Category: New Research Page 24 of 66

For decades, we anole scientists have been fascinated by the marvelous throat fans, called ‘dewlaps,’ characterizing Anolis lizards. A bunch of brilliant studies have therefore focused on the origin, evolution, diversity and function of this structure, revealing important pieces of what seems to be a complex ‘dewlap puzzle.’ And I think you all agree that the answer to what might look like a rather simple question at first, i.e., ‘what does the dewlap say,’ is not evident at all. So, with our study we aimed to add an ‘extra’ piece to the puzzle, which may help to further unravel the exact nature of information conveyed by the Anolis dewlap.

We specifically focused  on what is signaled by various  components of the dewlap in the brown anole and whether diverse aspects of dewlap signaling provide additive information or highlight different characteristics of the sender. We therefore  measured several dewlap components involving design (i.e., dewlap area, patterning, and color) and use (i.e., dewlap extension frequency during intersexual interactions), and linked these to information a sender may need to transmit in order to increase its fitness (i.e., sexual identity, individual quality, and social status). We used several performance (i.e., bite force, sprint speed, and clinging capacity) and health state parameters (i.e., immunocompetence, hematocrit, and swelling response) as a measure of individual quality, whereas mirror-motivated aggressiveness was used as a measure of social status. Due to their fundamentally divergent reproductive roles, we expected males and females to differ with respect to what is signaled by the dewlap, and  therefore performed separate analyses per sex. For the male sex, we additionally distinguished between the color of the dewlap center and edge region.

What did our results show?
First, we found that body size together with relative dewlap area and color act as redundant messages in the advertisement of sexual identity. Depending on the distance between signaler and receiver and prevailing environmental conditions, recognizing a potential mating partner based on the estimation of its body size only may be a hard task to fulfill. We therefore suggest that repeating the same message in different ways using body size together with dewlap traits is a highly appropriate strategy to get information about sexual identity accurately across.

Second, we found that dewlap coloration is primarily responsible for signaling aspects of individual quality, but only in males. Our results show that individual health state parameters are reflected  in the color components of both male dewlap center and edge and that multiple different messages are conveyed by dewlap color. Specifically, we found that males bearing dewlaps with higher amounts of yellow and  red and lower amounts of UV show higher body condition indexes, and that individuals with generally brighter dewlaps have lower immunocompetence. In addition, males with more yellow and UV chroma  in dewlap edge only exhibit higher hematocrit values.

Surprisingly, none of the tested components of dewlap design in A. sagrei males conveyed  information on performance capacities and aggressive behavior, and the same result was found for dewlap use. Also, no links were observed between components of dewlap design and use during intersexual interactions.

Third, for females, neither dewlap design nor use were related to any of  the tested  individual quality measurements and mirror-induced aggression. However, in contrast to males, correlations between components of dewlap design and use were found in A. sagrei females. Female individuals with larger dewlaps showed higher dewlap extension frequencies during intersexual interactions only and the same is true for individuals with less bright dewlap centers, suggesting an important signaling function of the female dewlap during courtship.

What can be concluded?
Our study confirms that the dewlap signaling device is a very complex integrated system consisting of different components transferring redundant (sexual identity) as well as non-redundant information (individual quality). We found that both the dewlap center and edge bear a signaling function, but this was only tested in males. As expected, male and female dewlaps differ in the messages they convey and further research is absolutely necessary to get insights in ‘what the dewlap exactly says’.

Driessens, T.,  Huyghe, K., Vanhooydonck, B. and Van Damme, R. (2015). Messages conveyed by assorted facets of the dewlap, in both sexes of Anolis sagrei. Behav. Ecol. Sociobiol. DOI 10.1007/s00265-015-1938-5

Parallel evolution and convergent evolution are big themes within anole biology, so our lab was excited to discuss a new paper by Thorpe et al looking at these concepts in Lesser Antillean anoles. The paper focused on evidence for parallel evolution across seven small islands that contained both xeric and montane habitats with at least one species of anole split between the two habitats. Xeric habitats tend to occur along island coasts and are hotter, drier, and have less canopy cover, while montane habitats occur in the interior of islands and are cooler and wetter. There are many physical differences consistently found between the anoles associated with each type of habitat, even within a species; perhaps the most obvious examples are the repeated differences in skin color and pattern between habitats, beautifully illustrated in the first figure of the paper.

