Sunday, August 12, 2007
It's better to be fat
We named her meatloaf. She was rounded on top and squared off on the bottom edges. She looked solid. Actually, there were several meatloafs in the campground, all adult mantled ground squirrels.
There were also smaller ones, perhaps yearlings . . .
and certainly some of the most trim squirrels were young of the year.
I want to talk about fat squirrels. It's a relevant topic in light of the American addiction to junk food, rampant childhood obesity, and our obsession with dieting. They can teach us something. They have a good reason for getting fat, and they have a successful way of dieting.
First, a few words for those of you not familiar with this charming rodent.
Mantled ground squirrels inhabit the open coniferous forests of the Sierra Nevada and Rocky Mountains. These are great places to live in the summer when it's dry, sunny, and warm. In the spring the mountains offer nutritious fodder. Tender young plants are high in protein and fiber, so the squirrels don't gain much weight, and rest assured they have no complaints about "irregularity". As the forbs and herbs set seed and dry up, the mountains yield their gourmet luxuries. The squirrels then root about for hypogeous fungi, which we know as truffles. Still, they keep their mesomorphic physiques.
Then in late summer the conifers drop their cones, and our squirrels gorge on the protein and oil-packed manna from heaven. During the month of August they start to "plump out". By September, adults have three times the body fat they had in the summer. Juveniles on the other hand only gain about twice their summer fatness.
The seasons change. The dog days of August give way to freezing night-time temperatures, and then moisture-laden air from the Pacific drops a blanket of snow -- often as early as September. The mountains are a hard place to live when snow covers the countryside and temperatures are below freezing. Making it through the winter depends on getting enough food.
Squirrelly creatures have made two evolutionary choices: stay active and feed daily, or hole up and hibernate. Tree squirrels opted for an active athletic winter; they get bigger appetites, burn more energy, and tough it out. Chipmunks and mantled squirrels have opted for a restful comatose condition known as hibernation. Their appetite and metabolism shut down. They sleep and slowly burn fat. Chipmunks also have a lot of larders so they can snack periodically through the winter. Mantled squirrels tend not to hoard larders, and seem to rely almost entirely on their body fat.
Mantled squirrels undergo another mysterious change. As winter approaches their body water decreases. The point is this -- a critter whose body temperature hovers dangerously close to freezing may protect itself by lowering its own freezing point.
And what about the skinny mantled ground squirrels?
They have a problem, because without fat their bodies don't tell them it's time for winter sleep. Hunger drives them to continue their quest for food. With limited food they can't make a go of it. They are doomed.
We can draw only one conclusion: sometimes it's better to be fat.
Bartels, M.A. and D.P. Thompson. 1993. Spermophilus lateralis. Mammalian Species No 440:1-8.
Blake, B.H. 1972. The annual cycle and fat storage in two populations of golden-mantled ground squirrels. Journal of Mammalogy, 53(1):157-167.
Jameson, Jr. E.W. and R.A.Mead. 1964. Seasonal changes in body fat, water, and basic weight in Citellus lateralis, Eutamias speciosus and E. amoenus. Journal of Mammalogy, 45(3):359-365.
Jameson, Jr. E.W. 1964. Food consumption of hibernating and nonhibernating Citellus lateralis. Journal of Mammalogy, 46(4):634-640.
McKeever, S. 1964. The biology of the golden mantled ground squirrel, Citellus latralis. Ecological Monographs 34(4)383-401.
Pengelley, E.T. and K.C. Fisher. 1963. The effect of temperature and photoperiod on the yearly hibernating behavior of captive golden-mantled ground squirrels (Citellus lateralis tescorum). Canadian Journal of Zoology, 41:1103-1120.
Tevis, Jr., L. 1956. Observations on chipmunks and mantled squirrels in northeastern California. American Midland Naturalist 53(1):71-78.
Tevis, Jr., L. 1956. Invasion of a logged area by golden-mantled squirrels. Journal of Mammalogy 37(2):291-2.