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A pair of deep-diving dwarf sperm whales (Kogia sima). Photo credit: Bahamas Marine Mammal Research Organisation, July 2018.

Hunting with echolocation in the deep sea is a completely different kettle of fish from seeking quarry nearer the surface. Open ocean-going behemoths tend to produce loud clicks pitched between 10 and 50 kHz that carry well in water to penetrate the depths. In contrast, smaller porpoises and dolphins, which swim in shallower waters, tend to have softer, higher pitched (over 100 kHz) voices that don't carry as far, to avoid interference from the echoes produce by their cluttered surroundings. However, when Peter Teglberg Madsen from Aarhus, University, Denmark, investigated the clicks of an ocean-going dwarf sperm whale that had been recovering at an aquarium after stranding, he was surprised. The whale's echolocation clicks were more in keeping with the rapid high-pitched sounds produced by smaller species that hang out nearer to the surface. How are dwarf sperm whales able to hunt in the open sea when their high-pitched echolocation clicks don't carry as far?

Chloe Malinka and Pernille Tønnesen from Aarhus University joined Charlotte Dunn and Diane Claridge at the Bahamas Marine Mammal Research Organisation to listen in on dwarf sperm whales. ‘We needed to go where the animals are commonly observed, where deep waters close to shore allow for easy access, and where there are no other creatures around to produce sounds that overlapped with the dwarf sperm whales’, says Malinka. Out in the Atlantic Ocean, the team lowered a newly designed string of underwater microphones capable of picking up and locating the animals’ clicks. ‘It was always nerve wracking to do an initial analysis at the end of the day to see whether any of our recordings contained Kogia [dwarf sperm whale] clicks’, says Malinka, recalling how they could only venture out on calm days. Impressively, after 5 weeks of pursuing the cruising animals, the team had recorded 8636 echolocation clicks over distances ranging from 134 to 718 m. Yet, after months of patient analysis in Denmark, they realised that the whales were only in the ideal position for the string of microphones to record the clicks in full detail on 46 occasions.

Compared with sperm whales, which are larger and produce clicks ranging from 10 to 20 kHz, the dwarf sperm whales were clicking at frequencies around 123 kHz, more in line with porpoises and dolphins that remain near the surface. However, when the team analysed the time interval between the dwarf sperm whale's clicks, they produced fewer clicks per second (∼4), than those of the smaller surface species, which click at ∼10 clicks s^–1. In addition, the dwarf sperm whales’ clicks were softer (∼180–192 dB) than the researchers had anticipated. ‘Given their body size, Kogia should be able to produce sound levels of more than 220 dB’, says Malinka, adding that it is possible that dwarf sperm whales have sharper ears than their larger cousins, which may allow them to hear softer echoes in the relative peace of the uncluttered open ocean.

The team also noticed that dwarf sperm whales project their beams of sound with extreme precision. And when they investigated a series of clicks recorded from a dwarf sperm whale in Cape Town harbour by Tess Gridley and Simon Elwen, from Stellenbosch University, South Africa, it was clear that the animals are extremely versatile, adjusting their volume and the rate at which they produce clicks, and altering the range of frequencies in each click as they explored their environment. The team also suggests that it may not be necessary for the animals to depend on loud long-range echolocation to locate lunch; dwarf sperm whales may simply descend to the depth where their prey reside in large numbers, making their short-range high-frequency beams of sound more effective.