Figure 1 from Thorpe et al, showing the repeated evolution of characteristic xeric and montane color patterns in the Lesser Antilles

Thorpe et al. set out to conduct tests of parallel evolution among seven anole species using 18 phenotypic traits that vary between habitats, including both morphological and pattern measurements. In addition, they used mitochondrial DNA sequencing to produce a new phylogeny of these species and control for phylogenetic interference in their comparisons. The authors first used a principal components analysis to confirm that the major source of climatic variation is found within each island and between different habitats, rather than across different islands. The authors found convincing evidence for parallel morphological evolution in multiple phenotypic traits, especially those associated with skin pattern and hue: anole populations in xeric habitats consistently converge on a grey skin color and those in montane habitats converge on green. Thorpe et al. also go on to suggest that divergence in coloration may be the result of signal optimization in environments with different chromatic backgrounds (characterized by variance in background vegetation or sun exposure). The authors describe a possible evolutionary scenario in which an anole population first colonizes the coastal areas of each island after a dispersal event, and then rapidly expands into the interior montane areas of the island and adapts to new conditions there. Given the constant concern of climate change, repeated evolution in response to different climatic conditions may offer hope that anole populations can respond to rapid environmental change.

The most famous story of parallel evolution in anoles is the convergent evolution of ecomorphs across the islands of the Greater Antilles. This paper offers the tantalizing possibility of another type of convergent evolutionary pattern, this time within species but across habitat types. The smaller islands of the Lesser Antilles may be too constrained to allow for speciation driven by ecomorph specialization, but could still promote significant population divergence across habitats. More information on the adaptive differences between these xeric and montane populations, along with characterization of their genetic structure, could shed light on these possibilities. Based on these results in the Lesser Antillean populations, there is also the possibility that this type of xeric and montane divergence exists within species in the Greater Antilles, and fine-scale studies of population structure could reveal another level of convergent evolution in those species.

Thorpe, R. S., Barlow, A., Malhotra, A. and Surget-Groba, Y. (2015), Widespread parallel population adaptation to climate variation across a radiation: implications for adaptation to climate change. Molecular Ecology, 24: 1019–1030. doi: 10.1111/mec.13093

Left: A. apletophallus. Right: Decline in abundance of A. apletophallus on BCI

Monitoring populations over long time scales is one of the most important endeavours in ecology, but maintaining funding over decades is a huge challenge when the tenure of most research grants is only 3 years. The Smithsonian Tropical Research Institute (STRI) has made a concerted effort to address this problem and established long-term monitoring of animals (including an anole), plants and environmental variables on Barro Colorado Island (BCI) and the nearby forests surrounding the Panama Canal. These data provide a rare glimpse into the long-term changes in populations and climate in the tropics.

Recently, we used these data to investigate how population abundance of the anolis lizard Anolis apletophallus has changed over time and whether climate was related to abundance and population growth rate. The study recently published in PLOS ONE identified a decline in lizard abundance over the 40-yr study period. We also observed boom and bust fluctuations in population abundance and found that cycles in population growth rate were related to global weather cycles known as el nino and la nina. Specifically, population growth rate was lower one year after el nino (warmer-drier) events. This decline in abundance and the negative relationship of population growth rate with el nino events is alarming, as el nino events are expected to increase in frequency and severity in the future. Changes in the abundance of this lizard may also have knock-on effects to many other animals in the forest because these lizards are eaten by a range of animals including birds, snakes, other lizards, spiders, ants, bats, monkeys and opossums.

The long-term decline in abundance that we identified is consistent with findings of another long-term study of amphibians and reptiles in Cost Rica by Whitfield et al in 2007. In their study they identified a decline in the leaf litter amphibians and reptiles and suggest this is due to a climate driven reduction in leaf litter. In a more recent follow-up study they provide further evidence of this. Although, we did not measure leaf litter, there is no evidence of a reduction in leaf litter on BCI. The parallel declines that were observed in Panama and Cost Rica are worrying and emphasize the importance of long-term data to help us understand how anole populations are coping with climate change.

Most of the hundreds of researchers that visit STRI’s research station on BCI scarcely notice the anoles. Some are drawn to the monkeys or bats, but most are there to study tropical forest ecology making use of the famous 50ha plot: a forest plot where every free standing tree has been measured every five years since 1980. I can understand how some might overlook the anoles in the forest, they can be extremely well camouflaged, but as readers of AA know, anoles are also highly conspicuous.

