The tweet-o-sphere is full of news that there was some sort of anole question on Jeopardy late last week, and that apparently none of the contestants got the answer. But I can’t find any specific online. Does anyone have the inside skinny?
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!
Among Anolis lizards, sexual opportunities are typically monopolized by males and female mate choice is low. One way for female anoles to gain back some control in the mating process is through their specialized sperm storage system and selective fertilization. In her talk titled “Females bite back: Sexual conflict and the evolution of venom proteins in the reproductive tract of female anole lizards,” M. Catherine Duryea described her investigation into the genetics of sperm storage in anoles.
First, Duryea asked which genes are expressed in the female reproductive tract after copulation. Duryea extracted tissue from recently mated and virgin female A. carolinensis and generated cDNA libraries. From these libraries, Duryea found that over 160,000 genes were expressed in the reproductive tract, and that 5,153 of these genes were expressed differently in mated versus virgin females. Using a gene ontology analysis, which groups genes by function, Duryea found that many of the genes that showed increased expression in mated females were related to catalytic activity, protein binding, and nucleotide binding. The Anolis genetic response to mating is similar to that reported in Drosophila, suggesting that similar processes may be occurring across distantly related lineages.
Enzymes expressed after mating in anoles may be related to enzymes in snake venom (Image: Kendall McMinimy/Getty)
Next, Duryea looked for evidence of selection in a subset of the genes identified in the previous experiment. Specifically, she focused on the serine proteases, which are known to be important in sperm storage in Drosophila. Using a BLAST search, Duryea found eight serine protease genes in her A. carolinensis data. She then sequenced the orthologous genes in A. sagrei and compared the sequences to those of A. carolinensis. One serine protease gene showed evidence of positive selection, indicated by a large number of synonymous changes shared between species. This gene displayed striking similarity to a snake venom gene. Snake venom genes have a deep origin in squamates, including in non-venomous lineages; thus, Anolis reproductive serine protease may be derived from a venom serine protease. Compared to Drosophila, in which reproductive serine proteases are derived from digestive enzymes, this would represent a novel origin of reproductive serine proteases.
While these fascinating results are an important first step towards understanding the genetic basis of sperm storage in anoles, much work remains to uncover the exact function of serine protease expression in post-copulatory processes.
Though we now understand that post-copulatory sexual selection (such as sperm competition and cryptic female choice) can be as important in determining variance in reproductive success as pre-copulatory sexual selection, and though we recognize that the expression of traits subject to pre-copulatory sexual selection is often condition-dependent, it turns out that we know almost nothing about the condition-dependence of traits under post-copulatory sexual selection.
Sperm of Anolis sagrei. Picture by Ariel Kahrl.
In a session devoted to post-copulatory sexual selection, University of Virginia graduate student Ariel Kahrl described her research on the condition-dependence of sperm characteristics in Anolis sagrei. By feeding size-matched male lizards differentially for a period of four months, Kahrl not only generated differences in the body condition of these males, but also ensured that their sperm had developed under her imposed dietary regime. Kahrl predicted that male body condition would affect sperm morphology and sperm count. Pairs of males reared under different dietary conditions were also mated to a single female (making sure to control for mating order by using a reciprocal mating design), thus putting the sperm of two males with different body conditions into direct competition. Kahrl predicted that the fertilization success of males would depend on sperm morphology and count.
Not surprisingly, males with higher body condition had higher fertilization success. It turns out that variation in fertilization success may be influenced by a tradeoff between sperm mid-piece size and sperm number. This situation is interesting, because one can reasonably predict that males on either end of the tradeoff could have high reproductive success—having many sperm per ejaculate could increase the odds of fertilization, akin to purchasing multiple lottery tickets, but having sperm with larger mid-pieces, and thus potentially more mitochondria, perhaps might provide sperm with the burst of energy necessary to win the fertilization race.
In fact, Kahrl found that males with high sperm counts but small midpieces achieve high reproductive success. Intriguingly, she also found that high-condition males had sperm with less variable morphology than low-condition males, and hypothesizes that the dimensions of these uniform sperm match the dimensions of the tubules in females where sperm is stored. Kahrl’s results link pre-copulatory to post-copulatory sexual selection through condition-dependence, and represent a sizeable piece in the puzzle of how sexual selection works in Anolis lizards.
