Anole Annals

Ariel Kahrl, Christian Cox, and Bob Cox.

Sperm morphology is highly variable across animals and is a great model for studying the evolution of sexually selected traits.

Ariel Kahrl, a Ph.D. student in Bob Cox’s lab at the University of Virginia, gave a talk on a study which she and coauthor Cox did just that. They sampled sperm from native and introduced populations of Anolis sagrei, A. distichus, and A. cristatellus to look at variation in morphology.

Variation in sperm morphology between native and introduced populations of three Anolis species.

Interestingly, they found that introduced and native populations often varied in sperm morphology (i.e., head, midpiece, and tail lengths). Moreover, these effects were consistent between the three species tested!

Kahrl also pointed out that the variation observed in sperm morphology between males of a single species was often as large as that observed between different species. This study suggests that sperm morphology is highly plastic and/or is capable of rapid evolution in response to environmental change. Further work is needed to elucidate what selective pressures are driving the variation observed between introduced and native populations of these three species.

Brittney Ivanov at SICB 2016

Color changing behavior has been widely documented in many lizard taxa.For example, the green anole (Anolis carolinensis) can rapidly transition from a uniform green to brown-colored. In those taxa where color change is rapid (including the green anole), such behavior has been attributed to communication of socially relevant information such as aggression or dominance.  However, what information is conveyed through color change in A. carolinensis during social interactions remains an open question. Brittney Ivanov, a research technician in Michele Johnson’s lab at Trinity University performed experiments in captivity using lizards captured from the wild to examine this question.

Brittney captured 12 lizards of each sex, which were checked daily for coloration to assess the predominant coloration of each individual. She then paired lizards from the opposite sex and placed them together for 2 weeks to determine if coloration is used differently between naïve and novel pairs. Lastly, same-sex trials were performed in both male and female lizards to determine if coloration indicates higher social status.

Brittney found that males spent more time being green compared to females and that their predominant body color was consistent across social context or housing condition (living alone versus with a female). Predominantly green males also “won” more often in same-sex trials. Female coloration was not associated with the results of the same-sex trials, but females were found to be green more often when housed alone than when housed with a male. Brittney’s research suggests that coloration may be used differently between male and female green anoles and that for males, coloration may determine social status or competitive ability.

Austin Garner, an undergraduate at the University of Akron.

Anoles, geckos, and some species of skinks have adhesive toepads that allow them to cling to substrates. This adhesive ability is remarkable – anoles, for example, can hang from a glass pane using just one toe. Gecko adhesion is particularly well studied, but most research has focused on how these animals cling to dry surfaces. In their natural habitats, however, geckos often have to contend with wet surfaces.

Austin Garner, an undergraduate at the University of Akron working with Peter Niewiarowski, wanted to know whether geckos could move effectively on wet substrates. He measured sprinting performance in two species of gecko, Gekko gecko and Chondrodactylus bibronii, across a 2-meter vertical racetrack that was misted with water. Average sprint velocity on wet substrates did not differ significantly from the average sprint velocity on dry substrates, indicating that geckos can sprint equally fast on slippery surfaces. The substrate material, however, influenced how often geckos slipped. Geckos slipped more on glass substrates compared to acrylic substrates. Austin hypothesized that this is likely due to the surface chemistry of glass. Glass is a hydrophilic substrate, meaning that water is attracted to its surface more so than the surface of acrylic. Interestingly, the frequency of slipping differed among species. Chondrodactylus bibronii, a species of gecko from an arid habitat, slipped more often than G. gecko, a gecko found in the tropics. Although C. bibronii slipped more on wet substrates, this species did not suffer a decrease in average sprint velocity on wet substrates. This suggests that C. bibronii is somehow compensating for the slipping observed on wet substrates, but Austin is unsure of the mechanism behind this compensation. Overall, his study suggests that geckos can travel on wet substrates up to 2-meter without a reduction in their adhesive ability, and that at least one species of gecko can compensate for any loss of traction caused by the presence of water.

Many anoles have prolonged breeding seasons spanning from the late spring until the early fall. For part of his Master’s degree Phillip Pearson, a student in the Warner lab at Auburn University, asked whether the timing of oviposition is adaptively matched to a season’s thermal environment and if there are fitness consequences of early or late developmental temperatures in Anolis sagrei. They predicted that eggs laid early in the season (April-May) and were incubated under ‘early season’ temperatures would have higher hatchling fitness than under ‘late season’ (July-August) temperatures, and that late-produced eggs would have higher fitness in ‘late season’ temperatures than ‘early season’ temperatures.

To test this hypothesis Phillip collected adult males and females from the wild and brought them back to the lab to breed. He then collected eggs from March-April as the ‘early’ cohort and from July-August as the ‘late’ cohort. Each of these cohorts was then divided into two treatments with ‘early season’ and ‘late season’ incubation temperatures, resulting in four groups. Each hatchling was weighed, measured and assessed for sprint performance.

Phillip found that both the time of oviposition and the incubation temperature significantly affected the development of the hatchlings in several ways. First, eggs in both the early and late cohorts that were incubated under early temperatures had significantly longer incubation durations. Temperature also interacted with the season cohorts, so that the ‘late season’ cohort incubated under the late season temperatures had the shortest incubation duration (Figure 1A). Second, Phillip found a significant effect of season, incubation temperature and their interaction on egg survival, where the late season cohort that was incubated under late season temperatures had the highest survival (Figure 1B). However, he did not find a significant effect of either incubation temperature or season cohort on hatchling survival. Third, eggs that were laid in the late season cohort were significantly larger in mass, snout-vent length, and tail length at hatching than early-season eggs (Figure 1C). Finally, hatchlings from the ‘late season’ cohort had marginally faster sprint speeds, with more stops (Figure 1D).

