Killer whales colonised in 200,000 years

Killer whales, like humans, spread around the world thanks to small “founder” groups that pushed the species into new environments, according to a genetic study published in Nature Communications.

Andrew Foote from the University of Bern in Switzerland, Jochen Wolf from Sweden’s Uppsala University and colleagues from around the world sequenced genomes of 50 killer whales from five different groups in the North Pacific and Antarctic regions.

They found whenever a group split off to venture into new territory, their founder population initially declined but bounced back quickly.

Killer whales (Orcinus orca) are the largest species of the dolphin family. Different groups enjoy different menus, from penguins and seals to fish and reptiles.

Killer whales that live in Antarctica, for instance, have a menu very different to those in the warmer Pacific Ocean.

There are also differences in the diets of resident and transient populations. In North Pacific coastal waters, transient killer whales tend to feast on mammals while resident populations eat fish.

They each diverged from common ancestors around 68,000 and 35,000 years ago respectively.

Foote, Wolf and colleagues analysed genomes from skin biopsies of 10 individuals each from the North Pacific transient and resident populations, seven Antarctic mammal-eaters, 11 penguin-eaters also from Antarctica, and 10 of the smallest killer whales, which happen to dine on fish.

They found the killer whales’ most recent common ancestor lived around 126,000 to 227,000 years ago.

Whenever a founder group split off, it appeared their population dropped for a short time, then grew quickly. This was seen across all five groups.

A more in-depth genomic examination of a North Atlantic female and North Pacific resident male found a similar genetic trajectory.

A few genetic quirks were uncovered. Antarctic killer whales have “enriched” genes involved in developing fatty tissue compared to their temperate counterparts.

The mammal-eating specimens (from the North Pacific and Antarctica) shared genes that played a role in the methionine cycle, which breaks down the amino acid methionine into other compounds such as cysteine.

This is likely because mammal meat is high in methionine, so those killer whales need to break it down more efficiently.