More often than not, it turns out the obvious answer is right. A study published in the most recent issue of Nature Communications provides an explanation for why warm, surface water-dwelling devil rays possess a mass of blood vessels in the head, typically found in cold habitat to animals keep the brain warm.

Devil rays

It turns out that devil rays are actually excellent divers, and although they typically spend the day lolling near the surface, the majestic creatures spend most of their nocturnal hours making deep dives down to the cold sea floor to feed.

The mystery of devil rays’ reta mirabile

It was discovered over 30 years ago that the front of a devil ray’s skull is filled with a sponge-like mesh of arteries. This network is called a rete mirabile, and is known in other cold water organisms.

“It was a mystery as to why they had this system, which is a way of keeping brain activity high, even in a cold environment,” said Dr. Simon Thorrold, an ocean ecologist at the Woods Hole Oceanographic Institute, the lead author of the paper.

In fact, the scientists who discovered the rete mirabile in devil rays were so baffled by finding it in a warm-water species that they even hypothesized it might somehow work to help cool the brain while the rays basked in the warm water and sun.

However, it appears Thorrold’s new study has solved the mystery. “We looked at the data and of course, it made complete sense,” he told BBC News in a recent interview, admitting to a little sheepishness.

Rays are deep divers

The depth data from the research confirmed that devil rays regularly make incredibly deep dives, usually lasting 60-90 minutes and reaching depths of 1,848 meters.

Moreover, devil rays are fast divers. The rays swam downward at close to 22 km/h (13mph), which is a great deal faster than sharks and whales can dive. The rapid diving speed was also a surprise to researchers. It turns out that because devil rays are negatively buoyant, they can just allow themselves to sink, and glide downward with minimal effort to their deep-sea floor dinner.