Category: New Research Page 17 of 66

Fig 1. Photographs of the housing conditions used in the experiment. (a) One of the experimental enclosures (with an artificial tree) surrounded by blinds on all sides (note, the front blind was pulled back to reveal the tree and cage). (b) Close-up of the available horizontal perches. (c) Juvenile Anolis sagrei with its identification number on the lateral body surface for visual identification.

For many animals, optimal habitats vary across age classes, and individuals shift habitat use as they age. While many studies have documented such age-specific habitat use, most are observational and do not identify the causal factors. In addition, we know that competition between species has been an important driver of habitat use in Anolis lizards. However, less is known about the role of competition on habitat use within species of anoles, especially between age classes.

Dan Warner and I previously found that adults use higher and thicker perches than juveniles at our field site in northeastern Florida (Delaney and Warner 2016). We hypothesized that this variation was a result of adults forcing juveniles to suboptimal habitat. Thus, we altered the density of adult males in mesh enclosures (Fig. 1) in the lab and monitored changes in juvenile microhabitat choice.

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Anoles are probably best known for the ecomorph story: the presence of specialized species adapted to the same sets of structural microhabitats on different islands. Anoles in the Greater Antilles have contributed hugely to our understanding of both the evolutionary history and the contemporary ecology of communities of specialists.

While they are better known for specialization of species in communities, anoles have also contributed to our understanding of within-species ecological diversity. Around the same time that Ernest Williams was developing the ecomorph concept, Roughgarden (1972) used data from Lesser Antillean anoles to introduce a new framework for investigating the extent to which a population’s niche width (i.e. the diversity of habitats it uses or prey it eats) is determined by variation among individuals versus variation within individuals. For example, individuals in a population of Anolis roquet differ in the size of prey they consume, mainly because larger individuals can catch and ingest larger prey items. While Roughgarden’s early work set the stage for an explosion of studies of individual specialization over the past decade or two (reviewed in Araújo et al. 2011), surprisingly little work has been done to revisit individual specialization within species of anoles. In particular, we don’t know enough about how much individuals specialize in important aspects of microhabitat that differentiate ecomorphs, especially perch height and perch diameter.

“Gar” lived alone on my desk, so I don’t know if he was an individual specialist or not

Anole Annals contributors Ambika Kamath and Jonathan Losos have helped to fill this gap with a study just published online in Evolution. Ambika and her team spent a summer observing microhabitat use of a population of brown anoles (Anolis sagrei) in a forested park in Gainesville FL. They marked lizards with colored beads, and repeatedly recorded individual lizards’ perch height and diameter, compiling a total of over 1000 observations of 80 anoles. They grouped perch heights and perch diameters into classes, then compared the distribution used by each individual to the distribution used by the whole population (or to the distribution available to that individual) using a proportional similarity index. The mean value of this index gives a measure of the overall degree of individual specialization in a population, as lower overlap values tell us that individuals are specializing on a subset of the available perches.

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Photo courtesy of Catalina Mantilla

This post was written by Brittney Ivanov, research technician in Michele Johnson’s lab at Trinity University.

Catalina Mantilla, a Ph.D. candidate at Florida International University working with Tonia Hsieh of Temple University, is interested in how anoles use their toepads and claws when they run. For most animals, movement on vertical perches such as tree trunks or buildings usually requires specialized morphologies to adhere to these substrates. While many species have evolved adaptations for moving through complex arboreal habits (e.g., prehensile tails or feet, sticky pads, spines), anoles evolved enlarged toepads and distinct claws, presumably to allow for better adhesion. The morphologies of these specialized structures can greatly impact performance; for example, greater toepad area is associated with greater clinging ability. Catalina wanted to better understand how toepads and claws work together to enhance running performance.

Catalina collected 17 males from four Anolis species (A. carolinensis, A. sagrei, A. cristatellus, and A. distichus). Each male was tested in four different running courses to test performance at difference inclines and on different substrates. Two of the courses were positioned at a 45° incline and two at a flat (0°) incline. Plexiglass covered one course at each incline to allow the use of toepads and eliminate the use of claws. Nylon mesh covered the other course at each incline to test the use of both toepads and claws. Performance was evaluated using mean relative sprint speed, relative stride length, and stride frequency.

