Since the 1970s, scientists in the Puget Sound region have been studying the differences between two types of killer whales: the so-called residents, which eat salmon, and the Bigg’s or transient killer whales, which eat seals, sea lions and other marine mammals. Even though the two groups of whales have different behaviors, vocal calls and social structures — along with distinct body styles and genetic traits — the two types are still considered a single species, Orcinus orca.
In fact, killer whales in every ocean of the world are grouped together as a single species. Until now, scientific discussions and debates have yet to resolve whether any orca population or group is different enough to be declared its own species.
This single-species convention could soon undergo a decisive change, thanks to advanced genetic techniques used to discern evolutionary patterns. Following years of study and consideration, scientists with the National Oceanic and Atmospheric Administration have proposed two new species of killer whales, Orcinus ater for residents and Orcinus rectipinnus for Bigg’s.
The new names were suggested in a peer-reviewed article published this week in the journal Royal Society Open Science. Next, the proposal will be reviewed by the Society for Marine Mammalogy’s Committee on Taxonomy. If approved by the committee and generally accepted among experts, these new names could launch a worldwide effort to name other species of killer whales under specific criteria, said Phillip Morin, a molecular geneticist at NOAA’s Southwest Fisheries Science Center who has been working on killer whale genetics for more than 20 years.
“This has been one of the most difficult projects I’ve ever done,” said Morin, the paper’s lead author. He noted that the evolutionary origins of Bigg’s and residents has been slowly revealed with the advance of genetic techniques, which can now spell out the entire genome of killer whales. That includes genetic material called mitochondrial DNA, which tells a special story, because this DNA is passed down only from mothers, generation after generation.
Aerial images comparing the sizes of adult male Bigg’s and resident killer whales, both taken in the Salish Sea off southern Vancouver Island and scaled to lengths. Images were collected using a non-invasive drone authorized by research permit 19091 issued by the National Marine Fisheries Service. Photos: John Durban and Holly Fearnbach/SR3 SeaLife Response, Rehabilitation, and Research used with permission
Why new species?
According to Morin, the latest genetics work serves to cement the idea that resident and Bigg’s killer whales should be considered entirely separate species — not just sub-species, the lowest level of division in a taxonomic system in use for nearly 300 years. In addition, traditional lines of evidence, such as geographic distribution, coloration, body size and shape, along with behavior, all support the idea of separate species for the two groups of whales, he said.
Surprising to some observers are the distinct differences between Bigg’s and resident killer whales, which do not interbreed even though they share the same waters. The range of both proposed species spans the West Coast of North America from California to Alaska, across to Asia, along the coast of Russia and south into Northern Japan. One difference is that Bigg’s also occupy waters farther north, even into the Arctic Ocean.
Expected range for Bigg’s (larger area in pink) and resident (narrower area in blue) killer whales. Maps: NOAA; whale illustrations: Uko Gorter, used with permission
Despite sharing similar areas, the two types of whales never interact, even avoiding each other on sight. Based on genetic evidence, Bigg’s killer whales became a separate group between 200,000 and 300,000 years ago, while residents separated from the original killer whale population about 100,000 years ago.
Understanding the relationships among all killer whales in the world could help humans protect the genetic diversity represented by individual species, subspecies and populations, Morin said. Southern resident killer whales, which frequent Puget Sound, have been defined by NOAA as a “distinct population segment” with unique characteristics. As such, the 74 whales that make up this population are considered at high risk of extinction, and they have been listed as endangered under the federal Endangered Species Act.
On the other hand, the International Union for Conservation of Nature, a worldwide environmental network, takes a broader perspective. Its “Red List” of threatened animals operates at the species level. If multiple species of killer whales are identified, then more public attention could be brought to bear on genetically distinct groups at risk of being lost, Morin said.
“I believe there will be management issues, not for the southern residents which already have a management unit,” he explained, “but what other management units might there be?”
One group of orcas, known as “offshores,” are not well known because they stay in the open Pacific Ocean for the most part. Based on limited studies, they seem to eat fish, particularly sharks and rays. Genetically, offshores are more closely related to residents than to Bigg’s, but experts say more information is needed to decide if they should be considered their own species.
