How to sex a dolphin

28 October 2008

Assessing the ratio of males to females in endangered populations is vital for conservation work. But sexing a dolphin is tricky — not least because the crucial parts of the mammal are usually concealed beneath the waves. Researchers generally have to rely on time-consuming observations, either inferring a female's sex from its close association with a calf, or noting genitalia when animals leap from the water or are captured on underwater video. The alternative is a biopsy that is potentially unpleasant for the animal. Lucy Rowe and Stephen Dawson, marine biologists at the University of Otago in Dunedin, New Zealand, have now come up with a accurate alternative that could spare their fellow researchers, and the dolphins, this inconvenience. In a recently published paper in Marine Mammal Science the pair report successfully using fin photographs to determine the sex of bottlenose dolphins in a well-studied population in New Zealand's Doubtful Sound. "Our technique allows bottlenose dolphins to be sexed from characteristics measured solely from dorsal fin identification photographs, which are routinely collected as part of non-invasive population monitoring," Rowe and Dawson told Nature News in an email.

The pair teamed an over-the-counter digital camera with a pair of laser pointers, which project two reference spots precisely 10 centimetres apart onto the fin, allowing an accurate determination of its size. After taking a series of digital photographs of the Doubtful Sound dolphins, the authors compared them with existing fin and sex records for the population. Rowe and Dawson found that male fins had significantly more scars than female fins, probably as a result of fighting. Male fins had a median of 15% scar tissue, whereas in females this was just 3.9%. Conversely, female fins tended to have a greater number of patchy skin lesions than male fins, with a median of 12.1% coverage compared with males' 6.8%. The two then used a statistical analysis of scarring, number of fin nicks and fin size to correctly predict the sex of 93% of the 43 dolphins in the group. This laser-sighted technique could potentially be applied to other, less-studied populations of dolphin or even to other species, the scientists say.

"The best candidates are species in which there is slight sexual dimorphism, but not enough to reliably sex individuals without a photogrammetric tool," say Rowe and Dawson. "There are many long-term cetacean research projects that conduct photo-identification and have gathered sex data on a number of individuals. It may be possible to use those photographs to develop a species-specific sex-classification model following the technique we used." The authors are currently applying their technique to another population of dolphins in the nearby Dusky Sound, and "initial signs are good". Shannon Gowans, a dolphin researcher at Eckerd College in St. Petersburg, Florida, and the associate editor at Marine Mammal Science responsible for the paper, says, "If it's applicable to other populations it would be very helpful. In the population I work with, very few of these individuals have been sexed. If this kind of thing applies far beyond this one population it would be great."

Nature
October 28, 2008

Original web page at Nature