Anolis carolinensis from Miami. Photo by J. Losos.
I’m often a little skeptical of studies that suggest how lab results can have implications for natural systems without actually examining the problem in nature, and studies that address the same question in both lab and field are still rare. That it investigated the same broad question in the lab, in enclosure-style experiments, and in the field is what made Trinity University student Jordan Bush’s poster remarkable.
Bush was interested in the traits associated with dominance, and began by running a tournament of agonistic interactions between a set of male Anolis carolinensis. She used a number of different Markov Chain Monte Carlo algorithms, widely used in predicting winners of sports tournaments, to convert pairwise fight outcomes into individual ranks of “fight-winning ability.” Bush found that rank could be predicted by size-corrected head length, as well as the propensity for aggressive behaviours such as push-upping and crest-raising.
But do these same traits predict dominance in nature? Not exactly. Bush took the question to the field, and found that none of the traits that predict dominance rank in the lab are correlated with territory size in the wild. In the final piece of the puzzle, Bush constructed experimental enclosures to measure the territory sizes of males with known ranks (by conducting another dominance tournament). As predicted by the first two parts of the study, territory size was not correlated with rank.
By harnessing the power of both lab and field experiments to observations made in nature, Bush’s study will change the way behavioural ecologists think about territory formation. I’ve always assumed that winning agonistic encounters is the means by which anoles increase the sizes of their territory, but like everything in nature, it’s more complicated than that!
Modifying behaviour is often an animal’s first line of defence against a changing environment. We know from Martha Muñoz’s research that high elevation relatives of Anolis cybotes—A. shrevei and A. armouri–modify their perch use to better thermoregulate in colder climates. In a talk entitled “Behavioural divergence along an altitudinal gradient in a clade of tropical lizards,” graduate student Katie Boronow investigated a number of other behavioural traits, asking whether shifting to high altitudes necessitates a suite of behavioural modifications in the cybotoid anoles.
Boronow measured basking, display, escape, and locomotor behaviour in four anole populations–a high-elevation and a low-elevation site in each of two mountain chains in the Dominican Republic. Sites differed substantially in habitat openness–high-elevation sites had a higher proportion of exposed substrate and lower canopy cover than low-elevation sites.
In both mountain chains, individuals basked more and fled more readily at high-elevation sites than at low-elevation sites. The first result is easily attributed to variation with altitude in thermoregulation–it’s not surprising that lizards bask in direct sunshine more when it’s cold. While the differences in escape behaviour might also be driven by high-altitude lizards being thermally disadvantaged, Boronow found no differences in lizard body temperature between high- and low-elevation sites. Predation risk (as measured by attacks on clay models) also did not differ between sites at high and low elevations, so this variation in escape behaviour remains a mystery.
Given that locomotor behaviour is thought to be tied closely to ecomorph and that both high- and low-elevation cybotoids are still trunk-ground anoles, it is also unsurprising that rates of locomotion (measured by movements per minute, a common metric of foraging behaviour) did not vary by elevation. Patterns of display rates were interesting–while there was no altitudinal effect on display rate, the variation in display rate among populations of cybotoid anoles was comparable to the extent of variation seen across ecomorphs in previous studies. Boronow’s results suggest that differences in macrohabitat can be an important driver of intra-ecomorph behavioural diversification in anoles.
Readers of the Anole Annals know that the Caribbean radiation of Anolis is a classic example of evolutionary convergence: different ecomorphs have evolved repeatedly on islands in the Greater Antilles and show convergent microhabitat use and morphology. Thus, anoles are a great candidate with which to test a different type of evolutionary convergence: convergence in the physiological mechanisms underlying behavior. If anoles do show convergence in these traits, then there should be a common relationship between physiology and behavior across distantly related species. If not, then different species are using different mechanisms to achieve similar functional outcomes. Michele Johnson of Trinity University addressed this question using a comparative approach in her talk, “The Evolution of Muscle Physiology and Social Behavior in Caribbean Anolis Lizards.”
Species of Anolis that copulate more frequently tend to have a larger RPM muscle.
