Commercial whaling brought whales close to vanishing from the world’s oceans within living memory. It drove numerous large species towards extinction. Their populations collapsed, leaving only a small proportion of the original animals in some regions.
In recent decades, humpback whales have recovered impressively. Their populations are increasing across several areas of the world.
Yet recovery involves more than a higher whale count. It also affects which individuals can pass their genes to the next generation. That change is now becoming clearer.
New research indicates that age has become more influential in male humpback whale reproduction than it was in the first years of population recovery.
The study uses almost 20 years of information on humpback whales breeding in New Caledonia, in the South Pacific.
The recovery of humpback whales
Whaling altered more than the number of whales: it disrupted their age structure. Hunters frequently removed the largest and oldest animals in great numbers, leaving populations disproportionately made up of younger whales.
During the initial recovery period, young males were predominant on New Caledonia’s breeding grounds.
As survival improved and whales grew older, however, this pattern changed. The population came to contain a more balanced spread of young and old males.
Researchers identified a significant trend. Once older males became more numerous, they increasingly fathered calves more successfully than younger males.
Scientists at the Sea Mammal Research Unit at the University of St Andrews led the work, using long-term monitoring records gathered by the NGO Opération Cétacés.
Across almost two decades, the researchers examined male humpbacks’ age structure, behaviour and paternity.
Unravelling a long-standing mystery
No one has ever observed humpback whales mating in the wild. For many years, it was therefore unclear which males were the fathers of particular calves, creating a major gap in knowledge of whale reproduction.
The researchers addressed that question through genetics. They took small skin samples from whales and used DNA to establish paternity.
They also used an epigenetic molecular clock, which estimates an animal’s age from chemical alterations in its DNA. This information could all be obtained from a minute skin sample.
Male humpbacks are recognised for their lengthy, intricate songs. These vocal performances can travel across breeding areas and are believed to assist in attracting females.
Males also accompany females and can engage in fierce physical contests with competing males. Their behaviour changed as the age structure changed.
Dr. Ellen Garland, the study’s senior author, specialises in cetacean behavioural ecology and bioacoustics at the University of St Andrews.
“Mating behavior, and who was successful at mating, changed with these shifts in age structure. As the population recovered, there were more older males than expected singing, escorting females, and successfully fathering calves compared to younger animals,” said Dr. Garland.
Older males were not merely present more frequently; they exceeded the reproductive success of younger rivals.
The far-reaching effects of whaling
Why does age have such an impact? The results indicate that male humpback whales may require time to develop and perfect both their songs and competitive strategies. Their singing is not simply random sound.
Humpback songs have structured patterns and may change through time. Males could spend many years enhancing their performances.
Competition is also likely to matter. Bigger, older males may possess greater strength and endurance in physical confrontations. Their experience may enable them to interpret social signals, choose the right moment for displays and remain close to receptive females.
As populations expand, females might also become choosier. A busier breeding ground provides more options, potentially favouring males with more powerful songs, superior tactics or greater experience.
“It is only now, as whale populations recover and new analytical tools become available, that we are beginning to understand how far-reaching the consequences of whaling truly are,” said study lead author Dr. Franca Eichenberger.
“The impacts extend beyond population size – they shape behavior, competition, and reproduction.”
A changing baseline for humpback whales
There is a further complication: scientists did not study humpback whales in large numbers before whaling transformed their populations.
Most contemporary research was conducted during or after intense exploitation. Consequently, researchers have worked from a so-called shifted baseline, where the “normal” condition they recorded had already been changed.
Fresh patterns are appearing as whale populations rebuild. Some could signal a return to earlier social structures, while others may be wholly new dynamics resulting from historic losses.
“Virtually all populations of whales have changed due to whaling; our work shows that they continue to change as they recover. This is why the continued long-term monitoring of previously exploited whale populations is so important,” said Dr. Eichenberger
“Humpback whales have shown a remarkable comeback over recent years. Now is the time we can learn so much more about their behavior and life history. We just need to keep looking.”
The complete study appeared in the journal Current Biology.
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