Pink dolphins? Who knew?

The BBC has an article by Lesley Evans Ogden about some unusual river dolphins:

Reputation: Botos, or Amazon river dolphins are pink, quiet, solitary, blind mythical mammals. They stalk the rainforest at night in the guise of exotic and attractive men, who seduce and then impregnate innocent local women. Or, at least, so says one popular myth about the Amazon river dolphin (Inia geoffrensis), known locally as the boto. The legend of their nocturnal human transformation "has been used to cover up adultery and sexual misconduct", says Gabriel Melo Alves dos Santos, a doctoral student at Brazil's Federal University of Para.
Reality: They are real, not mythical, but they are the subject of interesting myths. Some are pink. Others are grey. They are anything but quiet. In some regions they are very sociable, and not just amongst themselves. Controversy swirls around how many species there are, and how many of these unusual animals remain.
The scientific view of the dolphin is a little more mundane, but not without its own mysteries. The truth is that the boto, one of the few remaining freshwater dolphin species on Earth, is still something of an enigma.
We do not know how many there are in the wild or how many species they fall within. We do not even know for sure why this strange dolphin is often a delicate shade of pink.
Not all botos are pink. "Coloration varies," says Vanessa Mintzer, a postdoctoral researcher at the University of Florida at Gainesville. "Typically, younger animals are grey, and they get pinker as they age. Usually the adult males are the pinkest."
Pink is surely a way to match the particulate red mud that occurs in some of the rivers following heavy rains
One recent hypothesis suggests the pinkness develops because botos fight a lot, and it is their scar tissue that is pink. There is certainly a lot of aggression between the dolphins, especially the males, which are about fifty percent heavier than females.
"Almost the entire body surface of adult males is often covered with multiple overlapping tooth-rake marks," writes Anthony Martin of the University of Dundee and Vera da Silva of the National Research Institute of the Amazon in Brazil.
Mammal coloration expert Tim Caro of the University of California at Davis offers an alternative explanation. "Pink is surely a way to match the particulate red mud that occurs in some of the rivers following heavy rains," he says. Some individuals, he adds, have grey backs and pink undersides. Such countershading, dark on top and light underneath, is common in dolphins, whales, and porpoises. It is thought to be a way to hide from predators.
Scientists are still arguing about how many species or subspecies of boto there are. Some argue for the existence of a southern species in Bolivia, distinct from botos in more northerly Amazonian reaches.
In 2014, Tomas Hrbek at the Federal University of Amazonas at Manaus and colleagues presented evidence for a third species in Brazil's Araguaia River basin. The Marine Mammal Society is not yet convinced of this proposed new species.
Also an enigma is exactly how many botos there are in the Amazon's rivers. The International Union for Conservation of Nature lists them as "data deficient", but says they may be at risk in some parts of their range. One challenge to visually counting the numbers of these dolphins is that their riverine waters are often murky with silt.
Botos do not exactly go out of their way to make scientific counts easy, either. Typically, "they just put the tip of their head out of the water", says Alves dos Santos.
Even when seen at the water's surface, they lack the prominent dorsal fin used to photo-identify individuals in other dolphin species. Botos have a subtle ridge on their back, so it is still possible to "fingerprint" individuals using the unique pigmentation patterns, nicks, curves and wounds on its skin. But it is tricky.
Their shy, elusive behavior and the amount of time they spend underwater means that even the tally of dolphins counted from the same boat, with some observers at the front and others at the back, does not always match.
Paradoxically, though, some botos go out of their way to make themselves incredibly easy to spot. This can be seen at a market in the town of Mocajuba in Para State, Brazil. There, botos play with local children taking a break from helping their family with sales at the market. The water is clear, and Alves dos Santos has seen the dolphin behavior in detail. One of the fascinating things about this behavior, he says, is the lack of aggression shown by the dolphins. "If they wanted to they could rip those children apart because they are pretty big and they have pretty strong jaws," he says.
But the botos do nothing of the sort. The children stroke and play with the dolphins almost every morning. Locals report that this interaction has been happening for thirty years.At the market, botos are always in groups, never alone. That is unusual, says Alves  dos Santos' research advisor Laura May-Collado at the University of Vermont. "Many scientists have assumed these animals are solitary," she says, the only groups being a mother and young.
Alves dos Santos has discovered that some individuals come to the market repeatedly, year after year. Is that to take advantage of an unusual situation, humans feeding them scraps from the market, or are they in stable groups because they are close relatives? He hopes to find an answer by exploring the dolphins' family tree.
Attracting dolphins with fish to take tiny tissue samples for his DNA family tree analysis, "you see how intelligent they are", says Alves dos Santos. The dolphins are often assumed to be blind because their eyes are small. But they are not; instead, they are curious, and "they look into your eyes".
Although botos are difficult to visually count, there may be another way to work out how many individuals remain in the wild: recording their vocalizations.
Once thought to be a quiet species, botos turn out to be anything but. Rather like bat clicks, the clicks that make up the bulk of boto communications are inaudible to humans. Slowed down by about ninety percent, the clicks sound like popcorn popping in a microwave.
Marie Trone of Florida's Valencia College, along with two retired acousticians from the US Navy and two researchers at the University of Toulon in France, is studying individual differences in boto sounds.
Clicks have been successfully used to distinguish between individual bats. Trone hopes to do the same for botos. If the researchers can distinguish between the sounds of individuals, they might provide a useful tool for counting how many of these hard-to-see animals there really are. So far, their method shows promise.
As fish eaters, botos sometimes have a challenging relationship with human fishers. In parts of the Peruvian and Brazilian Amazon, botos are killed for use as catfish bait. Though this fishing method has been officially banned, the practice continues.
Sneaky fishermen have made up fictional names like douradinha for the boto-baited Zamurito catfish they sell. This practice has been revealed by DNA evidence at six markets in Manaus in the Brazilian Amazon.
Even when botos are not caught for use as fish bait, they are sometimes intentionally killed as perceived competitors for fish, or killed accidentally in fishing nets and boat strikes. The hope is that research into these secretive river dwellers will improve our understanding of their ecology, and perhaps improve their uneasy relationship with locals, who see them as competition for fish.
It is less clear whether that research can also dispel the myth that botos steal more than just fish. "A lot of unwanted pregnancies in the Amazon are blamed on the boto," says Mintzer.

Rico says he'll never see them, but it's nice to know they're out there, and providing excuses for unwanted pregnancies...