Marathon runners and elite sprinters, like Usain Bolt, have dramatic differences in their muscle physiology that allow them to specialize in their respective track-and-field events. Whereas sprinters have lots of muscle fibers that produce high force but fatigue quickly, marathon runners have lots of muscle fibers that produce less force but allow much longer activity because of their reliance on aerobic respiration. Might this be true for our beloved Caribbean anoles, too? Faith Deckard of Michele Johnson’s lab at Trinity University tried to answer that very question. She studied six species of anoles in the Dominican Republic to test whether anoles that have higher rates of dewlap extension and extend their dewlap for a longer duration have dewlap muscles with a higher proportion of slow-twitch muscle fibers that can be used for endurance. Surprisingly there was no significant correlation between the two behavioral traits and the proportion of slow-twitch fibers! However, this scrutinizing attendee feels pretty strongly that there is a relationship that is just yet to be teased apart statistically. The raw data Faith presented looked very convincing to me, so we’ll see what the future holds for this question. Faith’s results are a very interesting clue to the still-elusive mechanisms that underlie anole behavioral diversity.
All of the gumbo, Po boys, and beignets consumed by attendees of SICB 2017 have to go somewhere after consumption. Much of the energy contained in those delicious foods is used for very important maintenance functions in your body: metabolism, cell repair and replacement, and your immune system. What’s left over after maintenance costs can then be divided amongst other tasks, such as reproduction, movement, and wide variety of other tasks. Unlike humans, anoles do not have unlimited access to gigantic portions of gumbo, so their energetic investments require much harder decisions. Once energy from a cricket, for example, has been put into the immune system, it can no longer be used for making eggs or patrolling a territory a little bit longer. Andrew Wang of Jerry Husak’s lab at the University of St. Thomas was interested in what mechanisms are involved with anoles making these investment “decisions.” He did this by forcing allocation of resources to an energetically expensive trait (endurance running) by exercise training lizards to see what would happen to everything else that they might invest in.
Previous work showed that exercise training and diet restriction results in dramatic trade-offs with reproduction and the immune system. He suspected that what might explain this suppression was the hormone leptin, which is made by fat cells (yours make it, too). Since bigger fat cells means more leptin in the body, leptin can be thought of as a signal to the brain and body of how much resources are available for investment. Indeed, without sufficient leptin, reproduction grinds to a halt from the brain downward. Much like elite athletes, Andrew’s marathon lizards have little to no fat stores in their body, thus suggesting a role for leptin. To address this question, he supplemented half of the lizards with leptin (the rest got only saline as a control) to see if he could “rescue” immune function and reproduction. Interestingly, he found that leptin did rescue his measure of immunity, but it did not rescue reproduction. He attributes this latter finding to either (1) a lack of energetic resources to produce eggs even if there is a leptin signal or (2) the stress of the leptin injections over-rode the leptin signal in the brain where reproduction is controlled. His results suggest some very complex interactions in physiological pathways that can result in the trade-offs observed in many animal species.
What does it take be a good sprinter? How about a marathon runner? One might think that the traits responsible for such performance traits would be the same in males and females. If you are a green anole, that just isn’t true. Annie Cespedes, working in Simon Lailvaux’s lab at the University of New Orleans, explored the multivariate predictors of seven performance traits (sprint speed, bite force, cling force, exertion, endurance, jump power, and climbing power) in male and female green anoles. Annie explained how animals in nature rely on lots of different performance traits in their daily lives, and the large difference in body size and shape between male and female anoles might mean that the two sexes use different means to be successful in life. To add to this complexity, some individuals are just better overall at ALL performance traits than others (imagine a couch potato versus a very fit athlete), and one must account for this to understand what shapes anole performance.
