For over a century, whalers hunted South Pacific humpbacks to near-extinction, and decades later, scientists discovered older males now control who fathers the next generation

The story of humpback whales is often told as one of nature's greatest recovery successes. After being hunted almost to extinction during the period of commercial whaling, populations have increased significantly thanks to protective measures enforced in recent decades.

However, recent research has shown how whaling is still impacting these giants of the ocean in a different manner.

In fact, researchers have found that, among the humpback whale population of the South Pacific, older males increasingly participated in behaviours associated with reproductive success. This new research offers some rare insights into how whales reproduce and demonstrates the long-lasting impact that human interference has on the species.

According to a study published in the journal Current Biology, reported by Phys. org, changes in the age structure of recovering humpback whale populations have altered which males achieve reproductive success.

The largely unnoticed legacy of industrial whaling
The commercial exploitation of humpback whales for oil and meat was particularly intense across the Southern Hemisphere in the 19th and 20th centuries. This caused dramatic population declines among the species before the advent of international whaling quotas.

Humpback whales are said to be among the most negatively affected species by industrial whaling,with some Southern Hemisphere populations declining by as much as 90 percent.

Scientists have suggested, since the increased numbers seen in whale populations in recent years, that the recovery has been more than simply an increase in numbers; the age of the whales is also believed to be a crucial factor.

Two decades of whale observations show a pattern
The study observed humpback whales in the region of New Caledonia, and compiled almost 20 years of data collected by the conservation organisation Opération Cétacés. Utilising skin biopsies, the age of individual whales was estimated by using a so-called "epigenetic molecular clock" and then comparing paternity in different family groups using the information from skin biopsies.

Researchers found that while numbers were recovering, younger males appeared to reproduce in the first years of recovery, and after this time, more and more older whales joined the population, which led to a larger proportion of breeding-age males being older.

More surprisingly, older males eventually gained a reproductive advantage and produced more offspring.

Older males get a reproductive advantage


Older males were found to have greater reproductive success than younger males: they sang more, escorted more females and produced more offspring. According to a University of St Andrews study, mating behaviour shifted as the recovering humpback whale population aged, with older males increasingly dominating key reproductive behaviours and fathering more calves than younger rivals.

As whale numbers have increased, older males have become more successful reproducers, which suggests this may be due to greater experience of mating, as well as more advanced singing abilities when competing for a mate.

The role of experience in male courtship
Humpback whales are world-renowned for their beautiful and, indeed, complex songs, which can be heard over many miles of ocean. These songs are thought to be important in attracting females, though observations of humpback whale mating have always been notoriously difficult. Researchers suggest that factors such as song performance, competitive ability, or accumulated experience may contribute to the advantage seen in older males.

Researchers propose that greater population density may allow stronger sexual selection, potentially favouring traits associated with older males. Scientists suggest this may help explain the pattern for the greater number of older males now being selected for reproduction by females.

Wildlife recovery can take generations


This study also indicates that wildlife rehabilitation does not only happen when populations increase in size. The findings suggest that human impacts may continue to influence species behaviour and population structure for decades.

Lead author Dr. Franca Eichenberger noted that scientists are only now beginning to understand how deeply whaling altered whale populations.

For scientists, this study shows the importance of long-term data collection, as for the most part, all whale studies have been carried out in populations that are already altered. The ongoing recovery of humpback whales has given researchers a rare opportunity to observe how species rebound from such extensive human impact.