Why do Hammerheads swim on their Side?

Source.

Awesome!
But first, watch this video at 1:23ff.

Those are Scalloped Hammers off Mozambique.

See how several individuals are swimming on their side? Seen it a squillion times in the Galapagos, Malpelo and Cocos and always thought they were doing it in order to better watch me.

Possibly - but possibly not!

Have a look at this astounding paper about GHH!

Turns out that for those Hammerheads with large cephalofoils and large dorsal fins, swimming on the side helps conserve energy. It looks like the development of extended hammers for better predation and large dorsal fins for better maneuverability/tighter turns did come at an energetic cost which is being (partially?) offset by the rolling swim pattern - e.g. see this video of a GHH.

Or as one of the authors writes,

My theory is: Hammers are about 5 million years old – young for sharks, and the winghead and great hammer are the oldest species of the hammers, and they also have the biggest heads. The younger species have much more reduced heads, so Im guessing that hammers as a group first developed this big ole head, which is great for hunting, and a big dorsal to also help with maneuvering while hunting – but then realised that this head is actually pretty shit for cruising about (not efficient). Hey, but when I swim on my side that works much better.

Then as the newer species evolved, the head is reduced – a tradeoff between the hunting benefits and transport. 

Like I said, simply awesome! 

Kudos to the authors for a) observing the behavior, b) developing the hypothesis and c) providing for compelling evidence, even going as far as to build a model and test it a wind tunnel. This is exactly how good science should happen!

Well done folks - very impressive!