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How to identify and determine the sex of dolphins in the wild.

Most studies of wild dolphin behavior study the animals at the surface from regular vessels. This is how we started as well. From this vantage point, you mostly get to see their backs and dorsal fins, which fortunately offers a great opportunity to identify individuals, since they often have nicks in the trailing edge of the fin. Sometimes, there are also marks in the leading edge. There can also be variations in the shape of the fin, which also provide opportunities for individual identification.

By identifying individuals, you can estimate the size of the population and put together a picture of who spends time with whom and how stable the social groups are, in other words the social organization of the animals you are studying. However, the dorsal fin generally does not allow you to determine whether the individual is male or female - a very basic piece of information for any behavioral study. For some dolphin species adult males tend to have larger, more prominent dorsal fins than adult females, but this is not always the case. Subadult (sexually but not socially mature - think human teenagers) and juvenile (not sexually mature) males generally do not have dorsal fins that stand out.

The only way to definitively determine the gender of a dolphin is a close look at its genital area: males have two openings, or slits in their ventral (belly) area, the anus closer to the tail region, and a slit further up on the belly where the penis has been folded in for the animal to be more streamlined. Females, on the other hand, have one central genital slit, containing both the anus and the vagina, with 2 small mammary slits on either side, where the nipples are folded in. Females of some cetacean whale species have 4 mammary slits, one pair on each side of the genital slit.

This is all well and good, if you find a dead dolphin stranded on a beach, but how do you get a look at the genital area of dolphins in the wild? Sometimes, you are lucky and have a dolphin roll over near your boat, or you get a photo of a leaping dolphin showing the ventral area, but these are rare occasions. For this reason, it is not uncommon for researchers to make assumptions about their subjects based on what they can see. For example, an adult animal associated with a calf is often assumed to be a female.

Our research on Hawaiian spinner dolphins was carried out under the auspices of our advisor, Dr. Ken Norris. He had developed a series of underwater viewing vessels (see this link) to overcome this challenge and we were lucky enough to help develop, build and use the last version in that series of vessels. The underwater viewing boat we used, the “Smyg”, had a viewing chamber that could be lowered through the hull so that the windows ended up just below the hull, putting the observer below dolphins swimming near the surface around the boat. This allowed us to determine the gender of dolphins by eye, or during frame-by-frame analysis of videos taken from the chamber.

We were also able to do focal animal follows, video-taping a specific dolphin, or group of dolphins for an extended period of time from the chamber, while other people on the team photographed them at the surface for individual identification purposes. Back on land, a frame-by-frame analysis of the interactions between all individuals on camera while a transcription of the narration done during filming provided additional information about animals and events off camera, providing the information needed to interpret actions and reactions by the dolphins.


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