Perrin's beaked whale (Mesoplodon perrini) is part of the toothed whale suborder and is one of over 90 known cetaceans in existence today. Beaked whales are part of the family Ziphiidae, which are the second most diverse group out of all marine mammals with over 20 species currently recognized. Although diverse, little is understood about these timid, deep divers that can dive for up to two hours. The whales are partially named after their beak shaped jaw, which extends from their small head. The genus name Mesoplodon comes from the Greek meanings of meso- (middle), - hopla (arms), - odon (teeth), and may be translated as 'armed with a tooth in the center of the jaw'. Perrin's beaked whale was described as a new species in 2002 by Dalebout et al. based on five animals stranding on the coast of California between 1975 and 1997, which were initially identified as other species. The common and specific names of Mesopledon perrini are a tribute to cetologist William F. Perrin. As of May 2019, only six specimens have ever been examined. The first two specimens were found stranded on the California coast in May 1975, other specimens were found in 1978, 1979, September 1997 (a strong El Niño year), and October 2013. The first four individuals were initially identified as Hector's beaked whales (Mesoplodon hectori), but the mtDNA sequence database of beaked whales revealed the specimens were genetically distinct. The fifth was assumed to be a neonate Cuvier's beaked whale (Ziphius cavirostris). Description Perrin's beaked whales are closely related to pygmy beaked whales and likely represent pygmy beaked whales' Northern Hemisphere sister species. Perrin's beaked whales have not definitively been recorded alive by scientists; however, its appearance is known from beached specimens. Perrin's beaked whales cannot be identified with absolute certainty at sea. However, the combination of small size, appearance, and a small presumed geographical range makes uncertainty unlikely. Stranded specimens can be identified as this species by either DNA sequence data and/or anatomical details of the skull. Morphology The morphology of the Perrin's beaked whale Mesoplodon perrini resembles the Hector's beaked whale M. hectori. Morphological characteristics that set these two species apart include minor differences in the cranium, teeth, and mandible. External appearances of Perrin's beaked whales are typical of Mesoplodon beaked whales, with a relatively small head, long thorax and abdomen, deep peduncle, and short tail. Only the original five