Do changes in body mass alter white blood cell profiles and immune function in Australian cane toads (Rhinella marina)?

Variation in food resources can result in dramatic fluctuations in the
body condition of animals dependent on those resources. Decreases in body
mass can disrupt patterns of energy allocation and impose stress, thereby
altering immune function. In this study we investigated links between
changes in body mass of captive cane toads (Rhinella marina), their
circulating white blood cell populations, and their performance in immune
assays. Captive toads that lost weight over a 3-month period had increased
levels of monocytes and heterophils and reduced levels of eosinophils.
Basophil and lymphocyte levels were unrelated to changes in mass. Because
individuals that lost mass had higher heterophil levels but stable
lymphocyte levels, the ratio of these cell types was also higher,
partially consistent with a stress response. Phagocytic ability of whole
blood was higher in toads that lost mass, due to increased circulating
levels of phagocytic cells. Other measures of immune performance were
unrelated to mass change. These results highlight the challenges faced by
invasive species as they expand their range into novel environments which
may impose substantial seasonal changes in food availability that were not
present in the native range. Individuals facing energy restrictions may
shift their immune function towards more economical and general avenues of
combating pathogens.