BLACK-BELLIED SANDGROUSE (PTEROCLES-ORIENTALIS) AND PIN-TAILED SANDGROUSE (PTEROCLES-ALCHATA) - CLOSELY RELATED SPECIES WITH DIFFERING BIOENERGETIC ADAPTATIONS TO ARID ZONES

Sandgrouse of the genus Pterocles are good models for studying adaptations to deserts, with 14 species living in different parts of a habitat spectrum ranging from semiarid to extreme desert. To evaluate possible differences in bioenergetic and thermoregulatory ability within the group, we studied two species, black-bellied sandgrouse (Pterocles orientalis) and pin-tailed sandgrouse (Pterocles alchata). These two species are partly sympatric, but the latter also occurs in hotter and more arid regions than the former. Black-bellied sandgrouse bad a mean resting metabolic rate (RMR) similar to the allometric prediction (5.53 mW g-1 by day, 5.04 mW g-1 at night; 97% and 110% of allometric prediction, respectively), whereas mean RMR in pin-tailed sandgrouse was lower than predicted (4.04 mW g-1 by day; 62% of allometric prediction). Similarly, the pin-tailed sandgrouse was the better thermoregulator of the two species, especially at high temperatures, with a well-developed evaporative cooling ability. At 40-degrees-C, evaporative heat loss (H(e)) dissipated 89% of metabolic heat production (H(m)), and at 25-degrees-C, H(e) was 152% of the allometric prediction. This compares to 53% at 40-degrees-C and 101% at 25-degrees-C for black-bellied sandgrouse. However, both species had high lower and upper critical temperatures, and metabolism at high ambient temperatures (T(a)'s) was relatively insensitive to increasing temperature. Black-bellied sandgrouse showed greatly increased thermal conductance (C(d)) at high temperatures facilitating nonevaporative beat loss. This species also demonstrated labile body temperature (T(b)), especially at low T(a), allowing energy savings under cooler conditions. It was not possible to measure T(b) for pin-tailed sandgrouse because of the birds' behavior.