Johannes Foufopoulos

Advisor: Tony Ives

PhD Abstract: Host-Parasite interactions in the mountain spiny lizard Sceloporus jarrovi.

Sublethal parasites are exceedingly common in nature, yet their effect on their hosts is not well understood. I examined host-parasite interactions in a natural population by experimentally manipulating parasite burdens in a free-ranging population of mountain spiny lizards, Sceloporus jarrovi (Phrynosomatidae, Iguania). These lizards harbor several macroparasite species such as gastrointestinal nematodes and ectoparasitic mites, as well as microparasites such as the lizard malaria Plasmodium chiricahuae. Macroparasite and microparasite burdens were respectively reduced by administering an antihelminthic and an antimalarial compound to the hosts. Preliminary pen experiments demonstrated the ability of the drugs to significantly reduce both nematode and ectoparasite burdens as well as malarial parasitemias. Administration of the antimalarial treatment in females resulted in significant mass and body size (snout-vent length) increases as compared to the control group - no such results were observed in males. Macroparasite treatment led to significant mass and body size increases in males but not in females. Neither treatment had significant effects on host survival as inferred from recapture rates. These sex-specific effects of parasitism were best explained by the differing energetic and physiological demands put upon males and females as the result of different reproductive schedules. Infection with Plasmodium also reduced female clutch size. Comparing infected and uninfected lizard populations using Leslie matrices indicated that infection with Plasmodium may result in a reduction of population growth rate of approximately 12%. Hence infection with malaria may have the potential to significantly reduce population growth rates for free-ranging lizards.

During mating season male lizards carry approximately four times as many mites as females. Past studies have indicated that changes in levels of steroid hormones may be responsible for such changes in parasitism levels. To explore this pattern, I conducted an experimental manipulation of testosterone levels in male S. jarrovi by selectively castrating or implanting male lizards with testosterone. Increases in testosterone produced strong increases in ectoparasite burdens and a weak concomitant dicrease in gastrointestinal helminths numbers. I propose two mechanisms, involving changes in the innume system or changes in host behavior, to explain these results.