17 June 2012

Silence of the hunter

The killer whale, Orcinus orca, is one of few species of toothed-whale and is closely related to oceanic dolphins, which first appeared around 11 million years ago. Often being called 'the wolves of the sea', killer whales are highly intelligent and are extremely successful predators that can be found throughout the world's oceans; being able to feed off a wide range of prey species that they hunt using echolocation.

The main prey that a pod of killer whales hunts appears to be culturally specific and typically, pods hunt either mammals, sharks, fish or birds. Cultures of killer whales are highly specialised and efficient at hunting their specific type of prey and use a variety of advanced and sinister tactics. These tactics can be very different depending on what animals the whales are hunting, but usually rely on the stealth or the strength and speed of the whales. Furthermore, these tactics are usually highly coordinated and individual whales have been seen to be 'talking' with each other and adjusting their behaviour accordingly!

Killer whales living at the poles can break up through ice in order to reach the surface and breathe. In order to this they swim upwards like a battering ram and strike the ice with enormous force. The power of such strikes is staggering and was highlighted by the famous British Antarctic explorer Sir Ernest Shackleton, who recorded fragments of ice being scattered for 20 - 30 feet around their holes! This behaviour is also used to knock seals off an ice floe into the water where another whale finishes it off - a fact that was of great concern for Shackleton and his expedition while they were stranded in Antarctica for nearly 2 years!

However, this behaviour has long baffled scientists who study the pods of killer whales that hunt mammals such as seals and sea lions, since all marine mammals have excellent hearing and the whiskers of these animals can detect even very subtle vibrations in water. Judging by this then, it would be expected that their prey should hear the very vocal whales coming and flee the area or climb out of the water to safety. However this isn't the case and often, their prey doesn't even know that the whales are there. New research conducted in partnership between St. Andrews University in the UK and North Carolina State University in the USA has helped to shed some light on this, finding that when they are hunting, killer whales "go into silent mode" and neither speak to each other nor use their echolocation to help them find their prey.

Instead, the whales seem to use a very sophisticated 'search and destroy' method in order to hunt where the members of a pod spread out over great distances and then close in again. This is repeated in silence until an animal is found, whereupon the whales close in on the unfortunate animal and kill it - remarkably, still in complete silence! It is only after they have killed their prey that they begin to talk again, a behaviour that Dr. Deecke from St. Andrews relates to humans: "it's a bit like us in a dinner party - they communicate while they eat then gradually wander off and go quite again".

Killer whales have a highly complex social organisation that is comparable to that of elephants and even humans. At its most basic level, their society is made up of units of closely-related females called matrilines that typically consist of 5 or 6 individuals. These matrilines then form 'pods' with around 4 other matrilines, with which they regularly interact and spend time with. Multiple pods regularly mingle with other pods to form 'clans' and finally, multiple clans interact forming larger aggregations called 'communities', which can be spread over huge areas of ocean.

Exactly how killer whales manage to conduct this very sophisticated behaviour in silence isn't definitively known, but it is believed that they rehearse their hunting tactics beforehand so that an individual whale knows exactly where they are supposed to be and what they supposed to be doing, as well as where the rest of their pod is supposed to be. Scientists at the two universities plan to continue their research by fitting sound recording devices and satellite tracking tags to killers in order to follow their behaviour much more closely and it is hoped that by implementing such technologies into their research, that they can determine for certain whether or not the whales are hunting in set patterns and conduct 'practice' runs.

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