22 July 2013

Face off: the great white shark vs the killer whale

Our oceans are full of predators, ranging from tiny carnivorous fish to poisonous snakes; from poisonous snakes to electrocuting eels; from electrocuting eels to murderous dolphins. In short, predators are everywhere and none are more feared than the great white shark and the killer whale. Both of these animals are titans – huge animals that can hunt and kill almost anything they want to (and yes, humans are included). But which of these is the top predator? Which of them would best the other in a fight?

The great white shark, Carcharodon carcharias, is one of the most feared animals in the oceans. The sharks often hunt by taking advantage of their huge strength, ambushing the seals they hunt from underneath while they rest on the surface. Unfortunately, surfers resting on their boards can look very similar to seals from underwater, which is believed to be the main cause of great white shark attacks on humans.

I’m sure many of you are thinking that this would be the great white. And why not? Great whites have been known to kill humans and there is a huge ‘fear culture’ around sharks after Peter Benchley’s Jaws was televised in a film. You would be wrong, however, and there is a case that to show that it is in fact orcas that rule the oceans.

In the first instance, you have to remember that orcas are social and hunt in packs (which has actually earned them the nickname of ‘the wolves of the sea’). Obviously, this provides killer whales with a huge advantage over the solitary great white and orcas can bring down prey much larger than themselves. In fact, orcas have even been known to go after small female sperm whales (which are actually the largest predator alive), so picking off a lone great white shouldn't be too much of a problem for them!

Like humans and other primates, killer whales are extremely social and live most of their lives in tight-knit social groups of close family members. To prevent the inbreeding that this would inevitably lead to, male orcas usually leave their maternal pod once they sexually mature and actively seek another pod to live with for the rest of their adult lives.

Secondly, you have to consider the intelligence of killer whales, which are part of the same family of oceanic dolphins (Delphinidae). All of the members in this family are extremely resourceful and have been seen to use complex, highly sophisticated tactics while hunting their prey. In fact, killer whales have actually been observed practicing to hunt (click here for more information) and it is one such tactic that leaves little doubt that orcas are hierarchically above great whites.

As you may or may not already be aware, orcas have different cultures (in much the same way as humans do) and different pods in various locations around the world specialise in hunting certain animals. Common examples of their sources of prey include fish and water mammals, such as seals. But it is the pods that specialise in hunting sharks that are really relevant for this post. Such orcas have realised and taken advantage of two very simple aspects of shark anatomy:
  • Firstly, most species of shark go into a hypnotic state of sleep called tonic immobility when they are turned upside down
  • Secondly, sharks have to keep swimming in order to breathe and, as soon as they stop moving, water ceases flowing through their gills and they begin to suffocate

Since great white sharks have to keep swimming in order to breathe, they have developed a remarkable method of sleep where they rest each half of their brain separately. This allows the neurotransmitters in their brain to recover to normal levels after waking activity, while simultaneously allowing the shark to keep moving and remain alert for prey and danger.

By taking advantage of these two simple facts, orcas can kill sharks extremely easily and with very little risk of injury to themselves (i.e. the sharks can’t fight back). To do this, orcas take advantage of their extreme agility and ambush sharks from above. As they near the unassuming shark, they bite it just above their dorsal fin and use their strength and body weight to roll the shark upside down, meaning that it falls asleep and stops struggling. Then, it’s just a simple case of the killer whale holding its breath while it waits for the shark to suffocate and die!

This tactic is merciless and brutal, but it is extremely effective and scientists have actually seen killer whales using it on great whites off the coast of California! What is also interesting, other than orcas being able to kill great whites so easily that is, is that great whites (like many species of shark) release chemicals called ‘death signals’ into the water when they are killed. These signals drive off all other great whites for miles and, for many years, scientists were baffled by why the massive predator suddenly vanished from Californian waters. It was only when they realised that the exodus of great whites coincided with the seasonal arrival of a certain pod of orcas from Antarctica that they put two and two together and began to observe how the two predators interacted. Just imagine their surprise!