Catalina found, unexpectedly, that when the lizards ran on the level plexiglass, they ran slower, took shorter strides, and increased their stride frequency compared to when they ran on the inclines. These results suggest that anoles are less stable when they can’t use their claws! in addition, these data support the idea that the combination of toepads and claws is important for their running performance. In the future, Catalina hopes to increase the number of species in this study to determine the effect of ecomorph on claw and toepad interactions during running, and to evaluate limb function changes when running across different inclines.

Kathleen Foster presents her work to a packed room at SICB.

Regular readers of AA will be familiar with the differences in microhabitat use that define the Anolis ecomorphs, but do species with such distinct structural habitats move differently on their specialized perches? In other words, does muscle function differ between the ecomorphs? In the very last session at this year’s SICB, Kathleen Foster, currently a postdoctoral researcher at the University of Ottawa studying the biomechanics of fish locomotion (come back to anoles, Kathleen!), presented a portion of her graduate work in Tim Higham’s lab at the University of California, Riverside, to address this question. She used high speed video to film five species of anoles running on broad and narrow perches at two angled inclines, combined with electromyography to record fore- and hindlimb muscle activity during running.

Photo courtesy of Kathleen Foster.

Kathleen found that all five species had greater motor unit recruitment on steeper inclines than on horizontal perches, and that muscle activity is shorter but begins more abruptly on inclines. Further, recruitment of the gastrocnemius (a “calf” muscle) was greater on broad perches, because the way lizards sit on narrow perches limits the function of this muscle. If you’ve seen how anoles position their feet on both sides of narrow perches, it’s easy to understand how this posture prevents effective propulsion by ankle extension. Kathleen also found several intriguing differences that distinguish trunk-ground species’ muscle function from trunk-crown and crown-giant species. The activity of the caudofemoralis (a limb retractor muscle in the hindlimb) changes more in trunk-ground species as a function of incline, and trunk-ground species use the biceps and gastrocnemius more in the early stance phase of propulsion than trunk-crown species.

Overall, these data help us understand how specialization in neuromuscular function can allow different anole species to successfully move through their varying habitats, and offer insight into how behavioral differences depend on the muscles that underlie them.

Above: Faith Deckard presenting her research on how muscle physiology may explain variation in social behavior among Caribbean anoles.

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.

Above: Andrew Wang presenting his research on how leptin may be a mechanism underlying life-history trade-offs in green anoles.

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.

Leptin is best known as a satiety hormone, but it has important roles as a signal to the body of adequate energy stores. Image from wiki.brown.edu.

Green anole image from reptilesmagazine.com.

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.

(c) OwenMartin12, some rights reserved (CC BY-NC)

The abilities of certain animals to navigate and home on a specific location over long distances are some of the most fascinating behaviors that scientists study. However, studying homing behavior, especially experimentally, can be a major challenge, as many animals home over long distances (thousands of miles), in difficult-to-study environments (underwater, high in the sky), on in ways that are technically difficult or very expensive to monitor. As we know, anoles can be relatively simple (and cheap!) to study. So what if anoles could be developed as a model system for studying homing behavior?

On the surface, the presence of homing behavior in anoles might seem unlikely, as many species are highly territorial and may not travel long distances during their lifetimes. David Steinberg and Manuel Leal showed that, while seeming unlikely at first glance, at least one species of anole, Anolis gundlachi, does indeed show strong homing behavior.

Anolis gundlachi, the yellow-chinned anole, is a denizen of cool, closed forests in Puerto Rico. Because these lizards stick close to their small territories, they likely have little specific knowledge of their surrounding habitats, potentially making navigation through unfamiliar areas difficult. Steinberg displaced anoles 40 and 80 meters from their home territories and then monitored their territories to see how many anoles returned. Surprisingly, 40-60% of females returned and 80% of males returned, even when taken 80 meters from their homes. Simulations of these movements show that it is highly unlikely anoles would be able to return to their territories in this way via random searching. Steinberg then tested whether two common mechanisms that support homing, use of magnet fields and visual detection of polarized light, were responsible for homing, but found that the homing abilities of these anoles do not depend on either of these two senses.

Finally, Steinberg tracked anoles through the Puerto Rican forest using radio transmitters, and found that anoles returned to their home territories with a high degree of accuracy, in some cases making a beeline home within 24 hours! These results suggest that homing ability may be more common in anoles than has previously been considered, and that strong selection for territory ownership in anoles may support spatial memory and navigation in these animals.