Leukocyte allometries in birds are not affected by captivity

Body size affects many traits, but often in allometric, or disproportionate ways. For example, large avian and mammalian species circulate far more of some immune cells than expected for their size based on simple geometric principles. To date, such hypermetric immune scaling has mostly been described in zoo-dwelling individuals, so it remains obscure whether immune hyper-allometries have any natural relevance. Here, we asked whether granulocyte and lymphocyte allometries in wild birds differ from those described in captive species. Our previous allometric studies of avian immune cell concentrations were performed on animals kept for their lifetimes in captivity where conditions are benign and fairly consistent. In natural conditions, infection, stress, nutrition, climate, and myriad other forces could alter immune traits and hence mask any interspecific scaling relationships between immune cells and body size. Counter to this expectation, we found no evidence that immune cell allometries differed between captive and wild species, although we had to rely on cell proportion data, as insufficient concentration data were available for wild species. Our results indicate that even in variable and challenging natural contexts, immune allometries endure and might affect disease ecology and evolution.