| Summary: | Insights in the variation in pigmentation patterns and other external features in Hyperoodon planifrons are few, in particular, variability contributed by each of individual, ontogenetic, sexual, and geographic factors. A preliminary assessment is based on two close-up sightings of juveniles during IWC-SOWER Cruises and stranded specimens from South Africa, Australia and New Zealand. Bold, cream-white facial fields separated by a distinct dark blowhole stripe are diagnostic of all juvenile/neonates examined and may persist in subadults. Dark eyepatches may be pronounced or almost absent, a light nuchal band behind the blowhole may be prominent or muted, and the dorsal aspect of flippers may be light or dark. A light flank patch on the tailstock is consistent in juveniles. With maturation the head pattern gradually blurs, apparently because facial fields and the nuchal band gain pigmentation. The slate-grey or dark brownish-grey spinal field in juveniles is appreciably darker than in most adults, in which it varies widely, ranging from brownish-grey (most common), bluish-black to yellowish. Diatoms have been blamed as cause for the brown hue, however the evidence is unclear. Large, almost-white animals have been reported but no such specimens were available for study. Much of the external variation seen may be individual and ontogenetic, however sexual dimorphism and geographic variation remain unassessed due to small samples. A comprehensive morphological study is indicated. The bold features in juvenile H. planifrons are diagnostically distinct from juvenile Tasmacetus shepherdi and Indopacetus pacificus . Non-priority status and long dive durations have allowed scant time for data collection on H. planifrons during Antarctic surveys. This study shows that maximizing opportunistic encounters through allowance for some flexibility in schedules, valuable data can be obtained. A rule-of-thumb is recommended for avoiding ziphiid species identification at distances exceeding 0.8 nmiles.
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