What You Know* About the Difference in Dolphins and Porpoises is Wrong**

*Mostly  **Some of the time

Last weekend I was lucky enough to speak at a math workshop for high school teachers put on by some of the University of Hawaii Match Department faculty.  And let me just start out by saying that it was AWESOME.  As anyone who has ever read "How to Win Friends and Influence People" knows, one of the best ways to get people to like you is to ask them questions about themselves.  For scientists, this extends to talking about our research. Beware asking a scientist good, intelligent questions about their work: you may be there for hours!

So, talking to the math teachers was an incredible treat.  They asked great questions, including one of my favorites, "What's the difference between a dolphin and a porpoise?"  Porpoises are from the cetacean family Phocoenidae, while dolphins are in the family Delphinidae.  I love this question, because there are so many misconceptions about what makes a porpoise.  This cartoon does a great job of showcasing those misconceptions:

Let's get the easiest misconception out of the way first: size.  I'll admit the average weight of porpoises is less than the average weight of dolphins.  However, there are many examples in which this just isn't true.  For example:

Dall's Porpoise (up to 200 lbs)

Hector's dolphin (up to 125 lbs)

So size of the animal isn't exactly the best diagnostic to use when deciding whether something is a dolphin or a whale.  How about rostrum (head) shape?

  Risso's Dolphin by Greg Boreham                             Finless Porpoise

Hmm, one of those is a dolphin and one is a porpoise, but they seem to actually have very similarly shaped heads.

OK, FINE! But if it has a bottlenose like Flipper, we DEFINITELY know it's a dolphin, RIGHT?

Northern bottlenose whale  (Hyperoodon ampullatus, in Family Ziphidae)

ARGHHHH!  It must be the fins, then. Dolphins HAVE to have pointy dorsal fins (the one on their back), and porpoises have rounded fins!

Chilean dolphin (rounded)                                                      Dall's Porpoise (pointy)

No dorsal fin at all (Southern Right Whale Dolphin).
The finless porpoise also has no dorsal fin, as the name implies.

OK, I give up, there must be SOMETHING I can use to figure out whether this random cetacean I got in the mail is a dolphin or a porpoise - but what????

TEETH!

Bottlenose dolphin                                |                             Harbor Porpoise

Dolphins have cone-shaped teeth throughout their mouth, while porpoises have what are known as spade-shaped teeth.  Here's a comparison, with A being the porpoise tooth and B being the dolphin tooth.

Dichotomous keys are tools that scientists use to figure out what species they are looking at.  These keys are kind of like those "build your own story" books that you had when you were a kid. You are asked a series of questions, and the questions lead to an ending.  It's also a lot like 20 questions, with each question further narrowing down the possibilities of which species you could have.  In fact, if you were using a dichotomous key to determine the species of a cetacean, the tooth question is what separates the porpoises from everything else.  

Try keying out whale, dolphin, and porpoise species
at the Marine Species Identification Portal!

In general, can you use these rules to distinguish between a dolphin and a porpoise?  Sure you can. But it's important to be aware of the many exceptions. Let's be honest, the likelihood of encountering Dall's Porpoise (found on the US west coast) and Hector's Dolphins (found in New Zealand) on the same day is infinitely small.  Even smaller is the likelihood that they'll give you a good look at their teeth. The best plan is to be knowledgeable about the species you are likely to encounter in your area.

Thanks for the great questions, Math Teachers of Hawaii.  If you'll notice, I haven't actually given much space to the actual workshop.  I'll get to that later. See what happens when you ask a scientist about something that interests them?!

If you ask a scientist a question...
they'll want a cookie.

Who am I kidding, I always want cookies.

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