In the Antarctic, a unique group of bulbous-headed killer whales, known as Type D orcas, appears to be small in number and highly inbred, according to genetic studies. This could make them a candidate when considering new species around the world.
(Left) Type D killer whales from South Georgia Island in the South Atlantic. These distinctive whales, which are small in number and highly inbred, may be a candidate for a new species. (Right) Comparison of a more common adult male killer whale (top) and the rare Type D killer whale (bottom) showing a tiny eye patch, more rounded head, and more narrow, pointed dorsal fin. Photo: J.P. Sylvestre; illustrations Uko Gorter
Criteria for deciding when a population is different enough to become its own species have been evolving. They now include genetic as well as physical differences, although rules for defining a species remain somewhat flexible.
As proposed at this time by Morin and colleagues, only the Bigg’s and residents would become new species, Orcinus rectipinnus and Orcinus ater, respectively, based on names suggested in 1869. At that time, a whaling captain described what he thought were two different types of orcas in the Pacific Ocean — although his observations are somewhat confusing. All the other known groups will remain Orcinus orca until their unique characteristics — especially genetics but also physical and otherwise — are better understood.
Movement for change
In April 2004, 48 marine mammal experts from six countries gathered in La Jolla, Calif., to discuss the complex relationships among whales, dolphins, and porpoises, collectively known as cetaceans in the taxonomic system. Consideration of killer whales received maximum attention because of the pending legal status of the southern residents in the Northwest.
Just two years before, in 2002, NOAA Fisheries had declined to list the southern residents as threatened or endangered under the Endangered Species Act, even though agency officials acknowledged that the population was failing and could be in crisis. Since all the killer whales in the world were considered a single species, available data could not prove that the southern residents were “significant” — a key test for listing under the ESA.
For the next 20 years, Morin and many other researchers worked to develop the genetic rationale and mathematical formulas to describe the genetic relationships among orcas.
Supporters of the ESA listing took the case to court. By the end of 2003, a federal judge had ruled that the agency must reconsider its decision about whether southern residents qualified for listing. As marine mammal experts gathered for the La Jolla workshop, they confronted questions about fitting the various groups of whales into a taxonomic framework with real-world implications. They also wrestled with fundamental concepts, such as how to define “species” in the context of modern genetics.
A southern resident killer whale (J16) with a juvenile (J50) swimming alongside under water. Photo: Copyright Melisa Pinnow, used with permission
“The killer whale in the northeastern Pacific provides an example in which, by some points of view, taxonomy is inappropriate and inadequate for meeting conservation needs,” states a report from the workshop. “Not only is there genuine uncertainty about the taxonomy of killer whales within the region, but also data from all over the globe were thought to be needed before this could be resolved, and it was not possible to obtain such data in a timely way.”
Morin, author of the new paper, attended the workshop and was inspired to search for ways to use genetics to help define independent species. He had just taken a new job a year earlier as a molecular geneticist at NOAA’s Southwest Fisheries Science Center. For the next 20 years, Morin and many other researchers worked to develop the genetic rationale and mathematical formulas to describe the genetic relationships among orcas. They were able to show that Bigg’s and resident killer whales are on independent, evolutionary paths that are diverging. It is a system that he believes could work for defining other species of killer whales or even other cetacean species.
Fortunately for the southern residents, it was not necessary that they be declared a species to gain special protections from the Endangered Species Act. The law allows ESA listings for “distinct population segments” at risk of extinction. During the court-ordered reconsideration, NOAA officials looked at the evidence anew and decided that the orcas were distinct and significant and should be declared “endangered.” They concluded that if this unique population were to go extinct, it would leave a significant hole in the ecosystem.
Following a formal process of review, the southern residents became officially listed in 2005. Still, when it comes to species designations, all the killer whales in the world were still the same. Whether and how to organize the many groups of orcas into separate species remained an open question until this new proposal by Morin and associates.
