Anole Annals

Anolis sagrei has impressive sexual size dimorphism, but what causes it? (Photo by Bob Reed)

Sexual dimorphism is always a hot topic at SICB, and this year it was no exception for anoles (1, 2). Christian Cox, a postdoc in the laboratory of Bob Cox (no relation) at the University of Virginia, sought to explain how testosterone might lead to phenotypic divergence in a number of sexually dimorphic traits. As many of us are aware, sexual dimorphism varies widely among lizard species, and evolutionary shifts to and away from dimorphism are common, including in anoles. Testosterone has been shown to be an important regulator of growth in several lizard species, so Cox experimentally tested this effect in Anolis sagrei.

Both males and females were given a testosterone or blank implant and allowed to grow to maturation. One group was manipulated as juveniles, just as phenotypic divergence was beginning, and the other group was manipulated as subadults after divergence. Testosterone addition increased growth in body size and mass, increased metabolic rate, increased dewlap size, and changed dewlap coloration in both sexes and both juveniles and subadults. Fat storage was reduced as expected, in both sexes and age classes. These results are intriguing, because a sex difference in testosterone production may play a role in the degradation of between-sex genetic correlations. The next question is how that happens, as both sexes produce testosterone, just to different extents.

Numerous variables can affect an organism’s survival, including its age and sex, the demographics of the population in which it resides, and environmental conditions like climate, and habitat. However, the relative importance of these factors is poorly understood. Dan Warner described his investigation into factors affecting natural selection in wild populations of anoles in his talk titled, “Spatial and temporal variation in phenotypic selection in the lizard Anolis sagrei.”

Warner measured directional selection on A. sagrei on six islands in the Matanzas National Estuarine Reserve in Florida. These islands were intentionally founded with populations having unequal adult sex ratios. Half of the islands were founded with more males than females (male-biased), and the other islands received more females than males (female-biased). This manipulation was done to strengthen the effects of male-male competition on the male-biased islands. Warner measured survival selection on adult and juvenile body size by marking and recapturing individuals over the last three years.

Warner found a lot of variation in the strength of directional selection on adult and juvenile body size both across islands and within each island in different years. However, there was no relationship between the strength of selection on each island and either habitat structure (represented by canopy openness) or island size. Thus, the probability of survival at a particular body size does not seem to depend on environment.

However, population demographics did seem to affect survival at different body sizes. There was a negative correlation between the strength of selection on body size and the density of adult lizards, indicating that smaller body sizes are favored at high population densities (and vice versa). This trend was observed in both adults and juveniles, but was more pronounced in juveniles. Warner hypothesized that it was the density of adult males in particular, rather than the total density of adults, that was driving the observed trend. To test this idea, he tested for a correlation between the strength of selection on juvenile body size and the adult sex ratio. He found a negative correlation, indicating that large juveniles are favored in more female-biased populations while small juveniles do better in male-biased populations. One possible explanation is that on islands with male-biased sex ratios, large juveniles are more likely to come into contact with territorial adult males, are more likely to be perceived by these males as a possible competitor, and are therefore more likely to be harassed by these males. The presence of adult males might even reduce recruitment, as evidenced by slower population growth rates on male-biased versus female-biased islands.

These results suggest that patterns of natural selection on individuals can depend on characteristics of the population. Only with long-term field studies such as this one can we begin to unravel the many factors affecting selection in wild populations.

Although sexual dimorphism is found in many animal species, the mechanisms by which it evolves remains a hot topic. Selection may favor different phenotypes in the two sexes, but sharing a genome may put constraints on if and how sexual dimorphism might evolve. Many anoles have sexual dimorphism, of course, but the degree to which they are dimorphic varies quite dramatically. Robert Cox studied how between-sex genetic correlations in Anolis sagrei, a very dimorphic species, might degrade over ontogeny to result in divergent male and female phenotypes.