Left: Spot the A. apletophallus on the forest floor. Right: Male A. apletophallus displaying

Thankfully, BCI’s anoles have not always been overlooked. The most abundant anole on BCI is Anolis apletophallus (previously limifrons), so abundant that Stan Rand, STRI’s world-renowned herpetologist, described it as the ‘most abundant vertebrate in the forest.’ Thanks in part to Stan’s interest in this little brown anole, the species was the focus of much research on BCI in 70-80s most notably by Robin Andrews. Robin’s research on the ecology, physiology and life history of A. apletophallus remains some of the most detailed knowledge of a mainland anole today. Her work also had a lasting legacy at STRI, and the population monitoring that she began still continues today, some 44 years on.

The annual census, which has been continually funded by STRI, has been able to persist largely because of the efforts of STRI scientists.

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A slide from Danielle Klomp’s talk showing how color is used in communication by some species of lizards. Check out the quick guest appearance by an anole.

The diversity of anole dewlap shapes, colors, and patterns is one of their most distinctive features. But anoles are not the only squamates with flashy dewlaps. When it comes to such accoutrements, anoles have some stiff competition from their Agamid cousins in the Indo-Pacific region, the ‘flying’ dragons (Draco). Draco lizards don’t really fly, of course. Rather, they can laterally expand their ribs and the connecting membrane to create a ‘wing’, which they use to glide between trees in their habitats. If you haven’t seen how they do this, it’s more than worth a watch. Lest you think anoles get left behind in this respect, we do know that some anoles glide, as well, even if they don’t exhibit the impressive wing-like structures that Draco lizards have.

Slide from Danielle’s talk showing Draco lizards and their geographic distribution.

As I learned at Danielle Klomp’s talk at ASH 2015 last week, their dewlaps are almost as impressive as their gliding ability. Danielle is a PhD student working with Devi Stuart-Fox and Terry Ord and her dissertation has focused on studying the evolutionary ecology of Draco lizards. This past week she presented her work on these lizards’ dewlaps and what role they may play in sexual selection. Danielle examined dewlap size and coloration in 13 species of flying dragons. Overall, she found a strong negative correlation between color contrast (meaning they stand out relative to background coloration) and dewlap area in male lizards. Thus, she found that male dragons either had big dewlaps or conspicuously colored dewlaps, but not both. These results suggest that sexual selection for male conspicuity is occurring, but why can’t lizards exhibited large, conspicuously-colored dewlaps? Danielle suggested that having dewlaps that were both conspicuous in color and size were either too risky (meaning that they would be considerably more vulnerable to predation) or too costly to produce or maintain, though the precise mechanism underlying this pattern remains uncertain.

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Emma Sherratt gives her talk on fossil anoles

The annual meeting for the  Australian Society of Herpetology (ASH) is wrapping up here today in the lovely town of Eildon, Australia. Just because we’re a continent away from the native distribution of anoles doesn’t mean that anoles were not represented at the meeting. Yesterday afternoon Emma Sherratt, new faculty at the University of New England in Armidale, Australia, presented some of her post-doctoral work on fossil anoles preserved in amber. Emma began by saying that Caribbean anoles represent one of the oldest examples of extant adaptive radiations. Despite the age of this radiation, most of the work on the Caribbean anoles (and other adaptive radiations, for that matter), has focused primarily on living species, with historical inferences drawn from DNA analyses. She pointed out that historical insights based on analyses of extant species only should be treated with caution, unless there is corroborating information from the fossil record.

We know, she said, that islands are typically inhabited by a single lineage of ecomorphs (with subsequent diversification within ecomorphs). The fact that most ecomorph groups are represented by a single lineage on an island suggests that once an ecomorph niche is filled, it cannot be replaced, an idea known as ‘niche incumbency’. She argued that we can use fossils to assess that hypothesis – if fossil anoles pertain to same lineages of ecomorphs (e.g., the cristatellus clade of trunk-ground anoles, or the carolinensis group of trunk-crown anoles), then that would support the idea that ecomorph niches were only filled once. If extinct anoles fell into different lineages of ecomorphs, distinct from those that are extant today, then that would support the idea that ecomorphs could be replaced on islands, which would suggest that niche incumbency need not be occurring. Of course, it could also be possible for niche incumbency to have occurred if there were two lineages of the same ecomorph present on the same island, as long as the incumbent lineage drove the more recent one to extinction. But the hypotheses proposed by Emma were certainly a reasonable first pass to understand the origin of ecomorphs on the Caribbean islands.

