Fishing in the Ganges

The most spectacular cetaceans are, arguably, the really big ones - sperm whales, blue whales, and so on. However, while our knowledge of the exact numbers is a little hazy, somewhere around half of the cetacean species alive today represent much smaller animals. The majority of these belong to just two taxonomic families.

By far the most numerous are the "true" or "oceanic" dolphins, a family that also includes killer whales and pilot whales - small in comparison with the like of humpbacks, but fairly large by most standards. The second family are the porpoises, which are exclusively small, by cetacean standards, and usually slightly smaller than dolphins.

But there are a few small-sized cetacean species that fall into neither group. These oddities share one thing in common: they don't live in the sea. While they are, of course, just as fully aquatic as their better-known kin, this has lead to them receiving the common collective name of "river dolphin", thus distinguishing them from the "oceanic" sort that most people are more familiar with.

First and Last of the South Asian River Dolphins

South Asian river dolphin skeleton -
note the strange shape of the skull

Although the whales are undeniably spectacular, the majority of cetacean species are much smaller; the sort of animals we generally refer to as "dolphins". The great majority of these belong to the family Dephinidae, variously termed the "oceanic dolphins", "pelagic dolphins", or even "true dolphins". This is a large family, with nearly forty species, including killer whales and pilot whales alongside their smaller kin.

Whatever we call this family, it, in turn, belongs to the larger group of the "delphinoid cetaceans", which also includes the six species of the porpoise family and two other whales - the narwhal and beluga. Taken together, these animals and their extinct relatives have dominated the count of cetacean species across the world for millions of years, forming a key part of the ocean ecology. Of all the other cetacean groups, only the mysterious deep-sea beaked whales come close in terms of the number of species, and even they don't appear to be as varied.

A New Kind of River Dolphin?

Amazonian botos (probably)

I could, perhaps, have titled this post "I Told You So".

Back in August, at the end of my post on the newly discovered olinguito, I said: "Should we expect more new species of mammal to be found, perhaps even large and interesting ones? You can count on it." I also suggested in that post that remote jungles were exactly the sort of place you'd expect to find one. Well, since I wrote that, we have discovered at least one, and possibly two, new species of large mammal that fit exactly that description.

The first was the kaboman tapir (Tapirus kabomani), which lives in the Amazon jungle, and which was described a couple of months ago. I'll refer you to Darren Naish's blog if you want to know about that, since I have nothing useful to add to what he's already said. There is, so far as I can tell, no dispute or argument about the reality of this tapir as a new species - the first new tapir to be described since 1865. The acceptance of other one, published just last week in the online journal PLoS One, appears to not quite be so clear cut.

The animal in question is a new species of river dolphin, and it, too, was discovered in the Amazon jungle. But, before we look at the details, it's probably best if I start by describing what a river dolphin actually is.

Dolphins, on the whole, live in the sea; this is surely not a great surprise. Yes, they sometimes travel a short way up particularly large rivers, and some of them even do it on purpose. But, generally, the sea is where you expect to find them. Now, there is an exception, and I discussed it back in 2012: it's the tucuxi (Sotalia fluviatilis). This spends its entire life in freshwater, and, as I said at the time, it's the only true dolphin that does this.