Comparative morphology and distribution of the aduncus and truncatus forms of bottlenose dolphin Tursiops in the Indian and Western Pacific Oceans

Two morphological forms of the bottlenose dolphin, Tursiops truncatus, are recognised in Indo-Pacific waters; a coastal form referred to as T. cf. aduncus and an offshore form, T. truncatus. The two are distinguished primarily on the basis of ventral spotting, present in adult T. cf. aduncus and absent in T. truncatus. We compared the morphology of specimens obtained from parts of their range where both forms are found; south-east Africa, the East and South China Seas, and eastern Australia. Across its range, T. cf. aduncus has a shorter body and skull length and on average more teeth than T. truncatus from the same areas. No difference in body length was noted between sexes in T. cf. aduncus, while male T. truncatus were larger than females. T. cf. aduncus from tropical waters were distinctly smaller than in subtropical/ temperate regions. Differences in the pattern of the dorsal cape between forms from eastern Australia enabled their geographic distribution to be investigated. T. cf. aduncus was found in estuarine and near-coastal oceanic waters and T. truncatus in near-coastal oceanic and offshore waters. Differences in morphology, and likely niche separation in this partially sympatric distribution of the two forms suggests two species, but there are arguments both for and against the assignment of species status to each morphotype.