Why are these great white sharks swimming in tandem?

The waters off Mexico's Guadalupe Island are visited by some of the largest great white sharks on earth, so divers flock here for exciting encounters. Just recently, two great whites in the area put on a more subtle show for onlookers – but this bout of tandem swimming is actually an interesting glimpse at shark behaviour.

Guadalupe is a hotspot for shark diving, so it's possible that the allure of hang bait brought these two animals together. Areas of high human activity can create false aggregations as the animals temporarily group around food – so the behaviour we're seeing in this clip may not occur naturally. Still, the interaction is unique enough to have sparked curiosity among experts.

In some cases, sharks that come in to investigate the same site will battle for dominance – we've seen this play out in South Africa's Neptune Island – but great white communication isn't all tooth and tussle. They do send out more subtle warning signals, too.

If you look closely at the shark on top, you'll notice a behaviour known as "gaping" (not to be confused with spy-hopping). This underwater opening of the mouth is thought to be an agonistic display, a way for sharks to tell each other to back off. Diver Julian Gunther has seen this on numerous occasions, and likens it to a dog's warning growl.

By drawing water quickly into the mouth, sharks can force water over the gills at a faster rate than normal. This causes the individual gills to flutter in a wave-like pattern.

We see this gill-flaring in the Guadalupe clip as well, so chances are the smaller shark got too close for comfort. Still, there did seem to be a period of tolerance from both sides before this tea-for-two moment came to an end.

It's possible we're looking at two sharks sizing each other up, neither one feeling threatened enough to risk injury (and waste energy) in a fight. The larger animal, which appears to be female, comes out on top. Local researcher Maurcio Hoyos Padilla, who first filmed great white heavyweight Deep Blue, has catalogued a number of such interactions. And yet, even he was taken aback by the footage, as most close encounters of the white shark kind occur side by side.

"Wow," he said upon viewing the clip. "I thought you were talking about parallel swimming, but this is not! I have never seen something similar."

So, what's going on? Fisheries biologist Dr Gregory Skomal, who is working on a five-year white shark population study with the Atlantic White Shark Conservancy, offers one possibility.

"Guadalupe is thought to be a mating area for white sharks," he says. "While the top shark does look like a female, I can't see if the other shark is a male. An interesting scenario, which I am unable to prove, is that a smaller male (if it was a male) approached the larger female for potential mating and he was rejected."

After checking in with a few local dive operators, we were able to confirm that the smaller shark is in fact a male named "Luca", who is easily recognisable by his scars.

Of course, the presence of Luca's twin "penises" in this scenario doesn't mean we're seeing a mating attempt – but even the possibility is an intriguing one.

We still have many unanswered questions about white shark reproduction, but we do know that females cover great distances to mate and give birth.

The sharks return to Guadalupe every year, and baby whites have been seen near mainland Mexico. Given a gestation period of around 18 months, the annual migration supports the idea that mating takes place at or around Guadalupe. Previous tracking studies have also shown that some sharks exhibit what could be a courtship ritual at an offshore hotspot known as the "White Shark Café", which sits halfway between Baja and Hawaii.

But University of California Long Beach shark biologist Dr Chris Lowe points out that for all our suspicions about Guadalupe, the evidence isn't entirely compelling. For starters, known shark-mating behaviours, like pectoral fin biting, have yet to be documented here.

So what is Lowe's take on the tandem-swimming sharks? "I would lean more towards some form of agonistic display than mating," he says.



For predator-prey ecologist Michelle Jewell, meanwhile, who has done extensive work on white sharks, the clip stands out for other reasons, too. "This video is very interesting for me since I have always wondered about the idea of electric sense 'blind spots' in really large, wide sharks," she says.

These animals are known for their keen sense of smell (we've all heard the drop-of-blood-in-the-ocean adage), and their sight and hearing are also sharp. But it's the electrosensory system that makes sharks such astute hunters.

Like other fish, they can detect the faintest movements and vibrations in the water thanks to what's known as the lateral line. Special pores (ampullae) near the head and jelly-filled canals along the body work together like finely tuned radar. Based on the position of the pores, we know the line broadcasts out from the sides, and forward from the head – but what happens at the rear?

"If you get right underneath and just behind a wide shark (like buddy number two does) is he essentially in her electro-blind spot?" Jewell wonders. "You can see that as soon as he falls away and goes to the right of her, she seems to pick up on him and change direction."

Let's look once more:

This isn't unlike what happens on a freeway: wide load, no peripherals. You'd think millions of years of shark evolution would have taken care of such a design flaw, but Jewell wonders whether a blind spot could actually be beneficial in some way.

Shark sex can be rough, so for a male attempting to dock (yes, that's the technical term) with a large partner, the element of surprise could help.

"If you approach a large female from the side, she's going to detect you and buzz off," says Jewell. "If you approach from above or head on, she's going to detect you and buzz off. But, if you approach a wide female from just below and behind, your risk of being detected or attacked is lowered. You are effectively invisible to her and able to manoeuvre into striking/mating position."

By that logic, small females, who may not be as physically fit or mature, would be less likely to mate as they'd still be "seeing" in 360°. In such a scenario, 'blind spots' could be helping male sharks get the best mating deal.

While much of this is brainstorming and speculation, it shows just how much we can learn from each encounter with these misunderstood animals. If this many questions can be sparked by one 30-second clip, imagine what's coming next!

UPDATE (21 January, 2017): Based on the pelvic fin pattern, some have speculated that the top shark in this clip may be an individual known as "196". This animal is male, so if the suspicion proves true, that would certainly toss out the mating hypothesis!

UPDATE (06 April, 2017): Shark 196 has been named "Walter Whiteshark".

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