth hormone (GH) is secreted by the pituitary gland and has many functions in the body, including promoting muscle and bone growth and increasing protein synthesis (among many, many other things!). It seems a logical candidate mechanism to investigate when it comes to explaining variation in body size. Mueller looked at GH levels in three anoles of varying size:  A. equestris, A. carolinensis, and A. sagrei. GH was higher in A. equestris and A. carolinensis than A. sagrei, supporting his hypothesis. However, there was no difference in GH levels between A. equestris and A. carolinensis despite dramatic differences in adult body size. Looking within species, GH levels were positively correlated with SVL only in A. equestris, and not the other two species.

Although differences in circulating GH may explain some size differences among anole species, as in other studies of anole hormones, things don’t seem to be simple. Mueller hypothesized that other aspects of the GH pathway may be more important. For example, GH receptors, Insulin-like Growth Factor (IGF) levels, and IGF-binding proteins should be examined for a full picture. The GH-IGF axis also interacts with other hormone pathways, such as testosterone, making this a very complex issue. Since endocrine systems are so multi-faceted, and multiple components have the possibility to evolve independently, there is lots of potential for future research that seeks to explain species differences in body size.

It was a real pleasure to see Dr. Ray Huey give a presentation that was inspired by research he and his collaborators began in the 1970s on seasonality of reproduction and behavioral thermoregulation in Puerto Rican Anolis cristatellus. Almost 40 years after the publication of that work, Huey and many of the same colleagues (and some new ones) returned to the same areas in Puerto Rico to examine how very fine-scale variation in thermal environment (a few meters!) might lead to variation in reproduction. The investigators (Otero, Huey, and Gorman) studied how reproduction differed between open areas (where lizards carefully thermoregulate) and forested areas (where lizards are thermoconformers) and found striking differences between them. Females in open habitats reproduced most of the year, whereas females in the neighboring forest decreased reproductive in a much more seasonal manner. Differences were largest from October – December, with females in forested habitats essentially shutting down reproduction during those months. This finding was true at two different sites.

These striking differences in reproductive phenology are similar in magnitude to differences seen along elevational gradients, but the difference here is the scale. The females that Huey compared were literally only a few meters away from each other. One important take-home message from these data is that reproduction can vary at the microgeographic scale just as it can at larger geographic scales. While the latter type of study is now common, the former isn’t. Future work should consider how small-scale variation in microhabitat use might influence reproduction so that we can better understand how general this phenomenon is.

One final point that Huey made was how collaborations can not only be an integral part of research, but also a source of personal reward as those collaborations continue over time and result in great friendships. He encouraged young investigators to keep this in mind as they progress through their academic careers.

Editor’s note: this research project has been the subject of previous posts [1,2].

Anoles display a staggering amount of phenotypic diversity, even in their genital morphology. Traditionally research has focused on characterizing the diversity and function of male genitals, or hemipenes, but females also possess paired genitals, or hemiclitorises, and yet almost nothing is known about them. In fact, female genital morphology is poorly understood across all reptiles. To date, we know that in some species hemiclitorises appear as miniaturized versions of hemipenes, whereas in other species they are unique structures. Further, the timing of sexual differentiation of genital structures appear to differ among lizard clades. Clearly, we need a broader understanding of the form, function, and evolution of female genitalia in reptiles.

In a fascinating poster, Casey Gilman, a graduate student at the University of Massachusetts, Amherst, presented her work on the development and morphology of hemiclitorises in the bark anole, Anolis distichus. Here’s the abstract:

Genitalia are extraordinarily diverse and show remarkably rapid evolution, relative to other morphological traits, across a wide range of animal taxa. Male and female genitalia in many animal groups begin as the same embryonic structures and later go through hormone-mediated differentiation. Surprisingly, little is known about the genetic mechanics of these processes. Even less is known about external genitalia differentiation in reptiles. Unlike other amniote groups, lizards and snakes possess a set of paired reproductive intromittent organs, called hemipenes. In a number of lizard species, females retain miniaturized versions of the male genitalia, called hemiclitorises. In these species, hemiclitorises can be used for taxonomic purposes, as they retain many morphological characteristics of the male genitalia, which are often species-specific. In lizards, the external genitalia of both sexes grow at the same rate until approximately halfway through embryonic development. Following this period, the hemipenes of the males continue to grow while the hemiclitorises of the females regress until they are about half the length of their male counterparts. We investigated the development of male and female external genitalia in Anolis distichus to determine the timing and patterning of growth and regression of these structures using histology, immunohistochemistry and whole mount in situ hybridization.

