Epic breaching shows might be on most whale-watching wishlists, but some trips out on the water bring encounters of a more mysterious kind.

This tail poking up above the surface belongs to a humpback whale, and the sight has onlookers in the boat feeling puzzled. Is the animal sleeping? Showing off? Perhaps it has its nose stuck in the sand?

Sleeping may seem like an odd guess at first. Depsite that "big fish" reference in the video, whales are, of course, mammals, and they need to surface in order to breathe. So wouldn't it make more sense for them to sleep horizontally at the surface to give the blowhole access to air? Some whales do indeed sleep that way, but believe it or not, others have been known to snooze vertically. In fact, the whale in this video has its tail flopped over in much the same way as another sleeping humpback caught on camera a few years ago.

And it's not just humpbacks. One Discovery Channel documentary includes bizarre footage of an entire pod of vertically sleeping sperm whales. (My cat will be relieved to know she's not the only mammal who dozes in odd positions.)

But not all tilted whales are necessarily snoozing. Back in March, a NOAA research drone caught a humpback doing a headstand near Hawaii while her calf swam around her. The animal stayed that way for at least ten minutes while the drone filmed her. Ed Lyman, resource protection specialist for Hawaiian Islands Humpback Whale National Marine Sanctuary, admits that this behaviour – which he calls "tail sailing" – is mysterious, but he does offer a few possible explanations. 

Because the behaviour is quite common in females with calves, Lyman suggests that hanging around vertically might be a good way for the mother to get some rest while keeping an eye on her calf underwater. The vertical posture might also work well for nursing. But why the tail in the air? Lyman points out that a wet fluke in the air might be a trick for cooling off. Whales have huge bodies covered in thick blubber, so a nice breeze probably feels pretty good.

In the end, the best answer is that whales are mysterious, and we simply don’t understand everything they do. Perhaps the whale in this new video is taking a snooze, sticking its tail in the air the same way you might stick your feet out of your blanket on a warm night. Then again, whales and dolphins are known to be playful and social, so maybe some of these tail-sailing whales are just goofing around. 

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Top header image: Alexandre Roux, Flickr