A. Incubation duration, B. egg survival, C. hatchling mass, and D. sprint speed for eggs oviposited in ‘early season’ and ‘late season’ cohorts raised under two different incubation temperatures.

Overall, Phillip’s results suggest that eggs laid later in the season and incubated under warmer late-season temperatures seem to have higher performance and fitness (in some cases). Currently, Phillip has released these hatchlings onto an island in Florida near the site of the parent population. He and the Warner lab will be going back this spring to assess survival of these hatchlings to get field-relevant data on survivorship under these two developmental treatments.

Slide from Michele Johnson’s SICB talk.

Muscles used for short, rapid movements should experience different physiological demands than those used for slow, stalking movements. Fortunately, lizards display a wide range of movement patterns from sit-and-wait foraging to slowly stalking prey. Thus, they are ideal for addressing questions on the evolution of muscle morphology, physiology, and behavior.

Dr. Michele Johnson and colleagues of Trinity University addressed such a question which Johnson presented during a talk at the SICB meeting in Portland. Although most studies of locomotion focus on the hindlimb, Johnson and colleagues wondered if forelimb muscle physiology is associated with lizard locomotor behavior. To address this, they made 30 minute observations on a minimum of 40 males of 6 species and recorded the frequency and type of locomotor behavior and social display. This information allowed them to classify lizards as “short-burst” species that often run, jump, and perform push-ups as a component of their social displays (green anole, Texas spiny lizard, northern curly tail) or “endurance” species that more frequently crawl (little brown skink, Mediterranean house gecko, spotted whiptail).

They found that short-burst species have more tonic fibers (involved in maintaining posture and balance) in the forelimb musculature, and endurance species have more twitch fibers (used during quick movement). In addition, species with more frequent locomotion had more twitch fibers. Relative fiber size increased in species that ran often and decreased with crawling behavior. Their study suggests that the evolution of forelimb fiber type is associated with the frequency of locomotion and that fiber size is associated with the speed of locomotion.

A male brown anole basking on a tree.

Theory predicts that as environmental temperatures change, animals that function better at the new temperatures will be favored by natural selection. Thus, we might expect that climate warming will select for animals with higher thermal optimums (T_opt). In addition, thermal performance curves are also characterized by the breadth of temperatures that animals can function. Theory predicts that increases in environmental temperature variation will select for animals with larger thermal breadths (Tbr). Previous work has shown that brown anoles transplanted to a warmer environment experienced strong directional selection favoring individuals with higher T_opt and Tbr (Logan et al. 2014). However, it is unclear if selection acts on these two traits independently or if they might be genetically constrained.

Mike Logan, an NSF postdoctoral fellow at Stellenbosch University, gave a talk on a study that he and coauthors (John Curlis, Ingrid Minnaar, Joel McGlothlin, Susana Clusella-Trullas, and Bob Cox) conducted to test this question. They brought brown anoles into the lab and found a significant negative correlation between T_opt and Tbr, suggesting that increases in one trait lead to reduction in the other. To test the generality of their findings, they brought ladybugs into the lab and conducted similar trials. Interestingly, they found the same results for ladybugs. This study suggests that these thermal adaptations are evolutionarily constrained in two very distant relatives.

Logan, M. L., Cox, R. M., & Calsbeek, R. 2014. Natural selection on thermal performance in a novel thermal environment. Proceedings of the National Academy of Sciences 111(39):14165-14169.

Leah Selznick presents her poster at SICB 2016.

Muscle and jaw morphology is highly variable among lizards, which could be driven, at least in, part by a species’ diet, intraspecific combat, or both. Leah Selznick of the Johnson lab at Trinity University collected data on the head dimensions, jaw muscle mass,  diet data (prey count), and estimates of sexual dimorphism (SSD) for seven species of lizards. Four of the species she examined – the leopard gecko, Northern curly tail, Texas spiny lizard, and the Carolina green anole – are saurophagous, meaning that they eat other lizards. The other three species – the Mediterranean house gecko, little brown skink, and spotted whiptail – do not eat other lizards. Leah predicted that saurophagous species and those with a higher variance in prey diet would also have larger heads and larger jaw muscles. Additionally, she predicted that species with higher sexual size dimorphism (SSD), a proxy for the strength of pre-copulatory selection on male body size, would be associated with larger jaw morphologies. First, she tested for phylogenetic signal in all of her traits and found strong signal for jaw muscle mass (λ = 0.99) and head size (λ = 0.65). She then tested for an association between both head size and jaw muscle mass (standardized by body mass) with species prey count and SSD. She found no correlation between any of the jaw morphologies and SSD or species prey count. Leah suggests that (1) there may be other traits that are experiencing selection due to prey size and combat and/or that (2) these traits may be experiencing evolutionary constraint. Leah is going to continue exploring the evolution of jaw morphology by examining the histology of the jaw muscle in these species to test for an association between muscle fiber composition and type with prey count and SSD.

Selznick compared two groups of lizard species: four saurophagous species, and three exclusively insectivorous species. Here, she shows the prey count, and SSD for each species used in her analysis.