Humpback whales are known for their complex and beautiful songs, which they use to communicate, attract mates, and assert dominance. But a new study has revealed that these songs are not fixed, but rather change according to the density and distribution of the whales.
The study, conducted by researchers from the University of Hawaiʻi and NOAA’s Hawaiian Islands Humpback Whale National Marine Sanctuary, used underwater listening devices and visual surveys to track the movements and acoustic patterns of humpback whales in their high-density breeding grounds near Maui.
The researchers found that the whales exhibited an intriguing daily movement pattern: they tended to drift away from the shore during the day, and return as evening approached. This pattern was not random, but rather a strategic choice by the whales to optimize their acoustic display.
Humpback whales sing less when there are more of them
The researchers also observed that the number of male singers decreased as the population of humpback whales increased. In 2004, when the population was around 6,000 whales, about 2 in 10 males were singers. In 2015, when the population reached 27,000 whales, only 1 in 10 males were singers.
The researchers speculated that singing played a more important role in attracting mates when the population was severely depleted due to commercial whaling. When there were fewer whales, it was harder to find other whales, so singing was a way to broadcast their presence and location.
But when there were more whales, singing also had a downside: it alerted other males to the presence of potential rivals, who could interfere with mating attempts. Therefore, the whales reduced their singing and relied more on physical jostling and chasing to compete for females.
Humpback whales adapt to the changing acoustic environment
The study not only revealed the dynamic movement and singing behavior of humpback whales, but also the reasons behind them. The researchers suggested that the whales were responding to the changing acoustic environment, which was influenced by the density of whales, the noise from human activities, and the natural sounds of other marine animals.
The researchers explained that the whales moved away from the shore during the day to avoid the noise from boats, ships, and other human sources, which could mask their songs. They also moved away from the areas where other whales were singing, to reduce the competition and interference.
On the other hand, the whales moved towards the shore around sunset, to avoid the offshore evening chorus of fish, dolphins, and other marine animals, which could also drown out their songs. The researchers said that the whales were trying to find the optimal acoustic space and time to sing, to ensure that their songs were heard by other whales.
Humpback whales are a conservation success story
The study, published in the journal Nature Communications Biology, is the first of its kind to use specialized acoustic sensors to localize individual singers close to shore, and to understand their daily variations in distance, spacing, and movement behavior.
The study also highlights the remarkable recovery of the humpback whale population, which was once on the brink of extinction due to commercial whaling. The eastern Australian humpback whales, which breed near the Great Barrier Reef, have rebounded from around 200 whales in the 1960s to nearly 30,000 whales in 2020, approaching their estimated pre-whaling levels.
The study shows that humpback whales are not only resilient, but also adaptable, as they adjust their behavior to the changing conditions of their environment. The study also provides valuable information for the conservation and management of humpback whales, as it helps to understand how they use their habitat and how they cope with human impacts.