Recovering populations following the end of massive whaling have led to altered mating behavior in male humpback whales. The bulls are increasingly doing without their characteristic song, as Rebecca Dunlop and Celine Frere from the Australian University of Queensland write in the journal Communications Biology. They had studied singing and non-singing whales in more detail and analyzed data from 117 males collected over a total of 123 days off the coast of eastern Australia from 1997 to 2015.
A well-known mating tactic used by male humpback whales is their distinctive song. The researchers write that there is a discussion among experts about the exact functions of these sounds. However, there is general agreement that singing has something to do with attracting the females and with the competition between the bulls. Males attach themselves to a female and then accompany her on her way to be able to mate with her later.
Another tactic used by humpback whales is to approach a female without singing and then escort her as well. When other males then join one of these pairs, there is usually a physical struggle, urging and competition between the whalemen, with the larger one often prevailing.
Dunlop and Frere wanted to find out how the population recovery after a whaling moratorium in the 1960s affected the animals' mating behavior. During the study period, the number of East Australian humpback whales climbed from around 3,700 to 27,000 animals. The males were proportionally in the majority, which consequently led to greater competition.
The researchers were able to show that the male humpback whales sang less and less often to attract a mate over the years after whaling was stopped. They also come to the conclusion that most singing male whales also stop singing the more competitors are in their vicinity. Instead, these animals also switch to the silent mating tactic.
Dunlop and Frere suspect that the animals with the more silent and often more aggressive mating tactics want to avoid that other males willing to mate are attracted by their sounds and then challenge them for their potential mate. And this strategy seems to be working: according to the study, the non-singing whales were 4.8 times more successful in finding a partner in 2014/2015 than those that started their underwater song. For comparison, in 1997, when the whale population was much smaller, the singing bull whales still had an advantage.
The authors point out that humpback whales are not the only wild animals capable of individually adapting their mating behavior to changing circumstances. According to experimental studies, male ungulates, elephants, fish and amphibians would also use different tactics such as territorial behavior, guarding the partner or fighting in order to be able to reproduce successfully.
Here, too, it depends, among other things, on how many female or male animals belong to a population. However, empirical studies on wild animals are rare because the study requires a large change in population density.
Whaling, which killed around 1.8 million great whales in the Southern Ocean in the 20th century, was primarily aimed at adult whales at the time. The number of East Australian humpback whales, which also search for food in the Southern Ocean, shrank to just 200 animals. After whaling ceased, the population recovered by 11 percent annually. According to Dunlop and Frere, it is likely that changes in male mating behavior also helped prevent the extinction of the East Australian humpback whales.
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