10 March 2012

Hunting in the depths

The sperm whale, Physeter macrocephalus (previously Physeter catodon), is a fearsome hunter and frequently dives to depths as deep as 3km in search of its main source of food - giant squid. Even though 3km doesn't sound like much, it is so deep that we struggle to build submarines that can withstand the extreme pressures even now; making the whales an amazing feat of evolutionary engineering, being one of the few animals that can survive the radical pressure change between the oceans' surface and its depths. Sperms whales can live for more than 70 years and males, which are more than 40% more massive than females, can grow to over 20 metres in length, weighing up to 57, 000kg. They have the largest brain of any animal and are also the largest living toothed species, although only males actually possess teeth, which can weigh up to a kilogram each.

A male sperm whale breaking up out of the ocean. Note its slender lower jaw, which increases the pressure of its bite (making it more powerful) and the thin, conical teeth that fit into alcoves in the whale's upper jaw - helping to trap anything that it bites in its mouth.

Sperm whales are named due to the large and well developed spermaceti organ that forms most of their head, which at times, can make up a third of an individual's total body length. This gives the whales a distinct body shape, making it difficult to confuse them with another species. Unfortunately, this made them easy targets for whalers in the Industrial Revolution, where they were killed for the whale oil that was harvested from the organ itself and from their blubber. Sadly, their population fell dramatically and, as a result, sperm whales are now protected and the species is classified as 'vulnerable' by the ICUN. Even though the organ has largely brought about the downfall the species, it is critical for their survival. This is fairly obvious really, when you think about the organ's large size compared to the whale's body and the energy that it must take in order to produce and maintain it.

A sperm whale, with its box-like spermaceti organ clearly visible. The whale also has a very powerful, muscular body and thick, triangular flukes that allows it to dive to such extreme depths.

The oldest known function of the spermaceti organ is as a buoyancy aid. At the surface, whales reduce the blood flow to the organ by contracting the diameter all of the blood vessels it contains, in a process known as vasoconstriction. This causes the temperature of the organ to fall and the spermaceti wax solidifies, increasing in density. As the organ is in the whales head, it sinks and pulls the whale downwards head-first; thereby, helping it dive and stay submerged. When the whale wants to rise to the surface, usually to breathe (sperm whales can stay submerged for about 90 minutes at a time), the converse it true and the blood vessels dilate (expand), causing the wax to melt and decrease in density. Thus, the whale is pulled upwards head-first, which allows it to ascend more quickly. This could be potentially life-saving for a whale - imagine running out of breath 3km underwater...

Sperm whales live in small social groups, rather unimaginatively called units, which consist of multiple related females, one unrelated male and calves (both male and female - males leave the group once they sexually mature). In this photo, the unit is sleeping, which is identifiable since the whales always sleep in this vertical position; kept afloat by their spermaceti organs.

However, buoyancy aid is only a secondary function of the spermaceti organ, with it being primarily involved in echolocation. Water is an excellent absorber of light, so that 3km down it is completely dark - no sunlight penetrates to such depths. Interestingly, even if light did reach such depths, sperm whales would be unable to see as they shut their eyes very tightly to protect them against the pressure and cannot open them. Therefore sperm whales are completely blind at these depths, which isn't ideal for a predator. Echolocation, also seen in bats, is an ingenuous adaptation that allows the whales to form an image of their surroundings by generating a series of clicks using the phonic lips at the front of their nose. These clicks then pass into Junk Bodies in the spermaceti organ and are forced out into the water through the Junk's lens-like structure. The whales can then process these clicks as they are reflected back, bouncing off the objects in the water. This allows them to form a 'mental image' of their surroundings, which may be part of the reason why they have such large brains - forming the images requires a large amount of neural processing.

The spermaceti organ also has a third and much more sinister function. The broad-beam clicks that it uses to generate an image of its surroundings can be focused into a much narrower, more intense beam. This beam is used for hunting and is 'fired' at potential prey targets once they have been located. It is so intense that it stuns and completely disorientates its prey, allowing the whale to get close enough to bite it, which normally means game over for the unlucky animal. Further to this, the depths of the ocean are filled with bioluminescent phytoplankton and cyanobacteria, which light up when stressed. Thus, when they are hit by the narrow beam they light up and the sudden surge of flashing light can disorientate the giant squid, which can see at such depths, even more.

A photograph taken by a deep-water submersible that shows a male sperm whale attacking its prey, a giant squid. Giant squid are a formidable killer in their own right, possessing thousands of suckers that each contain razor sharp chitin rings and a great quantity of scars can be seen on the whale, as a result from past encounters.

As said previously, one of the most amazing abilities of sperm whales is their ability to dive down from the relatively low pressure of the surface, to the abyssal depths that are under enormous pressure. So, as you would expect, they have many physiological adaptations that allow them to do this. The main ones are that their entire body, including all of their viscera (internal organs) are designed to be crushed, yet continue to function. Their rib cage is highly flexible and can fold almost flat; their lungs secrete a unique pulmonary surfactant that contains detergents to stop their delicate alveoli sticking together; and their heart virtually stops beating during the dive, which is accomplished by a massive drop in their metabolism. The fact that their metabolism and heart rate slows down so dramatically allows them to hold their breath for much longer than they could have otherwise. However, this still wouldn't allow them to hold their breath for a dive of 90 minutes so, to increase this time, they have an enormous quantity of blood, which therefore, can hold an enormous quantity of oxygen. Sperm whales also have extensive reserves of a pigment in their muscles called myoglobin, which acts as an emergency store of oxygen. Thus, sperm whales are able to dive at great depths for an extended period of time.

Despite still being rare, the population of sperm whales is beginning to rise and it is faring much better than other endangered species of whale. Mainly because its hunting is now banned in almost every country, meaning that commercial whaling of sperm whales has ceased, it has no natural predators and giant squid are not fished by humans, so its food source is not declining. Therefore, it is likely that this truly amazing animal has survived industrial whaling and will grace our oceans for many years to come.

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