Chappell MA, Bech C, Buttemer WA (1999) The relationship of central and peripheral organ masses to aerobic performance variation in House Sparrows. Journal of Experimental Biology 202:2269-2279.

SUMMARY -- We evaluated the relationship between organ mass and the limits to aerobic metabolism in house sparrows, Passer domesticus. The results were used to test three models of performance limitation (the central limits, peripheral limits, and symmorphosis concepts). Basal metabolism (BMR) was determined during the rest phase. The maximum oxygen consumption during exercise (VO_2max) was measured in an enclosed wheel that allowed limited hovering flight. Both BMR and VO_2max were unaffected by gender, but adults had significantly higher VO_2max and lower BMR than juveniles. The masses of most central organs (gut, gizzard, liver, heart, kidney, and reproductive organs) differed significantly between ages. There were no gender differences in organ mass among juveniles, but liver mass differed between male and female adults. In the pooled data, BMR was positively correlated with the mass of three central organs (gut, liver, and kidney) and one peripheral effector (breast muscle); together these explained more than half of the variance in BMR (r2= .57). In adults, BMR was positively correlated to the mass of reproductive tissue. One peripheral effector (breast muscle) and one central organ (the heart) were positively correlated to VO_2max (r2 = .17 for the pooled data set). These results are consistent with a symmorphosis model of aerobic capacity. We found a significant positive relationship between BMR and VO_2max in juveniles, but not in adults. Taken together, our data indicate that house sparrows can achieve elevated VO_2max without paying a 'penalty' (fitness trade-off) in the form of an increased BMR.