As most connoisseurs of whale trivia know, all “toothed” whales echolocate — they hunt underwater prey via reflected sound waves. In contrast, all baleen whales — which hunt krill by straining them through seive-like mandibles — do not echolocate. Why the difference?

Marine evolutionary biologists have long assumed it’s because toothed whales dive down deep to hunt prey at night, whereas baleen whales stay near the surface. Since it’s so insanely pitch-black down there, the toothed whales had to evolve echolocation merely to be able to navigate.

For years, this explanation made perfect sense. Still, the biologists were puzzled by one unanswered question: How did the toothed whales know there was food down below?

Because they followed the squid. Many squid rise up to the ocean surface during the night, and plunge down deep during the day. In a recent paper, David Lindberg — the UC Berkeley professor of integrative biology — used fossil records of ancient whales and squid to argue that the toothed whales probably followed the squid down deeper and deeper until they realized there’s a massive buffet of squid kicking around in the ocean’s basement. At which point they also were like, damn, we’re gonna need us some echolocation to fully harvest this squid bonanza.

As a story in Science Daily reports:

Lindberg and Pyenson propose that whales first found it possible to track these hard-shelled creatures in surface waters at night by bouncing sounds off of them, an advantage over whales that relied only on moonlight or starlight.

This would have enabled whales to follow the cephalopods as they migrated downwards into the darkness during the day. Today, the largest number of squid hang out during the day at about 500 meters below the surface, though some go twice as deep. During the night, however, nearly half the squid are within 150 meters of the surface.

Squid: Noble pistons in the engine of marine evolution.