Winter World_ The Ingenuity of Animal Survival - Bernd Heinrich [91]
In Australia, the Lugong moth (Agrotis infusa) also migrates to cool mountain areas where it clusters in large numbers (and where it was once an important food for Australian aborigines). The principle of the moths’ migration is the same as that of the monarchs’—to conserve their fat reserves during a long quiescence—but rather than migrating to escape freezing, they migrate to escape hot conditions.
In bats, we are given a fine example involving escape from both low and high temperatures. Bats are, like the Daneidae and also the Hominidae, animals of the tropics. Those that live in the north are outliers (as are Danaus plexippus among the Daneidae, and Homo sapiens among the Hominidae). Like us, bats are now able to live in the north, not because they tolerate freezing, but because they manage to avoid it. Like monarchs, many bats migrate, but their ability to do so leaves them much more leeway as to destination.
We tend to mostly think of migration as north-south movement, but migration can be in any direction whatsoever. Blackcap warblers from central Europe, for example, have traditionally migrated south, into Africa, in the winter. But within several decades a part of their population has evolved, by natural selection, to fly east-west instead, wintering in Great Britain where the weather is milder and bird feeders have become available. Similarly, northern bats of many species also migrate to where they can keep in energy balance. But that energy balance is achieved without feeding. Like the monarch butterflies, they migrate to cold storage environments where they can both conserve their fat reserves and not be endangered by freezing. With many bats, that means overwintering in caves.
If no feeding is possible for months, then just any cave won’t do. Cave temperatures can’t be below about 0°C, or else the bats risk death by freezing or energy exhaustion by shivering to prevent freezing. At the same time, cave temperatures can’t be high if no food is available outside, because then even the animal’s idling or resting metabolism would eventually exhaust their fat reserves. In general, each 10°C rise in body temperature doubles the rate of resting metabolism (i.e., resting energy expenditure).
In the south, some bat populations migrate north where cave temperatures (and lowest possible body temperatures) are low enough for them to remain in extended torpor (McNab 1974). Few bats are able to hibernate at cave temperatures above 14°C. The exceptions are very small bats and those that don’t cluster, and thus enhance their ability to cool.
Bat in summer, resting on a leaf.
A bat entering a cave for the first time cannot know beforehand if the temperatures within will be suitable to maintain a positive energy until the end of the winter, any more than monarchs can actively choose specific mountain retreats where their energy balance will come out just right by spring. However, if the animal ends up surviving, then conditions were suitable—the energy balance came out favorable, and the bat’s survivors will likely return to that site the next winter. This is what bats do. It’s not only natural selection that’s operating on bats to survive winter; there is also selection for caves that become bat caves; in those caves that are not suitable, the populations never build up. Conversely, once having built up, they decline if conditions become unfavorable.
Bats are long-lived animals that learn by experience, returning year after year to caves that have proven themselves to be safe, probably for centuries. Little had disturbed the constant and specific environment of their traditional caves until humans came along. Not surprisingly, therefore, human disturbance of bats gathered in specific caves has been a big factor in some bats’ declines. Merlin D. Tuttle from the Milwaukee Public Museum reviewed the association of decline with disturbance by people in caves, specifically of the endangered gray bat (Myotis grisescens).
Like other bats, gray bats are restricted to specific caves. They cluster in densities