Move over Tom Daley! Cetaceans are real diving champions
The True Dive Champs
You’ve all heard of the Thames Whale? Most of us will remember the day the Bottlenose Whale (Hyperoodon ampullatus) warmed the nation’s hearts once the county’s media and special rescue teams scrambled to move towards the capital to cover or take part in the massive operation to try and save the homesick whale.
What if I told you that H. ampullatus is one of the natural world’s best divers? Infact most cetaceans (whales and dolphins) are highly adapted for diving with many physiological adaptations to allow for safe and long distance dives, with many even being able to dive for more than two hours at a time.
It’s true our distant cousin the Cetaceans truly are the best divers around! (Image credit; WikiCommons)
Sounds impressive; but how is it that these animals can dive for so long without getting ill? Us humans can dive for long periods of time with the aid of machines and compressed air tanks; cetaceans need air to breathe too but they carry an air tank of sorts with them by having a storage molecule called myoglobin which stores oxygen in the animal’s skeletal muscle releasing it when needed. This method of storing breathable oxygen has it’s benefits; oxygen is a gas and will expand and contract when the pressure is changed, this doesn’t effect us on land where the pressure is fairly constant but under water pressure can be many times what it is on the land. Storing oxygen in solutions like myoglobin therefore protect cetaceans from dangerous effects caused by air expanding as they surface.
But what about the Bends? Decompression sickness is an illness that effects thousands of budding and professional divers annually, it is caused by nitrogen and other gasses from the air forming bubbles in blood when under pressure this can cause some serious health issues so in order to avoid any unnecessary injury injury divers have learned to ascend slowly. Cetaceans have also developed their own ways of avoiding bubble formation, they have compressible chests which help to an extent to prevent gasses in their blood as they have somewhere to store all the gas bubbles, but this alone isn’t enough to explain their impressive abilities. They also have the ability to shut down air transfer across their lung alveoli when the depth is such that bubbles will begin to form, these two abilities make cetaceans feel at home in the deep sea.
A nitrogen bubble caused by decompression sickness (Image credit; WikiCommons)
Why do they need to dive so deep?
Food is scare in the ocean, with lots of competition for food animals have adapted to be able to eat different food sources and to find their food in different places. With humans now being one of the primary predators in the shallow waters, since the advent of trawler fishing, competition is higher than ever and most fisheries are in decline so areas where cetaceans used to rely on for food are no longer useful. This shortage of food and hunting from humans has lead to declines and extinctions which have been linked directly to human impacts. This means that in the modern oceans those who can tolerate the most environments tend to do the best, which has caused certain species to recover and even thrive in the ocean having evolved to utilise pretty much every food source available to them even other cetaceans. This coupled with their incredible intelligence allowed them to, for most of history, be the top predators in the ocean with species exploiting and using novel feeding strategies at many trophic levels.
Orca were named “whale killers” by Spanish sailors who supposedly would witness the act of them predating on larger cetaceans during the 18th Century (Image Credit; WikiCommons)
Humans and Our Impacts
It’s no secret that in the last hundred years mass strandings have increased in number as military and civilian sonar use has increased, but how does this work to cause these tragic events? The answer is not as simple as we’d like, due to incomplete data and a number of contributing factors we don’t exactly know how sonar can cause these events. These events are thought to cause panic in cetacean who use high pitched sounds to communicate to each other. Deep occurring sonar pulses could mimic feeding or distress signals forcing cetaceans to surface quickly. We see evidence of this rapid ascension in stranded individuals who often have livers full of gas filled lesions, in addition to the same lesions elsewhere. This causes liver failure and often multiple organ failure if the lesions are more widespread throughout the animal’s body. Organ failure resulted in the Thames Whale dying even after the huge rescue effort as it had strayed too far from it’s natural environment and had surfaced too quickly.
Lesions full of gas found in an autopsy of a common dolphin (Delphinius delphis.) a common sign of decompression sickness. These lesions have been seen in a number of recently stranded cetaceans (image credit)
In addition to military activity our fishing practices are also harming cetaceans. An increasing amount of our commercial fisheries are fishing for big predator species such as sharks and tuna (and rarely cetaceans) as they are currently very valuable, however removing these top level predators can often have wider implications for an ecosystem. Removing a top level predator can harm an ecosystem as they exert top down control over their ecosystems, preventing smaller fish from getting too abundant and overwhelming their food sources (which would lead to starvation). When these predators are removed fish populations can increase to a level which can damage the ecosystem causing a “trophic cascade”. Predators aren’t the only thing we fish for though and according to a U.N report we exploit, or over exploit over 90% of the fisheries on earth, and with the combinations of fishing out their prey, damaging ecosystems by removing predators, military activity and habitat damage by pollution or climate change we are seeing fewer and fewer of the cetaceans that once ruled the seas.