Multivariate statistics allowed Annie to show that males and females do indeed differ in performance, but only in clinging ability, sprint speed, bite force, and jump power. Even more interesting, the suites of morphlogical traits that explained performance ability differed substantially between the sexes. For example, small females with large leg muscles were better sprinters and jumpers than females who are smaller and are better biters and endurance runners. What is especially important about Annie’s research is her approach. When considering how animals evolve, one must do so by simultaneously looking at a multitude of traits that might impact their survival and reproduction. By knowing how morphology predicts performance, we can begin to better understand how evolution will shape that morphology when selection acts on those performance traits.
Frequent readers of Anole Annals are likely to recall the amazing convergent evolution of morphology related to habitat use in Caribbean anoles that coincides with similarly striking convergent evolution of social behavior. Most of what we know about behavior of Caribbean anoles is how males behave: there are major differences among ecomorphs in how often males use their colorful dewlaps and how often they mate with females. Such male-typical behavior seems intuitively linked to species differences in testosterone signaling. Previous work has shown, though, that these differences do not seem to be related to levels of testosterone in the blood, so Miguel Webber of Michele Johnson’s lab at Trinity University examined whether the receptors for testosterone varies in a manner consistent with the behavior for six Dominican Republic species of anoles and one U.S. species.
Hormones can only cause effects on tissues that have receptors for them, so Miguel looked at receptors for testosterone (androgen receptors) in the muscles responsible for moving those fabulous dewlaps (the ceratohyoid muscle), expecting to find a correlation across species between the number of androgen receptors in the muscle and the rate of dewlap extensions. Although the data are still preliminary, there was a trend for males with higher dewlap extension rates to have more androgen receptors in the ceratohyoid muscle. His next steps are to look for an association between rates of copulation and androgen receptors in the muscle used by males to copulate (retractor penis magnus muscle – yes, it does what you would guess based on the name…). He also wants to see if there is a correlation among species in the behavioral traits and androgen receptors in regions of the brain that are important for social behavior regulation.
It’s often said that winning isn’t everything. This may be true for humans and the games we play, but, unfortunately, for most animals losing a contest can have serious implications for whether they survive or reproduce. The study of animal contests has been thoroughly studied in males, and we know that losing to a rival can mean you get less or no mating success. However, we know far less about the consequences of winning and losing if you are a female. Jess Magaña and Matt Lovern (from Oklahoma State University) asked what happens to females after they win or lose a contest, and they had one of my favorite talk titles ever: “Small and large lizards agree in defeat but react differently to victory.”
They studied brown anole females, which are known to show aggression toward each other. Winners and losers were pre-determined by residency in a cage. Females who got to compete in their home cage were winners, and those who were placed into another lizard’s cage were the losers. They were allowed to interact, and then Magaña monitored their reproduction thereafter. Previous work had shown that losers laid eggs that hatched more quickly, suggesting that offspring were given less yolk and would perhaps be less successful because of it.
Comparisons between winners and losers reveleaed surprisingly little difference in most reproductive traits, such as egg size, time to hatch, and sex ratio. However, when they looked at the effects of body size on reproductive traits, there was a marked difference between winners and losers. In losers, investment in reproduction was unrelated to body size. In winners, though, size was related, and size reflects age in this species. Small (young) winners laid eggs that hatched quickly, but large (old) winners laid eggs that took longer to hatch. They interpreted this as different strategies of investing in potential future reproduction: old winners should invest in current offspring, whereas a young winners should invest in potential future offspring. This interesting finding highlights the fact that there is still much to be learned about the subtlety of how a mother’s environment and experiences can shape her offsprings’ life.
If you’ve ever been around brown anoles, you know that the males can be pretty aggressive. Part of that aggression involves the enlargement of a crest along the neck and back. The crest is caused by fluid rapidly rushing into the tissue of the crest. How this works has been discussed here before, but Matt Rand’s research group at Carleton College continues to try to unravel what hormonal pathways are responsible for crest formation. Ademi and Rand used an experimental approach to discover what molecular receptors are activated to cause crest formation. Body-wide and local injection of a variety of chemicals and drugs gave some tantalizing clues as to how it works.
