I spent most of last week battling a cold. One of those awful, end of the semester, got-a-taste-of-freedom-but-now-I’m-bedridden colds. If you’re anything like me, the first sign of illness sends you scrambling trying to recall what your mother said about coping with illness. I always seem to remember my mother explaining fevers to me.
I was shivering under a pile of blankets, influenza-ridden and miserable, suffering from the worst fever I experienced as a child. My mother described a war being waged inside me, self versus virus, and tried to convince me that this crummy feeling might actually be a good thing. Impossible! I spent that day wrapped up a little too tightly, flashing from hot to cold, cold to hot. The next morning, I was achy and exhausted—a war had taken place in there, after all—but miraculously, I felt better. I was amazed that my body’s response to the flu-y invader had contributed to its eradication!
“Fever” occurs when body temperature is elevated above the normal range, and we have long been intrigued by its function in staving off infection. In our own bodies, the brain can trigger the body to heat up when it detects infection. This temperature elevation seems to facilitate the immune system, and the warm temperatures make the internal environment less favorable for infectious agents such as bacteria or parasites attempting to live there (feverish to know more?). Endothermic creatures such as us, which produce and maintain body heat internally via metabolic activity, frequently battle infection with fever. But what about ectotherms?
Ectotherms, such as reptiles, depend on external heat sources to maintain their body temperature, and thus cannot produce fever the same way endotherms do. However, ectotherms may be able to behaviorally regulate their body temperature so that it rises above the normal range. By spending more time basking, reptiles fighting infection may raise their body temperature enough to harness the healing effects of elevated body temperature associated with fever. Elevated body temperatures have been observed in many reptiles fighting infection in the lab, but it remains unclear whether reptiles in the wild alter their behavior and body temperature in response to infection. We put this to the test in our recent paper!
Our team investigated whether two species of Anolis lizards from the Dominican Republic (Anolis armouri and A. cybotes) altered their basking behavior in response to infestation by chigger mites. The chiggers themselves irritate the lizards and may transmit blood-borne pathogens such as haemogregarines, or others, that can seriously harm them.
We predicted that heavily parasitized lizards would spend more time basking, and consequently have higher body temperatures, than lizards with fewer or no parasites. We studied lizard populations distributed throughout a 2000-m elevational transect in the Sierra de Barouco mountains in the Dominican Republic. We observed the basking behavior of free-living lizards, and then caught these individuals to measure body temperature and parasite loads. Many of these anoles were naturally parasitized by chigger mites, which reside in the folds of the collapsed dewlap, beneath the limbs, and near the cloaca. Several mites are visible on the extended dewlap of the A. cybotes on the right.
We found a strong altitudinal pattern in parasite infestation—whereas many lizards at mid-elevation sites were heavily parasitized (some lizards had 200+ mites, among the highest levels recorded for lizards!), most individuals at low elevations and high elevations had few or no parasites at all. We also observed altitudinal patterns in basking behavior, which increased with elevation. However, neither basking behavior nor body temperature was significantly correlated with parasite load.
In our study, chigger infestation did not elicit a febrile response in anoles. It is possible that anoles do not exhibit febrile responses to any infection, as has been suggested by some previous work. In fact, some studies report a hypothermic response to infection. Alternatively, chigger infestation may not impose sufficient costs to the lizards to trigger a febrile response. Chiggers and other ectoparasites parasitize many different host species, but it is often unclear whether these parasites negatively impact hosts. Even in cases in which ectoparasites are negatively associated with some aspect of host condition, we do not clearly understand the mechanisms through which ectoparasites elicit these effects.
Clearly, much remains to be learned about how wild animals cope with infection and infestation. Be sure to check out our paper for details on this cool study, and for a discussion on a possible mechanism underlying the altitudinal pattern we observed in parasite infestation!
Conover, Asa E., Ellee G. Cook, Katherine E. Boronow, and Martha M. Muñoz. “Effects of Ectoparasitism on Behavioral Thermoregulation in the Tropical lizards Anolis cybotes (Squamata: Dactyloidae) and Anolis armouri (Squamata: Dactyloidae).” Breviora 545, no. 1 (2015): 1-13.