By Dr. Becker
Orcas, or killer whales, are a fascinating group of whales and dolphins that span the waters of the globe. Many are separated geologically, so while they are distantly related, each group has its own appearances, behaviors, sounds and even genetics. Different types of orcas are known as "ecotypes," at least 10 of which have been described.1 In the Pacific, there are three primary orca ecotypes that have maintained their differences even though, in this case, they are not divided by geographical boundaries.
Although the animals could interbreed, it appears that they do not. This, coupled with the fact that each group prefers a distinct diet, has raised discussions that they could be in the midst of sympatric speciation, in which new species evolve from a single species while living in the same geographic locale.
Three Orca Ecotypes in the Pacific
According to the Whale and Dolphin Conservation (WDC), "Over the last few decades as wild orca research has expanded, researchers have described different forms or types of whales. Known as ecotypes, these orca communities look different and have different prey preferences, feeding habits and acoustic behaviors. Their ranges can often overlap but it appears they rarely interact and don't interbreed."2 Three communities of orcas call the northern hemisphere home, including:3
- Resident orcas: These killer whales prefer to eat fish, have small home ranges and live in large social groups called pods. Their dorsal fin is "falcate," meaning it's rounded at the tip, curved and tapered.
- Transient orcas: Also known as Bigg's killer whales, transient orcas primarily eat marine mammals such as seals, travel frequently over a large home area and live in smaller groups than resident orcas (typically 10 or fewer whales). Their dorsal fins are straighter at the tip than resident and offshore whales.
- Offshore orcas: Offshore orcas are similar to resident orcas but have an even larger geographic range and occasionally visit coastal and inshore waters. They eat fish, with recent research suggesting they may prefer sharks. They are smaller, overall, than resident whales and have rounded fins with nicks on the edge.
Offshore whales are among the least understood group of orcas, in part because they hunt underwater and may not approach the coast often. Video captured by a drone camera in California's Monterey Bay in 2016 revealed a clue to their eating habits, however, as it showed a group of offshore killer whales hunting a shark. In addition, they often have worn-down teeth, likely from biting into cartilage, further suggesting they may focus their diet on sharks.4
It's been suggested that the distinct dietary preferences among resident, transient and offshore orcas could be further cause for, and evidence of, speciation occurring. Such cultural differences would necessitate different social behaviors and hunting tactics, for instance, which could discourage interbreeding. Bangor University evolutionary biologist Andrew Foote told The Christian Science Monitor:5
"I think the picture of sharks forming a significant part of the offshore ecotype diet was already starting to emerge, thanks to many years of work by biologists working in the North Pacific … This video certainly offers a valuable data point … It seems like the residents and transients are close to being completely reproductively isolated."
'The Beginning of a Very Slow Process'
It's difficult to say for sure whether orcas are changing genetically in response to their dietary preferences, in large part because the process takes a long time to be completed, making related observations difficult. Foote told The Monitor:6
"Speciation is typically a process, not an event, and as such it takes time … So we rarely get to see the process from start to finish — the rare exceptions are species with very short generation times. Therefore, we have to try and infer the past and forecast to the future, when studying speciation in killer whales and other long-lived organisms. That is why the topic is so hotly debated."
In a 2013 study authored by Foote and colleagues, no genetic differences were found among orca populations in the North Atlantic even though there are distinct groups with different dietary preferences and hunting habits. "When hunting herring, the whales travel in large groups and vocalize a lot," Foote told Science. "But they travel in small groups, three to five animals, and hunt in complete silence when going after seals."7
Although no genetic differences were revealed, it doesn't rule out a speciation event. Foote noted that all the "ecological ingredients" necessary for such an event to occur are there and "it might be that the whales are at the beginning of what is a very slow process."8 That being said, they also suggested that the process may not reach completion, and animals that are considered fish eaters may also eat mammals, and vice versa, at least among North Atlantic whales.
In some whales, geographic separation may also be necessary for genetic divergence to occur, while in others, sticking to a strict diet of fish or mammals may be enough for them to form two distinct species. Interestingly, while killer whales develop their own acoustics within their communities, they are capable of learning to communicate with others, even dolphins.
They're also among the most intelligent animals, learning local and complex languages that are retained for many generations, and teaching difficult hunting techniques to their offspring. Some also use echolocation to find fish in murky water. It's easy to see how orca ecotypes, with their highly specialized cultures, could interact less and less until they ultimately diverge into separate species.