Ecoimmunological differences among congeneric tortoises in the United States

Comparative studies, especially of related species that span across ecoregions, have the potential to increase our understanding of different ecological or evolutionary pressures that may drive host–pathogen dynamics. We quantified differences in immune investment, via differential leukocyte counts and bacteria-killing assays, across four closely related species of Gopherus tortoises, found across a gradient from the desert southwest to the subtropical southeast of the United States. We further quantified differences in a commensal nasal microbe (Pasteurella testudinis) and tested for associations among immune measures, P. testudinis, and previously quantified levels of the pathogen Mycoplasma agassizii and upper respiratory tract disease (URTD). We also evaluated the potential influence of environmental variables on immune investment, P. testudinis, and disease. We found that tortoise species in more arid environments had a reduced investment in inflammatory leukocytes, possibly as a strategy to reduce water loss, and invested more heavily in innate anti-inflammatory leukocytes. Conversely, we found that species in moister, resource-rich environments may face greater pathogen pressure, likely due to increased population densities and transmission rates among host tortoises. These tortoises had increased investment in inflammatory cells and appeared to reduce their nasal microbes (including P. testudinis) when they exhibited URTD. Thus, we quantified two negative correlational patterns: (1) between inflammatory responses and water conservation and (2) between resource quality and transmission rates. We hypothesize that these relationships across species may reflect variable development or evolution of innate immune functions.