Whales may be able to develop decompression sickness, the same ailment experienced by scuba divers who surface from a dive too rapidly, according to a new study.
Researchers from the University of North Carolina Wilmington investigated how marine mammals’ tissues – specifically, fat deposits in the jaws of toothed whales that are used in echolocation – absorb nitrogen gas, one of the gases that contributes to the bends.
Decompression sickness (commonly known as ‘the bends’) occurs when an individual (in most cases a scuba diver) is in a situation that involves pressure rapidly decreasing around the body (like ascending from deeper water), causing a build up of nitrogen bubbles in the body. When we breathe, approximately 79% of the air we’re breathing is nitrogen and because air under high pressure is compressed, each breath taken at depth contains many more molecules than a breath taken at the surface. Because oxygen is used continuously by the body, the extra oxygen molecules breathed under high pressure usually do not accumulate, however the extra nitrogen molecules do accumulate in the blood and tissues. As outside pressure decreases during ascent from a dive, the accumulated nitrogen that cannot be exhaled immediately forms bubbles in the blood and tissues which can instigate a range of physiological problems in an individual, potentially leading to paralysis or death.
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. Although whales do not breath compressed air like scuba divers (so it is less likely for them to develop the bends), they possess a lung structure that can collapse under high pressure, as a defence against decompression sickness. This forces the air away from the alveoli and into the upper airways where the gas can’t enter the bloodstream, and that’s what keeps the blood from absorbing too much nitrogen at depth. This also preserves a reservoir of oxygen that becomes available again during the trip back to the surface.
At one time, scientists believe that deep diving marine mammals such as whales were completely resistant to developing decompression sickness. However, increasing evidence seems to suggest that this might not be true in some cases.
For example, in 2000, the U.S. Navy said its sonar exercises led six beaked whales to fatally beach themselves in the Bahamas, and stranded whales have died near sonar-testing sites in at least five other cases since then. In 2002 a mass stranding of 14 whales occurred in the Canary islands. The whales were found to have gas bubbles in their tissues, a sign of decompression sickness. The mass stranding coincided with naval sonar exercises in the area at the time and scientists have claimed that the anthropogenic, acoustic (sonar) activities and the stranding and death of the marine mammals were likely to be connected. It is suggested the loud noises may disorient whales or force them to surface too quickly.
To understand whether whales are capable of developing decompression sickness as a result of sonar activity, researchers have now began to investigate how much nitrogen whale tissues can absorb.
Gina Lonati, a graduate student at the University of North Carolina Wilmington, began collecting the heads of whales and other marine mammals, and extracting their fatty tissue. Nitrogen solubility in the blubber and mandibular fats of seven species of toothed whale across five families could then be examined. The researchers found that the makeup of the fat affected how much nitrogen gas dissolves in it—and that different species had different fat compositions, thus affecting their chances of potentially developing the bends in certain situations.
Lonati suspects high nitrogen solubility in the jaw fats of beaked whales and pilot whales combined with their deep-diving lifestyles leads them to accumulate nitrogen in the jaw fats, which leaves them prone to decompression-related injuries.
“In order to see if these animals may be susceptible to something like the bends or decompression sickness, we want to see how much nitrogen their fats can hold,” she says. “Because the more they can hold, it’s possible the more it may affect them when they surface.”
Investigations into the reasons behind why certain whale individuals are surfacing too quickly and developing the bends are ongoing, with sonar activity and attempting to escape predators being the most likely explanations at this time.
Header Photo Credit: A sperm whale (Physeter macrocephalus), the largest of the toothed whales, diving. By DOUGLAS HOFFMAN
Journal Reference: Lonati, G. L., Westgate, A. J. Pabst, D. A., Koopman, H. N.(2015). Nitrogen solubility in odontocete blubber and mandibular fats in relation to lipid composition. J. Exp. Biol. 218, 2620–2630