Why do Humpback Whales Sing?

Singing humpback whale by Tony Wu

There are many ideas for why humpback whales spend so many hours every day singing their complex songs on the mating grounds.

• Could it be related to how Red Deer (UK) / Elk (US) have roaring contests?

• Is it for males to evaluate each other's strength and endurance, to avoid a fight with a stronger, more fit male, thereby risking injury?

  • However, if that is the case, why do you often see surface active groups on the mating grounds, where many males follow a single female, fighting each other for the prime position behind the female.

• Is it to impress the females with their virtuosity?

• Could it depend on the density of whales on the mating ground or is it possible that there is more than one male strategy.

Animal behavior is very much affected by evolutionary pressures, that is, behavior that increases the number of offspring in the next generation is favoured. For example, male Red Deer (UK), also known as Elk (US), fight each other during the rut, or mating season. The smaller, weaker animal is likely to be injured in these contests, so it would behove males to be able to ascertain the size and strength of their opponent before a physical interaction.

Dr. Tim Clutton-Brock discovered that the roaring of male red deer allows them to do just that. The bulls roar repeatedly during the mating season, long before they see any other bull. They can hear each other and engage in roaring bouts, where two males take turns roaring at each other for extended periods of time. At the end of these bouts the bulls can be seen to breath heavily, clearly tired from the vocal battle. Not only that, but observations show that the strength of a bull, especially in its neck muscles, was directly correlated with how long they were able to keep a roaring contest going. In other words, the bulls can evaluate the strength of their opponent long before they see each other. Specifically, they can evaluate whether their opponent has greater strength and endurance in their neck muscles, used when engaging physically with their antlers, than they do.

If the bulls are fairly evenly matched, they approach each other and perform a parallel walk, where they walk side by side, back and forth, eying each other, apparently evaluating their opponents physical fitness. Only if they both appear to be of equal size and strength do they turn on each other for a physical contest, locking and twisting antlers, as well as pushing each other around. As a result there are several opportunities for a smaller, weaker bull to realize that his opponent is bigger and stronger, allowing him to bow out before getting hurt. Most competitions between males in most species in nature are ritualistic like this, designed to test the strength or endurance of the participants without the risk of severe injury. It is in neither competitor's interest to risk this, even though the males in many species have the weapons available to kill their opponent.

All whales have a common land mammal ancestor with ungulates, such as deer, so it is possible that some of the behavior of these two groups of mammals has the same origin in a common ancestor. That is, the song of the humpback whale may serve a similar purpose to that of the roaring of red deer. The song is mostly performed on the mating grounds, such as in Hawaiʻi, or in the Caribbean. It has even been described how male singers join other singers to briefly swim side by side before moving on. However, the actual mating between a male and a female has never been observed, since it presumably takes place during a long dive performed by the female and the “chosen” male.

If the goal of the singing males (females don’t sing) is to ensure more offspring in the coming generation, there should be a way for other whales to evaluate, or measure the singer, based on the song. So what aspects of a song can be measured from a distance? The first aspect that was discovered fairly early was that when a singing whale surfaces for a few breaths they have to stop singing, since they cannot breathe and sing at the same time. Furthermore, if a male ascends to breath while singing, but before blowing, the song will disappear for any listener, since it will be lost in the waves at the surface. In other words, it is possible to measure how long a whale can hold its breath, just by listening to its song.

So, the purpose of the song-bouts could be to measure how long the male can hold its breath, maybe not just for one dive but for a series of dives over a longer time period - a song session can last for hours. Since female humpback whales tend to be bigger than the males, theoretically they should be able to hold their breath longer than the males. So the long dive when the male and female supposedly mate, could be a way for the female to test whether the male has what it takes? So if the purpose of the song is to evaluate who has the most stamina, males should be visibly, measurably more tired after a long singing bout.

However, this does not take into account what happens in the surface active groups, when a large number of males (10 or more) follow a female, fighting each other for the position right behind the female, as the primary escort. These fights can be very severe and physical, with males ramming and hitting each other.

There are even some instances when a male has been killed in these skirmishes. The supposed “mating dive” takes place at the end of these surface active group interactions, with the primary escort (the one closest to the female) being the lucky male. This is very similar to how several other species of animals decide which male will mate with a particular female, including chickens and bighorn sheep. The females in these species will make sure that they have a large group of males duking it out behind them, by leading their group of suitors towards males that are not yet in pursuit, thus increasing their chances of mating with a high quality male.

