Diving mammals are superbly adapted to their submarine lives. With their hearing finely attuned to waterborne sound and lungs that can compress at extreme depth, they seem well protected from their high-pressure lifestyle. However, all is not well beneath the waves. In recent years there have been reports of mass strandings of whales and dolphins – some of the animals showed symptoms that resembled decompression sickness – that coincided with the Navy's use of sonar in the vicinity. Gina Lonati and colleagues from the University of North Carolina Wilmington, USA, suspected that the animals may have changed their diving behaviour, affecting how nitrogen is stored in tissues where it may have accumulated during a dive. Knowing that fat absorbs nitrogen and that the amount of gas that fat can hold depends on its chemical make-up, the team decided to find out how much nitrogen can be absorbed by the fats that pack around the jaws of whales and dolphins – which carry sound to the ear – to find out how vulnerable the animals may be to decompression injuries.
Collecting samples of jaw fat from small Atlantic spotted dolphins to mighty sperm whales, the team was impressed when they measured the nitrogen solubility and found that it ranged from 0.066 ml N2 ml−1 in pygmy sperm whales to an impressive 0.101 ml N2 ml−1 in the short-finned pilot whale – compared with 0.062–0.075 ml N2 ml−1 in the animals’ blubber. And, when the team tested the composition of the fats, they found that high levels of wax ester fats – which do not occur in other mammals – increased the nitrogen solubility. However, when they looked at the fat composition in more detail, they found that specific chemical components of the fats (such as fatty acids and alcohols) also contributed to the nitrogen solubility in the jaw fats, especially in the spotted dolphins, pilot whales and beaked whales.
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. She also hopes that these new measurements will help us to understand how nitrogen circulates in whales during dives to identify other species that may be at risk of decompression injury as a result of human activity.