Cutaneous and Respiratory Water Loss in Larks from Arid and Mesic Environments

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Abstract
Birds from deserts generally have lower total evaporative water loss (TEWL), the sum of cutaneous (CWL) and respiratory water loss (RWL), than species from mesic areas. We investi- gated the role of CWL and RWL as a function of air temperature (Ta) in hoopoe larks (Alaemon alaudipes) and Dunn's larks (Eremalauda dunni) from the Arabian Desert and skylarks (Alauda arvensis) and woodlarks (Lullula arborea) from tem- perate mesic grasslands. The proportional contribution of CWL to TEWL in all larks at moderate Ta ranged from 50% to 70%. At high Ta (40-45C), larks enhanced CWL by only 45%-78% and relied on an increase in RWL by 676%-2,733% for evap- orative cooling. Surface-specific CWL at 25C was 29% lower in the arid-zone species than in the mesic larks. When accli- mated to constant Ta ,1 5C-acclimated hoopoe larks increased CWL by 22% compared with 35C-acclimated birds, but the other species did not change CWL. This study is consistent with the hypothesis that larks from deserts have a reduced CWL at moderate and low Ta but provided no support for the hy- pothesis that at high Ta larks from arid regions rely more on CWL than larks from mesic environments. Interspecific dif- ferences in CWL cannot be attributed to acclimation to envi- ronmental temperature and are possibly the result of genetic differences due to natural selection or of phenotypically plastic responses to divergent environments during ontogeny. avenue of water loss in birds and of major importance in main- taining heat balance, especially for birds living in hot and dry environments (Bartholomew 1972; Dawson 1982; Williams and Tieleman 2001). Birds from deserts generally have lower TEWL than species from mesic areas (Williams 1996), although the mechanisms responsible for this finding remain unresolved. Understanding these mechanisms might provide insights into the evolutionary processes that underlie the correlation between TEWL and environment. The variation in physiological traits such as TEWL and their adjustment to environmental condi- tions may be the result of genetic differences or phenotypic plasticity. Forms of plasticity include acclimatization or phe- notypic flexibility of adult phenotypes in response to changing environments and developmental plasticity that can determine an individual's phenotype during ontogeny. Genetic variation in physiological traits may result from natural selection or ge- netic drift or may be a consequence of phylogenetic constraint