Flexibility in basal metabolic rate and evaporative water loss among hoopoe larks exposed to different environmental temperatures.

The 'energy demand' hypothesis for short-term adjustments in basal metabolic rate (BMR) posits that birds adjust the size of their internal organs relative to food intake, a correlate of energy demand. We tested this hypothesis on hoopoe larks (Alaemon alaudipes), inhabitants of the Arabian desert, by acclimating birds for 3 weeks at 15 degrees C and at 36 degrees C, then measuring their BMR and total evaporative water loss (TEWL). Thereafter, we determined the dry masses of their brain, heart, liver, kidney, stomach, intestine and muscles of the pectoral region. Although mean body mass did not differ initially between the two groups, after 3 weeks, birds in the 15 degrees C group had gained mass (44.1+/-6.5 g), whereas larks in the 36 degrees C group had maintained a constant mass (36.6+/-3.6 g; means +/- s.d., N=6). Birds in the 15 degrees C group had a mean BMR of 46.8+/-6.9 kJ day(-1), whereas birds in the 36 degrees C group had a BMR of 32.9+/-6.3 kJ day(-1), values that were significantly different when we controlled for differences in body mass. When measured at 35 degrees C, larks in the cold-exposure group had a TEWL of 3.55+/-0.60 g H(2)O day(-)(1), whereas TEWL for birds in the 36 degrees C group averaged 2.23+/-0.28 g H(2)O day(-1), a difference of 59.2%. Mass-independent TEWL differed significantly between groups. Larks in the 15 degrees C group had a significantly larger liver, kidney and intestine than larks in the 36 degrees C group. The total increase in organ mass contributed 14.3% towards the total mass increment in the cold exposure group. Increased food intake among larks in the cold group apparently resulted in enlargement of some of the internal organs, and the increase in mass of these organs required a higher rate of oxygen uptake to support them. As oxygen demands increased, larks apparently lost more evaporative water, but the relationship between increases in BMR and TEWL remains unresolved.