Killer whale nomenclature
The modern taxonomic system that classifies all living things was first presented in a 1758 publication titled “Systema Naturae” by Swedish naturalist Carl Linnaeus, who became known as the father of taxonomy. The original list of about 10,000 species has multiplied many times over with the addition of new species. The scientific world has embraced the two-part scientific name, with genus followed by species. For example, the scientific name for killer whales is “Orcinus” (genus) “orca” (species).
From the top, the taxonomic system represents orcas as members of the Kingdom “Animalia” (all the animals), Phylum Chordata (mostly animals with backbones), Class Mammals (warm-blooded creatures), Order Cetacea (whales, dolphins and porpoises), and Family Delphinidae (dolphins). Today, only one species, orca, occupies the genus Orcinus, but that would change if the two newly proposed species are added. (More recently, Cetacea has been placed within the larger order Artiodactyla, which includes pigs, deer and hippos.)
...residents find fish through echolocation and communicate a great deal through vocalizations. Transients find their prey by listening, and they remain generally quiet so as not to alert marine mammals to their presence.
On his original list, Linnaeus used the term Delphinus orca. This may have been a reference to the bottlenose dolphin, based on some earlier drawings, according to Tom Jefferson, a taxonomic expert affiliated with NOAA who contributed to today’s naming proposal. Any confusion was settled in 1768, when Johan Ernst Gunnerous, a Norwegian biologist, revised the list, settling on Delphinus orca for the killer whale and Delphinus tursio for the dolphin (Aquatic Mammals, September 2023).
A close relationship between killer whales and bottlenose dolphins was perceived even in those early years of taxonomy. Confirmation came years later through molecular genetics —although the names were changed along the way. Today, the genus Delphinus refers to the common dolphin, while the genus Tursiops goes to the bottlenose dolphin.
By the end of the 1800s, at least 23 new species names had emerged for killer whales in different parts of the world, often in concert with purported evidence such as skulls or roughly drawn illustrations. By the 1850s, the genus Delphinus (Latin for dolphin) gave way to the genus Orca (Latin for large-bellied pot) when describing most species of killer whales, but even that genus name would not last.
In 1963, marine mammal experts Victor Scheffer and Dale Rice, both working for the U.S. Fish and Wildlife Service, published “A List of the Marine Mammals of the World.” The 20-page list described Orcinus orca as a single species of killer whale throughout the world. In 1998, Rice published a 231-page book, “Marine Mammals of the World” (PDF) in which he explained that distinguishing characteristics among many of the so-called species appeared to be inconsistent and confusing. While genetic analysis was a promising taxonomic tool, it was not yet capable of resolving critical questions at the species level, he said.
“Each pod of killer whales, or local group of pods, is largely endogamous and differs in minor ways from neighboring groups in both morphology and genetics, as well as in traditions such as migratory behavior, prey choice, and dialects,” Rice wrote, noting that such differences may or may not represent separate species, but more proof was needed to separate potential species.
In 1992, marine biologist Robin Baird, who was with Simon Fraser University at the time, laid out the case for declaring separate species for transient (Bigg’s) and resident killer whales without using molecular genetics. Among other evidence, he argued that specialized food choices of the two groups — one eating marine mammals, the other fish — led to specialized behaviors and increased hunting success for both groups. For example, residents find fish through echolocation and communicate a great deal through vocalizations. Transients find their prey by listening, and they remain generally quiet so as not to alert marine mammals to their presence. These hunting methods tend to conflict with each other, so the two groups moved apart, leading to divergent populations growing into separate species. (Oecologia, 1992)
The idea that the two types of killer whales first evolved in the same area remains unproven genetically, according to some researchers who say it is also possible that the two types were physically separated hundreds of thousands of years ago during major geological changes in the Northern Hemisphere. (Heredity, 2015)
Scientists at the 2004 workshop in La Jolla recognized that species are groups of organisms that are distinct — genetically and physically — from other groups. The differences are the result of divergent ecological pathways. While physical and genetic differences can be examined separately, both concepts should be considered when determining species, the group concluded.
“Given the difficulties of knowing the degree to which geographical distribution and behavior actually reflect genetic divergence, these kinds of data should not be the primary basis of species delimitations, but can serve as useful lines of evidence,” according to a summary of the findings.
