Common bottlenose dolphins (Tursiops truncatus) in California waters: Cranial differentiation of coastal and offshore ecotypes

Coastal and offshore bottlenose dolphins in California waters are currently assessed and managed as separate stocks. Recent molecular studies (of mtDNA haplotypes and microsatellites) have shown the two populations to be genetically differentiated. This study investigated cranial osteological differentiation of the forms. The sample analyzed included 139 skulls from live captures, direct takes, fishery bycatch, and strandings; the skulls were assigned to form based on collection locality or mtDNA haplotype. The coastal form differs from the offshore form mainly in features associated with feeding: larger and fewer teeth, more robust rostrum, larger mandibular condyle, and larger temporal fossa. This suggests that it may feed on larger and tougher prey than the offshore form. Differences between the forms in other features of the skull may reflect differences in diving behavior and sound production. Approximately 86% of the stranded specimens were estimated to be of coastal origin; based on relative estimated sizes of the two populations and assuming similar mortality rates, this suggests that a coastal carcass is about 50 times more likely to beach than an offshore one. The morphological differences between the two ecotypes indicate evolutionary adaptation to different environments and emphasize the importance of conserving the relatively small coastal population and its habitat.