They found after several inhibitory and stimulatory drug manipulations that crest erection is likely stimulated by epinephrine acting on a Beta-2 like adrenergic receptor that stimulates cyclic AMP (cAMP) activity to cause vasodilation (enlarging of blood vessels) and fluid entrance into the crest. This activity that starts with the B2-adrenergic receptor is essentially the same function as that seen in mammalian circulatory systems, including us. They also stimulated cAMP activity without stimulating the B2-like adrenergic receptor and found similar results. You can see how dramatic the response was below, where they used local injection to cause crest formation only at the site of injection! The use of epinephrine binding to a B2-like adrenergic receptor as the molecule of communication makes the rapid time-course of crest formation make sense. There are still some unknown aspects as to how the vasodilation mechanistically causes the fluid release in the crest, but they are actively studying it.
When you’re used to living at low to moderate elevations, it can be challenging to visit high-altitude places. The declining partial pressure of oxygen at high altitude makes it difficult for your body to deliver the same amount of oxygen to tissues. This is why National Football League players often struggle to play in Denver (see playoffs next week!). However, organisms that live at high elevations, including humans, have evolved a number of ways to deal with living in such oxygen-challenged environments. We know less about whether the same aspects of the cardiovascular change in different organisms, even among relatively closely related species. Well, what better group of organisms to address such questions than anoles!
Virtually nobody reading this blog will be unfamiliar with the story of the Greater Antillean ecomorphs, and they are great to use for questions related to elevation and adaptations to deal with it. They live along steep elevational gradients within an island, and such gradients exist across islands. Although, the Caribbean anoles have been the subject of numerous studies that have shown convergent evolution in body size and shape, as well as locomotor performance and endocrine function, we know much less about how they deal with elevational challenges at the cardiovascular level.
Miguel Webber, an undergraduate in the laboratory of Michele Johnson at Trinity University, along with Brittney Ivanov, studied several blood physiology traits in 13 species across five ecomorphs in the Dominican Republic to determine whether elevation has been an important driving force in the evolution of oxygen delivery mechanisms. Although looking at an impressive number of traits that included hematocrit (the proportion of red blood cells), hemoglobin concentration, and red blood cell size, Miguel only found hemoglobin concentration to be positively related with elevation when looking across species.
One of the more interesting findings was that none of the blood physiology variables that Miguel measured were ecomorph specific. However, this makes sense because members of an ecomorph live across wide geographic areas and across elevational gradients. Physiological studies such as Miguel’s are offering interesting insights into how anoles have adapted to their environments and emphasizes that ecomorph membership does not determine everything.
The immune system can be costly, even for anoles. However, despite a large amount of work on natural populations examining when and why animals use their immune system, as well as what it energetically costs, it remains poorly understood whether a larger (more costly) immune response to pathogens offers more protection. In other words, is a major immune response worth all the cost? This is what Amber Brace, a graduate student in Marty Martin’s lab at the University of South Florida has been trying to test. Amber used experimental malaria infections in introduced brown anoles in Florida to determine whether the high costs of an immune response would result in better protection from the disease. Although malaria naturally occurs in Floridian brown anoles, Amber first had to develop an experimental protocol to successfully infect lizards. She gave one group a low dose of malaria, and another group a high dose. Interestingly, only the high-dose group became infected. Once this was worked out, she could then test how experimental infection would affect individuals.
Since malaria ultimately results in the bursting of red blood cells, she predicted that a higher malaria burden would be positively related to the change in number of immature red blood cells (from pre- to post-infection), and this is exactly what she found. This shows that individuals with greater malarial infections are compensating for lost red blood cells by producing more. Perhaps most importantly, she found a negative relationship between malaria burden and the change in number of white blood cells. This suggests that individuals greatly increasing one group of immune cells (white blood cells) are able to decrease their malaria burden. Thus, it appears that an enhanced immune response does, in fact, offer added protection, and the high costs of an activated immune system are worth the investment.
