A case study of coevolution: squirrels, birds, and the pinecones they love (related to Evolution Series – Part 3)

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

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.

Based on their hypotheses, the scientists who did this study made some predictions:

1.There should be geographic differences in the pinecones.

If the trees have evolved in response to their seed predators, we should observe geographic differences in pinecones: where squirrels are the main seed predator, trees should have stronger defenses against squirrel predation, and where birds are the main seed predator, trees should have stronger defenses against bird predation. This turns out to be true. Where there are squirrels, the pinecones are heavier with fewer seeds, but have thinner scales, like the pinecone on the left. Where there are only crossbills, pinecones are lighter with more seeds, but have thick scales, like the one on the right.

Lodgepole pine cones adapted to squirrels — easier for crossbills to eat.

Lodgepole pine cones adapted to crossbills — easier for squirrels to eat.

2. Geographic differences in the predators should correspond with differences in the prey.
If the crossbills have evolved in response to the pine trees, we should observe geographic differences in birds: where the pinecones have thick scales, birds should have deeper, less curved bills (below left) than where the pinecones have thin scales (below right). This also turns out to be true.

The bill is less curved on this female red crossbill

The bill is more deeply curved on this male red crossbill

So we have evidence that the trees have adapted to the birds (and the squirrels) and that the birds have adapted to the trees. (However, note that we don’t have evidence that the squirrels have adapted to the trees.) It’s easy to see why this is called a coevolutionary arms race: it seems possible for the evolutionary “one-upping” to go on and on…even thicker-scaled pinecones are favored by natural selection, which causes deeper-billed birds to be favored, which causes even thicker-scaled pinecones to be favored, and so on…

Source: Berkeley University

Read the related chapter: