Anole Annals

Frequently phenotypic evolution is rapid on islands, resulting in many ecologically diverse species. Most of what we know about the faster evolution on islands has been through the examination of morphological diversity. By utilizing species-rich Anolis lizards, Dr. Martha Muñoz and collaborators were able to examine the island effect with regards to rates and patterns of evolution through a different lens: thermal physiological trait diversity. Muñoz et al. examined the evolutionary dynamics of cold tolerance, body temperature, and heat tolerance in island and mainland anoles. They discovered faster heat tolerance evolution in the mainland lineages, and that island and mainland anoles are evolving towards separate trait optima with island lizards having a higher upper thermal limit. A higher optima and slower evolutionary rates are consistent with the Bogert Effect, in which organisms are shielded from selection because of behavioral buffering such as thermoregulatory behavior. Cold tolerance did not differ between habitats, not surprisingly due the fact that lower physiological limits cannot be behaviorally buffered against selection. Despite island and mainland anoles occurring in similar thermal environments, island lizards thermoregulate more. Consequently, the ecological opportunity (fewer predators and/or competitors) provided on islands may be reducing the costs of thermoregulation and slowing down, rather than accelerating evolution of certain traits. This study highlights the importance of other phenotypic axes that organisms diversify along in an adaptive radiation, such as physiological diversification, and that behavior can elucidate or drive patterns shaping evolution.

The effect of temperature on biological processes and systems is one of the most studied topics in ecology. Despite a wealth of existing research, we still have a relatively poor understanding of what factors contribute to the thermal tolerance of complex organisms. Much research suggests that oxygen limitation at extreme temperatures is what determines the thermal limits of complex aquatic life; however, this hypothesis (i.e. Oxygen-and capacity-limitation of thermal tolerance; Pörtner 2010) has not proven very useful in explaining the thermal limits of terrestrial organisms. One reason is that there is a comparatively greater amount of oxygen in air vs water. Moreover, terrestrial organisms tend to have very efficient systems of ventilation (e.g., air sacs of birds and tracheal system of insects). Terrestrial vertebrate embryos, however, rely solely on diffusion of oxygen through a hard shell and, thus, their thermal limits may be set by oxygen limitation (Smith et al. 2015).

Sylvia Nunez and Thom Sanger set out to determine the relationship between oxygen availability and temperature for brown anole (Anolis sagrei) embryos. Previous work shows that thermal stress can induce embryo mortality and severe craniofacial malformations at incubation temperatures above 33 °C (Sanger et al. 2018). They used a factorial design (2 incubation temperatures: 27 °C and 33 °C; and 2 oxygen treatments: 10% O₂ and 21% O₂) and dissected eggs at day 14 or day 20 after oviposition to measure the effects of these treatments during morphogenesis and the growth phase of development, respectively. They found that hypoxia did not lower survival during these periods at 27 °C; however, survival was reduced for embryos incubated at 33 °C and under hypoxic conditions (i.e. 10% O₂). Furthermore, high temperatures and low oxygen resulted in various craniofacial malformations and increased incidences of cerebral blood-pooling. It appears that oxygen supply may limit the thermal tolerance of anole embryos, and these data support the findings of previous work in other lizard species (Smith et al. 2015). The next steps for the Sanger lab are to determine the cellular mechanisms that drive the results discovered in their current study.

Pörtner, H.O., 2010. Oxygen-and capacity-limitation of thermal tolerance: a matrix for integrating climate-related stressor effects in marine ecosystems. Journal of Experimental Biology, 213: 881-893.

Sanger, T.J., Kyrkos, J., Lachance, D.J., Czesny, B. and Stroud, J.T., 2018. The effects of thermal stress on the early development of the lizard Anolis sagrei. Journal of Experimental Zoology Part A: Ecological and Integrative Physiology, 329:244-251.

Smith, C., Telemeco, R.S., Angilletta, M.J. and VandenBrooks, J.M., 2015. Oxygen supply limits the heat tolerance of lizard embryos. Biology letters, 11:20150113.

Different environments promote different life history strategies. The way an organism allocates resources in an environment can have large consequences on growth, reproduction, and immune function. Furthermore, energy allocation tradeoffs may differ between sexes. The energetic costs of immunity can differ between sexes due to differences in energetic demands and ecology. However, the reason some organisms exhibit sex-based energy allocation, and the causes of this phenomenon, remain enigmatic.

Zachariah Degon, an undergraduate student at Georgia Southern University, and colleagues examined the relationships between ectoparasite load, organ mass, fat body mass, and total body size in the Panamanian anole, Anolis apletophallus. Mainland populations of adult male (n=72) and female (n=34) A. apletophallus were sampled in Gamboa, Panama during the reproductive season. Each lizard was visually inspected for mites. The density of mites, and the location of each mite on the lizard body, were recorded. Lizards were dissected and all fat-storing organs were removed. Fat-storing organs included the fat bodies, livers, and gonads. Organs were dried and weighed before measuring organ mass. They found that overall males had more mites than females, and that this difference was driven by the high density of mites located on male dewlaps. Larger lizards, regardless of sex, had higher mite loads. Higher fat body mass was linked to decreased mite loads, although this was only true for male lizards. Liver mass had no effect on mite load in either species. However, mite load increased with ovary mass in females, but there was no relationship between testes mass and mite load in males.

Overall, sex-based differences in energy allocation may have important implications for maintaining immune function in variable environments. Male A. apletophallus had higher mite loads due to their heavily parasitized dewlaps. And interestingly, males with increased fat body masses had lower mite loads. This suggests that males may be allocating energy away from storage and towards increasing immune function.

Jenna in search of brown anole eggs.

Maternal nest-site choice plays an important role in determining the developmental environment of oviparous organisms, but even so, almost nothing is known about anole nest-site choice. Jenna Pruett (Ph.D. student in Dan Warner’s lab at Auburn) and company set out to examine what microhabitats female Anolis sagrei are choosing to lay their eggs in on the Warner Lab’s experimental intercoastal islands, and to experimentally manipulate eggs by altering nest microhabitat locations in the field. Jenna found 100% of eggs under some type of cover object (rocks, leaf litter, etc.)  and quantified nest habitats in comparison to what was available in the environment, finding that moms choose nest sites that were cooler and moister habitats than what was available on the island overall.

The second part of this study involved taking 400 eggs (200 placed in June and 200 placed in August) from the lab breeding colony and incubating them in the field across all island microhabitat types. The control eggs were sealed in a Petri dish to ensure a constant moisture supply throughout incubation. Survival was higher in the control eggs with constant moisture and higher in August. They found that survival probability decreased as the incubation temperature increased and that hatchling body condition improved with increased soil moisture.

To top off this very neat study, Jenna was the winner of the Division of Animal Behavior best student presentation award, the Marlene Zuk Award. Congratulations Jenna!

Parasites are an ever-present threat to the organisms they interact with. Reptiles, like anoles, are often heavily infested with mites, an ectoparasite that drinks the lizard’s blood and are often visible on the surface of the skin. Despite the ubiquity of mite infestations on reptiles, the fitness costs of these infestations and the factors that cause mite load to vary among individuals within populations are surprisingly understudied.

Adam Rosso, a masters student in Christian Cox’s lab at Georgia Southern University, studied the factors that drive variation in mite infestation among individuals in a population of slender anoles (Anolis apletophallus) in Panama. Slender anoles are sexually dimorphic; males have much larger dewlaps than females. Adam counted mite loads on hundreds of lizards and asked a series of questions, including: How does mite load differ between the sexes? Do the sexes differ in where they are being parasitized on their bodies? Can ecological factors such as habitat use and body temperature affect mite load?

First, Adam found that males have more mites than females, but this was due entirely to their larger dewlaps. In fact, females actually had more mites on other parts of their bodies (such as on their hind and forelimbs). But it gets even more interesting: Adam found that mite infestation increases with dewlap area in males but not in females, suggesting that mites prefer male dewlaps over female dewlaps. Neither field-active body temperature nor perch characteristics predicted mite loads in either sex, suggesting that dewlaps are the main factor influencing ectoparasitism in this population. Adam’s results suggest that there may be an important cost to producing a large dewlap in males. More generally, if parasite loads and dewlap sizes are seen as honest signals in anoles, the results of Adam’s work could have implications for understanding sexual selection and morphological evolution in this group of lizards.

Predicting the responses of species to current environmental and climate change is one of the largest duties of current biologists. Ectothermic species (including lizards) are particularly vulnerable because they lack the ability to metabolically generate heat and rely on environmental sources of temperature to maintain their body temperatures. For species that live in the tropics, this task is much harder because tropical environments experience less temperature variation both within and across seasons. Tropical lizards traverse these landscapes to try and maintain optimal and preferred body temperatures, but are all thermal environments equal in the constraints they impose on lizards?

To address this question, Lauren Neel, a student of Mike Angilletta’s at Arizona State University, collected an astounding amount of data from two species of anole: Anolis sagrei from their native range on Great Exuma in the Bahamas, and A. apletophallus in Panama. She collected environmental temperature data using biophysical models, thermal performance data by racing anoles at several different body temperatures and measuring their sprint speed, and preferred body temperatures by placing lizards in a thermal gradient. Despite both lizards living in tropical climates, she found distinct differences between the environments (and anoles!). Anolis sagrei thermoregulated more,  was active for longer periods of time than A. apletophallus, and exhibited warmer preferred temperatures. Neel and colleagues also found that A. sagrei is not likely to suffer a drop in performance capacity as environments warm over time, whereas A. apletophallus is likely to experience a significant reduction in their speed performance which might be a physiological precursor to population collapse and a rise in local extinction events. Great stuff coming from Lauren Neel; stay tuned for more!