An Examination of Sodium, Potassium and OSmotic Concentrations in Blood and Urine of Arid-Zone Rabbits in Seasonal Field Conditions and in the Labortary

Abstract

The sodium, potassium and osmotic concentrations of blood and urine, and serum protein and PCV of the blood were examined in rabbits living in stony-hill habitat and sand-dune habitat in the arid zone of far western New South Wales. Samples were collected in dry summer, dry winter, wet summer and wet winter seasons. Rabbits from the same region were brought into the laboratory and fed dry lucerne chaff with ad libitum or restricted water, or 5-10% salt-enriched chaff with ad libitum or restricted water, and their blood and urine parameters measured. Rabbits became acclimated to water shortage (restriction to 50% of total food and water intake) over a 50-day period. Similar restriction of water with the salt-enriched diet caused reduction of food intake to starvation level; the maximum mean sodium concentration in the urine of these rabbits was 476 mmoll-1. Habitat did not influence the degree of dehydration of field animals; the effect of season was marked, resulting in a range in mean plasma osaotic pressure between cold wet and hot dry seasons of 260-326 mosmoll-l. Generally urine concentrations were high under hot and/or dry seasons (maximum mean total oxmotic pressure 2719 mosmoll-1 and low only in the cold wet season (minimum mean total osmotic pressure 5 13 mosmol l-1). Inability to produce urine with a high osmotic concentration or high sodium content compared with rodents endemic to arid areas suggests that plasticity in behaviour towards minimizing water losses, and high grazing selectivity maximizing intake of food with a high water content and without a high salt load, enable rabbits to survive in arid habitats often devoid of free water for long periods.