Author: Chris Thawley

Caty Tylan measuring anole footpad thickness as a indicator of immune function.

A major challenge for organisms is to protect themselves from pathogens, things in the environment, including other organisms or toxins, that can cause disease and harm them. Animals, including anoles, have several different types of immune responses by which they can respond to pathogens. Ecologists are particularly interested in how these immune responses work in natural settings, how effective they are at protecting organisms, and how other aspects of an organism’s environment, including diet, stress, and reproduction, may positively or negatively impact immunity. However, immune systems are very complex, and measuring immune function, especially in the field, can be quite challenging!

Ecoimmunologists have developed various procedures to test different aspects of immune function, but ideally these procedures should be validated, or proved to be meaningful, in each organism they are used in. The phytohemagglutinin (PHA) skin test is one such assessment of immune function that is commonly used. This procedure involves injecting a small amount of phytohemagglutinin, a plant protein that provokes an immune response in animals, into the tissue of an organism of interest, waiting a given amount of time, and measuring the resulting swelling which is then used as an index of immune response. Researchers like this test because it is minimally invasive, works in almost any species, and is simple and easy to do in the field. Though this test has been used in multiple reptile species, it has never been validated in a reptile.

Enter Caty Tylan, a Ph.D. student and diploma-carry veterinarian from Penn State University, who addressed this issue in her talk “Local and systemic immune response to phytohemagglutinin: Validation of the PHA skin test in the green anole, Anolis carolinensis.” Caty chose to validate this test in green anoles (Anolis carolinensis), a model anole species, using two different types of PHA, PHA-L and PHA-P, which are available to researchers. She injected PHA into anole footpads and compared swelling in those feet over 48 hours post-injection. By comparing swelling in these feet to those which were only injected with sterile saline, Caty showed that PHA does induce a swelling response over 48 hrs, and that this response is the same for both types of PHA. She also examined the white blood cell counts in these anoles and found that PHA-L, a more purified and specific PHA, induced the stronger lymphocyte response, an immune measure that many ecoimmunologists look to quantify. In the future, Caty will examine histological sections of injected anole feet to examine the local immune response to PHA injections and fully validate this assay. This work should allow effective use of the PHA assay in future anole research and support investigations into how various environmental variables affect cell-mediated immune function in reptiles.

For many years, biologists believed that evolution was a process that played out over vast stretches of geological time and would not be observable during field studies. More recent research, however, has begun to show that evolution can occur very quickly and that experiments in the field can address evolution in action. Tom Schoener, eminent professor at the University of California, Davis, shed light on our evolving view of how evolution occurs in his talk, “Eco-evolutionary Aspects of the Lizard Anolis sagrei in an Island Metapopulation” at JMIH 2016.

By introducing a novel predator, the curly-tailed lizard (Leiocephalus carinatus), which devours anoles, to a series of small islands in the Bahamas, Schoener and colleagues were able to observe evolutionary responses in A. sagrei in fewer than 10 years. By preying on A. sagrei, curly-tailed lizards induced behavioral changes in perch height, and created selection for relatively longer limbs that increase anoles’ ability to escape this predator. Curly-tailed lizards also caused a variety of ecological effects, including reducing anole populations and changing arthropod abundance, which may affect the future evolution of anoles on these islands. Ongoing monitoring shows that these anole populations seem to be rebounding and that different types of selection may be acting on hindlimb length.

A curly-tailed lizard (Leiocephalus carinatus) displays its namesake in Florida. Photo by Ianaré Sévi.

Perhaps not surprisingly, many of the experimental islands were occasionally devastated by hurricanes which are becoming more frequent and more powerful in the Caribbean. While these extreme weather events interrupted some of Schoener’s planned research, they also provided a unique opportunity to study how hurricanes may cause natural selection. Schoener found that anoles which survived hurricanes had longer hindlimbs, and these lizards were better able to hold onto trees and other perches at high wind speeds, likely increasing survival of hurricanes by preventing lizards being blown out to sea! Taken together this body of research suggests that novel environmental changes, such as invasive species or increasingly extreme weather, exert selection on organisms and that we can observe these organisms evolving rapidly on ecological timescales.