Comparison of paired sex ratio estimates obtained from novel molecular assays and visual surveys in the sexually dimorphic Steller sea lion (Eumetopias jubatus)

Estimating sex ratios is important for understanding population demographics. Here, we present a direct comparison of paired sex ratio estimates from visual field surveys and molecular sex identification using scat in sexually dimorphic Steller sea lions (Eumetopias jubatus). For this purpose, we designed novel qPCR Taqman gene expression assays to determine sex from scat of both Steller and California (Zalophus californianus) sea lions targeting the zinc-finger X-linked gene (ZFX) and sex-determining region Y gene (SRY). The assay pair was validated using scat samples from California sea lions of known sex with 100% success rate of signal amplification. Our assays yielded a 94.1% success rate in identifying sea lion sex from field-collected scats. In addition, the sex ratio determined via molecular analysis was closely correlated to the adult male and adult female proportions observed at the haul-out site for each month scat samples and demographic counts were collected in tandem. While visual counts cannot determine the sex of juvenile sea lions, scat collections are assumed to represent all age classes present at the haulout, suggesting that the juvenile sex ratios likely mirrored the adult sex ratio. The protocol described here could be implemented to identify the sex of Steller and California sea lions from their feces in free-ranging populations in other non-invasive, scat-based studies. Further, the concordance with visual observations suggests that molecular sex identification from scat collected at haul-outs is a robust methodology in sea lions and possibly in other pinniped species that lack distinctive sexual dimorphism.