title: Sexual Dimorphism and Geographic Variation in Dorsal Fin Features of Australian Humpback Dolphins, Sousa sahulensis.
abstract: Determining the sex of free-ranging cetaceans can be challenging. Sexual dimorphism among external features may allow inferences on sex, but such patterns may be difficult to detect and are often confounded by age and geographic variation. Dorsal fin images of 107 female and 54 male Australian humpback dolphins, Sousa sahulensis, from Western Australia (WA) and Queensland (QLD) were used to investigate sex, age and geographic differences in colouration, height/length quotient and number of notches. Adult males exhibited more dorsal fin notches (p<0.001) and a significantly greater loss of pigmentation on the upper half of their dorsal fins (p<0.001) than did adult females. These differences likely reflect that males experience a higher frequency and/or intensity of intraspecific aggression than females. In QLD, heavily spotted dorsal fins were more frequent among females than males (p<0.001). Logistic regression analyses revealed that dorsal fin spotting and loss of pigmentation on the upper half of the dorsal fin provided the best model parameters for predicting the sex of sampled adults, with 97% accuracy. This technique offers a rapid, non-invasive method for predicting sex in Australian humpback dolphins, which could potentially be applied to populations throughout their range. In contrast to adults, presumed immature animals showed little or no loss of pigmentation or spotting; however, the rate of development of these features remains unknown. There were pronounced differences between QLD and WA in the intensity of spotting on dorsal fins and the extent of pigmentation loss around the posterior insertion and trailing edge of the dorsal fin. While based on a limited sample size, these geographic differences may have conservation implications in terms of population subdivision and should be investigated further.
authors.affiliation: Murdoch University Cetacean Research Unit, School of Veterinary and Life Sciences, Murdoch University, Perth, Western Australia, Australia. Electronic address: alex.brown@murdoch.edu.au.
authors.forename: Alexander M
authors.initials: AM
authors.lastname: Brown
authors.affiliation: Murdoch University Cetacean Research Unit, School of Veterinary and Life Sciences, Murdoch University, Perth, Western Australia, Australia.
authors.forename: Lars
authors.initials: L
authors.lastname: Bejder
authors.affiliation: Cetacean Ecology, Behaviour and Evolution Lab (CEBEL), School of Biological Sciences, Flinders University, Adelaide, South Australia, Australia.
authors.forename: Guido J
authors.initials: GJ
authors.lastname: Parra
authors.affiliation: Marine Ecology Research Centre, School of Environment, Science and Engineering, Southern Cross University, Lismore, New South Wales, Australia.
authors.forename: Daniele
authors.initials: D
authors.lastname: Cagnazzi
authors.affiliation: Cetacean Ecology, Behaviour and Evolution Lab (CEBEL), School of Biological Sciences, Flinders University, Adelaide, South Australia, Australia.
authors.forename: Tim
authors.initials: T
authors.lastname: Hunt
authors.affiliation: Murdoch University Cetacean Research Unit, School of Veterinary and Life Sciences, Murdoch University, Perth, Western Australia, Australia.
authors.forename: Jennifer L
authors.initials: JL
authors.lastname: Smith
authors.affiliation: Murdoch University Cetacean Research Unit, School of Veterinary and Life Sciences, Murdoch University, Perth, Western Australia, Australia.
authors.forename: Simon J
authors.initials: SJ
authors.lastname: Allen
mesh_terms: D063147:Animal Distribution; D058500:Animal Fins / Q000033:anatomy & histology*; D000818:Animals; D004289:Dolphins / Q000033:anatomy & histology* / Q000502:physiology*; D005260:Female; D008297:Male; D012737:Sex Factors; D013045:Species Specificity
keywords: Colouration; Discriminant function; External morphology; Geographic variation; Image analysis; Logistic regression; Photo identification; Pigmentation; Population structure; Sex determination