Anolis sagrei displays marked sexual dimorphism. (photo from Bob Cox’s website)

Using a large breeding colony of brown anoles from the Bahamas, Cox found that between-sex genetic correlations were lowest for traits that are the most dimorphic, like body size. Even more interestingly, the correlations change as the individuals get older. Whereas juvenile anoles have high between-sex genetic correlations for most traits, those correlations decrease around sexual maturation, most strongly in those traits that are dimorphic. This suggests that the pronounced divergence in phenotype seen in adults is associated with a degradation of the between-sex genetic correlations for those traits. Cox is currently exploring what mechanisms lead to this degradation, and is especially interested in whether testosterone is a major player.

Tropical ectotherms such as anoles are considered to be especially vulnerable to climate change. Given that tropical lizards already function near their upper tolerances, even a modest increase in ambient temperature can have disproportionately large negative fitness consequences. Most models that predict how climate warming will impact tropical ectotherms rely on ambient temperature. Michael Logan, a graduate student at Dartmouth College, presented a study suggesting that temperature alone is insufficient to predict the impacts of environmental warming on organismal fitness. He points out that other abiotic factors, such as humidity and wind speed, may be equally important in determining whether and how organisms will be impacted by climate warming.

For this study, Michael explored how daily variation in temperature, humidity, and wind speed interact to determine the abundance of two species of anole, Anolis allisoni and A. lemurinus, from the Bay Islands of Honduras (see map above). He deployed sensors that recorded temperature, humidity, and wind speed in a forest site, where A. lemurinus is found, and an open-habitat site, where A. allisoni is found.

Contrary to expectations, he found that environmental temperature alone is a poor predictor of lizard abundance in the open habitat. Rather, wind speed constrained lizard activity in the open habitat more than any other environmental factor. Further, environmental temperature predicted lizard abundance only when wind speed was low. Michael posits that there might be a trade-off between thermoregulation and evaporative water loss on windy days, such that the ability to achieve high body temperatures through basking may be counterbalanced by the ability to maintain water balance. Michael found that in the closed forest habitat, the variance in environmental temperature and the degree to which the temperature varied from the lizard’s optimal range were important predictors of A. lemurinus abundance. These results suggest that this species might thermoregulate more than was previously thought, as forest anoles are generally considered to be thermoconformers.

Together, Michael’s results suggest that factors besides temperature are important determinants of lizard abundance, and that they should be more explicitly considered in predictive models for the biological impacts of climate warming.

Both images from ganeshdhane’s flickr page: http://www.flickr.com/photos/ganeshdhane/

A non-anole regular on Anole Annals (e.g., 1, 2, 3) made an appearance at SICB this year. Not the species itself, but a fascinating presentation by Ambika Kamath on population variation in dewlap dimorphism in Sitana ponticeriana. Kamath presented information on display behavior for three color variants of Sitana: uncolored, colored, and intermediate. She wondered whether the three geographically separated variants display differently and whether the dewlap variation might be due to environment or sexual selection.

Coloured-fanned, intermediate-fanned, and white-fanned male Sitana ponticeriana. Photographs by Shrikant Ranade, Jahnavi Pai, and Jitendra Katre respectively.

By studying eight populations of this species, Kamath found that the three variants did indeed display differently. The colored variants had long displays with remarkable head turns and twists (wow, there was some amazing video!). The uncolored variants had body position changes, but no head turns and twists. Finally, the intermediate variants simply had short displays with no head turns or body position changes. Multivariate analysis of behavior clearly separated the populations based on color variant. Also, they flick that throatfan VERY quickly!

Based on the available data, it seems unlikely that environmental variation in habitat type or vegetation explains the variants, but sexual selection does appear possible. Colored dewlaps are associated with male-biased sexual dimorphism, whereas the uncolored variants have no dimorphism or female-larger dimorphism. Further, scaling of dewlap area to body size revealed that the colored and intermediate variants have evolved large dewlaps in different ways. This also supports Kamath’s proposal that there are multiple origins of large dewlaps and colorful dewlaps within the distribution of this widespread species. Future research will no doubt be of interest to us at Anole Annals and beyond!