Animals often have symbiotic relationships. Egrets hang out on the backs of many large animals, picking parasites in exchange for free food and transportation. Plovers act as dentists, eating the leftover food inside the mouths of crocodiles.

But this relationship is baffling. Sometimes dolphins hitch rides on the backs of humpback whales — and it's very possible that the only thing either party is getting out of it is a little bit of fun.

The above photo of a dolphin riding piggyback on a whale garnered lots of attention when it was posted a few years ago on Facebook by the Whale and Dolphin People Project and it's making the rounds again this week.

According to the description that came with the photo:

"This is one of the strangest cetacean photos I've ever seen. It was taken by Lori Mazzuca in Hawaii. She said that the dolphin and humpback whale were playing gently together. The game seemed to be about how long the dolphin could stay atop the whale's head while the whale swam. When the dolphin finally slipped off, it joined another dolphin and they began to leap with joy."

The creature lovers at Discovery News were a little suspicious that the image may have been Photoshopped or altered in some way. So they asked some experts to weigh in.

“Both dolphins and humpback whales can be extremely playful with each other and other species,” said Diana Reiss, a cognitive psychologist and dolphin researcher at Hunter College in New York. “It is very possible that this is play, but without seeing it first-hand, I really don’t know.”

“Based on the description, I believe play would be the best explanation,” agreed Ken Ramirez, vice president of animal care and training at Shedd Aquarium in Chicago. “If this were a video, there would be far more information to allow for better interpretation. But it is believed that the 'surfing' or bow riding that dolphins exhibit in front of boats may have had its genesis in riding in front or in the wake of big whales.

“What we may be seeing here is that type of surfing, but in this case the whale chose to give the dolphin a different type of ride.”

It's not quite as clear as the image above, but here's a video taken in Maui, Hawaii, of a bottlenose dolphin allegedly riding on a humpback whale.

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Mary Jo DiLonardo Mary Jo writes about everything from health to parenting — and anything that helps explain why her dog does what he does.