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

Abstract

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 %) while the Otidae have unusually large relative M h (1.6 %). Hummingbirds have extremely large relative M h (2.28 %), 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.

Non-universal interspecific allometric scaling of metabolism

“…In fact, it is equidistant from −0.25 and −0.286. It is worth mentioning that −0.27 was also predicted by other recent analysis of Bishop [35]. On the other hand, the respiration rate empirical exponents of mammals and birds have the majority of values within the predicted range.…”

The maximum oxygen consumption and aerobic scope of birds and mammals: getting to the heart of the matter

“…V O 2max of birds during £ight can be made based on data for heart mass and haemoglobin concentration (Bishop 1997). Therefore, it is possible to use published data for relative heart mass and haemoglobin concentration to normalize the measured values for .…”

“…As heart beat frequency is the only functional variable associated with the convection of blood that scales signi¢-cantly with respect to body mass (Weibel et al 1991;Bishop 1997), then the maximum heart beat frequency of birds and mammals should scale with a smaller negative exponent than that of the resting heart beat frequency, i.e. M b 70.12 AE 0.02 and M b 70.27 AE 0.04 , respectively.…”

“…Data are sources given in the text. maximum heart rate of mammals (n 24) scales as M b 70.16 AE 0.02 (Bishop 1997). …”

“…Yet most small terrestrial mammals have very limited aerobic capacities compared to those of bats and the latter were not considered by and Weibel et al (1991) The information available for the scaling of the . V O 2max of birds is even more limited than that for mammals (Bishop 1997). Schmidt-Nielsen (1984) again concluded that the aerobic scope of birds is probably a constant, but that the general value measured for birds during £ight is slightly greater, at around 15-fold, than that of the general value for mammals while running (i.e.…”