So if the humpback whale song, just as in red deer, allows a male to assess the size and strength of his “opponents”, younger and upcoming males would be able to assess their chances from a distance, enabling them to avoid unnecessary injuries. If this was the case, you would expect the average size of a male in a surface active group to be larger than the average solitary singer. This has been shown to be the case. It has also been shown that larger females tend to be followed by more males. Interestingly, larger females produce larger calves that grow faster and are more likely to survive to adulthood.

However, this does not take into consideration the whole aspect of the complexity of the humpback whale song, which was first described by Roger Payne and Scott McVay in 1971. A song session can go on for hours, where a song, about 12-15 min long, is repeated over and over again. The song is composed of several Themes, that in turn are composed of Phrases containing one or a few Subphrases that, in turn are made up of various Units. Parts of the song may change over the season and may even disappear. While new song components can be added from year to year. However, at any particular time during the mating season all singers in the area sing the same song!

The purpose of this complexity, changing from year to year, has defied an evolutionarily based scientific explanation for over 50 years, other than that it somehow would indicate male fitness. What would be the value of being able to perform this song exceptionally well? Is it to display a great memory, or could it be a display of great vocal ability, being able to display a great virtuosity, much like that of a blackbird? 

A 2023 study of blue tits found that “repeating the same pattern of notes with high precision – was the key song feature that made females sexually excited.” However, “while singing exact copies of the same note was sexy, it also became ‘boring’ for females” resulting in a gradually declining response “until it was reignited when males switched to a different song type” (Lancaster University 28 June 2023).

Lower pitched sounds travel much further than higher pitch sounds both in air and underwater, so a listener would be able to hear the low frequency sounds from farther away. It would take a lot more effort to produce high-frequency sounds that carried far, so maybe it is the relative strength of the high frequency sounds, compared to the low-frequency sounds that is the key? Especially if it was tied to greater strength, endurance, or breath-holding capacity. This could be tested by comparing the relative loudness of high-frequency song sequences with low-frequency song sequences and tie it to the size of the singer.

It has even been suggested that the complexity is just the result of boredom. If these animals need to keep singing for hours and hours to show their prowess, it may be that the actual content of the song is less important. This would leave the animals free range to invent new sound sequences. Although this would not explain why all the singers sing the same song.

In all these cases, it would be crucial to identify the singers, analyze their song and measure their size, as well as recording their terminal position near the female in surface-active groups. Much of this work has been done over the last several decades, so it is fascinating to know that we still don’t know exactly why these whales sing such elaborate songs. 

At the same time, recent information uncovered by Dunlop and Frere, using data from off Eastern Australia, where humpback whales were hunted almost to extinction, indicates that singing was a more successful tactic in the years following whaling, when whale populations were at their lowest, while non-singing behaviour became a more successful tactic in later years. This would suggest that singing as a male strategy to compare yourself to other, competing males may be on its way out, as humpback whale populations recover. Maybe it is more advantageous to join surface active groups when there are higher densities of whales in the area. But, if this is the case, then the initial question remains: why do the males sing? Is it for males to compare themselves to other males, or is it to entice the females? If the song entices females to approach and mate with the male, he would be able to avoid the whole physical competition of a surface active group - an occasional mating taking place during a long dive between a solitary singer and a lone female could easily be missed. Does the function of the song depend on the population size and density in the area? Or could it be that different males have different strategies - this is also a common theme in the animal world.

References

Clutton-Brock, T. H., & Albon, S. D. (1979). The roaring of red deer and the evolution of honest advertisement. Behaviour, 69(3-4), 145-170.

Dunlop, R. and Frere, C. (2023) Post-whaling shift in mating tactics in male humpback whales. Commun Biol. 2023 Feb 16;6:162

Pack, A.A., Salden, D.R., Ferrari, M.J., Glockner-Ferrari, D.A (2006) Male humpback whale dies in competitive group in Hawaiʻi. Marine Mammal Science 14(4):861 - 873

Payne, R. S. and McVay, S. (1971) Songs of Humpback Whales: Humpbacks emit sounds in long, predictable patterns ranging over frequencies audible to humans. Science 173: 585-597.

Sierro, J., de Kort, S.R. & Hartley, I.R. (2023) Sexual selection for both diversity and repetition in birdsong. Nat Commun 14, 3600.