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Seasonal Variation in Desert Tortoise Immune Function
AdvisorTracy, Richard C.
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Threatened Mojave desert tortoise (Gopherus agassizii) populations may be negatively impacted by upper respiratory tract disease. To more fully understand the general impact of diseases in wild populations, a more complete knowledge of the factors that contribute to immunocompetence in this species is necessary. We analyzed minimal seasonal fluctuations in innate immune function and white blood cell numbers in a colony of captive, healthy tortoises. Bacteriocidal assays measured the ability of constitutive natural antibodies and complement proteins to kill a standard strain of Escherichia coli. Ratios of heterophils to lymphocytes were used to infer relative levels of chronic stress and/or activity among the four seasons. Bacteriocidal capacity was significantly suppressed during winter relative to the other three seasons, but heterophil:lymphocytes did not vary significantly among seasons. These results suggest that tortoises may be inherently immunosuppressed during winter, but elevations in stress do not contribute to this immunosuppression.