Ordinarily, baleen whales feed by opening their mouths and lunging forward into the water. They then strain the captured water out through sieve-like baleen plates in their mouths, leaving prey such as krill and small fish inside to be swallowed. Recently, however, scientists have documented another more passive form of feeding in the whales, which is known as "tread-water feeding."

Led by Takashi Iwata, now at Scotland's University of St. Andrews, the scientists observed the behaviour in a population of Bryde's whales (which are a type of baleen whale) in the upper Gulf of Thailand.

Instead of lunging forward, the animals have been simply "treading water" and holding their mouths open at the surface for several seconds at a time. Large numbers of anchovies are carried into their mouths by the current, and subsequently eaten.

The behaviour has been seen on 58 separate occasions, involving 31 individual whales. It could be occurring because the water is oxygen-low at that location, due to sewage entering the ocean from nearby rivers. This would drive the fish to live at the surface, and thus force the whales to alter their feeding strategies.

That said, the whales may also be doing it simply because it requires less energy output than traditional lunge-feeding. It also likely that the whales are learning it from one another – eight pairs of adult whales and their calves have been observed performing the behaviour together.

"Our findings suggest that calves might learn tread-water feeding by imitating adults," says Iwata. "Moreover, because imitation is an important aspect of social learning, the tread-water feeding of the adult-calf pairs in this study implies social learning."

A paper on the research was recently published in the journal Current Biology.