New research from the University of St Andrews, published in Current Biology on February 27, indicates that the role of age in male humpback whale reproduction has changed as populations recover from historical exploitation. Centuries of whaling brought many large whale populations to the brink of extinction, and its effects continue to shape these populations by influencing not only numbers but also which males reproduce.
The study utilized nearly two decades of data from humpback whales breeding in New Caledonia in the South Pacific. Drawing on long-term monitoring by the NGO Opération Cétacés, scientists analyzed changes in age structure, behavior, and paternity among male whales.
During the early stages of recovery, the population was predominantly composed of young males. As overall whale numbers increased, the age structure shifted, resulting in a more even distribution between older and younger males. Significantly, as older individuals became more common, they exhibited increased success in siring offspring compared to younger males.
Unraveling Paternity
Mating in wild humpback whales has never been directly observed, meaning paternity remained largely unknown until this research. The international team, led by the Sea Mammal Research Unit at the University of St Andrews, applied genetic analyses to identify paternity and used an 'epigenetic molecular clock' to estimate the age of individual whales, all from small skin samples.
Male humpback whales are known for their elaborate songs, which are audible across breeding grounds and are believed to contribute to mating. Males may also escort females or engage in physical competition with rivals.
The Advantage of Experience
Dr. Ellen Garland, senior author of the study, stated that mating behavior and reproductive success evolved with these shifts in age structure.
As the population recovered, a greater proportion of older males were observed singing, escorting females, and successfully fathering calves compared to younger animals.
These findings suggest that male humpback whales may require time to develop and refine their singing and competitive strategies, thus providing experienced males with a notable advantage. Additionally, as the population grows, females may become more selective, potentially leading to increased success among males with advantageous traits or stronger performances.
Understanding Whaling's Legacy
The study underscores the importance of continued research on recovering populations to understand how the consequences of exploitation influence population dynamics and reproduction over time. It also highlights that much existing knowledge derives from studying populations after whaling, indicating a shifted baseline from unexploited conditions.
Dr. Franca Eichenberger, lead author, commented that the comprehensive impact of whaling, extending beyond population size to behavior, competition, and reproduction, is becoming clearer as whale populations recover and new analytical tools become available.
Dr. Eichenberger further noted that virtually all whale populations have been altered by whaling and continue to change during recovery, emphasizing the critical role of long-term monitoring. She added that the recent comeback of humpback whales provides significant opportunities to learn more about their behavior and life history.