A Case Study of Coevolution:
Squirrels, Birds, and the Pinecones They Love (2 of 2)

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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.
  1. 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.

Crossbill with a deeper and less crossed bill
The bill is less curved on this female red crossbill

Crossbill with a shallower and more crossed bill
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 donít have evidence that the squirrels have adapted to the trees.) Itís 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...
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The arms race


• Image of lodgepole cones courtesy of Ed Jensen, Oregon State University.
• Image of female red crossbill courtesy of Greg Lasley, Greg Lasley Nature Photography.
• Image of male red crossbill courtesy of Dennis Oehmke and the Illinois Raptor Center.

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