About the author
Emma Luck is an Alaska-based marine scientist and science communicator with experience working with killer whales and other marine mammals in Alaska and Norway. She enjoys communicating with the public and uses her photos, illustrations, and background in biology to highlight research on marine mammals, particularly killer whales, through various social media channels. Follow Emma on Twitter @emmaluck22 or Instagram @northern.naturalist.
Killer whales, also known as orcas, are one of the most widespread mammals on the planet, being nudged out of the top spots only by humans and perhaps the Norwegian rat (Reeves et al.2017). They are also one of the most recognizable marine mammal species and have been subject to numerous scientific inquiries over the last fifty years. Despite interest from scientists and the public, there are still many killer whale populations in the world that remain understudied, particularly those that inhabit remote regions of the planet. A new study published in the Journal of Heredity, led by Andrew Foote and his team of coauthors (Foote et al. 2023), is providing the first detailed glimpses into the genetic background of poorly known killer whales called type Ds.
In the 1970s, American and Canadian researchers began studying killer whales off the west coast of North America. They eventually discovered that there were actually two types of killer whales in the waters of the Pacific Northwest—the gregarious, fish-eating killer whales that traveled in large groups and a stealthy, mammal-eating form that only occasionally passed through the area. These two types, dubbed residents and transients, also had noticeable differences in size, shape, and coloration patterns (Bigg 1982). As killer whale research expanded in other parts of the world, scientists uncovered more “ecotypes,” unique forms of killer whales that differ in appearance, behavior, and genetics (though, notably, they are still classified as the same species).
A New Kind of Killer Whale
In 1955, before killer whale ecotypes were on anybody’s radar, a group of strange looking killer whales stranded on a beach in Paraparaumu, New Zealand. These animals were unlike any other killer whales seen before. They had large, bulbous foreheads, bearing more of a resemblance to pilot whales than other killer whales. Their dorsal fins were narrow, backswept, and sharply pointed. The most notable feature about these whales were their extremely small eyepatches—so small that they almost appeared to be absent on some individuals. Photographers documented the whales and two skulls were saved at the Museum of New Zealand Te Papa Tongarewa.
These killer whales retained a near-mythical status for several decades following this initial stranding. Were the seventeen whales found on the New Zealand beach just a family of morphological oddities? Or did they represent an unknown branch of the killer whale family tree? In 2005, photos were shared with marine mammal biologist Robert Pitman of some odd-looking killer whales seen taking fish off of long lines in the Crozet Islands (Pitman et al. 2020). He immediately recognized them as the same type of killer whale that had stranded in New Zealand fifty years earlier. That confirmed it—these were definitely a unique form of a killer whale. After reviewing a collection of photos of these killer whales taken from tour ships transiting to and from Antarctica, Pitman and his colleagues went on to write up a description of this new type of killer whale, naming it the “type D,” or “subantarctic” killer whale (Pitman et al.2011). It joined types A, B1, B2, and C on the existing list of southern hemisphere killer whale ecotypes.
Out in the Field
The limited information on type D killer whales comes primarily from opportunistic encounters, mostly from fishermen and Antarctic tourists. Their circumpolar distribution in the offshore waters of the Southern Ocean make these whales tricky to locate and organizing a dedicated research expedition to find them was going to be a challenge. Thanks to reports from commercial fishermen in Chile that type D killer whales were regularly taking fish off of their lines, Pitman was able to narrow down a region where scientists might have a decent chance of encountering these enigmatic whales: Cape Horn, Chile. An international team of researchers was assembled, including John Totterdell and Rebecca Wellard of Australia, Jared Towers of Canada, Robert Pitman and Lisa Ballance of the United States, and Mariano Sironi of Argentina. They were able to charter a small research vessel for a period of three weeks in January 2019 to seek out type D killer whales. The weather in this region of the world is particularly treacherous, and it became clear why type D killer whales had evaded scientists for so long. When you live in the most inhospitable waters on the planet, it is exceptionally easy to avoid being seen by curious human eyes. Nevertheless, the team caught a lucky break, and during a lull in a storm, they managed to find a group of approximately thirty type D killer whales. Even more remarkable, the whales were friendly! Their close approaches to the research ship allowed the scientists to collect biopsy samples—the top priority on this trip—from three different whales and they even obtained the first underwater footage and audio recordings of the type.
Analyzing the Samples
After some pandemic-related delays, the first fresh genetic samples from type D killer whales were being processed in the lab. The team also wanted to revisit genetic material from skulls collected in 1955 from the original stranding of type D killer whales in New Zealand. Samples from these skulls had been analyzed once before in a 2013 study (Foote et al. 2013), but they would be useful for comparison with the new samples from Chile. The New Zealand whales likely represent a different family group than the whales sampled off Chile. Since the three sampled Chilean whales are relatives and share similar genetic backgrounds, comparing them to the New Zealand whales, which were from a different region and time period, would offer broader insight into the genomics of the ecotype.
