Robust age estimation of southern sea otters from multiple morphometrics

• Published in: Ecology and evolution Vol. 10; no. 16; pp. 8592 - 8609
• Main Authors: Nicholson, Teri E., Mayer, Karl A., Staedler, Michelle M., Gagné, Tyler O., Murray, Michael J., Young, Marissa A., Tomoleoni, Joseph A., Tinker, Martin Tim, Van Houtan, Kyle S.
• Format: Journal Article
• Language: English
• Published: Bognor Regis John Wiley & Sons, Inc 01-08-2020; John Wiley and Sons Inc; Wiley
• Subjects: Age; Age determination; Aging; Aging methods; Aquatic mammals; Body length; Chronology; Coloration; Computer simulation; dentition; Endangered & extinct species; Enhydra lutris; Estimates; Gestation; growth plate fusion; Life expectancy; life history; Life span; Marine mammals; Morphometry; Ontogeny; Original Research; Pelage; Scars; Schedules; Teeth; tooth eruption; tooth wear; Wear; Wildlife; Wildlife management; United States > US; California
• ISSN: 2045-7758; 2045-7758
• DOI: 10.1002/ece3.6493

Reliable age estimation is an essential tool to assess the status of wildlife populations and inform successful management. Aging methods, however, are often limited by too few data, skewed demographic representation, and by single or uncertain morphometric relationships. In this study, we synthesize age estimates in southern sea otters Enhydra lutris nereis from 761 individuals across 34 years of study, using multiple noninvasive techniques and capturing all life stages from 0 to 17 years of age. From wild, stranded, and captive individuals, we describe tooth eruptions, tooth wear, body length, nose scarring, and pelage coloration across ontogeny and fit sex‐based growth functions to the data. Dental eruption schedules provided reliable and identifiable metrics spanning 0.3–9 months. Tooth wear was the most reliable predictor of age of individuals aged 1–15 years, which when combined with total length, explained >93% of observed age. Beyond age estimation, dental attrition also indicated the maximum lifespan of adult teeth is 13‒17 years, corresponding with previous estimates of life expectancy. Von Bertalanffy growth function model simulations of length at age gave consistent estimates of asymptotic lengths (male Loo = 126.0‒126.8 cm, female Loo = 115.3‒115.7 cm), biologically realistic gestation periods (t0 = 115 days, SD = 10.2), and somatic growth (male k = 1.8, SD = 0.1; female k = 2.1, SD = 0.1). Though exploratory, we describe how field radiographic imaging of epiphyseal plate development or fusions may improve aging of immature sea otters. Together, our results highlight the value of integrating information from multiple and diverse datasets to help resolve conservation problems.
By synthesizing more than two decades of southern sea otter physical examination data from strandings, captive‐rearing for release, and field studies, we provide strong support for adopting minimally invasive aging techniques based on metrics obtainable during brief field capture. These techniques provide the means to gather age‐specific information from a living, wild population to inform management decisions focused on restoring sea otters and their ecosystems throughout California. Our results also highlight the value of integrating information from multiple and diverse datasets to help resolve conservation problems.