texified

Three days ago as I walked down my favorite road to the nearby beach, I came across this teeny-tiny little grey creature lying on its back on the side of the road. This little mammal was a shrew, apparently the Wandering or Vagrant Shrew (Sorex vagrans) as far as I could make out. I often find shrews lying dead in a path or a road. They never show any traumatic injury, and apparently they have simply died of some subtle cause. They usually don’t live long in the wild: one year or so, rarely surviving the second winter, but in captivity they average 2-3 years. I walked on down the road and found a mole (probably Townsend’s mole, Scopanus townsendii) lying on the side of the road just like the shrew–no mark or any sign of trauma. Both of these creatures had apparently died in the night judging by the freshness of their little bodies. The night wasn’t especially cold or wet, and I couldn’t think of any factor that might account for their dying.

A few minutes later I saw an Anna’s hummingbird. The temperature was in the upper thirties or lower forties and there were few Spring blossoms available for the hummingbird to gather nectar.

All of these creatures are homiothermic (warm-blooded) animals, and because of their small size they face some unusual problems. Homiotherms must maintain their body temperature within narrow limits in order to function properly. If you are very small AND warm-blooded like the shrews, you have a large surface area/volume ratio, and consequently you radiate a great deal of heat through this large surface area. Shrews have to eat almost constantly to replace the energy that is pouring from their bodies like water down an open drain. In one day they can eat their entire body weight. These little creatures are like small furnaces burning at a furious rate. On cold wet days the demands upon such animals are even higher.

When a creature doubles its linear dimensions, say it’s length, the surface area increases by 4x (squares) whereas it’s volume (wt) increases by 8X (cubes)! The ratio between the surface area and volume halves each time the linear dimension doubles.  Thus there is more volume in proportion to the surface area which makes it easier to retain heat.  Likewise these parameters diminish proportionately as the linear dimensions become smaller. There are all sorts of biological ramifications from this physical fact.