Vibrissal growth parameters of southern elephant seals Mirounga leonina: obtaining fine-scale, time-based stable isotope data

Stable isotopes provide a powerful, indirect approach to assess the trophic ecology of individuals on a spatial and temporally integrated basis (especially when combined with telemetry). However, using stable isotopes requires accurate, species-specific quantification of the period of biomolecule deposition in the sampled tissue. Sequentially sampled vibrissae (whiskers) provide a chronology of biogeochemical data, although knowledge of vibrissal growth is required for temporal interpretations. We sampled vibrissae from southern elephant seals Mirounga leonina (hereafter SES) at Marion Island, southern Indian Ocean, to address the following aims: (1) define the prevalence and timing of their vibrissal replacement, (2) determine the vibrissal regrowth rate and temporal resolution of isotopic data captured along the length of sequentially sampled vibrissae, and (3) explore assumptions regarding their vibrissal growth. Contrary to the previously described asynchronous vibrissal shedding pattern of SES, 71.1% of individuals displayed vibrissal shedding during the annual pelage moult. Furthermore, vibrissal growth ceased once the asymptotic length was reached, and the vibrissae were retained before being replaced. Vibrissae with known growth histories were resampled at multiple known intervals to control for unknown growth starting dates. Vibrissae followed a von Bertalanffy growth function as the growth rate decreased near the asymptotic length. The resolution of the isotopic data obtainable per 2 mm section ranged from 3.5 d at the vibrissal tip to >40 d at the base. Using these defined growth rates and shedding patterns, researchers can prudently apply timestamps to stable isotope values along vibrissae.