Delphinids: The Freshwater Dolphins of Brazil

Tucuxi

In this series so far, I have generally been referring to the Delphinidae as the "dolphin family". That's a literal translation of the name and serves to distinguish it from, say, the porpoise family. However, as I mentioned in the first post, not all animals commonly referred to as "dolphins" belong in the family. Thus, when zoologists want to distinguish the family from those other animals, but want to avoid saying "delphinids", the more common term is "Oceanic dolphins". Oceans are, after all, where they are found.

With, it turns out, one exception.

The tucuxi (Sotalia fluviatilis) is unique among Oceanic dolphins is being an exclusively freshwater animal. It lives in the Amazon River and its major tributaries, mostly in Brazil, but also further upstream into Colombia, Peru, and Ecuador. Indeed, it was first formally described, by Paul Gervais in 1853, from an animal sighted in Peru, about 2,500 km (1,500 miles) from the mouth of the Amazon... and they are known to get further upriver than that, until they are stopped by features such as waterfalls.

Obviously, the locals knew about it well before this, and the word "tucuxi" comes from the native Brazilian Tupi language. (Because it uses the Portuguese spelling of Tupi words, the "x" is pronounced as a "sh" sound). It is, perhaps unsurprisingly, one of the smaller species of dolphin, measuring about 150 cm (5 feet) in length and weighing up to 52 kg (115 lbs). Physically, they look much like the larger bottlenose dolphins, with a distinct beak and grey bodies that fade to a paler shade on the underside.

While living in freshwater makes it the most shallow-water of all delphinid species, the tucuxi still prefers rivers at least 3 metres (10 feet) deep, although it is willing to tolerate lakes as shallow as 1.8 metres (6 feet). They prefer slow-moving water, which probably explains their common presence in lakes, but they are evidently willing to travel through faster-flowing stretches, or they wouldn't reach so far upstream. They seem unconcerned as to whether the water is muddy or not, although places where muddy "blackwater" rivers join clear "whitewater" ones are apparently especially popular, presumably because there are also good places to find fish.

They will eat almost any fish that is less than about 35 cm (14 inches) long, but are said to prefer croakers, curimatids (toothless relatives of piranhas), and small catfish. Compared with marine dolphins, they do not live in large pods, with one survey suggesting an average group size of just four, while groups of more than six are rare - although larger aggregations may form temporarily in suitable lakes. Although they tend to stick to particular stretches of river, seasonal flooding means that they move to different places within that stretch over the year, as deeper channels open up or close.

They appear to be active both day and night, often entering lakes in the morning and heading back to the main river in the evening. Dives last up to two minutes, and, while they do leap from time to time, mainly to get a good veiw of their surroundings, they do so less than many marine species. They are vocal animals, although perhaps with less complexity to their calls than might be expected; there may be some variation in the whistles used by populations in different parts of the river system. 

Although we know little about their reproduction, most births seem to take place when the water levels are at their lowest in October and November and, with a gestation period of 11 months, mating must take place shortly after. Perhaps its when they are most likely to be gathered together in a single part of the river and less likely to be wandering off into temporary lakes. The males have unusually large testicles for their body size, which suggests that the females mate with multiple partners in something of a sexual free-for-all.

One of the reasons for them first having moved into the Amazon may be that there is very little there large enough to eat them, but, since human fishermen tend to look for the same prey that they do, and consequently focus on the same stretches of river, many end up tangled in nets and this may currently be the largest ongoing threat to the species, followed by the construction of hydroelectric dams. The total population is unknown, although there is a surprising degree of genetic diversity across their range, which is a positive sign. Nonetheless, it's very likely that the population is declining and that it was not especially high to start with, given their restricted habitat; they are therefore formally listed as an endangered species. 

Now that I've said all that, I should point out that the tucuxi is not the only "dolphin" in the Amazon, since one of those non-delphinid species, the boto, also lives there. The Amazon is large enough to host no fewer than two species of mammal that we'd normally associate with the sea. That's been clear for a long time, but whether the tucuxi is or is not exclusively freshwater turns out to be more complicated.

The genus Sotalia, to which the tucuxi belongs, was one of many named by John Edward Grey in 1866 as part of a catalogue of specimens in the British Museum, and initially referred to an animal caught off the coast of Surinam. Over the following decade, three specimens from different parts of the Amazon River were added to the genus as new species, followed by a fifth species from southern Brazil. By the end of the century, the naturalists of the day had decided that the three riverine species were really all the same thing, but the question of the two caught in coastal waters remained.

For most of the twentieth century, we therefore had two species, one in the river system, and one not. In the 1970s, it was decided that even these were just too similar to be regarded as separate species. Thus, most references to the "tucuxi" from the late 20th century also include dolphins that are not at all freshwater, because it was assumed that the animal was equally happy in both environments. True, not everyone was convinced, but it took the development of genetic and molecular studies in the 21st century to prove the point. 

