Two years ago while documenting a drive of Risso’s dolphins, I noticed an unusually-colored individual. It was mostly white with patches of gray pigment – a piebald. The dolphin wasn’t a true albino, due to the gray, but it was a rare capture, nonetheless.
By Jane J. Lee National Geographic
Believe it or not, how dolphins can swim so fast has been something of a riddle for researchers since the 1930s.
But a new study has laid to rest one of the most vexing questions plaguing scientists about dolphin speed: How can their muscles produce enough thrust for such high speeds?
“It’s been controversial for a while,” said Frank Fish, a marine biologist at West Chester University in Pennsylvania.
Now he has the answer: Bottlenose dolphins can produce the power they need to swim circles around whatever they wish by using their powerful tails, new experiments show.
The paradox began in 1936 with a British researcher named Sir James Gray, who conducted the first study on dolphin swimming, said Fish, a co-author of the study published online January 15 in the Journal of Experimental Biology.
Gray had observed a dolphin swimming around a ship at 33 feet (10 meters) per second for seven seconds, and wondered how the animal could move so quickly. (See National Geographic’s videos of dolphins and porpoises.)
Physics theory states that for something the size of a dolphin—and for the speed with which it travels through the ocean—water flow over the animal should be turbulent rather than smooth, Fish said. That turbulent flow creates a lot more drag that needs to be overcome than smooth flow does.
But when Gray input his variables into his equations and assumed a turbulent flow, “he found the animal didn’t have enough muscle mass to produce the power it needed to swim at that speed,” said Fish.
“This became Gray’s paradox,” Fish said—sparking a decades-long search for an explanation of how dolphins powered through the water.
Gray assumed that the dolphin must have been doing something to turn the turbulent flow over its body into a smooth flow. But scientists hadn’t been able to figure out how the mammals did it.
Bubbles + Lasers = A Solution
Part of the problem was that researchers weren’t able to directly measure the forces dolphins produce as they move through the water.
Obtaining that kind of data requires scientists to seed the water with visible particles, such as the tiny glass beads that are used in engineering experiments, explained Fish. Those beads are then illuminated with a sheet of laser light.
By filming how the illuminated beads move in reaction to an object moving through the water, experts can determine the forces generated.
But you can’t do this with a dolphin. Since it could injure the animal, “no one’s going to let you put little glass beads into a tank with a dolphin,” said Fish. And researchers certainly can’t shine potentially harmful laser beams at the mammals.
Wei had devised a bubble curtain to stand in place of the illuminated glass beads so that he could determine forces generated by human swimmers.
So Fish and colleagues created a bubble curtain at the University of California, Santa Cruz (UCSC), where they performed experiments with two captive bottlenose dolphins.
“Dolphins tend to be afraid of everything the first time they see something,” said study co-author Terrie Williams, a marine biologist who works with the animals at UCSC.
One of the dolphins seemed a little more skeptical of the bubble curtain than the other, but with some coaxing from trainers, the marine mammals soon got used to it.
“Once [the dolphins] got a feel for [the bubbles] on their skin, we were home free,” said Williams.
The results showed that a dolphin’s tail, or fluke, is more than capable of producing enough thrust to speed the mammal through the water. (Also see “DNA Discovery Reveals Surprising Dolphin Origins.”)
“The flukes are essentially wings,” said Fish. “[They] generate a lift force that is directed forward, on both the upstroke and downstroke.” This produces the thrust that pushes the dolphin through the water.
The flukes are also flexible, which is key to enabling the dolphin to maintain a highly efficient way of swimming over a broad range of speeds.
“The dolphin may have the ability to control that flexibility,” Fish explained. It could be that the fluke becomes stiffer the faster the dolphin swims, increasing its swimming efficiency at high speeds.
Or maybe the dolphins can actively control fluke stiffness by changing the tension of tendons in their tail, he said.
Fish isn’t sure how they’re doing it, but the marine biologist and colleagues are in the midst of trying to figure that out.
Either way, “we can abolish Gray’s paradox,” he said.
