As their population has recovered, competition has increased and males now have to fight to attract females, according to a study.

The song of humpback whales (or rorquals), emitted by males, is among the most complex in the animal kingdom.

If its exact role is debated among scientists, it is accepted that this music allows a dominant male to monopolize a female to become what is called the main escort.

Their parades are also made up of fights, jostling... So many strategies that make it possible to keep competitors away and to maximize the chances of reproduction within a group.

A study conducted near the eastern coasts of Australia, where these huge cetaceans migrate during the mating season, shows that over time, cetaceans have been able to modify these tactics to adapt to the evolution of the population.

Commercial hunting practiced during the 20th century had brought the species "to the brink of extinction", recalls the study published Thursday in the journal Communications Biology.

Singing individuals

In the waters of eastern Australia where the Great Barrier Reef is located, their population fell from 26,000 individuals to around 200 in the 1960s.

The international ban on whaling in the 1980s led to a spectacular rebound in their numbers: from 3,700 individuals recorded in 1997, the population rose to 27,000 in 2015, detail this work carried out by two researchers from the university. from Queensland to Brisbane.

They studied the site of Peregian Beach, a "unique" place for the observation of large groups of whales which, coming from Antarctica, come to mate very close to the coast, explains to AFP Rebecca Dunlop, one of the authors.

The song of humpback whales (or rorquals), emitted by males, is among the most complex in the animal kingdom © JOAQUIN SARMIENTO / AFP/Archives

The behavior of cetaceans has been studied there since the 1990s, using tracking systems, both acoustic - to identify singing individuals - and visual to monitor the behavior of rorquals whose life expectancy is estimated to be around 60 years old.

The researchers analyzed this data collected over 18 years, combined with biopsies to identify the sex.

And discovered "that during the first years, a large proportion of males used song as a signal to attract females", reports Rebecca Dunlop.

Breeding success

It was then very rare to see groups in competition.

But in the early 2000s, as the population recovered, scientists noticed a decline in the use of singing as a mating strategy, with the proportion of singing whales dropping to just one in ten in 2015.

This can be understood: when a suitor sings, he takes the risk that his sweet music will attract competitors at the same time and interrupt him in his seduction business.

The male can then leave the breeding area, disappointed at having "made so many efforts in vain", analyzes the biologist.

In the early 2000s, as the population of humpback whales recovered, scientists noticed a decrease in the use of singing as a mating strategy © EITAN ABRAMOVICH, EITAN ABRAMOVICH / AFP/Archives

So the more competitors he has around him, the less this tactic pays off.

Hence the change in strategy observed: faced with the increase in density, the whales opted for physical combat.

The male who has managed to become the main escort for his potential mate will then charge his competitor.

"You can see headbutts on the surface where one whale tries to hit another, whales chasing each other...", describes Rebecca Dunlop.

While some continue to prefer the peaceful method, no doubt for fear of injury, the majority have converted to physical confrontation.

Thanks to this flexibility in their behavior, the whales increased their reproductive success, which may have helped repopulate the seas and thus avoid extinction, the study suggests.

Other animals have seen their sexual behavior change with the evolution of their population, in particular the saiga antelopes victims of poaching, or certain rodents.

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