The effects of annual cycle, source population, and body condition on leukocyte profile and immune challenge in a basal reptile, the tuatara (Sphenodon punctatus).

Leukocyte profiles are broadly used to assess the health status of many species. Reference intervals, and an understanding of the factors that may influence these intervals, are necessary for adequate interpretation of leukograms. Using a data set that spans over three decades, we investigated variation in leukocyte profile in several populations of the evolutionarily unique reptile, the tuatara (Sphenodon punctatus). To do this, we first established reference intervals for each leukocyte type according to best practices. Next, we determined that source population and sampling date were the two most important predictors of leukocyte makeup. We found significant differences in the ratio of heterophils: lymphocytes (H:L) between populations, with tuatara on the more resource‐stressed sampling island having a significantly higher ratio of H:L. Finally, we found that sampling location, sex, and life stage did not explain variation in the responses of tuatara to stimulation with Concanavalin A and lipopolysaccharide in both 3‐(4,5‐dimethylthiazol‐2‐yl)‐2,5‐di‐phenyltetrazolium bromide and Griess assay experiments. Our results offer important insight into the function of leukocytes in reptiles. Research Highlight: We established leukocyte reference intervals for tuatara and found that island and sampling date were the most important predictors of leukocyte makeup. Tuatara in more resource‐stressed populations had higher H:L ratios, but were able to mount immune response despite poor body condition. [ABSTRACT FROM AUTHOR]