Date Published: October 10, 2018
Publisher: Public Library of Science
Author(s): Martha P. Rosas-Hernández, Claudia J. Hernández-Camacho, Eduardo González-Rodríguez, David Aurioles-Gamboa, Myra E. Finkelstein.
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.
Many recent studies have employed stable isotope analysis in order to assess the diet, nutritional status, and feeding ecology of both wild and captive animals . Nitrogen (15N/14N or δ15N) and carbon (13C/12C or δ13C) isotope ratios allow us to infer an organism’s trophic position and potential feeding areas, respectively. This is possible because the tissues of predators are enriched in 15N and 13C relative to the prey they consume [2–4].
For most of the vibrissae from wild adult female CSLs from SEI in the central Gulf of California, Mexico, a peak in the δ15N profile represented six months of vibrissa growth. The approach used in this study to estimate the growth rate is recommended for species or populations that lack an annual periodicity in their N isotope values. It is still unclear whether all CSLs throughout the species’ entire distribution also lack an annual periodicity; thus, this approach only applies to the population of CSLs from the central Gulf of California. The growth rates obtained here using the linear model are consistent with those reported for adult females of this and other otariid species. Although the pattern of vibrissa growth is similar for all otariids (linear), we recommend that the age- and sex-specific growth rates be determined for each species and individual whenever possible. Moreover, researchers also should consider whether animals are captive or wild in order to more accurately assign periods to the isotopic information contained in their vibrissae.