Scammon's hand-drawn sketches and wildly incorrect understanding of killer whale biology are a window into the process of discovery by observation of animals which, back then, could only be glimpsed in passing.

Scammon didn't understand the difference between marine mammal-eating orcas and those who eat fish, and thought killer whales with taller fins were a different species, when they were simply different genders.

But then, science is an iterative story, built generation by generation, that depicts an ever more accurate — but never complete — understanding of our world. And so it is that Scammon's hunch that there might be more than one species of orca out there was correct — just not quite in the ways he thought.

How they differ

Scientists used to think that it took physical separation, or an inability to breed together, to produce separate species. But, it turns out, profound cultural traits can differentiate species, too. After all, Bigg's and residents overlap in some of their range. And they could physically interbreed. But they have never been witnessed to do so in 50 years of scientific observation.

They do not interact, or even speak to one another. Yet they tick multiple boxes that indicate differentiation as species, both from one another, and all other killer whales, scientists argue in the paper.

Consider:

—Bigg's eat marine mammals, while residents eat mostly salmon, especially Chinook. Never does either even sample the other's diet.

—Bigg's of both sexes are physically much larger than residents. John Durban, associate professor of fisheries, wildlife and conservation at the Marine Mammal Institute of Oregon State University, can measure killer whales to the centimeter using a camera on a drone, flown high over the whales. From these images has emerged the fact that Bigg's killer whales are more robust than the residents. They are bigger and longer, by nearly a half a meter on average. The Bigg's also have a different jaw structure. Both their jaw and bigger body size make sense for an animal that has to take on big prey.

—The Bigg's dorsal fin is wider at the base, more triangular, and pointed at the tip, and the saddle patch — the marking behind the dorsal — is nearly all white. Resident saddle patches include black in their pattern.

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