Mating between two distinct dolphin species created the clymene dolphin, a genetics study shows.
By Charles Q. Choi for National Geographic
Also known as the “short-snouted spinner dolphin,” the clymene dolphin (Stenella clymene) grows to nearly seven feet (2.1 meters) long and dwells in deep waters in tropical and temperate parts of the Atlantic Ocean.
Evolutionary biologists have seen other such hybrid species elsewhere in the animal kingdom. The new discovery, reported in the journal PLOS ONE by a team led by marine biologist Ana Amaral of Portugal’s University of Lisbon, adds to increasing evidence of such cross-breeding commonly leading to new species, even in the wide-open oceans.
Clymene dolphins feed mostly at night when squid and fish come to the surface of the water. The short-snouted dolphin gets its name from the ocean nymph Clymene of Greek mythology. (See “Dolphins Have ‘Names,’ Respond When Called.”)
Researchers initially thought the clymene dolphin was a subspecies of the spinner dolphin (Stenella longirostris). However, in 1981, a closer look at the clymene’s anatomy revealed it was a distinct species.
But experts remained uncertain about the clymene’s relationship with its close relatives. Although its outward appearance and behavior are more similar to those of the spinner dolphin, its skull features closely resemble those of the striped dolphin (Stenella coeruleoalba).
To help solve this mystery, the study scientists analyzed skin samples from 15 clymene dolphins, as well as from 21 spinner and 36 striped dolphins. They collected the DNA from free-ranging dolphins—using special tissue-collecting darts—and from dead, stranded dolphins.
The investigators looked at nuclear DNA, which is found in the cell’s nucleus and comes from both the mother and father, as well as DNA from their mitochondria—the cell’s powerhouse—which possesses its own genes and is passed down solely from the mother.
“When I was first analyzing the data I collected, it was very confusing,” said Amaral.
That’s because Amaral and colleagues discovered that while the nuclear DNA of the clymene dolphin most resembled that of the spinner dolphin, the mitochondrial DNA was most similar to that of the striped dolphin. (See pictures of other hybrid animals.)
This is strong evidence that the clymene dolphin is a naturally occurring hybrid of the spinner and striped dolphins, said Amaral. Continued hybridization may still occur between the clymene dolphin’s parent species, although at low levels, the study authors added.
The birth of a new species, known as speciation, is often thought to happen when one species splits into two or more isolated populations that diverge as they amass differences over time due to a lack of interbreeding.
Hybrids are usually infertile, with the mule—a cross between a male donkey and a female horse—being the most familiar example of this.
Hybrid Species: How Rare?
Past studies have shown that hybridization could occasionally lead to fertile offspring and even new species in plants, fishes, insects, and birds.
To get a hybrid species, two things need to occur, said evolutionary ecologist Pamela Willis at the University of Victoria in Canada, who did not take part in the study.
“You need to have hybrids be as fit as the parental species, able to carve out their own ecological space,” she explained.
Then they somehow have to mate with only each other, rather than with either parental species, “hence allowing them to spin off onto their own, independent evolutionary trajectory and become a species of their own,” Willis added. “Both of these conditions are hard to meet.”
Scientists had thought hybrid speciation was exceptionally rare in mammals. “Mammals generally are less capable than other types of animals to produce healthy, fertile hybrids,” Willis said.
Still, hybrids were not unheard of in cetaceans such as whales and dolphins—both in captivity and in the wild. Since cetaceans have very similar numbers of chromosomes across species, researchers had speculated they could produce viable hybrids more easily than other mammals.
“Ironically, one translation of clymene can mean notorious or infamous, and now this dolphin turns out it’s living up to its name by being the first marine mammal known to arise through hybrid speciation,” said study co-author Howard Rosenbaum, a marine biologist at the Wildlife Conservation Society and American Museum of Natural History in New York.
Future research will analyze the DNA of these dolphins in greater detail to help deduce how long ago the clymene dolphin arose, Amaral says.
This study “adds to an ever-increasing amount of recent research that indicates that hybridization is a common and important part of animal evolution, facilitating the formation of new species,” Willis said.
