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."