Magazine article American Scientist

Computing Dolphin Fin Photo IDs

Magazine article American Scientist

Computing Dolphin Fin Photo IDs

Article excerpt

Before biologists can understand the movements and behavior of dolphins, they must know which animal is which. Luckily, adult bottlenose dolphins, Tursiops truncatus, have fingerprints of sorts: uniquely shaped dorsal fins. For decades, biologists have used photographs of fins to identify individuals in the wild. Image-processing software has sped up the procedure. Faculty and undergraduate students at Eckerd College in St. Petersburg, Florida continue to improve their open-source Digital Analysis and Recognition of Whale Images on a Network software (DARWIN), which debuted in the 1990s. And biologists keep finding new uses for DARWIN, Eckerd College computer scientist Kelly Debure explained to American Scientist contributing editor Catherine Clabby.

How are adult bottlenose dolphin dorsal fins unique?

The dorsal fins of young dolphins are essentially unmarked. During childhood and adolescence the fins acquire identifiable damage marks from attacks, vigorous play, accidental collisions with obstacles, boat strikes and entanglement. These marks, primarily nicks and notches along a fin's edge, mostly don't change in adulthood.

How did biologists initially use photographs of dolphin fins?

Originally, everyone worked with film cameras, and biologists maintained physical catalogs of slides or print photographs of the animals. Researchers were clever about how they organized the images into categories, but they still could have a couple hundred images in one category of fin type. Someone had to manually flip through many pages to make an ID.

How does DARWIN work?

We originally used what's called a centroid signature for encoding the outline. Imagine a dorsal fin as a big triangle. If we place a point in the center of the triangle and compute distances from that centroid to the edge of the dorsal fin at regular angular intervals, we obtain a list of distances that describe the fin shape. The centroid method worked pretty well - especially with odd-shaped fins. As it was used, however, it became clear it was susceptible to variation in user input of fin edge length. So we moved to what's called a chain code representation.

Starting at a point at the beginning of the leading edge of a fin, we now compute the angular direction to subsequent points along the fin edge at regular length intervals. Changes in these directions occur at features of interest, the fin tip and various nicks and notches along the edge. We use the feature points to solve for a transformation matrix that allows us to map one fin triangle to the other but maintains the original measurements of ratios of distances between points. …

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