Māori, the Indigenous people of New Zealand, have a strong relationship with whales and dolphins. As descendants of Tangaroa (the Sea God), whales and dolphins are considered kin to Māori and as such, they have an obligation to care for them and respect them, as well as utilize them as a gift from Tangaroa. Colonization has severed and disrupted the traditional relationships that Māori maintain with whales and dolphins and their role as caretakers and kin. To uphold the rights of Māori, Te Tiriti Waitangi (the Treaty of Waitangi) requires consultation with local Indigenous tribes—iwi—before taking samples or utilizing parts of taonga (treasured) species. This had not been done with the museum sample prior to being exported out of New Zealand and sequenced, so the authors reached out to Ngāti Toa Rangatira, the iwi of the rohe (area) in which the type D specimen came from, to correct this oversight and to start a conversation about the best way to proceed going forward. Part of this process included acknowledging Ngāti Toa’s relationship with the individual type D killer whale used in the study, translating the abstract of the paper into te reo Māori (the Māori language), and storing the data from the sample in the Aotearoa Genomic Database Repository.
The Long-Awaited Results
Led by geneticist Andrew Foote, the newest study published in the Journal of Heredity—titled “‘Type D’ killer whale genomes reveal long-term small population size and low genetic diversity‘” examines the nuclear genomics of type D killer whales. The analysis revealed that not only are type Ds the most inbred killer whales on the planet, they have some of the highest levels of inbreeding recorded in any mammalian species. They have similar levels of genetic diversity as the vaquita, an endangered porpoise species that numbers less than ten individuals. Type D killer whale genomes have long runs of homozygosity—these are stretches of the genome that contain identical alleles. This occurs when an individual inherits two copies of the same allele from its parents, something that is more likely to occur when close relatives reproduce. Inbreeding can start to happen when a population declines rapidly and remaining individuals have a limited selection of available mates. However, type D killer whales appear to have been inbreeding over a long time period and they have probably always had a small population. Inbreeding can lead to a decrease in genetic variation and make it more likely for recessive traits to pop out. This may explain the strange appearance of type Ds. Their tiny eyepatches and rotund heads might ordinarily be features that are masked by more dominant traits inmost killer whales. The long-term inbreeding of type D killer whales may have given these characteristics the opportunity to emerge and become permanently fixed in the population.
A New Species?
For many years, scientists have wondered if type D killer whales represent a unique killer whale subspecies or species. It can be tricky to define where one species ends and another begins, but the social isolation and high level of inbreeding in type D killer whales are indicators these whales may be distinct enough to warrant classification as a new species. As with other killer whale types, social behaviors probably provide the initial barrier that prevents type Ds from breeding with other killer whales. Killer whales are not particularly friendly with outside groups and typically avoid contact with one another. When type Ds congregate with other killer whales around fishing boats to depredate longlines in the Crozet Islands, social boundaries are maintained—the other killer whales will chase off type Ds if they come too close (Tixier et al.2016). Even if a type D killer whale managed to break killer whale social norms and mate with anon-type D, the genetic barriers might already be too large; if type D genomes contain too many deleterious alleles from inbreeding, it could reduce the fitness of resulting mixed-ancestry offspring or even prevent viable pregnancies all together.
Future Research
In November 2022, a dead type D killer whale was found stranded on a beach in Chile right as the current study was being wrapped up. A remarkable coincidence considering this is the first stranding of a type D killer whale since the discovery of the ecotype sixty-seven years ago! Samples from this animal have been saved and will be analyzed using the same methods described in the new study. The addition of this new individual may help answer remaining questions about the demographic history of type D killer whales. For example, while the type D killer whales from the New Zealand stranding do appear to be a different family group than the ones sampled in Chile, the high levels of inbreeding and small sample size make teasing out the genealogy of the population more difficult. The stranded specimen will help confirm that the three Chilean whales and the New Zealand whales weren’t actually related families being sampled by chance. Additionally, with the availability of a complete skeleton, scientists will be able to learn more about the ecology and morphology of this elusive killer whale. It may even help scientists make the final determination of whether type D killer whales will become the newest killer whale species.
As I write this sentence, wrapped up in a blanket while an Alaskan winter storm howls outside my house, I think about the fact there are type D killer whales cruising around right now a world away down in latitudes with weather conditions so perilous they have earned the nicknames the “Roaring 40s” and “Furious 50s.” For thousands of years, they have eked out a living in these turbulent southern waters, evading detection by humans until relatively recently. Scientists are finally starting to unlock the secrets of the type D killer whale and I can’t wait to see what else they might find.
References
Bigg M (1982) An assessment of killer whale (Orcinus orca) stocks off Vancouver Island,British Columbia. Reports of the International Whaling Commission. 32:655-666.
Foote AD, Morin PA, Pitman RL, Ávila-Arcos M, Durban JW, van Helden A, Sinding MS,Gilbert MTP (2013) Mitogenomic insights into a recently described and rarely observed killerwhale morphotype. Polar Biology. 36: 1519-1523.
Pitman RL, Durban JW, Greenfelder M, Guinet C, Jorgensen M, Olson PA, Plana J, Tixier P,Towers J (2011) Observations of a distinct morphotype of killer whale (Orcinus orca), type D,from subantarctic waters. Polar Biology. 34: 303-306.
Pitman RL, Ballance LT, Sironi M, Totterdell J, Towers JR, Wellard R (2020) Enigmaticmegafauna: type D killer whale in the Southern Ocean. 101: 1.Reeves R, Pitman RL, Ford JKB (2017)Orcinus orca.The IUCN Red List of ThreatenedSpecies.
Tixer P, Gasco N, Duhamel G, Guinet C (2016)Depredation of Patagonian toothfish(Dissostichus eleginoides) by two sympatrically occurring killer whale (Orcinus orca) ecotypes:Insights on the behavior of the rarely observed type D killer whales. Marine Mammal Science.32(3): 983-1003.