Birds in the tropics are thought to have high adult survival but low reproductive success, accompanied by low rates of metabolism, whereas birds living in temperate regions are considered to have lower adult survival but higher reproductive success and higher rates of metabolism.
Due to constant exposure to high temperatures, these animals need to regulate their body temperatures, to carry out the various processes that are important for their survival. Previous research showed that hoopoe larks from the Arabian Desert reduced CWL when acclimated to 35°C for 3 weeks, compared with individuals of the same species at 15°C; but skylarks and woodlarks from the Netherlands, and Dunn's larks, also from Arabia, did not (Tieleman and Williams 2002b). We especially thank Stephane Ostrowski, Mohammed Shobrak, Abdul Khoja, and Patrick Paillat, not only for unwavering support but also for friendship. A major task for researchers will be to find commonalities in these ecological factors that might select for the same coevolved suite of traits. We reasoned that if nasal turbinates significantly reduce TEWL, then occluding nares and forcing birds to breathe through their open bill should result in a marked increase in TEWL. Based on body chemistry and metabolism, physiological adaptations usually don't show from the outside. Perhaps the most often cited examples of physiological adaptation to deserts come from work by Schmidt-Nielsen and later by Walsberg on small mammals, kangaroo rats (Dipodomys; Schmidt-Nielsen 1979, Walsberg 2000). PW, Ricklefs
Hence we suggest that natural selection has operated on CWL in desert birds, and that this is the factor responsible for their reduced TEWL. However, when variation in body mass was taken into account, we could not show that organ sizes differ among larks from deserts and mesic regions (Tieleman et al. High values of K, the growth constant, indicate rapid growth; low values indicate slow growth. In a separate study, we found that the decrease in TEWL among larks along our aridity gradient cannot be attributed to the acclimation of adults to thermal environment, food availability, or photoperiod (Tieleman et al. Wyse
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Another factor that has been suggested as a cause of reduced TEWL in desert birds is a decrease of respiratory water loss as result of counter-current heat exchange in their nasal passages (Schmidt-Nielsen et al. The decrease in parental effort for larks in arid areas might reflect a lower fitness value of a single brood for desert species, suggesting that the probability of adult survival is higher in arid than in mesic areas among larks of the Old World, if patterns are congruent with those of birds in the New World. Cutaneous water loss (CWL) in larks (in milligrams water [H2O] per square centimeter per day) as a function of the percentage of lipids in the stratum corneum that were (a) ceramides and (b) free fatty acids (FFA). Species within the family of larks (Alaudidae)—all ground-foraging birds with similar diets, similar behaviors, and a common phylogenetic history—represent a model system to test hypotheses of physiological adaptation, because different members of the family occur in environments ranging from the Arctic to deserts (Williams and Tieleman 2001). We call into question the idea that birds have not evolved unique physiological adaptations to desert environments. IH, Schmidt-Nielsen
Interspecific phenotype–environment correlations can indicate either genetic differences brought about by natural selection or phenotypic plastic responses to environmental conditions. The skin of birds is composed of a thin outer nonvascular epidermis and a thicker inner vascularized dermis (Lucas and Stettenheim 1972).
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Hence, these physiological differences are probably the result of genetic differences, although we have not yet ruled out developmental plasticity during nestling development.
When the nasal apertures in the bills of crested larks and desert larks were occluded with rubberized plastic, changes in TEWL were insignificant for desert larks (the arid-zone species), but for crested larks, TEWL was 27%, 10%, and 6% higher at air temperatures of 15°C, 25°C, and 35°C, respectively, than when birds could breathe through their open nasal passages. B, Tieleman