“Traditionally, biologists have viewed hybridization as rare and either insignificant, evolution-wise, or serving only to meld species together into one,” she said. “We’re undergoing a paradigm shift in recognizing the creative role hybridization plays in contributing to animal evolution and diversity.”
“Dolphins could help us better understand this rare form of speciation,” added Rosenbaum. “We hope this work highlights the importance of conserving dolphins.”
Two species of dolphin can be indentified from one another by analysing the clicking sounds they make, new research suggests.
Previously, experts have distinguished white-beaked dolphins from Risso’s dolphins by their whistles.
Hebridean Whale and Dolphin Trust used a technique called passive acoustic monitoring to record the clicks.
Scottish Natural Heritage said click studies could be an additional aid to indentifying the species.
HWDT correctly identified 90% of its encounters with white-beaked dolphins and 100% of Risso’s from their clicks.
SNH has also suggested that further research could be done into how clicks might identify different groups of the animals.
An estimated 80% of the European population of white-beaked dolphins are located in the waters off Scotland and north-east England.
SNH said little was known about the distribution of Risso’s dolphin in UK waters or about population structure.
It added that it has been proposed that Risso’s dolphin were present off the UK coast all year round.
Five newborn Risso’s dolphins were spotted off the Isle of Man in September.
The Manx Whale and Dolphin Watch said the sightings highlighted the importance of the island’s waters as a breeding area for the species.
Credits: BBC News
A new species of humpback dolphin is found cruising the ocean.
Jane J. Lee National Geographic
Hiding in plain sight, researchers have discovered a new species of humpback dolphin living off the northern coast of Australia.
The discovery came when scientists with the Wildlife Conservation Society (WCS) tried to settle a decades-old argument among marine mammal researchers.
“For many years, there’s been this debate about the number of species of humpback dolphins,” said Howard Rosenbaum, director of the WCS ocean giants program. Scientists have proposed everything from two to four species within the group’s genus Sousa.
But there was never enough good evidence supporting claims of more than two species, Rosenbaum said. So about ten years ago, the community decided that until they had more information, they’d recognize only two species—the Atlantic humpback dolphin and the Indo-Pacific humpback dolphin.
Rosenbaum and colleagues decided to revisit this old argument, and started collecting physical and genetic samples from humpback dolphin populations throughout their range. This included samples from West Africa, the Indian Ocean, the Pacific Ocean, and off the coast of Australia.
“From a management standpoint, the marine mammal community has specified that they need at least two different forms of evidence to justify different species [designations],” said Rosenbaum.
So he and his colleagues tried to collect as comprehensive a data set as possible to get the best chance of putting this argument to rest. Usually, genetic analyses into the question of new species consider only DNA from an organism’s mitochondria—the cell’s battery pack.
This is because mitochondrial DNA is inherited only through the mother and is easier to work with than DNA from a cell’s nucleus, said Martin Mendez, assistant director for the Latin American and Caribbean program at WCS.
But Mendez and colleagues looked at DNA from both parts of the cell. That, combined with physical characteristics including the length of the dolphins’ beaks and the number and position of their teeth, suggested there were four species of humpback dolphin. Not two.
Three of those species were ones researchers had previously proposed. They encompass a species off of West Africa (S. teuszii), one in the central and west Indian Ocean (S. plumbea), and one in the eastern Indian and west Pacific Oceans (S. chinensis).
The fourth species, an as-yet unnamed group off the north coast of Australia, was a pleasant surprise, said Rosenbaum. (Related: “From Darth Vader to Jelly Doughnuts, Weird Species Names Abound.”)
In some ways, this species is new to science, said Mendez. But in other ways, it isn’t because researchers have known about this group down in Australia for a while. They just didn’t realize it was a different species.
It’s rare to find a new species of mammal, said Mendez. “[But] it’s also not crazy to find new species when you’re using the kind of [genetic] information we’re using.
“One of the reasons we’re finding new species is because we’re finding new tools,” he explained. “Genetics opens a new window into these kinds of questions.”
