Humpback whale sexual selection is undergoing a profound transformation as populations recover from decades of commercial whaling, according to a new study published in Current Biology.
Using an innovative epigenetic molecular clock applied to skin samples, researchers tracked the ages and reproductive success of 485 individual male humpback whales (Megaptera novaeangliae) representing about 24 % of the local male population over 19 years (2000–2018) on the New Caledonia breeding ground in the South Pacific.
The study reveals that as the population increased in size, the age structure shifted dramatically and with it, the patterns of who sings, who escorts females, and who actually sires calves.
From youth-dominated populations to a balanced age structure
In the early recovery phase (2000–2008, low abundance), the male population was strongly left-skewed, dominated by younger individuals in the 2–9 and 9–16 year age classes. By the later period (2009–2018, high abundance), the age distribution became significantly more even, with a clear increase in males aged 16–23 and ≥23 years (Fisher’s exact test, p < 0.001).
Ages were determined using the Humpback Epigenetic Age Assay (HEAA), an updated DNA methylation clock calibrated for this population. The model, based primarily on methylation levels at CpG sites in the TET2 and GRIA2 genes, achieved a mean absolute error of just 4.48 years and was validated with known-age samples (R² = 0.58).
(A) Regression of predicted versus known ages using CpG methylation measurements at TET2 and GRIA2 from 78 samples representing 68 whales.
(B) Leave-one-out cross-validation (LOOCV) results showing predicted versus known ages when each sample is estimated using a model trained on the remaining 77 samples.
Dotted lines indicate the 95% confidence interval of the regression. Circles represent samples from the New Caledonia dataset, while triangles represent samples from the Polanowski et al. dataset (Gulf of Maine, USA, and Australia).
Paternity was assigned through microsatellite genotyping at 15 loci, identifying 56 aged fathers responsible for 68 calves.
Paternity was assigned through microsatellite genotyping at 15 loci, identifying 56 aged fathers responsible for 68 calves.
Changing mating strategies as whale populations recover
Male humpbacks use three main reproductive tactics: singing complex songs to attract females, solitary escorting of receptive females (especially mother-calf pairs), and participating in competitive groups.
During the low-abundance period, the age distribution of singers and solitary escorts closely matched the overall population (χ² p ≥ 0.05). But in years of higher abundance, males aged 16 years and older became significantly over-represented in both singing and solitary escorting behaviours (χ² test, p < 0.001; strongest for ≥23-year-olds in escorting), while the youngest males (2–9 years) were strongly under-represented. Principal escorts in competitive groups showed no age bias in either period.
“As the population recovered, there were more older males than expected singing, escorting females, and successfully fathering calves compared to younger animals.”
Dr Ellen C. Garland, University of St Andrews
Older males regain reproductive dominance
The paternity data confirmed the behavioural shift. In the low-abundance years, fathers were skewed toward younger age classes (9–16 years sired well above random expectation), and no males aged 23 years or older sired any offspring. In contrast, during the high-abundance period, males aged 16–23 and particularly those ≥23 years achieved significantly higher reproductive success than expected by chance (10,000 randomisation simulations: 97 % and 63 % of iterations respectively showed observed success exceeding expectation).
Mean age at first paternity was 15 years, rising to 17 years for a second calf. Reproductive skew (multiple paternity) remained similar across periods.
“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. The impacts extend beyond population size they shape behaviour, competition, and reproduction.”
Lead author Dr Franca Eichenberger
How population collapse may have protected genetic diversity
The authors conclude that when the population was severely depleted and density was low, the relaxation of age-related sexual selection allowed younger males greater reproductive opportunities.
The paper states: “A relaxation of age-related sexual selection at lower densities could have facilitated a larger effective population size and acted to buffer genetic diversity loss during a time of lower abundance, when this was most critical.”
This mechanism may have helped maintain genetic variation during the post-whaling bottleneck. Now that older males have returned, classic age-based sexual selection has reasserted itself.
These results show that the impacts of heavy whaling did not end when the hunts stopped. In long-lived animals like humpback whales, the effects can echo across generations, shaping not only how many whales exist but also how they compete, reproduce, and evolve. The study also highlights the value of long-term research. By tracking individual whales over decades and combining those observations with modern molecular tools, scientists can finally begin to see how past human exploitation continues to influence wildlife today.
Reference:
Eichenberger, F., Carroll, E.L., Garrigue, C., Jarman, S., Steel, D.J., Robbins, J., Rendell, L., Garland, E.C. (2026). Changes in age-related sexual selection in a humpback whale population recovering from exploitation. Current Biology.
