The heart rate - breathing rate relationship in aquatic mammals: A comparative analysis with terrestrial species

Abstract

Aquatic and semi-aquatic mammals, while resting at the water surface or ashore, breathe with a low frequency (f) by comparison to terrestrial mammals of the same body size, the difference increasing the larger the species. Among various interpretations, it was suggested that the low-f breathing is a consequence of the end-inspiratory breath-holding pattern adopted by aquatic mammals to favour buoyancy at the water surface, and evolved to be part of the genetic makeup. If this interpretation was correct it could be expected that, differently from f, the heart rate (HR, beats/min) of aquatic and semi-aquatic mammals at rest would not need to differ from that of terrestrial mammals and that their HR-f ratio would be higher than in terrestrial species. Literature data for HR (beats/min) in mammals at rest were gathered for 56 terrestrial and 27 aquatic species. In aquatic mammals the allometric curve (HR=191·M -0.18 ; M= body mass, kg) did not differ from that of terrestrial species (HR=212·M -0.22 ) and their HR-f ratio (on average 32±5) was much higher than in terrestrial species (5±1) (P<0.0001). The comparison of these HR allometric curves to those for f previously published indicated that the HR-f ratio was body size-independent in terrestrial species while it increased significantly with M in aquatic species. The similarity in HR and differences in f between aquatic and terrestrial mammals agree with the possibility that the low f of aquatic and semi-aquatic mammals may have evolved for a non-respiratory function, namely the regulation of buoyancy at the water surface [Current Zoology 61 (4): 569-577, 2015].

Heart and breathing rate variability in the avian perinatal period: The chicken embryo as a model

“…29,33,37 By comparison, the adult Gallus gallus averages 190 beats per minute 46 and 22.5 breaths per minute. 47 Hence, the normoxic fH/fB in the embryos, ~6 beats per breath (Table 3), was lower than in the adult (~8) and higher than in most mammals (~4.5 48 ). These differences should reflect the fact that in adult birds, the cross-current respiratory system is a more efficient gas exchanger than in mammals and requires lower pulmonary ventilation, 33,49 an efficiency probably not yet reached by the embryo at hatching time.…”

Cardiorespiratory coupling in cetaceans; a physiological strategy to improve gas exchange?

“…2). Past work has shown that the f H f R −1 is significantly greater not only in cetaceans but also in a larger range of marine mammals (Mortola, 2015). It was suggested that this is a non-respiratory adaptation to provide additional buoyancy.…”

Ecophysiology of mammals

“…Aquatic mammals have special adaptations of the respiratory and cardiovascular systems that are associated with diving (Mottishaw et al 1999;Bostrom et al 2008;Mortola 2015), and that confer increased hypoxia tolerance (Sergina et al 2015;Hoff et al 2017) and oxygen storage (Nery et al 2013). Interest in conservation of marine mammals, in regard to anthropogenic stress, has also generated studies of their stress physiology (Wright et al 2007;Atkinson et al 2015), and the effect of climate change on these mammals (Thiemann et al 2008).…”