Shark-eating orcas: A window into how new species can arise?

Whale watchers on California’s Monterey Bay last week witnessed an impressive, if grim, spectacle: a group of orcas hunting a lone shark.

A drone camera captured the pod, which included two females and two calves, killing and eating what was probably a sevengill shark. The unlucky prey was about five feet long – small compared to the orca adults, but somewhat larger than the calves. The orcas themselves belonged to an elusive and genetically distinct population of “offshore killer whales.” It's unusual to see them surface at all, much less to feed. 

The footage is more than just a record of underwater bloodshed. It could also be a rare window into the ecology of a splitting species.

There are three main types of orca in the Pacific: transient killer whales, resident killer whales, and offshore killer whales. Transients, which are frequently sighted along the coastline, mainly eat seals and other marine mammals. Resident orcas are typically found in the Pacific Northwest and tend to prefer fish.

Offshore whales may travel as far as Alaska’s Bering Sea, but they usually only approach the coast in large groups during winter. They hunt underwater, so less is known about their feeding behaviors. But their teeth, which are often worn down from biting into cartilaginous flesh, suggest they might have a taste for shark.

All three ecotypes are still capable of interbreeding, so they are still considered part of the same species. But that may not be the case for long. 

“It seems like the residents and transients are close to being completely reproductively isolated,” Andrew Foote, an evolutionary biologist at Bangor University who has authored several studies on orca genetics, tells The Christian Science Monitor in an email. 

There’s evidence, marine biologists say, that the orca are undergoing speciation, the formation of a new species through evolution. There are clear morphological and genetic differences between types, and though they could theoretically interbreed, they don’t do so often. If that trend continues, the Pacific killer whale could split into three separate species.

“I would say they are in the early stages of speciation – various stages really,” Phillip Morin, a molecular geneticist with the National Oceanic and Atmospheric Administration, tells the Monitor in a phone interview. “The fact that they are found in virtually the same place, and yet are not mixing to any observable degree, is fairly unique.”

Speciation typically occurs when populations are separated by geological boundaries. But for all their differences, there’s almost nothing physically stopping transient whales from interacting with offshore whales. So what’s causing the rift?

Some biologists suggest that killer whales are diverging over dinner preferences. According to this thinking, the populations could have learned social behaviors and hunting tactics based on their preferred food source. These “cultural” differences could, in theory, prevent interbreeding long enough for the species to splinter. The notion that new species could arise from a single originator while inhabiting the same geographic region – what’s known as sympatric speciation – has met some resistance in the scientific community.

In a 2015 study published by the journal Heredity, an international team of evolutionary biologists argued that Pacific orca were likely undergoing sympatric speciation. That same year, Bangor Univerisity's Dr. Foote wrote to the same journal to express some skepticism – the killer whales are definitely diverging, he suggested, but it’s still not clear if food preferences are the main cause.

“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,” Foote tells the Monitor. “This video certainly offers a valuable data point.”

Others suggest that since speciation usually occurs among geographically separated populations, it’s possible that killer whales started that way.

“We’ve tried to tease apart what’s going on from the genetics,” NOAA's Dr. Morin says. “From our results, it seems like there have been periods of allopatry, when the populations were geographically separate before coming together. And they maintain those differences, and perhaps differentiate even more to avoid competition.”

Whatever the cause, these distinct ecotypes are on a trajectory to become even more different.

 “One theory is that a period of geographic separation may be the key ingredient. We should know more over the coming years as genomic data improve the resolution with which we can model the ancestry of these ecotypes,” Foote says.

But since orcas in the wild typically live 50-60 years, and as much as 100 years, the window to observe changes is relatively long.  

“Speciation is typically a process, not an event, and as such it takes time,” Foote adds. “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.”