In 2008, the IUCN listed the worldwide population of killer whales as “data deficient” while noting that some groups, if defined as species, might meet the organization’s “Red List” criteria for threatened species.
“Experts agree that the present taxon likely includes more than one subspecies, and possibly multiple species,” according to an assessment statement. “Although considerable effort continues to be made on improved understanding of the taxonomy of the genus Orcinus, the taxonomic issues have not been fully resolved. This is especially problematic due to the occurrence of sympatric, non-interbreeding ecotypes in the Eastern North Pacific, Antarctic, and possibly elsewhere.”
Since 2010, the Society for Marine Mammalogy’s Committee on Taxonomy has been regarded as the primary arbiter on changes in marine mammal taxonomy, including new species and names. But there is no real authority, according to Jefferson, an independent researcher who serves on the committee.
“That’s one thing about taxonomy; there is no supreme court,” he said. “Biologists are free to use whatever taxonomy they want. We just put together a list of what we think is the most correct taxonomy and review it every six months or a year, as needed.”
The committee started with the list of marine mammals reported by Rice in 1998. Changes are based on new research and analyses. Because Jefferson was involved in today’s proposal for the new killer whale species, he must recuse himself when the committee deliberates on the proposed additions to the taxonomy.
Observations of a whaling captain
Among the names submitted during the 1700s and 1800s for new killer whale species throughout the world, two species were proposed for the West Coast of North America, based upon the observations of Charles Scammon, a whaling captain who saw a scientific need to describe and sort the multitude of marine mammals in the Pacific Ocean.
“Being on the coast of California in 1852, when the ‘gold-fever’ raged, the force of circumstances compelled me to take command of a brig bound on a sealing, sea-elephant and whaling voyage, or abandon sea life, at least temporarily,” he wrote in the preface to his book “The Marine Mammals of the Northwestern Coast of North America.” “The objects of our pursuit were found in great numbers, and the opportunities for studying their habits were so good, that I became greatly interested in collecting facts bearing upon the natural history of these animals.”
Despite years of observations by many knowledgeable whaling captains, “there was no one who had contributed anything of importance to the natural history of the Cetaceans,” he said, explaining why he chose to write detailed descriptions for more than 30 whales, dolphins and pinnipeds, including seals, sea lions and walruses.
Whaling captain Charles Scammon (left) and illustrations from his field notes and sketches of Orcinus ater (center) and Orcinus rectipinnus (right) in the 19th century. Photos: Public domain
Although Scammon identified two types of orcas in the Pacific Ocean, he did not provide the kind of details needed to distinguish one from the other, according to the authors of the new paper. He seemed to think that the size of the dorsal fin was a defining characteristic, referring to whales with a tall dorsal fin as Orca rectipinna and those with a shorter dorsal fin as Orca ater. But experts now know that the taller dorsal fins generally belong to male orcas while females have shorter fins.
“Scammon obviously did not understand this sexual dimorphism and thought that both sexes were present in both of these ‘species,’” the paper states. “Scammon called them ‘high- and low-finned orcas.’”
Scammon also seemed to believe that the two types of killer whales he encountered were consuming both marine mammals and fish, depending on what the whales found available. In Latin, “rectipinnus” means “straight” and “ater” means “black.”
Actual specimens, such as bones or at least careful drawings or photographs, can be used to define new species. This kind of permanent representation is known as a holotype. Although bones were often collected and used by biologists for describing new species, holotypes were not considered essential for taxonomic consideration until the late 1800s, according to Jefferson.
Scammon had collected the skull of an orca, associated with “rectipinna,” which he presented to the California Academy of Sciences in San Francisco, Jefferson said. The skull remained in the archive until it was destroyed in 1906 during the massive San Francisco earthquake and subsequent fire that killed 3,000 people and left the city in ruins.
Scammon also may have had a jawbone from the other “species” of killer whale among his personal possessions, Jefferson said, but that specimen appears to be lost.
