Two groups of whales, the Humpback Whale and the subspecies of Blue Whale found in the Indian Ocean, are known to produce a series of repetitious sounds at varying frequencies. This is known as whale song. Marine biologist Philip Clapham describes the song as "probably the most complex in the animal kingdom".[7] Male humpback whales perform these vocalizations only during the mating season, and so it is believed the purpose of songs is to aid sexual selection.[2] Whether the songs are a competitive behavior between males seeking the same mate, a means of defining territory or a "flirting" behavior from a male to a female is not known and the subject of ongoing research.[citation needed] Males have been observed singing while simultaneously acting as an "escort" whale in the immediate vicinity of a female. Singing has also been recorded in competitive groups of whales that are composed of one female and multiple males.[citation needed] Interest in whale song was aroused by researchers Roger Payne and Scott McVay after the songs were brought to their attention by a Bermudian named Frank Watlington who was working for the US government at the SOFAR station listening for Russian submarines with underwater hydrophones off the coast of the island.[8] The songs follow a distinct hierarchical structure. The base units of the song (sometimes loosely called the "notes") are single uninterrupted emissions of sound that last up to a few seconds. These sounds vary in frequency from 20 Hz to 10 kHz (the typical human range of hearing is 20 Hz to 20 kHz). The units may be frequency modulated (i.e., the pitch of the sound may go up, down, or stay the same during the note) or amplitude modulated (get louder or quieter). However the adjustment of bandwidth on a spectrogram representation of the song reveals the essentially pulsed nature of the FM sounds. A collection of four or six units is known as a sub-phrase, lasting perhaps ten seconds (see also phrase (music)).[citation needed] A collection of two sub-phrases is a phrase. A whale will typically repeat the same phrase over and over for two to four minutes. This is known as a theme. A collection of themes is known as a song.[citation needed] The whale will repeat the same song, which last up to 30 or so minutes, over and over again over the course of hours or even days.[citation needed] This "Russian doll" hierarchy of sounds has captured the imagination of scientists.[weasel words][citation needed] All the whales in an area sing virtually the same song at any point in time and the song is constantly and slowly evolving over time.[citation needed] For example, over the course of a month a particular unit that started as an "upsweep" (increasing in frequency) might slowly flatten to become a constant note.[2] Another unit may get steadily louder. The pace of evolution of a whale's song also changes—some years the song may change quite rapidly, whereas in other years little variation may be recorded.[2] Whales occupying the same geographical areas (which can be as large as entire ocean basins) tend to sing similar songs, with only slight variations. Whales from non-overlapping regions sing entirely different songs.[2] As the song evolves, it appears that old patterns are not revisited.[2] An analysis of 19 years of whale songs found that while general patterns in song could be spotted, the same combination never recurred.[citation needed] Humpback whales may also make stand-alone sounds that do not form part of a song, particularly during courtship rituals.[9] Finally, humpbacks make a third class of sound called the feeding call.[citation needed] This is a long sound (5 to 10 s duration) of near constant frequency. Humpbacks generally feed cooperatively by gathering in groups, swimming underneath shoals of fish and all lunging up vertically through the fish and out of the water together. Prior to these lunges, whales make their feeding call. The exact purpose of the call is not known, but research suggests that fish know what it means.[citation needed] When the sound was played back to them, a group of herring responded to the sound by moving away from the call, even though no whale was present.[citation needed] Some scientists have proposed that Humpback Whale songs may serve an echolocative purpose,[10] but this has been subject to disagreement.[11]