A Porpoise Primer

April 2^nd and the Beagle is swiftly approaching Rio.  Darwin’s entry today is a virtual “stew” of pelagic wildlife:

“A rainy, squally morning, very unusual at this time of year in these Latitudes; being now about 130 miles East of Rio. A large flock of Mother Carys chicken are hovering about the stern in same manner as swallows do on a calm summer evening over a lake. — A flying fish fell on the deck this morning; it struck the mast high up near the main yard: sticking to the fish was a crab, the pain of which caused perhaps this unusual degree of action.”  (Apr 2)

From following Darwin’s diary entries, it is clear that time at sea consists of some very busy days (such as in the Abrolhos Islands) interspersed with a lot of slow days (where Darwin arranges collections, writes letters, watches the sea, and describes the weather.) Today was one of those slower days. (For more on the sea life Darwin notes today see posts on Mother Carey’s Chickens and Mail “Trucks” and Flying Fish.)

The lazy days continue tomorrow, too, where Darwin notes:

“We have seen great quantities of shipping; & what is quite as interesting, Porpoises, Sharks & Turtles; altogether, it has been the most idle day I have spent since I left England.” (Apr 3)

Video of spinner dolphins off of Fernando de Noronha (from 3rd Culture Children):

So I thought I’d say a little about porpoises and dolphins today, while giving a nod to my new friends in Brazil who write the blog 3^rd Culture Children. Check it out – I’ve found the informative posts and wonderful photography a great way to get to know Brazil. The connection to today’s entry – 3^rd Culture Children recently posted photos from a trip to Fernando de Noronha, including some dolphin shots.

Spinner dolphins (Stenella longirostris) in front of one of the classic landmarks of Fernando de Noronha,, Morro do Pico (from 3rd Culture Children – follow the link for more images and revisit the the Beagle Project post on Fernando de Noronha for a refresher on the location.)

There is a lot to say about dolphins, porpoises and whales (the cetaceans). In fact, they are one of my favorite stories in the evolution of life on Earth – the great migration of mammals back into the oceans about 50 million years ago. (You can bet I’ll be writing about that one of these days – but that will require a post of it’s own.) Today, I thought I might just start with a few words about porpoises – the porcopiscus, or “pig fish”.

There are actually only 6 species of porpoise living today, and of these, only 1 – the Burmeister’s porpoise (Phocoena spinipinnis) – has a range that comes anywhere close to Rio (technically it falls a little short, but there are always some outliers).  In addition, Burmeister’s porpoises are reported as being shy and they typically avoid ships. So the reality is, that what Darwin probably saw were dolphins – maybe even spinner dolphins.  No criticism of his abilities as a naturalist – the term porpoise was used more generically back the 1830’s. (But if he did indeed see a Burmeister’s porpoise then he missed his chance to have naming rights. It was only discovered and named about 30 years later by Hermann Burmeister.) But more likely than not, Darwin was watching dolphins.

Postage stage of a Burmeister’s porpoise:

Porpoises (family Phocoenidae) and dolphins (family Delphinidae) are two very distinctly different types of sea mammals.  Both are cetaceans and have a common ancestor that made the journey back into the sea, but the two families split and evolved separately.  Porpoises typically are smaller and stouter than dolphins, lack a “beak”, and have spade-shaped teeth (rather than conical teeth). Being the smallest cetaceans, the porpoises have to manage their heat more carefully (a smaller body size means that they have a higher surface to volume ratio and radiate heat more quickly). The stouter body size may be an adaptation to this (it conserves het more efficiently), but porpoises also have to eat a lot to maintain a constant source of heat.

The fossil evidence suggests that the porpoise and dolphin families diverged from each another about 15 million years ago. So although they are closely related and look similar, keep in mind that they would say the same thing about humans and chimps.  And, in fact, they would be more accurate in saying so, as our lineage diverged from chimpanzees a mere 5 million years ago. (RJV)

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