Phylogenetic allometric scaling of near basal breathing frequency in terrestrial, semi-aquatic and aquatic mammals.

We measured the BASAL breathing frequency following an overnight fast in adult, non-pregnant/non-lactating, inactive mammals ranging in body mass from 15 to 5520 kg. The data included results from 338 individual animals from 34 species that were divided into terrestrial, semi-aquatic (Otariidae and Phocidae) and aquatic mammals. Following attempts to limit the collection of breathing frequency using a basal definition and to correct the analysis phylogenetically, our results suggest that there are differences in the allometric mass-exponent between terrestrial and aquatic/semi-aquatic mammals. An allometric regression model, whereby both body mass and breathing frequency were transformed using log10, suggested that the allometric mass exponent for terrestrial mammals (-0.303) was different from both aquatic mammals (-0.124) and semi-aquatic mammals (-0.091). For semi-aquatic mammals, the breathing frequency was lower in water, but we detected no association between the breathing frequency and the temperature of the medium (water or air). We propose that allometric studies of cardiorespiratory function should, if possible, adhere to the basal definition during data collection, similar to that used for metabolic rate. Such data will provide valuable information for comparative medicine of large species that are difficult to study, for which controlled baseline data might be difficult to obtain.