Anoles are no strangers to urban environments. In fact, many anole species seem to do just fine in cities. However, they do face a number of different challenges not present in their native environments. One example is the perches on which anoles move. Andrew Battles, a graduate student in Jason Kolbe’s lab at the University of Rhode island, was interested in exploring how the perch use of two anole species differed between natural populations and urban populations, and what that habitat use might do to their running performance. Andrew studied Anolis cristatellus and A. stratulus on Guana Island in the British Virgin Islands to measure perch smoothness/roughness, perch use, and sprinting performance on various perch types.
Lizards were found most often on artificial perches, instead of natural perches, in urban environments. This is interesting, because such artificial substrates tend to be vertically oriented and significantly smoother compared to natural perches like tree branches and trunks. As predicted, lizards ran more slowly on substrates that are smooth and more vertical, and this was most pronounced in the larger male A. cristatellus compared to the smaller female A. cristatellus and both sexes of A. stratulus. Thus, while optimal substrate use might be inclined, rough, natural perches, these anoles are using smoother, more vertical, artificial perches in urban environments. This fits into a theme present at this year’s SICB meeting that animals often move in ways that seem counter-intuitive at first. How such perch decisions might influence fitness remains an open question. Future work will investigate how availability of perches and alternative escape strategies influence perch selection.
It’s happened to us all: you try so hard not to break the tail when you catch an anole, but inevitably it happens to one. As readers of Anole Annals know, many species of lizards, including anoles, lose their tails as a defense mechanism. While losing a tail, called autotomy, has known detrimental effects on social status in males and reduced locomotor capacity, we know less about other potential costs for a strategy that is intended to keeps lizards alive to reproduce another day. McKenzie Quinn, an undergraduate in Michele Johnson’s lab at Trinity University, wanted to know how losing so much tissue, and then replacing it, might take away available resources from other important processes. She measured changes in egg number, egg size, body size, and fat mass in the liver over the course of three weeks after experimental removal of the tail in green anoles. These females were compared to a control group that did not have their tails removed.
Lizards who had their tails autotomized re-grew their tails over the course of the experiment, whereas control groups that had intact tails had minimal tail growth. Surprisingly, there was no difference between the two groups in any of the traits measured. Females with autotomized tails had just as much growth, just as many eggs of the same size, and just as much fat accumulated in the liver. This suggests that in a laboratory setting females are not taking resources away from growth and reproduction to re-grow a tail. Field studies and additional manipulations of resource availability in the future may help us understand what costs are associated with such an intriguing and seemingly costly defense strategy.
Readers of Anole Annals are likely familiar with the amazing convergent evolution of habitat use and morphology in Caribbean anoles, but the corresponding divergent and convergent evolution of social behavior has recently captured the interest of anolologists. The species differences in social behavior would seem to be due to differences in how much testosterone, a steroid hormone that regulates behavior in many other vertebrates, but this does not appear to be the case. Bonnie Kircher, formerly of Michele Johnson’s lab at Trinity University and currently at the University of Florida, examined what other aspects of hormone signaling might be responsible for the diversity of social behavior seen in Hispaniolan anoles. Since hormones can only act on tissues that have receptors for them, it is possible that variation in hormone receptors might explain behavioral differences independent of hormone levels circulating in the blood. Since the behavioral differences in anoles involve variation in pushup displays and dewlap extensions, it seems intuitive that there may be differences in receptors for testosterone (androgen receptors) in the muscles responsible for these displays.
Bonnie studied six species of anoles that vary in pushup and dewlap display frequency: A. bahorucoensis, A. brevirostris, A. carolinensis, A. coelestinus, A. cybotes, and A. olssoni. After measuring display frequencies in these six species, the investigators quantified the number of androgen receptors in two muscles that are important for pushup displays (biceps) and dewlap displays (ceratohyoid). As predicted, the results showed that species with higher rates of pushup displays have more androgen receptors in their biceps than species with lower pushup frequencies. Interestingly, this was not the case for the ceratohyoid muscle, which controls dewlap extensions. There was no relationship between androgen receptor density of the ceratohyoid and dewlap display frequency. These results are a tantalizing clue to the still-enigmatic mechanism(s) that underlies anole behavioral diversity.
