Physics and the life of the squirrel

ELISABETH BROWN

Suffering through a late summer heat wave in Palo Alto, I noticed a

tree squirrel on top of a sturdy redwood fence, acting strangely. It

took a few steps, then flung itself down onto the wood beam on top of

the fence, rubbing its belly against a shaded section of the beam.

Was this animal sick? No, just doing something I had read about

but not seen: hot squirrel behavior.

Squirrels don’t go to school or take physics, but they know

instinctively to flatten themselves against cool ground when they’re

too hot. In physics class, they’d have learned that air is a poor

heat conductor, and that the ground is a better one, so heat

conduction is far more efficient between animal and ground than

between animal and air. As it is, this behavior is so advantageous to

hot squirrels everywhere that it’s hard-wired in their genes.

Desert ground squirrels in their fur coats are in danger of

overheating when they run around in the sun. They don’t want to waste

water by panting like dogs, or hide out in the shade under rocks and

bushes -- a favorite hiding spot for snakes. So when they get too hot

they retreat underground and flatten themselves, belly down, against

the cool soil of the burrow. Contact with the cool substrate carries

away their body heat more efficiently than sitting in the shade and

radiating into the air.

You can bet this goes on in our local Beechy ground squirrel

burrows as well, during the long dry summer. Seeing the same behavior

in tree squirrels indicates just how widespread it is; probably all

squirrels in warm climates do it.

Lizards know about heat conduction too. As cold-blooded creatures,

their modest metabolic rate doesn’t allow them to reach operating

temperature on their own; they have to pick up heat from their

surroundings. They are experts in the physics of heat conduction.

After about mid-morning in the desert, sand and rock are hotter

than the air.

To avoid overheating, the lizard positions its body with the

smallest edge facing the sun, and arches its body away from the hot

ground. It stands on the lizard’s equivalent of tip toes (actually it

balances on the point of its heels, toes off the ground), minimizing

contact with the ground. If this doesn’t work, the lizard reluctantly

goes underground to cool off.

Squirrels and lizards, and other animals out looking for food,

have to constantly adjust their behavior according to the physical

laws of the world. By working the angles, they maximize their active

hunting time above ground and minimize idle time.

At night, the ground stays warmer than the air for a while,

allowing snakes to hunt well into the night. Black asphalt roads,

especially newly-surfaced ones, are the warmest objects around. Being

darker, they absorb more heat during the day than lighter colored

sand and rocks. After dark, snakes and lizards are attracted to the

heat of the road, and they congregate there to sit and warm up.

Unfortunately, this makes them easy pickings for another animal that

understands just enough physics: “night drivers” hoping to collect

them for the reptile trade.

* ELISABETH BROWN is a biologist and the president of Laguna

Greenbelt Inc.