We investigated interindividual variation and intra-individual phenotypic flexibility in basal metabolic rate (BMR), total evaporative water loss (TEWL), body temperature (Tb), the minimum dry heat transfer coefficient (h), and organ and muscle size of five species of larks geographically distributed along an aridity gradient. We exposed all species to constant environments of 15°C or 35°C, and examined to what extent interspecific differences in physiology can be attributed to acclimation. We tested the hypothesis that birds from deserts display larger intra-individual phenotypic flexibility and smaller interindividual variation than species from mesic areas.
Larks from arid areas had lower BMR, TEWL, and h, but did not have internal organ sizes different from birds from mesic habitats. BMR of 15°C-acclimated birds was 18.0%, 29.1%, 12.2%, 25.3%, and 4.7% higher than of 35°C-acclimated Hoopoe Larks, Dunn's Larks, Spike-heeled Larks, Skylarks, and Woodlarks, respectively. TEWL of 15°C-acclimated Hoopoe Larks exceeded values for 35°C-acclimated individuals by 23% but did not differ between 15°C- and 35°C-acclimated individuals in the other species. The dry heat transfer coefficient was increased in 15°C-acclimated individuals of Skylarks and Dunn's Larks, but not in the other species. Body temperature was on average 0.4°C ± 0.15°C (mean ± 1 sem) lower in 15°C-acclimated individuals of all species. Increased food intake in 15°C-acclimated birds stimulated enlargement of intestine (26.9–38.6%), kidneys (9.8–24.4%), liver (16.5–27.2%), and stomach (22.0–31.6%). The pectoral muscle increased in 15°C-acclimated Spike-heeled Larks and Skylarks, remained unchanged in Hoopoe Larks, and decreased in 15°C-acclimated Woodlarks and Dunn's Larks. We conclude that the degree of intra-individual flexibility varied between physiological traits and among species, but that acclimation does not account for interspecific differences in BMR, TEWL, and h in larks. We found no general support for the hypothesis that species from desert environments display larger intra-individual phenotypic flexibility than those from mesic areas.
The coefficient of variation of larks acclimated to their natural environment was smaller in species from arid areas than in species from mesic areas for mass-corrected BMR and surface-specific h, but not for mass-corrected TEWL. The high repeatabilities of BMR, TEWL, and h in several species indicated a within-individual consistency on which natural selection could operate.