Tolerance and osmotic response to food deprivation and salt loading in the herbivorous non-drinking Moroccan Spiny-tailed lizard Uromastyx nigriventris (Sauria: Agamidae)

Abstract

Animals inhabiting arid environments use a variety of behavioural and physiological strategies to balance their water and salt budgets. We studied the effects of dehydration and salt loading on osmoregulatory capacities in a large herbivorous desert lizard, the Moroccan Spiny-tailed lizard Uromastyx nigriventris, the family Agamidae. These lizards select plants with a high K⁺ to Na⁺ ratio of 15 to 20, and like other herbivorous lizards, effectively eliminate the extra electrolyte load, mainly via a pair of active nasal salt glands, which exude the extra ions from blood. Here we present results of a series of laboratory experiments, which tested a five-week food and water deprivation and the excretory response of nasal salt glands, during a short period of five days, following salt loading by two separated injections of KCl or NaCl at a 5-day interval (4th and 9th days). During food-water deprivation, hypohydrated lizards lost 32% of their initial body mass with a substantial decrease of their Body Condition Index and the tail volume as an index of energy (fat and then potential metabolic water) storage. Plasma osmolality significantly increased by 20%. There were also significantly increased plasma sodium, chloride, and total protein concentrations. On the other hand, there was no significant decrease in the plasma glucose level. Most of the salt loaded lizards secreted far more K⁺ than Na⁺ via the nasal glands, even after NaCl loading. The K⁺/Na⁺ ratio decreased only after two to three repetitive NaCl injections but insufficient Na⁺ was eliminated. Two successive KCl injections were successfully eliminated, but daily natural average K⁺ administration induced progressive hyperkaliemia. These experimental data agreed with previous observations showing variations of plasma Na⁺ and K⁺ concentrations in free-living lizards. The nasal gland constitutes the main route of Cl⁻ excretion but the Cl⁻/(Na⁺ + K⁺) ratio may vary according to observations in other herbivorous species. Animals inhabiting arid environments use a variety of behavioural and physiological strategies to balance their water and salt budgets. We studied the effects of dehydration and salt loading on osmoregulatory capacities in a large herbivorous desert lizard, the Moroccan Spiny-tailed lizard Uromastyx nigriventris, the family Agamidae. These lizards select plants with a high K⁺ to Na⁺ ratio of 15 to 20, and like other herbivorous lizards, effectively eliminate the extra electrolyte load, mainly via a pair of active nasal salt glands, which exude the extra ions from blood. Here we present results of a series of laboratory experiments, which tested a five-week food and water deprivation and the excretory response of nasal salt glands, during a short period of five days, following salt loading by two separated injections of KCl or NaCl at a 5-day interval (4th and 9th days). During food-water deprivation, hypohydrated lizards lost 32% of their initial body mass with a substantial decrease of their Body Condition Index and the tail volume as an index of energy (fat and then potential metabolic water) storage. Plasma osmolality significantly increased by 20%. There were also significantly increased plasma sodium, chloride, and total protein concentrations. On the other hand, there was no significant decrease in the plasma glucose level. Most of the salt loaded lizards secreted far more K⁺ than Na⁺ via the nasal glands, even after NaCl loading. The K⁺/Na⁺ ratio decreased only after two to three repetitive NaCl injections but insufficient Na⁺ was eliminated. Two successive KCl injections were successfully eliminated, but daily natural average K⁺ administration induced progressive hyperkaliemia. These experimental data agreed with previous observations showing variations of plasma Na⁺ and K⁺ concentrations in free-living lizards. The nasal gland constitutes the main route of Cl⁻ excretion but the Cl⁻/(Na⁺ + K⁺) ratio may vary according to observations in other herbivorous species.