The relationship of central and peripheral organ masses to aerobic performance variation in house sparrows

We evaluated the relationship between organ mass and the limits to aerobic metabolism in house sparrows Passer domesticus, The results were used to te st three models of performance limitation (the central limitation, peripher al limitation and symmorphosis concepts). Basal metabolic rate (BMR) was de termined during the rest phase. The maximum rate of oxygen consumption duri ng exercise ((V)over dot(O2max)) was measured in an enclosed wheel that all owed limited hovering flight. Neither BMR nor (V)over dot(O2max) was affect ed by gender, but adults had significantly higher (V)over dot(O2max) and lo wer BMR than juveniles. The masses of most central organs (gut, gizzard, li ver, heart, kidney and reproductive organs) differed significantly between ages. There were no gender differences in organ mass among juveniles, but l iver 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 with one peripheral effector (breast muscle); together, th ese explained more than half the variance in BMR (r(2)=0.57), In adults, BM R was positively correlated with the mass of reproductive tissue. The masse s of one peripheral effector (breast muscle) and one central organ (the hea rt) were positively correlated with (V)over dot(O2max) (r(2)=0.17 for the p ooled data set), These results are consistent with a symmorphosis model of aerobic capacity. We found a significant positive relationship between BMR and (V)over dot(O2max) in juveniles, but not in adults. Taken together, our data indicate that house sparrows can achieve elevated (V)over dot(O2max) without paying a 'penalty' (fitness trade-off) in the form of an increased BMR.