Comparative Skull Morphology of California Sea Lions (Zalophus californianus) and Steller Sea Lions (Eumetopias jubatus) in the Pacific Northwest and Implications for Hybridization

Eumetopias jubatus (Steller sea lions) that breed on the Oregon coast are smaller than those found both in Alaska and California. During the breeding season, male Zalophus californianus (California sea lions) move past E. jubatus rookeries in Oregon on their way to their breeding rookeries in Southern California. Introgression potentially could explain the difference in size of Oregon E. jubatus relative to remaining populations. At Portland State University (PSU), the marine mammal collection of the Museum of Vertebrate Biology has over 200 skulls of Z. californianus and E. jubatus from throughout northern Oregon and southern Washington. To assess whether morphologically detectable hybrids occur between Z. californianus and E. jubatus, I used traditional morphometrics; in particular, Principal Component Analysis, to identify individuals with putatively intermediate morphological characteristics, and multiple range comparison to determine which groups differed significantly. The groups analyzed were based on genus, sex, and region (OR-WA female E. jubatus, AK male E. jubatus, etc.), and combinations of these groups (Z. californianus AK and OR-WA male/female E. jubatus). The data demonstrated the presence of specimens with significant cranial morphological variation within each of the groups examined. Intrapopulation clustering and the overlap of two Z. californianus with male E. jubatus and a female E. jubatus with male Z. californianus, suggested that they may have been hybrids. Because of the consequences hybridization poses for speciation, even for population extinction and conservation, it is important to detect possible hybrids, the frequency of occurrence and how this phenomenon may impact the populations of Z. californianus and E. jubatus in the Pacific Northwest