Sperm Whales: a deep-sea odyssey
If you think of deep-sea creatures, your mind might come up with angler fish, jellyfish and other mysterious creatures. Now, imagine a giant, air-breathing mammal roaming the deep-sea, far from the ocean surface. This is possible for sperm whales (Physeter macrocephalus). Sperm whales are the largest of the toothed whales (odontoceti) on earth, growing to lengths of 11-12m in females and 16m in males. These giants are found in most oceans, with the exception of the high arctic. Sperm whales are migratory, travelling due to water temperature, feeding and breeding. They are usually found in deeper water (>600m), above sea ridges such as the Mid-Atlantic ridge, where they can exploit the deep-sea environment. While average dives are around 400m, they can dive down to depths of over 1000m, lasting over an hour.
Diving into the deep is not an easy task. There are constraints with diving to lower depths of the ocean such as pressure change and oxygen supply. For every ten metres travelled down in the ocean, pressure increases one atmosphere. One atmosphere is equivalent to the weight of the atmosphere at sea level, which is approximately 14.6 pounds per inch squared. The deepest recorded dive by a sperm whale is 2,035m, as shown in picture below. This would mean they are subject to pressure of around 204.5 atmospheres. That is 204.5 times the pressure experienced at the surface, but how are sperm whales able to deal with this pressure change? If a human was to attempt and dive to the depths sperm whales achieve, their rib cage would be crushed. Sperm whales however are more flexible. The rib cage is able to collapse as a result of being bounded by cartilage. A sperm whale rib cage will collapse with increased pressure, rather than snapping under the pressure during a dive.
With sperm whales being air breathers, a good storage of oxygen is needed to dive for long periods in the depths. The lungs in sperm whales are collapsible. Lungs collapsing allows sperm whales to dive deeper, as a more inflated lung would make the whale more buoyant. Buoyancy is instead controlled by the spermaceti, located in the head. Sperm whales are therefore able to exhale 90% of the air in their lungs before diving. The oxygen however is not stored solely in the lungs. This because nitrogen is also stored in the lungs with oxygen, putting the whale at risk of high nitrogen tension at greater depths. The whale will use blood and skeletal muscles as the main oxygen store. This means 87% of the total oxygen reserve is distributed between the blood and skeletal muscle.
In skeletal muscle, myoglobin is the primary oxygen carrier. Myoglobin is an oxygen-binding protein. In marine mammals, the myoglobin are positively charged. Being positively charge allows whales to have more myoglobin when compared to other mammals such as humans, where it is 10 times higher. This is due to the positively charged proteins repelling each other, resulting in more myoglobin fitting into the muscle without sticking to each other. Larger numbers of myoglobin results in higher stores of oxygen, which in turn means more oxygen for longer and therefore deeper dives.
Now we know how sperm whales are able to dive to greater depths, it is time to ask why they dive to such depths. The answer to question is very easy, FOOD! The depths offer a food source which is not obtainable for other air-breathing mammals. The primary food source for a sperm whale is mesopelagic squid, however they are also known for feeding on giant squid and occasionally fish and sharks. The scars found on many sperm whales are often caused by encounters with giant squid, who use sharp-toothed suckers on their tentacles to fight off the sperm whale. The video below shows how sperm whales hunt when diving.
The last question is how can sperm whales hunt at such depths? Below 200m sunlight penetration is minimal and below 1000m (known as the midnight zone), there is no light at all. If there is low to no light source, how are sperm whales able to manoeuvre and hunt where they cannot see? Sperm whales use echolocation. This ability is found in many whale species such as the deepest diving mammal the cuvier’s beaked whale (Ziphius cavirostris), as well as terrestrial mammals such as bats.
The whale will send out ‘clicks’ created in the head of the whale by passing air from bony nares through the phonic lips, resulting in the lips closing to create a click. The click then passes through the spermaceti to the melon and out into the water and bounce back when they come into contact with prey and the environment. The rebounding sound will then be received by an oil filled channel located in the lower jaw, creating an image of the environment for the whale. This allows the whale to move around the deep without the need for a light source. This ability is also suggested as the primary weapon for hunting. The long-range echolocation is thought to help them detect deep prey patches and feed. The yearly biomass turnover in sperm whales is so high that it is considered to be comparable to the total yearly catches of human fisheries. With this untapped food source and global distribution, sperm whales are highly efficient and successful hunters.
Sperm whales take advantage of an environmental niche due to their ability to achieve great depths with dives. The adaption of echolocation, along with highly evolved anatomy allows them to move and hunt in an extreme environment; this combined with a lack of competition from other air-breathing mammals, sperm whales are highly successful deep-sea hunters. While some of the sperm whales diving behaviour is understood, there is still a need for more research; the lack of ability to actively watch sperm whales in-situ means that there is still a lot more to learn from this majestic giant of the sea. As technology continues to improve, the more discoveries will be made by scientists.