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By Root 1253 0

of over 1, 800 per square meter, and cave populations can be assessed by estimating the square meters of cave ceiling covered with bats. Their colonies are restricted to fewer than 5 percent of available caves, and in these caves the human disturbance has mainly been due to traffic by spelunkers and to vandalism, including by health authorities who have been known to torch a cave full of bats after receiving an erroneous rabies claim. The two most heavily disturbed caves in Alabama and Tennessee lost 90 percent of their bats, while the population in five of the rarely disturbed caves there remained stable.

In an attempt to stop the sometimes catastrophic declines, cave entrances were in many instances altered to restrict or limit human intrusion. Ironically, however, the results of these well-meaning measures were mixed; sometimes the population recovered, but in other cases improperly constructed gates resulted in the loss of entire colonies. Potential causes are illustrated in the endangered Indiana bats, Myotis sodalis. Female Indiana bats live nearly fifteen years and males slightly less (Humphrey and Cope 1977). Reproduction is slow. Females have their first pup at age two, and after that have only one per year.

This bats’ summer range covers most of the eastern United States, but about 85 percent of the population winters in seven caves; and half of the population can be found in just two. Since gaining legal protection in 1973, winter populations of the Indiana bat have decreased by about 28 percent until 1980–1981, and additionally by 36 percent in the next decade. A recent study by four researchers (Richter et al. 1993) from four different museums suggests that the bat’s perplexing declines were due to modification of cave entrances. For example, from in the early 1960s when entrance modifications were made, to the early 1990s the bat population of Hundred Dome Cave in Kentucky declined from 100, 000 to 50 bats. When the entrance of the Wyandotte Cave in Indiana was constricted by a man-made stone wall, the bat population declined from 15, 000 to 1, 400 bats by 1957, in twenty-five years. One thousand to 2, 000 bats continued to overwinter there until 1977, when the stone wall was removed. Immediately after that the population rebounded until fifteen years later, when it was back up to nearly what it had been originally. The researchers ultimately concluded that the modifications of the cave entrances had their main effect of restricting airflow so that temperatures inside had become higher. Those at Hundred Dome Cave, where there had been the most precipitous decline, had increased from 4° to 6° to 11°C. As a result, these hibernating bats, normally found in conditions of 4° to 8°C, were insufficiently cooled.

In this species the body temperature during hibernation is essentially identical to that of the air, from -3° to 30°C (Henshaw and Folk 1966). At the very lowest end of this temperature range the animals became aroused and exhibit mild shivering, to heat themselves up slightly above air temperature. (In a closely related species, Myotis lucifugus, the individuals cannot arouse from such low temperatures and they freeze to death at near -5°C.) However, the main danger to the Indiana bats in their traditional caves is not quick freezing, but slowly starving to death in temperatures above 10°C when their elevated resting metabolism eventually exhausts their fat reserves by the end of the winter.

To test the latter hypothesis, biologist Andreas Richter from Earlham College in Indiana, and three colleagues, compared body weight losses of bats in two caves of different temperatures. The body mass loss was 42 percent more rapid in bats roosting at higher temperatures, and the dead bats were emaciated bats. The mortality inferred by the conditions in the altered Wyandotte Cave, from 1953 to 1978, should actually have been high enough to eliminate the entire population at that cave. But bats attract each other, and the apparent stabilization of the cave population of one to three thousand individuals was more