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

<div><p>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 (<i>Zalophus californianus</i>) from San Esteban Island in the Gulf of California, Mexico. The δ<sup>15</sup>N and δ<sup>13</sup>C 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 δ<sup>15</sup>N profile and the length of each vibrissa, and 2) through direct comparison with the observed number of peaks in the δ<sup>15</sup>N profile. Cross-correlation confirmed that the two peaks in the δ<sup>15</sup>N 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<sup>-1</sup>, while that calculated based on the observed number of peaks was 0.10 ± 0.05 mm d<sup>-1</sup>. Both are consistent with the rates reported for adult females of other otariid species (0.07 to 0.11 mm d<sup>-1</sup>). 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.</p></div>