Via Treehugger and the Science Bulletin of the American Museum of Natural History comes an unusual report of bottlenose dolphins (Tursiops truncatus) sliding down the noses of humpback whales (Megaptera novaeangliae), an observation originally reported two years ago in a paper by Deakos et al. The behavior was seen twice (nice to get a paper out of about ten seconds of observation!). Here’s a video version of the report:
Here’s one observation described in the paper:
At 1427 h, two adult-sized dolphins (approximately 3 m in length) reversed direction and approached the humpback whales. The dolphins positioned themselves directly in front of one humpback still at the surface and appeared to surf the pressure wave created by the whale’s head as it swam. The two dolphins could be differentiated since one of them had a distinctive cookie cutter shark (Isistius brasiliensis) bite on the right side of the body and a notched dorsal fin. During the next two breaths by the same whale, each dolphin independently was seen lying across the whale’s rostrum as it surfaced, oriented perpendicular to the whale’s body. At 1430 h, the whale stopped and slowly raised its rostrum upward while lifting the well-marked dolphin out of the water (Figure 1a). Once completely clear of the water, the dolphin remained arched, on its side, balanced over the end of the whale’s rostrum (Figure 1b). The dolphin appeared to cooperate, with no discernible effort to free itself or escape. When the whale was nearly vertical, with its eye nearly breaking the water surface, the dolphin slid down the dorsal side of the rostrum (Figure 1c) while swinging its flukes upward (Figure 1d). This entire lift sequence lasted about 3 s, ending when the dolphin entered the water tail first.
What in the world is going on here? The authors suggest a number of hypotheses:
- The whale is pwning the dolphin. Dolphins are known to surf the “pressure wave” in front of swimming humpback whales, and this could piss off the whale. The lifting of the dolphin could result from a head lunge by the whale, and head lunges are known to be part of the whale species’ aggressive behavior. The authors discount this because the head lunge was so slow, and the dolphin didn’t appear to flee it.
- The whale is helping a distressed dolphin. This is called succorant behavior, and has been seen in other marine mammals, though it’s very rare in baleen whales. The authors discount this because the dolphin didn’t appear injured or distressed. Also, if the action was merely a maternal response by a misguided female humpback whale (sex was not determined), that wouldn’t explain the dolphin’s “cooperative” behavior.
- They’re playing! Both bottlenose dolphins and humpback whales have been reported to engage in play-like behavior. The authors suspect that this is the most likely explanation—that “social play” was initiated by the dolphin, perhaps stimulating a maternal effect on the part of the whale.
Deakos, M. H., B. K. Branstetter, L. Mazzuca, D. Fertl, and J. R. Mobley, Jr.. 2010. Two unusual interactions between a bottlenose dolphin (Tursiops truncatus) and a humpback whale (Megaptera novaeangliae) in Hawaiian waters. Aquatic Mammals 36:121-128.