Friday, August 24, 2007
A chipmunk between two worlds
I believe this is a lodgepole chipmunk (Tamias speciosus), but I am not certain. The old addage, "if it looks like a duck and sounds like a duck. . ." may apply to chipmunks in general, but it doesn't apply to species of chipmunks. Some chipmunk species are remarkably similar in coat color and size, and only diligent study reveals their true identify. This one, photographed at Lassen National Park among the Jeffrey pines, could also be the yellow pine chipmunk (T. amoenus).
Californians proudly boast 13 species of chipmunks. (Well, they should anyway.) Yes, 50% of the world's species of chipmunks live in the golden state. Most of the other species live in the intermontane parts of the western US, Canada, and Mexico.
Chipmunks are basically mini-tree squirrels with racing stripes. They occur from sea level to treeline, and fill an ecological niche between the terrestrial world of ground squirrels and the largely arboreal world of tree squirrels.
The sexes show variable dimorphism in body size. In more extreme habitats females tend to be larger than males, while males are the larger sex in other areas. This infers that big female chipmunks have certain advantages, perhaps in energetics and reproduction--though the subject needs more study. Their best known trait is their internal cheek pouches, which allow them to haul food from feeding areas to hoards and nests.
The two most widespread species, the eastern chipmunk (Tamias striatus) and the Siberian chipmunk (T. sibiricus) have the largest geographic ranges. They also have more generalized traits, so mammalogists believe they are a bit closer to the ancestral chipmunk. The hotbed of chipmunk evolution is western North America. Here where the forces of nature have created a quiltwork of habitats they have evolved into many forms.
The debate isn't settled on where exactly chipmunks came from--Eurasia, North America, or perhaps somewhere in between, like Beringia. If the ancestral chipmunk was Eurasian or North American, however, it had to cross the Bering land bridge, like a lot of other mammals in the past.
Contrary to popular belief, the word chipmunk does not owe its derivation to the resemblance of these rodents to cheerful Jesuit monks, or men of any particular monastic order. The word comes from the Algonquian "achitamon" for "one who descends trees headlong". In other words, chipmunks can rotate their hindlegs and climb down tree trunks headfirst. They share this trait with tree squirrels. Their small size gives them advantage when it comes to negotiating shrubby vegetation that bears nutlets, berries, and drupes. Thus they get around much better in the brushlands, chaparral and forest understory than ground squirrels and tree squirrels.
Like ground squirrels, they nest and hibernate in burrows, but apparently some species, like the lodgepole chipmunk, make tree nests in cavities or exposed on a limb close to the trunk. These are used by mother and young after the munklets develop their climbing skills. This practice of using both subterranean and arboreal nests is just another example of how chipmunks live between two worlds.
Best, T.L., R.G. Clawson, and J.A. Clawson. 1994. Tamias speciosus. Mammalian Species, No. 478:1-9.
Broadbooks, H.E. 1974. Tree nests of chipmunks with comments on associated behavior and ecology. Journal of Mammalogy, 55(3):630-639.
Levenson, H. 1990. Sexual size dimorphism in chipmunks. Journal of Mammalogy, 71(2):161-170.
Levenson, H., R.S. Hoffmann, C.F. Nadler, L. Deutsch, and S.D. Freeman. 1985. Systematics of the Holarctic chipmunks (Tamias). Journal of Mammalogy, 66(1):219-242.
Ralls, K. 1976. Mammals in which females are larger than males. The Quarterly Review of Biology, 51:245-276.
Sutton, D. A. and D.B. Patterson. 2000. Geogrpahic variation of the western chipmunks Tamias senes and T. siskiyou, with two new subspecies from California. Journal of Mammalogy, 81(2):299-316.
Wilson, D.E. and D. M Reeder. 1993. Mammal species of the world, a taxonomic and geographic reference. Smithsonian Institution Press, Wasgington D.C. and London.