Edward Cope, a zoologist and renowned paleontologist, reportedly used descriptions from Scammon’s unpublished book manuscript to write a scientific paper about the new species, published in the Proceedings of the Academy of Natural Sciences (PDF) in 1869.Although Scammon’s colorful written accounts were not much help to taxonomists, they do say something about his perception of the animals.
“Until recently,” Scammon wrote in his 1874 book, “we were under the impression that the short-finned Killers upon the western coast of North America were inhabitants especially of the frosty regions; but recent observations prove that they frequent both the high and low latitudes. Indeed, they may be regarded as marine beasts that roam over every ocean; entering bays and lagoons, where they spread terror and death among the mammoth balaenas (whales) and the smaller species of dolphins, as well as pursuing the seal and walrus, devouring, in their marauding expeditions up swift rivers, numberless salmon or other large fishes that may come in their way.”
Despite the apparent ambiguity, authors of the new paper gleaned enough information from Scammon’s accounts to align the resident killer whale with the name “ater” and the Bigg’s killer whale with the name “rectipinnus.” Because no holotypes are available, they designated a representative “neotype” for each of the proposed species. The specimen for Orcinus ater is the skull of a male collected from Yukon Harbor in Puget Sound. The specimen for Orcinus rectipinnus is the skull of a male collected near San Francisco. Both skulls can be found at the National Museum of Natural History in Washington, D.C.
Specimens from the Smithsonian Museum of Natural History selected to represent skulls for A) Orcinus rectipinnus (USNM 594671) and 1331 B) Orcinus ater (USNM 594672). Photos: Eric Archer/NOAA
Common names
In addition to their scientific names, the new paper proposes to adopt the common name “Bigg’s killer whale” for Orcinus rectipinnus. This would be in honor of the late Michael Bigg, a Canadian researcher who was among the first to document the true differences between the two types of killer whales that frequent the Salish Sea. Bigg called them residents and transients. At the time, during the early ‘70s, the fish eaters were more common in the inland waterways, and transients were observed only on rare occasions.
Those conditions no longer hold true, according to John Ford, a Canadian killer whale expert who has been promoting the common name “Bigg’s killer whales” as a replacement for “transient.” He noted that Bigg’s have become more resident — or at least more prevalent in inland waters — than the “residents” are. This turn of events is the likely result of more successful hunting in inland waters by Bigg’s orcas, particularly for harbor seals, which have seen a resurgence after their killing was banned around 1970, he said. At the same time, the resident orcas have been spending more time on the outer coast, probably because of declining salmon runs in the inland streams.
Michael Bigg’s contributions to early killer whale research in the Salish Sea are enormous, said Ford, noting how this careful researcher was the first to identify each individual whale by its unique markings, opening the door to a fuller understanding of their family structure and individual behaviors.
In the Spring 2011 issue of “Whalewatcher” magazine, Ford wrote that the use of the name “Bigg’s killer whales” was catching on among researchers as evidence grows in support of two new orca species.
“Of all the interesting facets of killer whale life history, Mike was particularly fascinated by the relationship between ‘residents’ and ‘transients,’” Ford wrote. “The notion that two different forms of killer whale could coexist in social and reproductive isolation, each with its own distinct diet and lifestyle to match, was without precedent and hard to explain. How could this situation have evolved and how was it maintained? Mike pondered such questions at length, discussing ideas with colleagues and writing copious notes summarizing his thoughts. Sadly, Mike was never able to write up his studies on transient killer whales. He died of leukemia in 1990, at the age of 51.
“The body of evidence that transient killer whales represent a distinct species from other killer whale lines is becoming compelling,” Ford wrote in 2011. “Although it may take some time before this is resolved and a new species is formally proposed, there is a growing movement among killer whale researchers that transient killer whales be called ‘Bigg’s Killer Whale.’ This would indeed be a fitting way of honoring the memory of this remarkable pioneer of killer whale science.”
The new paper published today not only supports the name “Bigg’s” for the one group, it also proposes replacing “resident” with a more appropriate name for the other group.
