We’ll get to weighty matters shortly, but first some photos of gratuitously cute mammals from my trip to Colorado. One of the more zoologically startling features of the western U. S. is the number of mammals wandering in plain view. The furry-faced, doe-eyed, jaunty-eared denizens of Tennessee tend to stay out of sight in the thick woodland, so mammalogists must be content with scat and sketchy tracks (not counting the white-tailed deer on the porch). Not so in the wilder parts of Colorado.
After a few days wandering among these animals and flipping through the field guide, a question emerges: Why so many squirrely creatures in the west? The Field Guide to the Mammals has twenty four pages of ground squirrels, almost all from the west. The one eastern species, the eastern chipmunk, has twenty two cousin species in the west. A rodent biodiversity bomb has flung fat-cheeked squeakers into every grassy glade, forest edge and rock pile west of the Mississippi.
A recent paper by Ge et al, evolutionary biologists working at the Chinese Academy of Sciences, helps us to understand what happened. The ground squirrels (Xerinae, a subfamily comprising the chipmunk, marmots and other ground squirrels within the rodent family, the Sciuridae) originated in Europe about thirty five million years ago. From there the animals spread to Africa and North America. The group’s explosive diversification happened later, about sixteen million years ago, and seems to have been triggered by climatic changes.
The same cooling drying climate that separated the American and Asian forests created a grand opportunity for ground squirrels. As forests retreated, open meadows, grassy plains and sparsely vegetated mountainsides took over, especially in the arid west. This change, combined with the highly variegated landscape (not too many other places in the world have such a complex mix of mountains, plains, deserts), caused the ground squirrels to speciate into the many species we see today. Some of these then colonized other parts of the world, probably after they moved west over the Bering land bridge. For the marmots, chipmunks and ground squirrels, Manifest Destiny did not end at the Pacific Ocean. They kept going and colonized Siberia, Asia and Europe: the far, far, Wild West.
We hear in this story echoes of our own. Homo sapiens is also a creature of the grasslands, brought into being by forests in retreat from a cool, dry climate.
Note how the ground squirrels survey their homes: on two legs.
Note their social systems: they often live in close-knit groups in stable villages.
We’re lucky that their thumbs are not quite as agile as our own and that their food didn’t take quite as much brainwork to catch.