Changes in age-related sexual selection in a humpback whale population recovering from exploitation

Whaling, one of the most extensive and prolonged global commercial hunting operations, pushed many large whales to the brink of extinction. To understand how evolutionary processes may be directly impacted by the consequences of exploitation, we assessed age-dependent population dynamics, reproductive tactics, and reproductive success of 485 male humpback whales (Megaptera novaeangliae) from a recovering breeding ground in New Caledonia. Over 19 years (2000-2018), the population age structure switched from a left-skewed, rapidly increasing low-density population toward a more evenly distributed age structure when abundance was higher in later years. Concurrent with the change in population age structure across time, there was a shift in age-related male mating tactics. Most strikingly, during years of higher abundance, the key reproductive tactics of singing and escorting were over-represented by older age categories when compared with the underlying age structure, suggesting age- and/or density-related tactic choice in males. This behavioral shift corresponded with an increase in the relative reproductive success of the oldest males during periods of higher abundance compared with lower abundance. Overall, our work suggests that at lower population densities, young males were less constrained by age-related restrictions on reproductive success. A reduction of age-related sexual selection could have acted to buffer genetic diversity loss during a time of lower abundance, when this was most critical. Integrating these changes in the pattern of sexual selection with population dynamics provides critical insights into the evolutionary consequences of exploitation and recovery in long-lived species.