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

Empirical studies of cardiovascular variables suggest that relative heart muscle mass (relative M_h) 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 on M_h can be used to provide quantitative estimates for the maximum aerobic power input (aerobic P_i,max) available to animals during intense levels of locomotor activity. The maximum cardiac output of birds and mammals is calculated to scale with respect to M_h (g) as 213 M_h^0.88+-0.04 (ml min^-1), while aerobic P_i,max is estimated to scale approximately as 11 M_h^0.88+-0.09 (W). In general, estimated inter-species aerobic P_i,max, based on M_h for all bird species (excluding hummingbirds), is calculated to scale with respect to body mass (M_b in kg) as 81 M_b^0.82+-0.11 (W). Comparison of family means for M_h indicate 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 relative M_h (0.25 per cent) while the Otidae have unusually large relative M_h (1.6 per cent). Hummingbirds have extremely large relative M_h (2.28 per cent), but exhibit significant sexual dimorphism in their scaling of M_h and 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 aerobic P_i,max (based on M_h and M_b in g) for male and female hummingbirds is 0.51 M_b^{0.83 +/-0.07} and 0.44 M_b^{0.85+- 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.