Deadly diving? Physiological and behavioural management of decompression stress in diving mammals

Abstract

Decompression sickness (DCS; 'the bends') is a disease associated with gas uptake at pressure. The basic pathology and cause are relatively well known to human divers. Breath-hold diving marine mammals were thought to be relatively immune to DCS owing to multiple anatomical, physiological and behavioural adaptations that reduce nitrogen gas (N(2)) loading during dives. However, recent observations have shown that gas bubbles may form and tissue injury may occur in marine mammals under certain circumstances. Gas kinetic models based on measured time-depth profiles further suggest the potential occurrence of high blood and tissue N(2) tensions. We review evidence for gas-bubble incidence in marine mammal tissues and discuss the theory behind gas loading and bubble formation. We suggest that diving mammals vary their physiological responses according to multiple stressors, and that the perspective on marine mammal diving physiology should change from simply minimizing N(2) loading to management of the N(2) load. This suggests several avenues for further study, ranging from the effects of gas bubbles at molecular, cellular and organ function levels, to comparative studies relating the presence/absence of gas bubbles to diving behaviour. Technological advances in imaging and remote instrumentation are likely to advance this field in coming years.

Figure 1.

Variability in diving behaviour of a range of marine mammal species. Dive traces are plotted to identical scales: 1500 m depth over a 16 h time period for each of: (a) Cuvier's beaked whale (Ziphius), (b) Blainville's beaked whale (Mesoplodon), (c) sperm whale (Physeter), (d) pilot whale (Globicephala), (e) killer whale (Orcinus), (f) northern elephant seal (Mirounga) and (g) Antarctic fur seal (Arctocephalus). Traces are coloured according to vertical speed (rate of change in depth). Insets to only 200 and 100 m depth are shown for 4 h portions of Orcinus and Arctocephalus plots, respectively. Histograms show percentage of time spent at depth (10 m bins, from 0 to 1500 m), with numerical display of percentage time at 0–10 m. Data sources: WHOI Dtag group (a–e), D. Costa (f), S. Hooker (g).

Figure 2.

Potential scenarios for the formation and resolution of gas bubbles.

Figure 3.

Selected mechanisms (in light grey) available to animals in managing gas loading (oxygen, O₂; carbon dioxide, CO₂; nitrogen, N₂) between lungs and different body compartments, with the physiological trade-offs (in dark grey) that might influence these.