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Tread-water feeding of Bryde’s whales -The discovery of passive feeding behavior in baleen whales-

Nov. 9, 2017

Takashi Iwata (University of St Andrews)
Katsufumi Sato (AORI, University of Tokyo)

Researchers at the University of Tokyo and University of St Andrews and their colleagues have discovered tread-water feeding of Bryde's whales in the upper Gulf of Thailand. This behavior is distinct from the lunge feeding which involves an open mouth at high speed and has been reported by many previous studies. By visual observations and using animal-attached data loggers, researchers indicated that Bryde's whales fed prey species with treading water which was opening the mouth until the lower jaw contacted the sea surface and waiting for the prey to enter the mouth. The energy cost of tread-water feeding, which is passive feeding behavior, may be much lower than that for lunge feeding.

The upper Gulf of Thailand is a hypoxic environment, except at the sea surface layer, by the eutrophication. This eutrophication may limit prey species to the water surface. Therefore tread-water feeding of Bryde's whales is considered efficient for catching prey at the sea surface.

Both solitary individuals and adult-calf pairs were observed to use tread-water feeding. The tread-water feeding of the adult-calf pairs in this study implies social learning. Because tread-water feeding has never been observed in Bryde's whales in any other area, this behavior is suggested group-typical behavior. The tread-water feeding of Bryde's whales might be a possibility of cultural behavior because the whales show social learning and a group-typical behaviour pattern in the upper Gulf of Thailand.

The research group showed this discovery of tread-water feeding in Bryde's whales represents the first report of passive feeding in baleen whales, which indicates their flexible capacity to modify their foraging strategy in relation to variable environments.

Bryde's whale during tread-water feeding
Bryde's whale during treading water is opening the mouth on the sea surface and waiting for the small fish species to enter the mouth.

Takashi Iwata, Tomonari Akamatsu, Surasak Thongsukdee, Phaothep Cherdsukjai, Kanjana Adulyanukosol, Katsufumi Sato, "Tread-water feeding of Bryde's whales", Current Biology, Online Edition: 2017/11/06 (US EST).

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