Understanding Evolution

A case study of coevolution:
squirrels, birds, and the pinecones they love

The scene: The Rocky Mountains Lodgepole Pines
The players: Lodgepole pine
Red Squirrels Crossbilled Birds
Red squirrel Red Crossbill

The plot:
In most of the Rocky Mountains, red squirrels are an important predator of lodgepole pine seeds. They harvest pinecones from the trees and store them through the winter. However, the pine trees are not defenseless: squirrels have a difficult time with wide pinecones that weigh a lot but have fewer seeds. Crossbill birds live in these places and also eat pine seeds, but the squirrels get to the seeds first, so those birds don't get as many seeds.

However, in a few isolated places, there are no red squirrels, and crossbills are the most important seed predator for lodgepoles. Again, the trees are not defenseless: crossbills have more difficulty getting seeds from cones with large, thick scales. But the birds have a mode of counterattack: crossbills with deeper, shorter, less curved bills are better able to extract seeds from tough cones.

The stage is set, but the question remains: has coevolution happened? In order to show coevolution, we need evidence that suggests that the prey (the trees) have evolved in response to the predator (squirrels or birds) and that the predator has evolved in response to the prey. Researchers Craig Benkman, William Holimon, and Julie Smith set out to see if their observations would support the hypothesis of coevolution.


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Photo of lodgepole pines courtesy of Ed Jensen, Oregon State University; Photo of red squirrel courtesy of Kevin MacAskill and the Nelson Naturalist's website; Photo of Red Crossbill courtesy of Dennis Oehmke and the Illinois Raptor Center.

Understanding Evolution © 2017 by The University of California Museum of Paleontology, Berkeley, and the Regents of the University of California