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

Abstract

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 the 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: define the prevalence and timing of their vibrissal replacement; determine the vibrissal regrowth rate and temporal resolution of isotopic data captured along the length 1 of sequentially sampled vibrissae; and 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, vibrissa growth ceased once the asymptotic length was reached, and the vibrissae were retained before being replaced. Vibrissae with known growth histories were re-sampled 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 days at the vibrissal tip to > 40 days at the base. Using these defined growth rates and shedding patterns, researchers can prudently apply timestamps to stable isotope values along vibrissae.

Use of 15N-enriched glycine to estimate vibrissa growth in free-ranging northern elephant seals Mirounga angustirostris

“…These estimates assume a constant growth rate, which likely does not occur in this species based on Beltran et al (2015), but allowed for direct comparison with available estimates in the phocid literature. Beltran et al (2015) and Lübcker et al (2016) reported that elephant seal vibrissa growth follows a von Bertalanffy pattern consistent with other phocids (McHuron et al 2016), in the form:…”

“…Observations of freeranging and captive seals, however, suggest that seasonal molting patterns may be partially or completely absent in leopard seals Hydrurga leptonyx (Hall-Aspland et al 2005), harbor seals Phoca vitulina (Greaves et al 2004), southern elephant seals Mirounga leonina (Newland et al 2011), and northern elephant seals Mirounga angustirostris (Beltran et al 2015). For example, 71% of southern elephant seals shed their vibrissae during the annual pelage molt (Lübcker et al 2016).…”

Morphological characteristics of harbor seal (Phoca vitulina) whiskers and their use in dietary reconstruction using stable isotope ratios

“…However, phocid whisker growth rates are nonlinear (Hirons et al 2001; Zhao and Schell 2004;Newland et al 2011) and vary by location across the muzzle (Greaves et al 2004;Beltran et al 2015;McHuron et al 2016). These sources of variability make it difficult to use stable isotope analysis of whiskers as a tool for quantifying foraging ecology because estimating stable isotope deposition date requires specificity in individual whisker initiation date, growth rate, final whisker length (asymptotic length), and number of days grown (Sadou et al 2014;Beltran et al 2015;Lübcker et al 2016;McHuron et al 2016). Previous studies have used repeat photogrammetry of growing phocid whiskers across the muzzle to describe asymptotic growth (von Bertalanffy 1968), and the timing of whisker shed once it reaches asymptotic length (Greaves et al 2004;Beltran et al 2015;Lübcker et al 2016;McHuron et al 2016).…”

“…These sources of variability make it difficult to use stable isotope analysis of whiskers as a tool for quantifying foraging ecology because estimating stable isotope deposition date requires specificity in individual whisker initiation date, growth rate, final whisker length (asymptotic length), and number of days grown (Sadou et al 2014;Beltran et al 2015;Lübcker et al 2016;McHuron et al 2016). Previous studies have used repeat photogrammetry of growing phocid whiskers across the muzzle to describe asymptotic growth (von Bertalanffy 1968), and the timing of whisker shed once it reaches asymptotic length (Greaves et al 2004;Beltran et al 2015;Lübcker et al 2016;McHuron et al 2016). Recently, these phocid whisker growth models have been used to quantify foraging behavior by using the von Bertalanffy (1968) organic growth equation to estimate the timing of stable isotope deposition (Lübcker et al 2017).…”

“…Correlations would suggest predictable variation among individual whiskers in growth rate, providing a tool for individually adjusting estimated dates of keratin deposition. We tested if the decreasing IBL along the whisker from the tip to the root was correlated with position-specific growth rates measured in captive phocid studies (von Bertalanffy 1968;Beltran et al 2015;Lübcker et al 2016).…”