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 previously cooled membranes of the nasopharynx is thought to be a mechanism that reduces respiratory water loss in mammals and birds. Such a mechanism could be important in the overall water economy of these vertebrates, especially those species occupying desert habitats. However, this hypothesis was originally based on measurements of the temperature of exhaled air (T(ex)), which provides an estimate of water recovered from exhaled air as a proportion of water added on inhalation but does not yield a quantitative measure of the reduction in total evaporative water loss (TEWL). In this study, we experimentally occluded the nares of crested larks (Galerida cristata), a cosmopolitan species, and desert larks (Ammomanes deserti), a species restricted to arid habitats, to test the hypothesis that countercurrent heat exchange in the nasal passages reduces TEWL. T(ex) of crested larks increased linearly with air temperature, (T(a)): T(ex) = 8.93 + 0.793 x T(a). Following Schmidt-Nielsen and based on measurements of T(ex), we predicted that crested larks would recover 69%, 49%, 23%, and -5% of the water added to the inhaled air at T(a)'s of 15°, 25°, 35°, and 45 °C, respectively. However, with the nares occluded, crested larks increased TEWL by only 27%, 10%, and 6% at T(a)'s of 15°, 25°, and 35 °C, respectively. At T(a) = 45 °C, TEWL of the crested lark was not affected by blocking the nares. In contrast to our expectation, occluding the nares of desert larks did not affect their TEWL at any T(a).