Ageing methods for common dolphins (Delphinus delphis) : a thesis submitted in partial fulfilment of the requirements for the degree of Doctor of Philosophy (PhD) in Zoology at Massey University, Albany, Aotearoa New Zealand

Abstract

Accurate age estimation underpins demographic inference, population viability analyses, and conservation management for cetaceans, yet remains challenging for most odontocete species. Reliance on invasive tooth-based ageing has constrained the contexts in which age data can be obtained, limiting demographic resolution for species exposed to ongoing anthropogenic pressures. This thesis develops an integrated suite of molecular and radiographic ageing frameworks for common dolphins (Delphinus delphis), expanding the methodological toolkit available for age and life-history assessment across post-mortem, free-ranging, and live-stranding contexts. The thesis establishes the first species-specific epigenetic clock for common dolphins using DNA methylation from skin samples of stranded and bycaught individuals aged via dental growth layer groups. The clock achieved a median absolute error (MAE) of 1.80 years (Pearson’s correlation (r) = 0.91, R² = 0.82) and remained reliable up to 16 years of age, with prediction errors increasing thereafter. In parallel, this thesis also establishes the first calibrated radiographic bone-ageing model for the species, demonstrating strong predictive performance using both univariate and multivariate approaches (canonical analysis of principal coordinates (CAP): females MAE = 1.35 years, Spearman’s rank correlation (ρ) = 0.90, R² = 0.85; males MAE = 1.80 years, ρ = 0.94, R² = 0.84; polynomial regression: females MAE = 1.25 years, ρ = 0.93, R² = 0.90; males MAE = 1.08 years, ρ = 0.95, R² = 0.86). Radiographic estimates outperformed epigenetic predictions and remained reliable to approximately 20 years of age. The thesis further demonstrates that radiographic skeletal ossification can be used to infer sexual maturity with high accuracy (95.8% of females and 98.4% of males correctly classified), allowing two fundamental life-history parameters — chronological age and sexual maturity — to be resolved within a single non-invasive imaging framework. Collectively, these approaches provide practical tools for obtaining age and life-history information from free-ranging, stranded and bycaught animals, as well as live-stranding responses. By improving access to reliable age data across sampling contexts, the frameworks developed here support future population assessments for common dolphins, including evaluation of fisheries bycatch and cumulative anthropogenic impacts. More broadly, this thesis demonstrates the value of integrating molecular, skeletal, and traditional ageing approaches rather than relying on any single method in isolation. It further highlights the importance of long-term curated biological archives for advancing cetacean life-history research and establishes a transferable foundation for developing species-specific ageing models across odontocetes.