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Tuesday, October 14, 2008

Porpoise Tails

Recently I was reading the Audubon Society's guide to Marine Mammals of the World, one of my favorite texts covering basic details of all cetacean species. From this I learned that you can supposedly tell the difference between male and female Dall's porpoise by the shape of their tails. The tail flukes on a female are relatively straight across, whereas the tail flukes on a male become convex along the trailing edge as they reach sexual maturity. Adult tails of both gender s have varying amounts of white "frosting" along the edges of the otherwise black flukes. Juveniles, by contrast, have uniformly gray tails that are concave along the trailing edge.

How cool! I decided to go through my photos of Dall's porpoises to see if I could spot some differences between males and females. This photo to the right seemingly shows an adult female. The tail is black with white frosting and the trailing edge is more or less straight across.

However, as I went through my photos, it looks like I only have pictures of female Dall's. This is entirely possible, since I have a fairly small set of photos where the tail shape is distinguishable, so maybe they just all happened to be females. This got me wondering, though: do females for some reason tend to bow-ride more? Or do males and females really not look all that different?

The graphic in the Marine Mammals of the World book is very distinct, but maybe it isn't that clear in wild animals. As I was researching this question further, I came across this paper, which suggests that while animals with convex tails are nearly always males, both males and females have straight tails, and little is known about how reliable this feature is at determining the age and sex class of wild animals. There are, however, other more reliable sexually dimorphic traits, such as the slope of the dorsal fin and the size of the hump in the caudal peduncle (tail stock) that gives the porpoise the look of having a "broken tail" when diving.

I thought that I should be able to differentiate between the sexes, then, using dorsal fin slant, so I went back through my porpoise photos. Unfortunately, whenever you're close enough to a Dall's porpoise to get a decent photo, they are usually speed-swimming as they bowride on the boat. This causes them to kick up a huge "rooster tail" splash, so your view of the dorsal fin is often limited, as you can see in the photo at right. where it is completley obscured by teh splash. Not exactly the best view. I guess that means I have a new challenge: photographing Dall's porpoise dorsal fins. In the meantime, as far as telling the two genders apart in the field, it's back to the drawing board.

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