Anoles have been fossilized in Hispaniolan amber, which we know to be about 15-20 million years old. All you folks who are anxiously awaiting the next installment of Jurassic Park be advised – this means that the famous amber used to get dinosaur DNA is far too young, as the dinosaurs (save for birds, of course) went extinct about 62 million years ago. For her study, Emma accessed an impressive 38 anole fossils preserved in amber. By far this is the largest data set of fossilized amber anoles ever examined. And, beyond their utility for understanding the process of diversification, anoles caught in amber are stunning fossils and the high resolution reconstructions that Emma makes using x-ray CT scans are equally impressive.

Emma found strong evidence that Hispaniolan fossil anoles fall into known ecomorph categories. To determine this she compared morphological details from extant species to the fossil anoles. Overall she found substantial morphological variation in the fossils, particularly in 20 of the best preserved and most complete fossils. Amazingly, Emma found that some of the fossils fell very clearly into the trunk-crown, trunk, trunk-ground, and twig ecomorph classes! She was further able to determine that the trunk-crown fossils fell into the chlorocyanus group of extant Hispaniolan lizards, and, with less confidence, evidence that the trunk-ground lizards fell into the cybotes group of extant Hispaniolan lizards. Thus, the results are suggestive that, once an ecomorph niche is filled, it prevents other lineages from evolving into it, which is consistent with niche incumbency. Obviously it is not possible to fully rule out the alternative – that species of other ecomorph lineages existed in the past – but certainly the results are a tantalizing glimpse into the processes that forged the current Caribbean fauna. In short, she found that most ecomorphs recognized today are not only present in the Miocene fauna, but also are represented by members of the same clades. Together, her results were consistent with the idea that niche incumbency occurred in the Caribbean radiation of anoles, which would indicate that interspecific interactions have regulated morphological diversity for millions of years.

Greetings from Eildon, Australia, where the 2015 meeting for the Australian Society of Herpetology (ASH) is currently underway. Today is the first full day of talks and posters and I’m excited to learn what’s new and exciting in herpetology. Although the focus is predominantly on Australian amphibians and reptiles, there are several presentations on non-Australian herpetofauna, as well. Anoles are also represented as I will be giving a talk on my work on Hispaniolan anoles and Emma Sherratt will be speaking about her work on fossil anoles. If you would like to see what’s going at ASH, feel free to follow the conference on Twitter using #ASH15.

Jake Stercula of the Johnson Lab and his 2015 SICB poster

Muscle fiber size can vary based on the frequency of use, or due to the fusion of multiple mononucleated myoblasts during development to form multinucleated fibers. To test if variation in muscle fiber size was due to frequent use or due to differences in development between species, Jacob Stercula of the Johnson lab examined the fiber size and number of nuclei for the ceratohyoid and the retractor penis magnus (RPM) of nine species of anoles. Most species exhibit a positive relationship between fiber size and the number of nuclei in both muscle types. Among species, this positive relationship between fiber size and the number of nuclei exists in the RPM muscle when accounting  for phylogeny using independent contrasts, whereas the ceratohyoid shows a positive trend, though the relationship was not significant. This suggests that for the RPM, muscle fiber size is evolutionarily conserved and is due to differences in development among species rather than differences in the amount of use. The size of the ceratohyoid muscle however, maybe be influenced by both the frequency of use and the fusion of myoblasts during development.

Michelle Oberndorf of the Johnson Lab (Photo courtesy of the Johnson Lab website)

Anolis visual display can come in two flavors: static and dynamic. Static displays are those that are involve permanent morphological structures, whereas dynamic displays involve movement of physical structures. Michelle Oberndorf of the Johnson lab asked if structures involved in both static display (tail crest) and dynamic display (dewlap size), were related to body condition or fighting ability (head size) in males and females of two species of anoles. She collected SVL, mass, head morphology, tail crest size, and dewlap size data from 50 males and 50 females of A. cristatellus and A. gundlachi. She found that in males, tail crest area was correlated with body condition in A. cristatellus. In male A. gundlachi, tail crest area was correlated with head size, and dewlap size, while dewlap size was correlated with body condition and head size. She found no relationships between any of the traits in females of either species. These results suggest that both the dewlap and the tail crest may communicate information about male quality and potential fighting ability.

Correlations between morphology and body condition for A. cristatellus and A. gundlachi males. Image from Michelle’s poster.