The bio-duck was first noted by submarine crews in the 1960s: a regular series of downswept pulses, around fifty to three hundred hertz, arriving in bursts through the Southern Ocean winter and spring. It appeared on hydrophone records year after year without ever being tied to a visible animal.
The answer came from tagging. Suction-cup acoustic recorders placed on Antarctic minke whales off the western Antarctic Peninsula captured the same pulse trains from the tagged animals themselves. Because the tags log depth and movement alongside sound, the calls could be attributed with confidence rather than inferred from co-occurrence.
What the sound is for is still open. The seasonal timing and its association with the ice edge suggest a role in navigation or in keeping contact within loose groups, but neither has been demonstrated. Identifying the caller was nonetheless useful in itself: the bio-duck can now be used to track where minke whales are through the dark polar winter.