Vibrissa growth rate in California sea lions based on environmental and isotopic oscillations

Pinniped vibrissae provide information on changes in diet at seasonal and annual scales; however, species-specific growth patterns must first be determined in order to interpret these data. In this study, a simple linear model was used to estimate the growth rate of vibrissae from adult female California sea lions (Zalophus californianus) from San Esteban Island in the Gulf of California, Mexico. The δ¹⁵N and δ¹³C values do not display a marked oscillatory pattern that would permit direct determination of the time period contained in each vibrissa; thus, time (age) was calculated in two ways: 1) based on the correlation between the observed number of peaks (Fourier series) in the δ¹⁵N profile and the length of each vibrissa, and 2) through direct comparison with the observed number of peaks in the δ¹⁵N profile. Cross-correlation confirmed that the two peaks in the δ¹⁵N profile reflected the two peaks in the chlorophyll-a concentration recorded annually around the island. The mean growth rate obtained from the correlation was 0.08 ± 0.01 mm d^-1, while that calculated based on the observed number of peaks was 0.10 ± 0.05 mm d^-1. Both are consistent with the rates reported for adult females of other otariid species (0.07 to 0.11 mm d^-1). Vibrissa growth rates vary by individual, age, sex, and species; moreover, small differences in the growth rate can result in significant differences over the time periods represented by the isotopic signal. Thus, it is important to assess this parameter on a species-by-species basis.