Lung function assessment in the Pacific walrus (Odobenus rosmarus divergens) while resting on land and submerged in water

In the present study, we examined lung function in healthy resting adult (born in 2003) Pacific walruses (Odobenus rosmarus divergens) by measuring respiratory flow ([Formula: see text]) using a custom-made pneumotachometer. Three female walruses (670-1025 kg) voluntarily participated in spirometry trials while spontaneously breathing on land (sitting and lying down in sternal recumbency) and floating in water. While sitting, two walruses performed active respiratory efforts, and one animal participated in lung compliance measurements. For spontaneous breaths, [Formula: see text] was lower when walruses were lying down (e.g. expiration: 7.1±1.2 l s^-1) as compared with in water (9.9±1.4 l s^-1), while tidal volume (V_T, 11.5±4.6 l), breath duration (4.6±1.4 s) and respiratory frequency (7.6±2.2 breaths min^-1) remained the same. The measured V_T and specific dynamic lung compliance (0.32±0.07 cmH₂O^-1) for spontaneous breaths were higher than those estimated for similarly sized terrestrial mammals. V_T increased with body mass (allometric mass-exponent=1.29) and ranged from 3% to 43% of the estimated total lung capacity (TLC_est) for spontaneous breaths. When normalized for TLC_est, the maximal expiratory [Formula: see text] ([Formula: see text]_exp) was higher than that estimated in phocids, but lower than that reported in cetaceans and the California sea lion. [Formula: see text]_exp was maintained over all lung volumes during spontaneous and active respiratory manoeuvres. We conclude that location (water or land) affects lung function in the walrus and should be considered when studying respiratory physiology in semi-aquatic marine mammals.