External features of the dusk dolphin Lagenorhynchus obscurus (Gray, 1828) from Peruvian waters

WAEREBEEK, K.V. 1993. External features of the dusk dolphin Lagenorhynchus obscurus (Gray, 1828) from Peruvian waters. Estudos Oceanológicos 12:37-53.

Abstract. Individual, sexual and developmental variation is quantified in the external morphology and colouration of the dusky dolphin Lagenorhynchus obscurus from Peruvian coastal waters. No significant difference in body length between sexes is found (p = 0.09) and, generally, little sexual dimorphism is present. However, males have a more anteriorly positioned genital slit and anus and their dorsal fin is more curved, has a broader base and a greater surface area than females Although the dorsal fin apparently serves as a secondary sexual character, the use of it for sexing free-ranging dusky dolphins is discouraged because of high overlap in values. Relative growth in 25 body measurements is characterized for both sexes by multiplicative regression equations. The colouration pattern of the dorsal fin, flank patch, thoracic field, flipper stripe and possibly (X², p = 0.08) the eye patch, are independent of maturity status. Flipper blaze and lower lip patch are less pigmented in juveniles than in adults No sexual dimorphism is found in the colour pattern The existence of a discrete "Fitzroy" colour form can not be confirmed from available data. Various cases of anomalous, piebald pigmentation are described, probably equivalent to so-called partial albinism. Adult dusky dolphins from both SW Africa and New Zealand are 8-10 cm shorter than Peruvian specimens, supporting conclusions of separate populations from a recent skull variability study.