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.
Because they all live in freshwater - specifically, in large river systems - these animals do have a number of things in common, and anatomically look quite similar. By the same token, however, they are quite clearly isolated from one another, since you can't get from one river system to a completely different one without crossing the ocean at some point. While it turns out that some of their fossil relatives could, in fact, live in salt water, which explains how they got into the rivers at the first point, our modern understanding places the six recent species of river dolphin into three different families, that turn out to be remarkably distant from one another in evolutionary terms.
Four of the species live in South America, where it's interesting to note that one species of the "oceanic" dolphin family has also entered the rivers. One of the others either is already, or very soon will be, extinct. The other is the South Asian river dolphin (Platanistes gangetica), the last survivor of a once much larger group, every other form of which seems to have been extinct for at least five million years.
Originally identified as a new species way back in 1801, this lives in the Ganges and Brahmaputra rivers (and their major tributaries) in India and Bangladesh, and in the main channel of the Indus River in Pakistan. While the Brahmaputra is a tributary of the Ganges, and so forms part of the same river system, the Indus River is on the other side of the subcontinent, and clearly doesn't. Partly on these grounds, the animal living in the Indus was raised to full species status in the 1970s, but, when nobody could find any other good reason to separate them, it was downgraded to subspecies again in the late '90s.
This was subsequently confirmed by genetic analysis of the two forms in 2014: there really is just one species, with two subspecies. This, of course, means that, at some point, a population of dolphins managed to get across the whole of northern India... a feat that makes a lot more sense when you realise that, for a while before the Ice Ages, the Indus really was a tributary of the Ganges.
Before humans came along and polluted the rivers and filled them boat traffic, the greatest challenge facing river dolphins was trying to find their food. This is because the kinds of rivers that are wide enough to support a population of dolphins also tend to be very muddy and full of silt, making it very difficult to see when you're underwater. Fortunately, in the case of dolphins, they had a secret weapon: sonar.
Cetaceans in general don't rely too much on sight when it comes to foraging for food, but this is particularly true for river dolphins, with the South Asian sort, in the mud-filled waters of the Ganges, one of the most extreme examples. While they do, of course, have eyes, and can presumably see something, those eyes are very small and have no functional lens, so it's hard to imagine that they can see images of any kind. Perhaps the most they can manage is to tell where the horizon might be when they're at the surface and whether or not it's daytime when they're beneath it; for all other purposes, they are effectively blind.
This means that they must be even more reliant on sonar - and, perhaps, other senses - than most cetaceans are, which makes their feeding habits a particularly interesting topic.
Despite the difficulty of observing South Asian river dolphins in their native habitat - it's no easier for us to see through the silt than it is for them - we have a pretty good idea of what they eat. Primarily, their diet consists of small fish and shrimp, although they do also consume a small quantity of water insects, snails, and even vegetation. They have unusually long, narrow, snouts, and weak jaws, and it's this that probably prevents them from eating larger fish, even though plenty are present where they live.
Specifically, creatures that are found close to the river bed, such as catfish, eels, and bottom-dwelling shrimp, seem to form about half their diet. Even where they are taking prey from above the bottom, it's often from areas with dense river weeds and the like, rather than simply open water.
Nonetheless, they do feed on some prey in the mid-river, and a reasonable amount that's found near the surface. One reason that this isn't their primary source of food, however, may be that sonar tends to be scattered at the water surface, making it difficult to echolocate for prey there. Observations of South Asian river dolphins feeding at the surface show them swimming round rapidly to churn up the water, even tossing some smaller fish into the air in the process. So far as we can tell, they appear to locate such prey simply by listening for the sounds it makes, a tactic known to be employed by bottlenose dolphins on occasion.
Echolocation really comes into its own, however, in the mid-part of a river (or, of course, the open sea), where there isn't much other than swimming animals for it bounce off. South Asian river dolphins employ a series of individual clicks and rapid stacatto bursts of sound to locate prey, and the high frequency of sound that they use has been estimated to allow them to detect fish from 20 metres (65 feet) away. It should also be good for identifying the sort of fine detail needed to pick animals out from surrounding vegetation, which may explain why so much of what they eat is normally found among dense weeds. The unusual shape of their skulls, with a large bulge above the upper jaw may also help to focus their sonar pings and keep them highly directional, so that they know exactly where their food might be.
Oddly, however, this might actually prove difficult in the muddy environment of the riverbed, which is where the dolphins seem to catch most of their prey. The supposition, therefore, has been that here they must be, at least partially, using something else. And it clearly isn't vision. The dolphins have been observed poking about in the mud with their snouts, and it has been suggested that, in addition to the obvious tactile clues that this might provide, the dolphins are actually able to sense the electrical currents produced by living creatures and detect them that way.
This seems remarkable, but, while it has never been shown in the South Asian species, it has been demonstrated in the Guiana dolphin (Sotalia guianensis), an "oceanic" species that lives along shallow coastlines and in brackish estuaries. That seems to perform the feat using whiskers on its snout, and, in this context, it may be significant that the South Asian river dolphin has, admittedly by the standards of animals that are normally entirely hairless, quite the largest number of whiskers on its snout of any known cetacean species. They're extremely short, and you have to look quite carefully to see them at all, but they are present, especially in young individuals that have yet to develop their full echolocation abilities.
Once they have located their food, the dolphins snap at it with their narrow jaws and fine, almost needle-like teeth. These are ideal for holding onto slippery fish, and anything that escapes the grip would rapidly die from the resulting blood loss anyway. The snout looks particularly inappropriate for the sort of suction feeding favoured by some other cetaceans, although they swallow it so rapidly after killing it, that it's possible suction is involved once the prey is already in their mouth, rather than a simple gulping motion; certainly, with those teeth, they can't chew anything.
A couple of other oddities about the dolphin may also be relevant to their feeding, or other habits. For one, they have a moderately large caecum in their intestine, something that is normally associated with digesting tough plant food (it's exceptionally large in, for example, horses and rabbits). Clearly, they don't eat anything of the sort, and it has been suggested that, perhaps, they use it to help digest the shells of prawns.
Away from the digestive system, the organs of balance associated with the ears are unusually complex. This suggests that the dolphins ought to be very agile, and, indeed, they do seem more flexible than one would expect of cetaceans. Uniquely, they have been observed swimming on their side, especially when they are in shallow side-channels off the main river, so this might have something to do with that. Quite why they would want to do such a thing in the first place is, however, much less clear.
All in all, these are really quite strange creatures, and their evolutionary history shows that they diverged from all other cetaceans further back than any other toothed whale. It's unlikely that any features they have now truly represent the original way that cetaceans lived back then, since they have surely been shaped by their unusual habitat. But they do show something of the variety that can exist among whales and dolphins, and how they can adapt to an environment that so few others can abide.
Threatened by pollution, the construction of barrages and dams, and both accidental and deliberate killing by fishermen, the South Asian river dolphin is today considered an endangered species. There may be as many as 3,000 or so in the Ganges and Brahmaputra, but there are probably only a few hundred in the Indus. A difficult life has been made much harder by the need to share major waterways with humanity, and we can but hope that they will not head the way of the baiji.
[Photo by "Babasteve", from Wikimedia Commons.]
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