A case study of coevolution:
squirrels, birds, and the pinecones they love
|The scene: The Rocky Mountains
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
Holiman, and Julie Smith set out to see if their observations
would support the hypothesis of coevolution.