A study shows that male humpback whales off the coast of Australia are far less likely to bewitch their "heart ladies" with songs than they were 20 years ago. Apparently, this is an adaptation of the mating behavior in the course of the strong population recovery after the whaling era: In the denser population again, the whale bulls therefore refrain from making loud courtship calls more often, so as not to draw the attention of competitors to an available female Explanation.
The greed for raw materials almost finished off the charismatic giants of the sea. In addition to many other whale species, humpback whales (Megaptera novaeangliae) were also heavily affected by commercial whaling in the 20th century. When the emergency brake was finally pulled in the 1960s, there were only small remainders. So there were only about 200 specimens left off the east coast of Australia. However, this small population was able to benefit surprisingly intensively from the protection efforts: According to estimates, there are now around 27,000 humpback whales off the Australian east coast.
This population has been the focus of research for decades. In the current study, Rebecca Dunlop and Celine Frere from the University of Queensland in Brisbane have now investigated whether changes in the mating behavior of the animals can be observed in the course of population recovery. Because, as the scientists report, other animal species are known to be able to adapt certain tactics, depending on how dense the populations are in their habitat.
Sing or fight?
It is well known that humpback whales have a particularly impressive element in their mating behavior: the males let out songs that can be heard from afar underwater. The full meaning is not fully understood, but it is obvious that the bulls use their sounds to attract and seduce females. In addition, singing can apparently play a role in competition between males - they engage in a singing contest while courting a female. Interestingly, not all bulls follow this tactic: some remain silent and rely only on physical combat, in which the animals hit each other with their heads.
For their study, Dunlop and Frere evaluated extensive investigation data on the mating behavior of humpback whales off the Australian east coast, covering the time frame from 1997 to 2015. These included underwater audio recordings and observations of strategies and individual successes in mating groups of males courting a female. This information indicated, for example, to what extent a bull that eventually got to the top position had or had not sung.
Change of strategy with a lot of competition
As the two researchers report, there was a clear shift in the tactics of the animals during the study period: "In 1997, the probability that a singing male whale was seen trying to mate with a female was almost twice as high as one that was not singing male,” says Dunlop. By around 2003, the ratio had reached roughly parity, and by 2015 it had reversed: "Non-singing males were observed attempting to breed nearly five times more often than singing males," reports Dunlop.
As the researchers explain, it is becoming apparent that this significant change in behavior took place gradually and was linked to the recovery of the population after the whaling era. As far as the period from 1997 is concerned, estimates revealed a further strong increase in the population: from 3700 to 27,000 animals. As a result, the conditions within the community have also changed significantly. “When competition is high, the last thing a male wants to do is announce that a female is around. Because that could attract more and more males who could challenge the singer for the female,” says Dunlop. "On the other hand, if the males switch to the non-singing behavior, they are more likely to keep the female," the researcher said.
As Dunlop and Frere sum up, a surprisingly pronounced flexibility in the use of mating strategies is also evident in the humpback whales. However, as the population of long-lived animals is only now slowly regaining its original structure, there could be further changes in their behavior, the scientists say with a view to further research: "It will be fascinating to see how the mating behavior of whales in will continue to develop in the future," concludes Dunlop.
Source: The University of Queensland, Article: Communications Biology, doi: 10.1038/s42003-023-04509-7