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Sexual dimorphism and geographic variation in dorsal fin features of Australian Humpback dolphins, Sousa sahulensis
Brown, A.M.; Bejder, L.; Parra, G.J.; Cagnazzi, D.; Hunt, T.; Smith, J.L.; Allen, S.J. (2016). Sexual dimorphism and geographic variation in dorsal fin features of Australian Humpback dolphins, Sousa sahulensis, in: Jefferson, T.A. et al. Adv. Mar. Biol. 73: Humpback dolphins (Sousa spp.): Current status and conservation, Part 2. Advances in Marine Biology, 73: pp. 273–314. https://hdl.handle.net/10.1016/bs.amb.2015.08.002
In: Jefferson, T.A.; Curry, B.E. (Ed.) (2016). Adv. Mar. Biol. 73: Humpback dolphins (Sousa spp.): Current status and conservation, Part 2. Advances in Marine Biology, 73. Academic Press: London. ISBN 978-0-12-803602-0. 326 pp., more
In: Advances in Marine Biology. Academic Press: London, New York. ISSN 0065-2881; e-ISSN 2162-5875, more
Peer reviewed article  

Available in  Authors 

Keywords
    Image analysis
    Population characteristics > Population structure
    Sex determination
    Marine
Author keywords
    External morphology; Colouration; Pigmentation; Photo identification; Logistic regression; Discriminant function; Geographic variation

Authors  Top 
  • Brown, A.M.
  • Bejder, L.
  • Parra, G.J.
  • Cagnazzi, D.
  • Hunt, T.
  • Smith, J.L.
  • Allen, S.J.

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.

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