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Zoologger: The great white shark cattle market

Species: Carcharodon carcharias
Habitat: Throughout the tropical and temperate oceans, acting mysteriously

Great white sharks are legendary. Ever since Jaws, we have known them as ferocious killing machines that sneak up on unsuspecting prey from the depths of the ocean.

But no animal can spend its entire life hunting and munching. Somehow, somewhere, the sharks must get together to mate.

Finally, we have a clue how they do it.

Because they spend so much time in remote waters, and don't survive in captivity, great white sharks are deeply mysterious creatures. But over the last ten years, biologists have been able to track them using electronic tags which record their position and depth, and the ocean temperature.

On the face of it, that information can't tell you what the sharks are actually doing. But Salvador Jorgensen of the Monterey Bay Aquarium in Monterey, California, and colleagues have developed a new statistical analysis that picks out patterns of behaviour from the tagging data.

It seems to confirm earlier suggestions that the sharks have a breeding ground in the east Pacific. What's more, it suggests that the males go there to show off side-by-side in front of the choosy females – cattle-market style.

On the prowl

Jorgensen looked at the electronic records of 53 great whites in the eastern Pacific, covering a period of 5571 days.

He found that the sharks spent a lot of time in two offshore habitats. One is around Hawaii; the other is an area between Hawaii and the Baja peninsula of Mexico, known as the White Shark Café, a region where they are already known to congregate. When travelling between these habitats and the coast of North America, the sharks swam just below the surface, only rarely diving deeper.

In Hawaii and the Café, the sharks spent more time diving – in two distinct ways. In both regions, they often spent the day in the depths and surfaced at night in pursuit of prey.

But sharks in the Café also went in for a different kind of diving: from the surface down to 500 metres and back again. "They go up and down, day and night," Jorgensen says, often completing 150 such cycles within 24 hours. "It's an astonishing behaviour." No other shark has been seen doing it.

Sharks made more of these oscillatory dives the closer they were to the centre of the Café, with males performing them much more than females.

Ever since the Café was discovered, researchers have suspected that it must be a mating ground since it has little prey to draw the sharks. Jorgensen says the sharks' peculiar diving strengthens the case.

Shark lek?

Many birds mate in a system known as a lek: the males establish territories next to each other, and the females move between them to pick their favourite. Essentially, the system ensures that females have the best possible choice of mates.

Jorgensen says the great whites may be doing the same thing. Rapidly diving and surfacing may be a way for males to display their strength and endurance to the females, just like a man might show off his skills on the diving board of a hotel pool.

Lek systems evolve in species where the males do not help to raise the offspring, so the females simply want to get the best possible genes for their young. That's very much the case with great whites, as males and females spend very little time together.

If the Café isn't a lek, it could simply be a designated mating zone: by entering it, females are announcing that they are willing to mate. In that case, the diving behaviour could be interpreted as the males searching up and down for females. "The fittest males will search the most and have the most success," Jorgensen says.

Violent encounters

Right now we don't know enough to distinguish between the two possibilities. We do know that females spend much less time in the centre of the Café than males do. That makes sense, because while we don't know exactly how great whites mate, we know it is violent. Females often have bite marks and other injuries, probably from males holding onto them by biting their fins. As a result, Jorgensen says females would want to slip into the Café, mate once, and then leave in a hurry.

Regardless of exactly what is happening, the Café area is clearly important for white sharks, says Jorgensen. "All the mature males [in that region] go there, every year."

Great whites are considered vulnerable to extinction due to the high amount of food needed to support each shark, coupled with the fact that people such as trophy hunters and sports fisherman are often eager to kill them. If the Café really is a mating zone, Jorgensen says we should consider making it a protected area.

Journal reference: PLOS ONE, DOI: 10.1371/journal.pone.0047819


This café is a bit of a dive <i>(Image: Reinhard Dirscherl/Getty)</i>

This café is a bit of a dive (Image: Reinhard Dirscherl/Getty)


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