Erythrocyte morphology of the Critically Endangered Geometric Tortoise Psammobates geometricus: effects of season, sex and age

Land transformation reduced the habitat of Psammobates geometricus to small isolated fragments, rendering the species vulnerable to extinction. Field biologists often use erythrocyte metrics to assess wildlife health but need baseline values to distinguish normal variations from disease or chronic stress. Here we characterise erythrocyte types and features of P. geometricus and assess if age, sex and environmental fluctuations influence their frequencies and metrics. We collected blood from males, females and juveniles over four seasons to prepare smears for cytological evaluation. We treated the smears with May–Grünwald–Giemsa stains and used imaging software to measure size, shape and colour intensity of erythrocyte types. The blood of all groups contained immature, mature and senescent erythrocytes, and the appearance of each cell type corresponded to descriptions for other chelonians. Season had a strong effect on the occurrence of erythrocyte types and features with few group differences. The frequency of immature erythrocytes was high in winter and spring, the wet season, reflecting increased erythropoiesis. Mature erythrocytes dominated in summer and autumn, but frequencies of senescent cells, poikilocytosis, cytoplasmic inclusions and cytoplasmic vacuoles increased in autumn, indicating that water and food shortages caused physiological stress. The small size and elongated shape of summer and autumn erythrocytes would facilitate oxygen exchange when high temperatures increase metabolic needs of ectotherms. Lower temperatures and energy demands in the wet season counter the lower oxygen capacity of round, immature cells. Our results provide baseline information on erythrocyte physiology and metrics for health assessments of geometric tortoises.