Haematology and plasma biochemistry reference intervals for the critically endangered western Santa Cruz Galapagos tortoise (Chelonoidis porteri)

Reference intervals (RIs) are an increasingly valuable tool for monitoring captive and free-living wildlife populations. Galapagos tortoises are one of the most emblematic species on Earth with 9 of the 12 extant species considered endangered due to human activities. As part of a long-term health assessment within the Galapagos Tortoise Movement Ecology Programme, we sampled a total of 210 free-living Santa Cruz Galapagos tortoises (Chelonoidis porteri). We collected blood from the brachial vein and performed packed cell volume (PCV), total solids (TS), morphological evaluation, white blood cell (WBC) count estimates and differentials and a VetScan biochemistry panel for each individual. We calculated 95% RIs and 90% confidence intervals (CIs) using the Reference Value Advisor tool and following international standard guidelines. Tortoises were categorized by estimated age and sex, with RI and CI reported here for 164 adult tortoises and 46 sub-adult tortoises. We found significant differences between sexes, with adult females presenting a lower PCV and higher values for both calcium and potassium. Among age groups, adult tortoises presented higher PCV, TS and albumin and lower WBC counts, aspartate aminotransferase and creatine kinase than sub-adult tortoises. We also found that tortoises presented higher numbers of lymphocytes during the dry season, but higher basophils, eosinophils, phosphorus, potassium and TS during the humid season. The heterophil:lymphocyte ratio did not differ between groups. To the authors’ knowledge, this is the first report of formal plasma biochemistry and haematology RI for free-living Galapagos tortoises. With the present study we provide an important diagnostic tool for captive-breeding programs in the Galapagos and zoological institutions globally that care for giant tortoises. The ultimate goal of this study is to contribute to the understanding of giant tortoise baseline health parameters and to inform local management decisions that help to conserve these emblematic species.