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The long, patterned 15–30-Hz ('20-Hz') vocal sequences of fin whales (Balaenoptera physalus) can reach intensities of 184–186 decibels (dB) relative to 1 µPa of sound pressure, and can be detected throughout the world's oceans3,4. However, the source of these vocalizations was hard to identify5 because of the inherent difficulty of studying wide-ranging pelagic species and in locating low-frequency sound sources in the ocean. Until now, their function was therefore unknown6, making it difficult to assess the effects of increasing levels of human-produced sounds on these whales7.

We investigated whether these vocalizations could be breeding displays8 by determining the sex of vocalizing fin whales (Fig. 1) and comparing the results to the overall sex ratio in Loreto Bay, Gulf of California, Mexico. We identified and tracked vocalizing fin whales by using a 120-metre-long towed array of hydrophones and beam-forming software to compute a series of cross-bearings to a vocalizing individual. Required confirmation criteria included the absence of other whales that could potentially match the acoustic cross-bearings, and asynchronization of vocalizations with respiration.

Figure 1
figure 1

B. R. TERSHY

Only boys make noise: the wide dispersal of populations of the fin whale, seen here surfacing off the Mexican coast, means that males have to sing extra loudly to woo females.

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Once the vocalizing animal was unambiguously identified, a skiff was deployed to obtain a biopsy sample in order to ascertain its sex9. To estimate the overall sex ratio in the study area, we took biopsy samples from all fin whales encountered during systematic weekly surveys.

Vocalizations were produced only by male fin whales, despite a 1:1 overall sex ratio in the area (vocalizing: males, 9; females, 0; binomial test, P < 0.001; population: males, 21; females, 22; binomial test with normal approximation, Z0.05,2 = 0.153, P = 0.879). This sexual dichotomy in vocal behaviour supports the idea that the patterned sounds of fin whales are male breeding displays.

We propose that these displays serve to attract females from great distances to aggregations of patchily distributed prey. This is supported by several observations. First, fin and blue whales (Balaenoptera musculus) do not aggregate in specific areas for breeding, a behavioural trait that distinguishes them from the closely related humpback whale (Megaptera novaeangliae)10. Second, we previously found that fin whales use the Loreto study area to forage on dense aggregations of krill11. Third, the low-frequency vocalizations of Balaenoptera spp. are optimal for long-distance communication in deep water12.

Our results help to focus growing concern over the effects of human-produced sound on Balaenoptera spp.13. Sound levels from commercial ships, military sonar, seismic surveys and ocean acoustic research are extremely high (190–250 dB relative to 1 µPa at 1 m; ref. 4) and, at least since the early 1960s, the amount of human-produced sound in the frequency range used by large whales has increased7. A sound is detectable if its received level exceeds that of background noise by enough to be detected by the animal. An increase in ambient noise could thus reduce the distance over which receptive females might hear the vocalizations of males. To the extent that growth of Balaenoptera populations is limited by the encounter rate of receptive females with singing males, the recovery of fin- and blue-whale populations from past exploitation could be impeded by low-frequency sounds generated by human activity.