The role of the nasal passages in the water economy of crested larks and desert larks

Condensation of water vapor in the exhaled air stream as it passes over pre viously cooled membranes of the nasopharynx is thought to be a mechanism th at reduces respiratory water loss in mammals and birds. Such a mechanism co uld be important in the overall water economy of these vertebrates, especia lly those species occupying desert habitats. However, this hypothesis was o riginally based on measurements of the temperature of exhaled air (T-ex), w hich provides an estimate of water recovered from exhaled air as a proporti on of water added on inhalation but does not yield a quantitative measure o f the reduction in total evaporative water loss (TEWL). In this study, we e xperimentally occluded the nares of crested larks (Galerida cristata), a co smopolitan species, and desert larks (Ammomanes deserti), a species restric ted to arid habitats, to test the hypothesis that countercurrent heat excha nge in the nasal passages reduces TEWL. T-ex of crested larks increased lin early with air temperature, (T-a): T-ex = 8.93 + 0.793 x T-a. Following Sch midt-Nielsen and based on measurements of T-ex, we predicted that crested l arks would recover 69%, 49%, 23%, and -5% of the water added to the inhaled air at T-a's of 15 degrees, 25 degrees, 35 degrees, and 45 degrees C, resp ectively. However, with the nares occluded, crested larks increased TEWL by only 27%, 10%, and 6% at T-a's of 15 degrees, 25 degrees, and 35 degrees C , respectively. At T-a = 45 degrees C, TEWL of the crested lark was not aff ected by blocking the nares. In contrast to our expectation, occluding the nares of desert larks did not affect their TEWL at any T-a.