Green anole eating a dronefly. Photo from Wikipedia.

The tremendous diversity in Anolis lizards is one of the major draws for researchers to work on this system. There are nearly 400 species of anoles and their distribution spans much of the New World. Most of Anolis’ distribution spans environments with very low seaonsality. One exception is Anolis carolinensis, whose range spans much of continental North America, and encompasses highly seasonal environments. Further, unlike most reptiles, A. carolinensis does not hibernate during the winter. Rather, lizards remain active during the cold North American winter months.

Today Shane Campbell-Staton, a graduate student at Harvard University, presented some of his thesis work examining how A. carolinensis adapts to the thermal environment, and how local adaptation influences patterns of gene flow. The work he presented was conducted in collaboration with Scott Edwards and Jonathan Losos at Harvard University and Zachary Cheviron and Anna Bare from the University of Illinois-Urbana Champaign.

Shane first asked whether differences in the thermal environment limit gene flow among populations of A. carolinensis. To answer this question, he examined variation in over 2000 loci for 131 individuals of A. carolinensis and its ancestor, A. porcatus, from Cuba. He leveraged the Anolis genome with double digest RADseq to discover these SNPs and used multiple matrix regression to assess the correlation between genetic distance among populations and geographic and climatic distance. He discovered a significant signal of isolation by temperature, but not isolation by geographic distance or isolation by precipation. This means that populations are likely structured by thermal habitat, and that differences in temperature among localities limit gene flow in A. carolinensis.

Next Shane asked whether there was a signal of local adaptation in physiological tolerance to the thermal environment. He measured heat tolerance (CTmax) and cold tolerance (CTmin) in nearly 200 individuals of Anolis carolinensis. He found a significant positive correlation between temperature seasonality and thermal tolerance (i.e., the difference between CTmin and CTmax), but that most (though not all) of this pattern was driven by variation in cold tolerance across habitats.

Finally, Shane wanted to understand the mechanism that limits cold tolerance for terrestrial ectotherms. Specifically, he wanted to test whether oxygen limitation plays a role in determining how cold tolerant lizards are. The oxygen limitation hypothesis suggests that the ability to transport and utilize oxygen is limited at cold temperatures, and that lizards lose their mobility at low temperatures because they can no longer effectively transport oxygen to their muscles. Under this scenario, lizards that are more tolerant cold should be more efficient at transporting oxygen at cold temperatures than less cold tolerant individuals. To test this hypothesis, he examined CTmin in lizards from different thermal extremes of the species range and found that lizards from more cold-tolerant populations (i.e.,: higher latitude) utilized less oxygen at colder temperatures. His results support the oxygen-limitation hypothesis, and suggest that lizards can achieve a greater tolerance to cold, at least in part, by becoming more efficient at transporting oxygen, thereby reducing their demand for oxygen at lower temperatures.

The six species examined by Robinson and colleagues.

Reptiles differ vastly in how they communicate. Some species are predominantly visually-oriented, whereas other species rely almost exclusively on chemical signals for communication. Despite such marked differences in communication modalities, there is surprisingly little known about how communication modalities translate into differences in neuranatomy among species. Chris Robinson, an undergraduate working with Dr. Michele Johnson at Trinity University, presented a study examining the relationship between sense perception and neural density in six species of lizards.

Chris predicted that visually oriented lizards should have larger and more densely packed neurons in two visual centers – the lateral geniculate nucleus (LGN) and the optic tectum (OT) – whereas lizards that employ chemical modalities should have a similar pattern in the nucleus sphericus of the amygdala (NS). He included three iguanid species in this study, the green anole Anolis carolinensis, the curly tail Leiocephalus carinatus, the Texas spiny lizard Sceloporus olivaceous, as well as the whiptail Aspidoscelis gularis, the skink Scincella lateralis, and the Mediterranean house gecko Hemidactylus turcicus. To determine which sensory modalities best characterized each species he performed focal behavioral observations. During these observations, he quantified the number of chemosensory behaviors (rubbing the cloaca on a substrate, licking the air or substrate) and visual behaviors (head bobbing, dewlapping, and tail curling). Chris amassed over 120 hours of behavioral observations, and 10-33 hours per species, which is no small feat.

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Results of mating trials in Ng et al.’s study. Top two rows are within-population crosses; bottom two are between crosses from different populations that differed in dewlap color. Note that in the top, individuals look like their fathers, whether at the bottom, dewlap colors are intermediate between that of the two populations.