Guiana dolphin

Since at least 2007, however, it has been clear to everyone that those marine animals represent a different species: the Guiana dolphin (Sotalia guienensis). 

The Guiana dolphin is by no means restricted to the region for which it is named. The species is found along the Atlantic coast from Nicaragua in the north to about 25°S in Brazil... the latter point being roughly the northernmost extent of the cold Falkland Current. However, this distribution is very patchy, with the dolphin being absent from many stretches of coast. This is probably at least partly due to it being picky about its habitat, with the dolphin preferring sheltered bays and estuaries to coastline more generally. Notably, the Amazon is sufficiently large that much of the coast nearby inhabited by the Guiana dolphin is technically freshwater and dolphins presumed to belong to this species have been spotted far upstream in the Orinoco River in Venezuela.

Visibly, about the only difference between the Guiana dolphin and the tucuxi is that the former is about 20% larger, typically around 180 cm (6 feet) in length. While it has been far better studied than its Amazonian cousin, therefore, its likely that much of what we know about it is applicable to both species, aside from those features directly relating to its (mostly) marine habitat.

For example, Guiana dolphins are a very shallow-water species. They rarely venture more than 100 metres (330 feet) from the shore, and while they do venture deeper in a few specific localities, most sightings suggest that they prefer water no more than about 5 metres (16 feet) deep. Their diving habits are similar to those of the tucuxi, with each lasting no more than a couple of minutes, and feed on similarly sized fish.

However, the exact type of fish they prefer seems to vary from place to place, suggesting that different populations have specialised in different prey. In some places, they prefer hairtails, in others anchovies, in others croakers or squid. Although they often feed alone, at times, groups of Guiana dolphins have been seen gathering together to herd shoals of fish towards the shore before feeding on them en masse. 

The basic social unit of Guiana dolphins seems to be small, with average group sizes often recorded as as low as three or four, but this disguises considerable variation because some groups can be much larger, with perhaps 60 to 90 members. This may be due to local conditions, or because of the smaller family-based social units randomly gathering into larger pods at certain times of day, with some reports indicating that they are more likely to socialise at high tide. There is also some suggestion that groups nursing calves may tend to be larger, perhaps because the larger community provides improved protection or a better learning environment.

Each family unit stays within an unusually small patch of water, perhaps 15 km² (5½ square miles), and over half of them staying in the same area year after year, although others do seem to travel further, perhaps in search of unrelated partners. Groups communicate using the same mix of whistles and calls as other dolphins do, in addition to the sonar clicks used for navigation, and while these are shorter and less complex than in other species, there is variation in their exact form across the span of their range from Nicaragua to Brazil.

There is no evidence of a particular breeding season, but the life of an adult female seems to consist of alternating 12-month periods of pregnancy and child-rearing, with them breeding again a year after their previous calf was born. They can live for at least 30 years in the wild, and longer in captivity. 

There is, however, one additional oddity about the Guiana dolphin and, on the face of it, it's very odd indeed. In 2000, researchers mapped the heat flow to the whiskers of harbour seals, showing that they were supplied by a dense network of blood vessels, keeping the whiskers warm so that they didn't go numb in cold water, allowing the seals to use them to sense their environment. More surprisingly, they found that the same thing was true with Guiana dolphins.

Which is odd, because dolphins don't have whiskers.

They do, however, have tiny vestigial structures in the skin of the snout, the remnants of the hair follicles that would have produced whiskers were they not... well, dolphins. The researchers' best guess was that the snout of these dolphins had a particularly sensitive sense of touch, perhaps to root around in the mud or to rub against one another in social settings. Perhaps, they said, those hair follicles aren't as vestigial as we thought. For various reasons, this is quite a plausible theory.

It's also wrong.

Because, ten years later, another group of scientists took a closer look at the cellular structure of those hair follicles. They are, indeed, supplied with plenty of nerves, so they're very clearly doing something. But the cells didn't look like the sort you'd expect to sense touch, instead resembling those that fish use to sense electrical currents in the water. Sure enough, when they tested a male Guiana dolphin, they discovered that he could sense electric fields of the sort that a fish's muscles might generate while it is swimming.

This makes the Guiana dolphin the only known placental mammal capable of sensing electricity. Which raises the question... is it really the only one? It's not as if it would be any less useful to other dolphins, although similar structures seem to be missing in porpoises. This has inspired scientists to try the same test on bottlenose dolphins, and, while the evidence here isn't so strong, there's a definite hint that they can pull off the same trick. Which, since they aren't especially close relatives of the Guiana species, means that it's probably widespread among dolphins.

While the tucuxi is the only purely freshwater "Oceanic" dolphin, there is another species that also does so, even if it's at least as comfortable in the sea. Next time, I will be heading over to the other side of the world to look at that one, and it's more coastal relative...

[Photos by Jan Willem Broekema and the Instituto Boto Cinza, from Wikimedia Commons. Cladogram adapted from Cunha et al. 2011.]