Johnson’s study focused on two different behaviors: copulation rate and dewlap rate. To quantify these rates, she first collected over 1,000 hours of behavioral observations on adult males across nine different species of anole. To address the mechanistic basis of copulation behavior, she then measured the sizes of the seminiferous tubules, renal sex segments, hemipenes, and retractor penis magnus (RPM, the muscle controlling hemipene retraction). Using phylogenetically independent contrasts, she found a significant positive correlation between the mean species rate of copulation and the mean species size of the RPM, but not with any other trait. Thus, species that copulate more frequently tend to have a larger muscle controlling hemipene retraction. This result supports the hypothesis that the size of a structure is related to how frequently it’s used.
To quantify the mechanistic basis of dewlap extension, she next measured the size and muscle fiber composition of the ceratohyoid muscle (which controls dewlap extension) and androgen receptor expression. There was no correlation between ceratohyoid muscle size or fiber composition and dewlap rate. However, there is preliminary support from four species for an association between androgen receptor expression and dewlap rate. This supports the hypothesis that higher sensitivity to the sex hormone testosterone increases dewlap rate. As the project proceeds, there are plans to add a fourth measure, size of the neuromuscular junction to the study, as well as increase the number of species included.
In conclusion, there appear to be some common physiological mechanisms underlying behavior across the Anolis radiation; however, there are also many physiological traits that may be employed differently among species in the production of behavior.
Anole Annals contributor Martha Muñoz of Harvard University won the second annual Raymond B. Huey Award for her presentation discussing the role of behavior in the evolution of Anolis cybotes. The Huey Award, sponsored by the Division of Ecology and Evolution of the Society of Integrative and Comparative Biology, is given for the Best Student Presentation in the division.
Behavior is thought to play two contrasting roles during evolutionary diversification. First, behavior can expose individuals to novel environments, thereby driving physiological and morphological change. Second, behavior can be used to compensate for environmental differences, thereby impeding organismal change. In her talk, Martha described how she tested these two contradictory hypotheses in a clade of trunk-ground anoles that span a wide environmental range.
The Anolis cybotes species complex occurs in Hispaniola from sea level up to 2,500 meters in elevation. By comparing two populations of a lowland generalist, A. c. cybotes, to two independently derived high-altitude specialists, A. c. armouri and A. c. shrevei, Martha was able to detect signatures of adaptation to high elevation. First, Martha asked whether physiological evolution had occurred. She found that body temperatures in the field were not significantly different at high and low elevations, despite the fact that lizards experience air temperatures 15 degrees cooler, on average, at high elevation. In addition, there were no significant differences in preferred body temperature (measured in the lab) among the four populations, and in each case the preferred body temperature matched the field body temperature. These results clearly support a lack of change in the thermal physiology of these lizards despite occupying very different thermal environments.
Martha then tested whether behavioral inhibition was the cause of the observed stasis in thermal physiology. By recording the perch sites of lizards in the field, Martha found that low-elevation lizards perch primarily on trees while high-elevation lizards have shifted to perching primarily on rocks. To quantify how this perch shift affects a lizard’s thermal environment, Martha deployed a series of copper lizard models at each site. The copper models closely mimic the thermal properties of a live lizard, so the temperatures recorded by the models are essentially those experienced by a non-thermoregulating lizard (i.e., the operative temperature). By placing the copper models on both rocks and trees at each site, she was able to assess the thermal properties of each perch type. Martha found that at low elevation, models on both trees and rocks achieved temperatures in the lizards’ preferred temperature range, and sometimes models on rocks got dangerously hot. At high elevation, however, only models placed on rocks achieved temperatures in the preferred range, while models on trees remained too cool. These results support the hypothesis that behavioral inhibition (perch switching) is preventing evolution in thermal physiology.
In a final twist, Martha asked whether evolutionary stasis is also observed in morphology. Morphology is known to correlate with microhabitat in Anolis lizards and is rapidly evolvable, and so stasis would be a surprising result. Martha found the high-elevation populations have significantly flatter and wider heads, a common feature of rock-dwelling lizards, compared to low-elevation populations. She found no differences in limb length or lamellae number. Martha hypothesized that for head morphology, perch switching was a form of behavioral drive that promoted evolutionary change.
Martha concluded by emphasizing that niches are multidimensional, and, therefore, evolution can occur along multiple niche axes simultaneously. By examining adaptation to both the thermal niche (body temperature) and structural niche (morphology) in this study, she revealed that behavioral drive and behavioral inhibition—previously thought to be incompatible—can in fact occur simultaneously in the same organism.
Congratulations, Martha, on your award-winning talk!