Mendez is hopeful that this discovery—reported this week in the journal Molecular Ecology—will help in the management of this IUCN Red List group. The Atlantic humpback dolphin is considered vulnerable, and the Indo-Pacific group is considered near threatened.
The legal framework used to protect vulnerable species is based on species designations, he explained. “We’re proposing that Australia has its own humpback [dolphin] species, which has implications for conservation strategies.”
“Countless dolphins die every year as bycatch in fisheries,” said Rosenbaum. The humpback dolphin is subject to particularly high rates of bycatch, and in some places is hunted directly.
“By describing these different species, we hope that this sets the stage not only for the appropriate conservation protections to be put in place by different countries, [but that] it also helps reduce threats like bycatch.”
Credits: National Geographic
Environmental degradation might be amplifying the effects of a measleslike virus, fueling infections that are propelling an alarming death count
VIRGINIA BEACH—Strolling along private beaches nearby, where waves lap against shores dotted with summer homes and volleyball nets, it’s easy to forget that the ocean has given up more than 250 dead bottlenose dolphins this summer. Sometimes dolphins wash up alive, emaciated and laboring with their final breaths. Some are missing small chunks of their fins—evidence that a shark took an exploratory bite when the animal was slowed by disease or after its organs gave out. Often the bodies are not in a state for scientists to know the difference.
The Virginia coastline hasn’t been the only place where dolphins are washing ashore, but it’s been hit the hardest. From New York State to North Carolina, reports of hundreds of dolphin deaths have accrued this past summer. So alarming was the toll that the National Oceanic and Atmospheric Administration (NOAA) declared the die-offs to be an “unusual mortality event” and released federal funds to shore up an investigation. The likely culprit, NOAA says, is an RNA virus related to measles. But the high death count and presence of myriad secondary infections have led some researchers to suspect a wider problem—namely, a coastal ecosystem possibly sickened by human activity.
Clues in the past
Twenty-five years ago morbillivirus ravaged the coastal bottlenose dolphins, wiping out some 50 percent of their estimated population by spring 1988. Scientists already fear what the loss may be this time, because 553 dolphins have stranded just in the past three months. The death toll for the same period in the 1987 die-off was only between 300 and 400 dolphins, climbing to 742 by the spring of the next year.
Scientists have no treatment for dolphins infected with the morbillivirus, an RNA virus similar to those that cause measles in humans, distemper in dogs and rinderpest in cattle. Nevertheless, they want to track the deaths and identify their causes because dolphins, atop the food chain, help serve as a barometer of ocean health.
In years past dolphins had apparently built up some level of antibody defense against the virus so that contracting it did not always lead to death. That was evident in the low mortality counts: In the past six years the average number of yearly strandings from New York to North Carolina was less than 160. Since July, however, more than 500 coastal dolphins have died in that same area. In Virginia, where the annual death toll for dolphins hovered below 80 for the past decade, the surge has reached 286 deaths. Researchers anticipate that, like the last die-off, this number will continue to rise through next spring.
What makes the disease so lethal this year is an enduring mystery. Researchers suspect coastal dolphins caught the virus from offshore populations this year and the former were unable to mount a strong response to this viral strain. Viruses continually mutate, swapping and rearranging genes, so the explosion of the virus could just be nature at work.
On the other hand, pathologists have noticed another factor that could be a handmaiden in the killings: a plethora of secondary infections by fungi, bacteria and parasites. So far, it’s unclear whether these infections could have been potent enough to kill the dolphins on their own. Their presence, however, has left some researchers wondering if humans are to blame—specifically, have poor environmental conditions fueled by agricultural runoff and other human activities made dolphins unable to weather the diseases?
The theory goes that some dolphins encountering morbillivirus would have been able to recover from the infection, but those secondary infections preyed on the vulnerable ones, finishing them off. Biotoxins that dolphins had accumulated in their blubber may also have been released as the weakened mammals broke down their fat for sustenance, flooding their systems with toxins that hamper an immune response.
Credits: Scientific American