The air we breathe is made up of 78% nitrogen - an inert, completely harmless gas - at the surface at least, but as we dive into the ocean depths it's another matter....

When diving, we experience pressure that is twice as great as the pressure on us at the surface (atmospheric pressure) at only 10 metres underwater. The pressure exerted on our bodies by the surrounding water column is known as hydrostatic pressure and it increases with depth. Hydrostatic pressure is equivalent to 3 times atmospheric pressure at 20 m (3 ATM, or 3 atmospheres), 4 ATM at 30 m and so on until at the deepest point in the ocean, at the base of Mariana Trench (roughly 2,550 km below sea level) it reaches over 1000 ATM - not something our poor bodies can stand!

Diving in the ocean, if not practiced safely, can lead to serious health problems. One of these is decompression sickness (DCS), also known as "the bends". Under the high pressures experienced at depth, nitrogen can dissolve into the bloodstream (because gasses are more soluble at high pressures). When divers ascend to the surface too quickly, the dissolved nitrogen comes out of solution, forming gas bubbles in the blood and body tissues, and causes local damage. Divers frequently experience this as joint or muscle pain when nitrogen bubbles form in these parts of the body. This is the most common symptom, but high proportions of bubbles can lead to numbness, paralysis and loss of brain function.

So what about marine mammals? Sperm whales dive down to 1500 m to feed on a daily basis; Cuvier's beaked whales reach similar depths when foraging and many other marine mammals are capable of diving to great depths with no ill effects on ascent to the surface. Why?

All vertebrates have a reflex response to diving (the ‘diving response'), that allows them to limit nitrogen uptake into the blood and body tissues and conserves oxygen, allowing them to dive for longer. The diving response has two main effects on the body: 1) reduced blood flow to muscles (peripheral vasoconstriction), and 2) reduced heart rate (bradycardia). Both of these limit gas transport around the body, conserving oxygen for the vital organs, and limiting nitrogen uptake by the blood and body tissues. Many marine mammals have the added ability to store gas in their windpipe (trachea) when diving. The trachea, together with smaller branches in the lungs, are reinforced by rings of cartilage, and when the air sacs within the lungs (alveoli) collapse under pressure, gas is forced out into these strengthened lung cavities, rather than the bloodstream. This prevents excess nitrogen entering the blood as they descend.

Whales, seals and dolphins all posses similar adaptations that prevent them from suffering decompression sickness as they go about their daily activities, but despite these, marine mammals have been seen with symptoms that could indicate decompression sickness. Many stranded animals have been found to have legions (tears) in their tissue that may indicate damage due to bubble formation. Determining whether or not decompression is the cause of this abnormal tissue is a difficult process as the tissues need to be studied soon after an animal has stranded, otherwise, we can't distinguish between damage due to decompression from that due to decomposition. To achieve this, the stranding has to be reported by a passer by almost immediately after the event and marine biologists need to be dispatched to take a closer look at the stranded animal ASAP - no easy task!

If marine mammals are well adapted to cope with diving to (and ascending from) great depths, what could cause them to suffer from decompression sickness?

Several factors contribute to the onset of decompression sickness. The main one is the reduction in pressure on the body as the diving mammal (or SCUBA diver) ascends to the surface. Bradycardia and the presence of specialised breathing apparatus allows marine mammals to cope with this, but other factors may make them more susceptible to getting “the bends”. Deep dives, long dives, cold water, vigorous exercise and rapid ascents increase the chances of decompression sickness occurring. Seeing as many marine mammals frequently dive to great depths and many species populate high latitudes (where the water is colder), they are well adapted to diving in these conditions. Given that, perhaps the most likely cause of decompression sickness in marine mammals is an uncharacteristically rapid ascent to the surface.

Do whales suffer from decompression sickness? It looks likely, but determining the cause is another matter entirely. If a rapid ascent to the surface is responsible for decompression, what initiates this change in behaviour is not yet understood. What do you think would make a marine mammal ascend rapidly to the surface? There were 76 marine mammal mass strandings between 1977 and 2001 in Florida alone - that's an average of 3 per year, not including smaller strandings, and only in Florida - imagine the number of strandings happening around the world!

Marine biologists are working hard to pin down the cause of these strandings. There is some evidence to suggest that noise pollution due to the use of sonar; seismic mapping and ships may be disruptive, causing mammals to make a speedy ascent to the surface. Another possibility is the presence of a threat at depth. Animals may ascend rapidly to the surface to escape a predator, for example, though it is difficult to comprehend a predator that would cause a large set of whales to ascend to the surface.

The main problem we face is being able to observe the behaviour of these animals in their natural habitat and without this, our understanding of their behaviour is limited. Marine mammals are difficult to track and while we can use data loggers to record their diving behaviour, what causes them to dive in a particular pattern remains unknown. If you wanted to find out more about how whales behave, how would you do it? Could you watch them in the wild, or would you have to observe them in an aquarium? How would this affect your investigation?

References

Acoustic Pollution and Marine Mammals. Scitable Spotlight, NPG Education, 2010.

Fernández, A. et al. "Gas and Fat Embolic Syndrome'' Involving a Mass Stranding of Beaked Whales (Family Ziphiidae) Exposed to Anthropogenic Sonar Signals. Vetetinary Pathology Online 42 446-457 (2005).

Hooker, S. K. et al. (2011). Deadly diving? Physiological and behavioural management of decompression stress in diving mammals. Proceedings of the Royal Society B 279 1041-1050 (2011).

Walker, R. J. et al. Environmental correlates of cetacean mass stranding sites in Florida. Marine Mammal Science 21 327-335 (2005).

Fancy a little extra reading? The response of marine mammals to noise pollution is a hotly debated area and we have a lot left to uncover. This is the most comprehensive review of the topic to date:

Southall, B. L. et al. Marine Mammal Noise Exposure Criteria: Initial Scientific Recommendations. Aquatic Mammals 33, 1-121 (2007).

Images

Sperm whale: True Wildlife Creatures. Sperm Whale (2012).

Stranding: Wikimedia Commons. 1986 beached whales in Flinders Bay (1986).