“We are planning on consulting with North American Indigenous tribal groups, and expect to eventually have a consensus common name,” the paper states, “but in the meantime, we suggest continued use of ‘resident killer whale’ so as to maintain consistency.”
Evidence for new species
To determine taxonomically if Bigg’s and resident killer whales should be considered separate species, Morin and his fellow researchers considered a variety of differences between these groups, both physical and genetic. Key questions include whether the two groups are genetically distinct, whether breeding takes place outside either group, and whether each group is evolving along a path that further separates it from other killer whales. Another major test is “diagnosability” — whether a single orca can be assigned its rightful place in the taxonomy structure based on features defined for that species.
Because both Bigg’s and residents travel in the same waters, they have the opportunity to interbreed. But careful studies over more than 50 years show that the two groups never interact and will even take measures to avoid each other, providing evidence that they are reproductively isolated.
Other differences: Residents often travel in large groups, while Bigg’s maintain smaller groups. Residents eat fish, while Bigg’s eat marine mammals. And, while both types seem to communicate with whistles and clicks, marine mammal experts can easily tell them apart by their distinctive sounds, including how often and how long each type makes their calls.
It appears that Bigg’s and residents have developed different physical traits based on their food choices, including the shape of their skulls and the curvature of their jaws. Biggs of both sexes tend to be longer and more robust than residents. Other differences include the size and shape of their dorsal fins and the white “eye patch” on their heads. Experienced observers can often tell them apart at first glance.
Genetically, it has been a challenge to distinguish Bigg’s from residents, even though many DNA samples were available. Early techniques were limited to analyzing relatively small portions of DNA used as genetic markers. While one could tell sperm whales from killer whales, the killer whales were too closely related to conclusively tell them apart — even if they were genetically distinct.
Advances in genetic techniques and lab equipment allowed for rapid gene sequencing in both nuclear DNA, inherited from both mother and father, and mitochondrial DNA, inherited only from the mother. (Mitochondria are double-membrane organelles within each cell that contain the biochemical machinery for producing energy. They also contain a single circular chromosome.)
In recent years, sequencing entire nuclear and mitochondrial genomes has allowed for comparisons between individuals and populations when even slight differences are present. Guidelines and quantitative genetic standards (PDF) for designating species and subspecies were developed in 2017 by geneticist Barbara Taylor at NOAA’s Southwest Fisheries Science Center along with other researchers.
As analyzed by Morin, these guidelines would confer the status of subspecies, but not species, upon residents and Bigg’s killer whales. But the developers of the guidelines agreed that standards should not be applied rigidly, particularly when population sizes are small or when culturally driven conditions could be accelerating evolution and genetic separation into distinct species.
These are the exact conditions for resident and Bigg’s killer whales, said Morin, who used a variety of genetic techniques for comparison and found “relatively large and significant differentiation” for the two groups of whales. Furthermore, regarding diagnosability, he said genetic information alone could be used to assign individual whales to their own communities with relative certainty. And with genetic information available for a good portion of the whales, the researchers could find no evidence of recent genetic mixing.
“These combined lines of evidence, taken together, support divergent evolutionary trajectories consistent with species,” the paper states.
While physical and acoustic differences allow trained observers to easily tell them apart, that does not mean they are different species. However, evolutionary changes in the skull and jaw, likely brought on by differences in diet, provide strong evidence that these groups are already on independent evolutionary paths and should be considered separate species.
Robin Baird, now with Cascadia Research Collective, called the long-awaited argument for new species “convincing.”
“They’ve done a good job putting together the evidence and comparing it to the criteria,” he said. “I have little doubt it will be accepted by the scientific community.”
Morin’s proposal mostly addresses just the two groups of killer whales, each of which he considers a separate species, and he acknowledges that this leaves a lot of orcas in the world in a sort of limbo. How many more species are there? New genetic techniques and guidelines for identifying different species could be put to use in resolving those questions, he said.
“This issue will continue to evolve,” Morin added, “because we are going to learn more about the taxonomy of killer whales. It’s no longer enough to just say that this group is different.”
Story text available under a Creative Commons Attribution-NonCommercial-NoDerivs license: CC BY-NC-ND 4.0.
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