External morphology of the short-beaked common dolphin, Delphinus delphis: growth, allometric relationships and sexual dimorphism


Sinéad Murphy, Sea Mammal Research Unit, Gatty Marine Laboratory, University of St Andrews, St Andrews KY16 8LB, UK. E-mail: snm4@st-andrews.ac.uk


Growth, allometric relationships and sexual dimorphism are described from measurements of 105 male, 149 female and 38 unsexed specimens of short-beaked common dolphin, Delphinus delphis, stranded along the Irish coastline (53.8% of the sample) or by-caught in fisheries (46.2% of the sample), from 1990 to 2003. For each dolphin, 24 external body length measurements were recorded. Ages were determined for 183 dolphins by analysis of growth layer groups in the dentine. Males ranged in total body length (TBL) from 105 to 231 cm and females from 93 to 230 cm, with a maximum age of 25 years obtained for both sexes. Using a single Gompertz growth curve, asymptotic values obtained for TBL were 211.6 cm and 197.4 cm for males and females, respectively. Asymptotic lengths were attained at 11 years in males and 9 years in females. The gestation period was estimated to last approximately 11.5 months. Sexual size dimorphism (SSD) was evident, with males being significantly larger than females for 20 of the characters measured, and an SSD ratio of 1.06 was obtained. Sexual shape dimorphism was lacking, except for the presence of prominent postanal humps in mature males.