Anole Annals

Morgan Gerace and her SICB 2015 poster on dorsal crest erection in Anolis sagrei

We perviously learned about new research on the mechanisms of dorsal crest erection in the brown anole, Anolis sagrei, being done by the Rand Lab at Carleton College. This is a interesting new research area with relevance to our understanding of anole signaling and anatomy that is being carried out by Rand and a team of impressive undergraduate students. Rand Lab student, Morgan Gerace, presented a second Rand Lab poster on this topic at SICB 2015. Following up on the first poster, in which the authors found no evidence of the involvement of muscles, cartilage, or vascular sinus in crest erection, Morgan tested whether crest erection is due to an inflammatory response. By injecting male anoles with the an adernegenic receptor  antagonist, interfering with the lizard’s fight-or-flight response, Morgan determined that crest erection may be the result of an inflammatory-like response. Conversely, injection of with epinephrine, essentially supercharging the fight-or-flight response, facilitated a crest erection response. This exciting work by this set of motivated undergraduates draws new attention to the physiological control mechanisms of this under-studied display mechanism.

Dorsal crest erection in  Anolis sagrei

Anolis equestris. Photo courtesy of Day’s Edge Productions.

The ability to move through complex arboreal habitats is critical to anoles, yet we know very little about the physiological mechanisms that underlie arboreal locomotion.  Kathleen Foster, a graduate student in Tim Higham’s lab at UC-Riverside, presented an outstanding talk at SICB this week on the kinematics of locomotion in Anolis equestris. (Yep, Kathleen gave both a poster and a talk at this meeting!)

The relationship between the force a muscle can produce and the length of that muscle determines how force is generated at different positions of a limb – at different joint angles, the muscle will have a different length.  But, this is extremely difficult to study in anoles, as standard surgical techniques aren’t possible in such small muscles. Kathleen is focusing on the muscles of the hindlimb, and she first determined that tendon strain did not contribute to the force-length relationship in these muscles. This result indicated that in vivo measures of muscle length should provide relevant information on limb kinematics. Next, she addressed how muscle function changes as lizards move on different substrates, using the gastrocnemius (a muscle in the “calf” of the hindlimb). Her preliminary data showed that this muscle is more active on a broad, flat perch than a small, narrow perch, that the active length of the gastrocnemius is more optimal on a flat perch, and that the maximum force generated by the gastrocnemius is greater on a flat perch. Together, these results indicate that the gastrocnemius contributes more to locomotor propulsion on a flat perch than a small perch.

And, because Kathleen has shown that tendon strain doesn’t significantly contribute to differences in muscle length in lizards, she can now study smaller species.  This will allow her to examine the physiological differences underlying locomotor adaptations among ecomorphs!

Brown anole photo by Amber Brace.

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.

Life cycle of malaria, showing infection of red blood cells (erythrocytes). Photo from malariasite.com

Anolis stratulus, one of the species studied. Photo by Jerry Husak.

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.

Jamal Murray. Photo from Johnson Lab website.

Activity is where the rubber meets the road in the interaction between organisms and their physical and social environments: in order to acquire energy, attract mates, and produce offspring you have to get up and move around. But what dictates how much activity an individual will engage in? We know a fair amount about what causes organisms to be more or less active, with temperature a particularly important one for ectotherms like anoles. Nonetheless, the physiological mechanisms that underlie activity variation are less well understood. Jamal Murray, an undergraduate in Michele Johnson’s lab, presented a poster at SICB that begins to address this question with the Puerto Rican Anolis stratulus. He put lizards into an enclosure with grids marked on all sides, and measured activity rates as the number of times individuals moved from one grid to another. Afterward, he measured blood glucose levels and found that individuals with higher glucose levels were more active. This suggests a proximate physiological mechanism driving differences in activity rates among individuals and, potentially, populations and species.

Image of an anole with a regenerated tail. The point of breakage (and regeneration) is shown with an arrow. Image from Wired.com

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.

Species studied by Kircher et al. Image credit to Bonnie Kircher.

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.

Photograph of Deep Shukla, courtesy of neuroscience.gsu.edu.

What effect does social rank have on display rate in Anolis carolinensis? Can individual display patterns predict social status? Deep Shukla, a graduate student at Georgia State University in Walt Wilczynski’s lab, addressed these questions during Tuesday’s poster session at SICB 2015. Green anoles often form dominance hierarchies in conditions with limited resources (such as those in captivity). Deep predicted that competition for these resources might also cause behavioral variation in display use between dominant and subordinate anoles. Using a mirror to induce display behavior and a female to induce courtship behavior, Deep counted the number of pushups performed by size-matched male anoles in isolation. He then housed two males together for 7 days to allow males to establish a dominance hierarchy within the cage. After the weeklong cohabitation, Deep again measured display frequency levels for each male.

In the baseline trials Deep found that the males who later were identified as dominant and subordinate males did not differ in the frequency of aggressive or copulatory displays. After cohabitation, display use frequency declined for both dominant and subordinate lizards overall, but dominant lizards showed higher levels of aggression relative to subordinates.  Deep also found that aggressive behavior was correlated with copulatory display before cohabitation; however, this result disappeared after cohabitation. These results suggest that dominance hierarchies in anoles can alter display behavior use based on social rank! This is exciting because it means that dominance hierarchies may be established and maintained in complex ways. Deep is interested in exploring these relationships further by measuring brain metabolic activity and morphology before and after the establishment of dominance hierarchies. Given his preliminary data, it seems likely that there will be interesting differences in the brain that accompany this suite of behavioral changes!

Note: This post was written by Bonnie Kircher, a graduate student studying anole development in Marty Cohn’s lab at the University of Florida.