Parotid and mandibular gland secretion by red kangaroos, Macropus rufus, in response to heat stress

Abstract

Salivary flow rates from ipsilateral parotid and mandibular glands were measured in conscious red kangaroos over a 70–90-min period during episodes of saliva spreading induced by heat stress. At the onset of saliva spreading, mandibular flow rose rapidly to plateau at 1.12 ± 0.10 mL min–1 for the collection intervals after the first 10 min of licking. Parotid flow increased more slowly and progressively, reaching secretion rates similar to those of the mandibular gland after 40 min of saliva spreading, exceeding mandibular flow after 70 min and showing no indication that it had reached maximum secretion at 90 min of saliva spreading. The ion concentrations of both parotid and mandibular salivas during saliva spreading were similar to those previously reported for parasympathomimetic stimulation. The low osmotic concentration of mandibular saliva relative to plasma (40%) makes it a functionally better evaporative coolant than parotid saliva, which was nearly isosmotic with plasma. The increased production of hydrogen ions associated with the increased secretion of bicarbonate by the parotid gland would tend to offset the respiratory alkalosis due to panting thereby helping to maintain acid/base balance during periods of prolonged heat stress.