Although there is a vast literature on how resource availability affects physiology, behavior, and reproduction (among many other things), we know surprisingly little about the composition of individual diets in nature. To truly know whether you are what you eat, you have to understand what it is you are eating. Dan Warner from the University of Alabama at Birmingham set out to do just that with some very interesting preliminary data on an island population of brown anoles in Florida. He trapped potential prey in two very different habitat types on the island: interior forest and open shoreline. The shoreline had mostly marine-sourced prey items (amphipods), whereas the forest had more terrestrial insects, like roaches. Dr. Warner then wanted to know if these differences in diet would affect body composition of anoles in those habitats.
The methods here are the best part. Dr. Warner used Quantitative Magnetic Resonance (QMR) technology, typically used for rodent lab animals, to determine body composition. He found that there was a very strong match between the QMR estimates of lean and fat mass compared to chemical carcass analysis of the same individuals. And, the QMR measures only take about 5 minutes to do! This non-invasive, non-lethal way to estimate body composition has huge implications for studies that seek to tie those characteristics to components of organismal fitness, namely survival and reproductive success. It doesn’t work to track survival on individuals sacrificed for chemical carcass analysis. He also suggests that this now-validated method will be important to test whether typical measures of body condition (such as mass-length residuals) are actually good estimates. It doesn’t sound good for our typical measures of condition, but he will tell that story soon!
Returning to diet’s effect on body composition, the results showed that lizards in the interior of the island had more fat mass and less lean mass than lizards found on the shoreline. He plans to continue the research by repeating it on replicate islands with similar habitat types, as well as look at long-term consequences of variation in body composition. This new approach will open the door for fascinating research to come, so stay tuned!
Anolis proboscis, showing the male-specific proboscis. Photo by D. Luke Mahler.
A long–time favorite here at Anole Annals, the Ecuadorian Horned Anole (Anolis proboscis) made an appearance at SICB. Diego Quirola and colleagues from Pontificia Universidad Católica del Ecuador described the use of the proboscis during social interactions. They captured male and female anoles and videotaped staged male-male and male-female interactions. From the videos they were able to quantify behavioral patterns of these fascinating lizards. They did some very anole-like behavior, but they definitely have a flair all their own! With such a fascinating, chameleonic appendage one would expect some important functions of the proboscis, and one would not be disappointed. Watching videos that Diego had on display revealed social behavior very reminiscent of chameleons, with males puffing up, curling their tails, and swaying while doing the more typical anole dewlap extensions.
Then there’s the proboscis. This structure, much like the dewlap, is used during both courtship and agonistic interactions. In both contexts, males actually lift the proboscis. Yes, they can move the proboscis up and down, something not seen in chameleons with rostral appendages (no, we don’t know how they do it!). Diego suggests that the proboscis is lifted to either stimulate females or allow the male to bite the nape as other lizards do while copulating. Males also display a behavior called “proboscis flourishing” where the proboscis is prominently displayed while moving the head side to side. During agonistic interactions it may serve as a dominance indicator, though they are still working on those analyses. Proboscis anoles seem to be at the low end of aggression for anoles, but males occasionally fight and lock jaws. During male fights the proboscis likely gets in the way, and it appears to be purposely lifted during these fights. It’s possible that they lift it to keep the rival male from latching onto their snout, or it could be moved so that they can get better bites in. I was very much looking forward to learning more about these anoles, and I was not disappointed. As more work is done on these fascinating anoles we’ll be able to better understand why it has evolved such an interesting, and un-anole-like appendage, as well as the unique behavior that is associated with it.
