Heart mass and the maximum cardiac output of birds and mammals: implications for estimating the maximum aerobic power input of flying animals

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Abstract
Empirical studies of cardiovascular variables suggest that relative heart muscle mass (relativeMh) is a good indicator of the degree of adaptive specialization for prolonged locomotor activities, for both birds and mammals. Reasonable predictions for the maximum oxygen consumption of birds during flight can be obtained by assuming that avian heart muscle has the same maximum physiological and biomechanical performance as that of terrestrial mammals. Thus, data onMhcan be used to provide quantitative estimates for the maximum aerobic power input (aerobicPi, max) available to animals during intense levels of locomotor activity. The maximum cardiac output of birds and mammals is calculated to scale with respect toMh(g) as 213Mh0.88±0.04(ml min−1), while aerobicPi,maxis estimated to scale approximately as 11Mh0.88±0.09(W). In general, estimated inter–species aerobicPi,max, based onMhfor all bird species (excluding hummingbirds), is calculated to scale with respect to body mass (Mbin kg) as 81Mb0.82±0.11(W). Comparison of family means forMhindicate that there is considerable diversity in aerobic capacity among birds and mammals, for example, among the medium to large species of birds the Tinamidae have the smallest relativeMh(0.25 %) while the Otidae have unusually large relativeMh(1.6 %). Hummingbirds have extremely large relativeMh(2.28 %), but exhibit significant sexual dimorphism in their scaling ofMhand flight muscle mass, so that when considering hummingbird flight performance it may be useful to control for sexual differences in morphology. The estimated scaling of aerobicPi,max(based onMhandMbin g) for male and female hummingbirds is 0.51Mb0.83 ±0.07and 0.44Mb0.85± 0.11(W), respectively. Locomotory muscles are dynamic structures and it might be anticipated that where additional energetic ‘costs’ occur seasonally (e.g. due to migratory fattening or the development of large secondary sexual characteristics) then the relevant cardiac and locomotor musculature might also be regulated seasonally. This is an important consideration, both due to the intrinsic interest of studying muscular adaptation to changes in energy demand, but also as a confounding variable in the practical use of heart rate to estimate the energetics of animals. Haemoglobin concentration (or haematocrit) may also be a confounding variable. Thus, it is concluded that data on the cardiovascular and flight muscle morphology of animals provides essential information regarding the behavioural, ecological and physiological significance of the flight performance of animals.