Adaptation to seasonal change in osmotic stressors in the Souss Valley tortoise Testudo graeca graeca, in an arid steppe-land of west-central Morocco: allostasis vs. homeostasis

We investigated for the first time, the seasonal patterns of iono- and osmotic responses of the Souss Valley tortoises, Testudo graeca graeca, in an arid steppe-land of west-central Morocco, a year-round (2011–2012). The mean body mass condition index varied significantly over time, with significantly higher values in males in summer and autumn 2011. All the plasma variables (osmolality and sodium, potassium, chloride, and urea concentrations) varied significantly among seasons, increasing during the summer dry period and decreasing after rainstorms. Osmolality and solute concentrations in voided urine were more variable than in plasma, with more pronounced decreases related to rainfall events. Soluble potassium was the major cation in voided urine. Potassium concentration decreased markedly following rainfall events, and rapidly increased in late summer and autumn 2011, when some green plants were available. As spring progressed and tortoises gained body mass while feeding on hydrated vegetation, urine osmolalities decreased, and then re-increased during the dry summer period, becoming essentially iso-osmotic with plasma. When summer thundershowers dropped rain, tortoises drank water, voided concentrated bladder urine, and stored dilute urine; body mass, plasma and urine concentrations returned to hydrated levels. Tortoises exhibited a tolerance to osmotic stressors that allowed them to osmoregulate opportunistically in their arid environments via allostasis, a process that allows maintaining homeostasis through change. When bladder urine becomes iso-osmotic to plasma, tortoises tolerate increases in osmolality and concentrations of plasma solutes. The success of facultative osmoregulation and variable ‘allostasis’ would depend on the occurrence of summer rain events as a source of drinking in osmotically stressed tortoises.