Revised Age Estimates for Northern Resident Killer Whales () Based on Observed Life-History Events and Demographic Discounting.

Long-term field studies have been invaluable in the study of ecology and evolution; however, for particularly long-lived species, even long-term studies often rely on estimated ages, for example when investigating demographic processes. One approach is to estimate unknown birth dates from the known timing of other life-history events. Building on previous methods, we update estimation techniques for Northern Resident killer whales (NRKW; ) as part of an ongoing long-term study that began in 1973. Despite almost 50 years of observation, many individuals were born before records began, and detailed understanding of NRKW life history relies on estimated ages. Our age estimation approach incorporates new data from photo-identification surveys into a framework that relies on accrued knowledge of demographic rates from known-age individuals. We use Bayes' law to determine conditional probability distributions from age-at-event data, incorporating mathematical descriptions of demographic patterns parameterised from the data. Key to our approach is the discounting of higher age estimates due to the increasing likelihood of mortality with age, a pattern not previously taken into account for NRKWs. We estimate ages for multiple age and sex classes of individuals, using related but tailored approaches, and we incorporate uncertainty into our estimates. Our revised age estimates suggest that individuals are often younger than previously thought (3.5 years on average across 73 individuals; range: 0-15 years). Moreover, the largest discrepancies appear for mothers with offspring at the onset of the study, a class of individuals instrumental for investigating menopause in killer whales-one of the few species other than humans known to exhibit this life-history feature. Our results will ultimately enable a refined understanding of the evolutionary forces that produce such patterns. We discuss the implications of our findings for the study of resident killer whales and for age estimation in other long-lived animals.