When most people think of vertebrate sexual dimorphism (differences between the sexes), they think of elephant seals or red deer. Most of us here, of course, think of the pronounced dimorphism in size and shape in many anole species. Indeed, anoles have served as excellent model systems for the study of sexual dimorphism, particularly the evolutionary forces that give rise to it. Although there has been significant progress since Darwin in our understanding of why sexual dimorphism evolves, we have made less progress in the HOW. That is, what mechanisms during development give rise to what are often extreme differences between the sexes when their genomes are so similar?
When we think of vertebrates where males are larger or shaped differently than females, and have weapons or ornaments, we almost immediately think of testosterone as a mechanism underlying the sex differences. Once sexual maturity happens, the testes start cranking out testosterone, thus causing a change in the male’s phenotypic trajectory. While there is certainly evidence for circulating testosterone to have this effect in some lizards, is this always the case, and does it apply to specific body parts and not just overall size? Aside from the circulating hormone, how are receptors involved in the development of dimorphism? In a new paper by Sanger et al., a novel developmental pathway of sexual dimorphism is described for lizards in the carolinensis clade, which are striking in their elongation of male faces relative to females.
Sanger et al. tested whether sex differences in several different pathways led to the observed head shape dimorphism in A. carolinensis compared to two non-carolinensis species (A. cristatellus and A. sagrei) that exhibit shorter male faces. They show, using a combination of developmental and molecular genetic techniques, that the extreme elongation of male heads in carolinensis lizards is not due to an androgen pathway (i.e., testosterone) or the somatropic axis (i.e., insulin-like growth factor). Instead, they found a significant shift in the estrogen pathway. Specifically, at sexual maturity, males decrease expression of estrogen receptors (erβ), which is the beginning of a signaling cascade, ultimately resulting in up-regulation of genes involved in skeletogenesis in the skull of males.
This identification of a novel mechanism for the development of sexual dimorphism will certainly stimulate further evo-devo research in anoles and beyond. For starters, is the same pathway responsible for male facial elongation in other species in the carolinensis clade, or are more ‘traditional’ mechanisms operating there? This important research highlights that investigators need to consider all aspects of signaling systems, including circulating hormones, their receptors, and signal cascades that result from activation of a particular pathway. Clearly this paper by Sanger et al. is an excellent step in the right direction for understanding how developmental pathways lead to adult difference in anoles, and it will also steer other investigators to consider a diversity of developmental mechanisms in their quest to elucidate how adults end up the way they do.
Sanger TJ, Seav SM, Tokita M, Langerhans RB, Ross LM, Losos JB, Abzhanov A. 2014. The oestrogen pathway underlies the evolution of exaggerated male cranial shapes in Anolis lizards. Proceedings of the Royal Society B 281:20140329.
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.
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.
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.
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.
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!
A figure from Quinn’s poster, showing alternative possible energy budgets in green anoles (click for a better view).
Animals allocate energy that they acquire to a variety of bodily functions and activities. Some of the more important allocations are those toward metabolism and growth, though the relative allocations to these is unclear. McKenzie Quinn, an undergraduate student working with Michele Johnson at Trinity University, presented her work in the third poster session on the dynamic energy budget of green anole lizards. She quantified food intake, excretion, growth, and resting metabolic rate (RMR, the energy required for basic maintenance) of individual lizards over 40 days to create a predictive model to describe how they allocate energy. If metabolism receives a large allocation, then RMR and/or body mass are expected to be significant predictors of energy use. On the other hand, if growth is more important, then aspects of body length (snout-vent length, SVL) are expected to be better predictors.
Interestingly, she found that RMR and body mass were not included in the best model of energy use. Instead, their model building (with AIC criteria, if you’re interested) showed that a decreasing nonlinear function of SVL was the best model. This suggests that metabolic functions are a small, non-significant part of these lizards’ dynamic energy budget. This work was conducted on adult males, so it will of course be interesting to see how this approach might apply to younger individuals or females. However, this is useful information to know for those who wonder how anoles allocate energy in their daily lives.
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?
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.