Lung size and thoracic morphology in shallow‐ and deep‐diving cetaceans

Abstract

Shallow-diving, coastal bottlenose dolphins (Tursiops truncatus) and deep-diving, pelagic pygmy and dwarf sperm whales (Kogia breviceps and K. sima) will experience vastly different ambient pressures at depth, which will influence the volume of air within their lungs and potentially the degree of thoracic collapse they experience. This study tested the hypotheses that lung size will be reduced and/or thoracic mobility will be enhanced in deeper divers. Lung mass (T. truncatus, n = 106; kogiids, n = 18) and lung volume (T. truncatus, n = 5; kogiids, n = 4), relative to total body mass, were compared. One T. truncatus and one K. sima were cross-sectioned to calculate lung, thoracic vasculature, and other organ volumes. Excised thoraxes (T. truncatus, n = 3; kogiids, n = 4) were mechanically manipulated to compare changes in thoracic cavity shape and volume. Kogiid lungs were half the mass and one-fifth the volume of those of similarly sized T. truncatus. The lungs occupied only 15% of the total thoracic cavity volume in K. sima and 37% in T. truncatus. The kogiid and dolphin thoraxes underwent similar changes in shape and volume, although the width of the thoracic inlet was relatively constrained in kogiids. A broader phylogenetic comparison demonstrated that the ratio of lung mass to total body mass in kogiids, physeterids, and ziphiids was similar to that of terrestrial mammals, while delphinids and phocoenids possessed relatively large lungs. Thus, small lung size in deep-diving odontocetes may be a plesiomorphic character. The relatively large lung size of delphinids and phocoenids appears to be a derived condition that may permit the lung to function as a site of respiratory gas exchange throughout a dive in these rapid breathing, short-duration, shallow divers.

Exercise at depth alters bradycardia and incidence of cardiac anomalies in deep-diving marine mammals

“…Baroreceptors 20 , pulmonary stretch receptors 21 , blood gases 4 , cardiovascular hormones (for example, natriuretic peptides) 22,23 as well as physical changes in lung volume and pulmonary shunting with the rise and fall of hydrostatic pressures during descent and ascent influence HR in diving humans and marine mammals 15,18 . The well-documented, progressive collapse and re-inflation of the lungs of diving dolphins 10,11 and pinnipeds 2,12 as the animals pass 40-150 m would also support a graded hydrostatic mechanism affecting HR 21 (Fig. 1a).…”

“…The inset illustrates a typical time-depth profile for a seal with normal (blue) and arrhythmic (red) periods indicated. The grey box denotes depths associated with lung collapse/expansion [10][11][12] .…”

“…Indeed, the greatest change (69% and 88% for seals and dolphins, respectively) in HR during a dive occurred between the water surface and depths (ca. 100-150 m) typically associated with rapid lung compression (descent) or expansion (ascent) [10][11][12] . HR declined asymptotically with instantaneous dive depth (that is, location in the water column), reaching a minimum as the animals descended to the dive bottom (Fig.…”

A comparative analysis of marine mammal tracheas

“…The relationship between pressure and volume gives an estimate of the compliance of the respiratory tract and has been successfully performed on excised lungs from terrestrial mammals (Bachofen et al, 1970), with few data documenting compliance of marine mammal lungs (Denison et al, 1971;Piscitelli et al, 2010;Fahlman et al, 2011). The compliance of the trachea has been suggested to affect the amount of air displaced from the lungs (Bostrom et al, 2008), and thereby the depth where the alveoli collapse and gas exchange ceases.…”

Lipid class and depth-specific thermal properties in the blubber of two species of odontocete cetaceans, the short-finned pilot whale (Globicephala macrorhynchus) and the pygmy sperm whale (Kogia breviceps)

“…In contrast, kogiids, physeterids, and ziphiids (sperm and beaked whales) store their blubber lipids primarily as wax esters (WE), a long-chain fatty acid attached to a long-chain fatty alcohol (Lockyer, 1991;Koopman, 2007). These latter odontocetes, which are pelagic deep divers (reviewed in Piscitelli et al, 2010), are apparently unique within mammals for using WE as their primary storage lipid (Koopman, 2007). Although the overall lipid content of the blubber in these species is similar to those that store their lipids as TAG, the thermal consequences of possessing 'waxy' blubber are currently unknown (Koopman, 2007).…”