Everybody loves a pretty dewlap, and recent years have seen a lot of interest in studying the factors that determine dewlap color, as well as the role dewlap color may play in species recognition, sexual selection and other processes. Many have suggested that the dewlap is a focus of sexual selection; some have even opined that it is an honest signal of something, maybe good genetic quality, maybe the ability to procure lots of color-inducing dietary items. Unfortunately, we know almost nothing about the genetic basis of dewlap color, nor about the effect of environmental variation.

Anolis distichus exhibits more variation in dewlap color and pattern than any other anole, and thus is the perfect choice for such a study. Julienne Ng just completed her doctoral research at U. Rochester on this species, documenting that variation in dewlap color correlates with environment among populations. Now she and colleagues report on laboratory studies to assess the extent to which variation is determined by genes vs. diet.

Why diet? Because reds and oranges are likely determined by carotenoids, which vertebrates cannot synthesize. Thus, it is plausible that the amount of carotenoids ingested by a lizard may correlate with its color. This hypothesis has only been tested once before, in a study on A. sagrei by Steffen, who failed to find evidence for a diet effect on the red-orange dewlap of this species.

This study had two components. First, to study genetics, lizards from two populations–one with an orange dewlap, the other with a plain whitish dewlap–were crossed in the laboratory. Second, lizards were fed lots of carotenoids.

The results: strong evidence for a genetic basis for variation in dewlap color. Purebred individuals looked like their fathers (top two rows in figure above), but crosses were intermediate in color (bottom two). Pretty strong evidence for a genetic basis for the trait. And the effect of diet? Not so much. No difference in color between lizards in the  carotenoid supplementation treatment vs. the control lizards.

The bottom line is that, at least in this species, genes control variation in dewlap color. Combined with Steffen’s study, there are now two negative results for a role of diet. Of course, work on other species is necessary to confirm the generality of these results, as well as additional investigation into the exact genes responsible for dewlap color.

The long-tailed Asian lacertid lizard, Takydromus sexlineatus. Photo by John White.

Tail loss (aututomy) is one of the more amazing things done by lizards, but for for me it’s a frustrating reality of studying the physiology of sprinting because rough handling (by me when I was a beginning Ph.D. student and now in my lab by some undergrads) results in a lost tail and thus changed locomotor mechanics. But this frustration turned to fascination when I began studying locomotion in Takydromus sexlineatus. This species is pretty special as it holds the distinction of having the longest tail (relative to snout-vent length) of any lizard.

So I had to pull the tail off and measure how locomotion was changed. This then snowballed into studying the effect of autotomy in Anolis carolinensis and then a collaboration with Philip Bergmann to more broadly address how autotomy influences locomotor performance in lizards by using a meta-analysis of the published literature. We showed that longer tails result in a more drastic change in performance for all lizards studied except the two Takydromus species…so we are still left wondering what that huge tail does!

The result was a talk at the World Congress of Herpetology in Vancouver and a publication in Physiological and Biochemical Zoology as part of a special issue on Tail Loss in Lizards, organized by Tim Higham and Tony Russell.

Grass anoles have really really long tails too….I wonder how those tails are used? Convergence between Takydromus and Grass Anoles? E.N. Arnold did a lot of work on Takydromus and hypothesized that the tail aids in grass-swimming. I have observed this species stand up bipedally and use the tail as a prop (like Varanus). Thoughts?

Almost nothing is known about Anolis macrinii, which is a little surprising because it is rather large (nearly 100 mm snout-vent length) and apparently locally moderately abundant. However, it’s small, localized range in Oaxaca, Mexico is no doubt the explanation. In any case, now a bit more is known, thanks to a recent paper by Gunther Köhler and colleagues in Breviora (freely available on the MCZ publications website).

The paper includes a detailed morphological description of the species, as well as notes on natural history and conservation status. Most interesting to me is the sexual dimorphism in dewlap size (males on top above, females below), which we have discussed in previous posts, and the aberrant patterning of one juvenile individual (right).

Here’s the abstract:

During three short visits to the coffee-growing region in the hills north of Pochutla (Oaxaca, Mexico), we observed Anolis macrinii in its natural habitat. The species appeared to be relatively abundant, and we collected 12 individuals, including several adult males. The holotype of this species was reported erroneously to be an adult male but actually is a female. The confusion might have arisen from the moderate-sized dewlap present in adult females. However, males have a very large dewlap and a pair of moderately to greatly enlarged postcloacal scales. We provide color descriptions in life for three individuals, color photographs in life, description and illustration of hemipenis morphology, and some natural history notes. Finally, we discuss the conservation status of this species.