Whale song

Abstract

What's special about whale song? People have known about whale song since at least the early whaling days. In the nineteenth century whalers were referring to ‘singers’ in their whaling logbooks. But it took until 1971 before a scientific description was published. Shortly thereafter, a commercial record was released that brought humpback whale (Megaptera novaeangliae) song home to stereo systems all over the world. Ever since, humpback whale song has been part of our culture, referred to often in books, movies and musical compositions. In the US, the National Aeronautics and Space Administration (NASA) even included whale song on a phonographic record sent into space aboard both Voyager spacecrafts. Whale song is scientifically interesting because of its distinct pattern of change in some species. In humpback whales, all males in a population tend to sing roughly the same song at any one time, but individuals that are within earshot of each other do not coordinate their songs to sing the same phrases at the same time. Intriguingly, this shared song changes gradually over the singing season, so that the shared song sung at the start of the season differs from the one we hear towards the end. This requires both vocal learning and coordination between whales, which is rare among mammals. Scientists using whale song for tracking and counting whales in the oceans have provided much new information on whale song. Whale song can inform us about population structure if we compare songs in different geographic areas. Songs of populations in different ocean basins are very different, while subtle variations within a basin can indicate relatedness. Do all whales sing? No, only some of the baleen whales have been found to be singers. Song is defined as a repetitive acoustic pattern that is highly predictable in its delivery and is often produced for long periods of time by single individuals. Humpback whales, blue whales (Balaenoptera musculus), fin whales (Balaenoptera physalus) and minke whales (Balaenoptera acutorostrata) are all in the same family (balaenopteridae) and all produce song. There are hardly any studies on the other species from that family, so they may turn out to be singers as well. All identified singers have been male. Among the other baleen whales only bowhead whales (Balaena mysticetus) produce songs, while gray whales (Eschrichtius robustus) and right whales (Eubalaena sp.) have been studied extensively and it is safe to say that they do not sing. Toothed whales, such as sperm whales (Physeter macrocephalus), killer whales (Orcinus orca) and beaked whales, also do not sing. What does it sound like? Whale song has been described as a composition of moans, snores, chirps and cries. They are all below 4 kHz in frequency. Blue and fin whale songs are so low in frequency that parts of them are inaudible to humans. Humpback whales produce the most complex song of the lot. It consists of repeated phrases arranged in themes in a hierarchical structure, and can last from 6–35 minutes. Individual whales have been found to sing for up to 22 hours, repeating the song over and over again. Bowhead whales produce slightly less complex sequences but, like humpbacks, also appear to have one shared song in each population that changes over the season. Other whales do not seem to change their song much. Blue whales have relatively short songs lasting only around one minute. In contrast to humpback whale songs, blue whale songs have been observed to remain stable for more than two decades. The most simple of whale songs is fin whale song. It consists of only very short, frequency-modulated pulses at around 20 Hz. Males have been found to produce such pulses over many hours in the breeding season. Recently, the minke whale has also been found to produce repetitive calling that looks like song. The call they use has been called a ‘boing’. It is frequently picked up on stationary hydrophones and scientists have wondered for over 50 years who or what produced it before the minke whale was identified as the culprit. What is the range of whale song? Fin and blue whales produce source pressure levels of up to 188 decibels in reference to 1 μPa. This is a measure of how loud the sound is. It is the same as 162 decibels in reference to 20 μPa sound in air, because the reference values used in the two media are different. To give you a better idea, this is roughly equivalent to the noise you hear from a jet airliner taking off when you are standing next to the runway. Sound travels much better underwater than in air, so that even the song of smaller whales can be heard at least ten kilometres away. The loud songs of blue whales have been detected successfully over more than a thousand kilometres from the singer. This has become apparent over the last decade when data from bottom-mounted hydrophone arrays designed for submarine detection were made available to biologists. Why do whales sing? Given what we know about bird song, it is tempting to conclude that whale song functions in the same way. Many features of whale and bird song are alike. It seems that only males produce song and the main singing activity is in the breeding season. As is the case for bird song, however, it is likely that whale song has more than one function. Humpback whales forage in higher latitudes in summer and migrate to lower latitude breeding grounds during the winter. Studies on breeding grounds have found that song has the strongest effect on other males. Singers are spaced further apart from each other than are quiet males, but males often approach or even attack other singing males if they are close. This suggests that song is an intrasexual signal to keep other males away, and possibly to form or maintain relationships with other males. During their migrations to and from the breeding grounds, singers seek out females and increase singing effort when joining females. It appears that migrating humpbacks take a more active role in approaching females while the breeding system on the wintering grounds has been described as a floating lek, where female choice may be the more relevant factor for mating decisions. Various authors have pointed out that whale song is also ideal for locating large obstacles and that whales might use these sounds as a form of echosensing. This may be a possible function for other whale sounds, but it seems an unlikely one for song since only males sing. The coordinated change over time in humpback and bowhead whale song is difficult to explain and remains one of the mysteries of whale song. It is possible that a coordinated acoustic display is necessary within a population to get females interested in mating while each male also tries to be sufficiently different to attract females to itself. Subtle changes introduced by one individual might then be copied by the next which could explain the shared song that slowly drifts in its structure over time. Have there been any cultural revolutions in whale song? Because humpback whale song is highly structured with very predictable transitions between themes within a song, it is relatively easy to map its change from one year to the next. While song structure changes considerably every year, it usually takes 15 years before a humpback whale song has changed completely within a population. In 1996, however, a handful of humpbacks from the western Australian population invaded eastern Australian breeding grounds and this resulted in a complete change of song within only two years. The eastern animals simply switched to the western song. Because vocal learning is required for such a change, this was called a cultural revolution. It is unclear why a large population of whales would change its song so quickly and what it was in the western song that helped it outcompete the eastern one. Are all whale songs this unstable? No, blue whale songs have been reported to remain the same for decades. Worldwide, there are at least nine different populations based on song differences. In the North Pacific, there are distinct Northeast and Northwest Pacific song types. Some of these whales are able to hear each other's songs in the central Pacific where more than one song can be recorded in the same area. It is unclear whether animals using different song types in the central Pacific breed with each other, but animals in the western and eastern North Pacific do not use the song of the other population and concurrent recordings of both types in the central Pacific are not very common. It remains to be seen whether some blue whales in the central Pacific have learned to use both song types or whether animals just keep the song identified from their population for life. Given the distribution of blue whales and the geographic and temporal variation of song types, it appears that vocal learning does not come into play. As in many cases with animals that cannot be studied in the laboratory, further data are needed to draw such conclusions with confidence.