How to sex a dolphin

Assessing the ratio of males to females in endangered populations is important for conservation work. Sexing a dolphin at sea is tricky, not least because the genital area of the mammal is usually concealed beneath the water. Researchers generally have to rely on time-consuming observations, either inferring a female's sex from its close association with a calf or taking sharp photos of the genital area (and the dorsal fin). The alternative is a biopsy sample, potentially unpleasant for the animal, again combined wih photos allwing for the individual identification of that individual.

Lucy Rowe and Stephen Dawson, marine biologists at the University of Otago in Dunedin, New Zealand, found an alternative. They recently published a paper reporting how photographs of dorsal fins were used to determine the gender of bottlenose dolphins in a well-studied population in New Zealand's Doubtful Sound. This technique allowed to sex the animals from features measured solely from dorsal fin identification photographs, routinely collected as part of non-invasive population monitoring.

The pair teamed an over-the-counter digital camera with a pair of laser pointers, which project two reference spots precisely 10 cm apart onto the dorsal fin. This procedure allowed an accurate determination of dorsal fin size. The digital photographs were then compared with existing fin and sex records for the population.

Applying this technique, the two researchers found that dorsal fins of males had significantly more scars than female's, probably as a result of aggressive behaviour among males. Fins had a median of 15% scar tissue, whereas in females this was just 3.9%. Conversely, the dorsal fins of females tended to have a greater number of patchy skin lesions, with a median of 12.1% coverage compared with males' 6.8%. Rowe and Dawson then used a statistical analysis of number of fin nicks, fin size and scarring to correctly predict the sex of 93% of 43 dolphins.

This laser technique could potentially be applied to other populations of dolphin or even to other species with slight sexual dimorphism. The authors are currently testing their technique to another population of dolphins in the nearby Dusky Sound, and "initial signs are good".

Annalise Petroselli

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Rowe L., Dawson S. 2008. Determining the sex of bottlenose dolphins from Doubtful Sound using dorsal fin photographs. Marine Mammal Science doi: 10.1111/j.1748-7692.2008.00235.x

More information: Nature.com