6 BULLETIN 1305, U. S. DEPARTMENT OF AGRICULTURE 



and with the first freezing nights outside they hang up for the long 

 winter's sleep. So far as known a temperature of 50 to 55° F. in 

 moderately dry air suits their needs for hibernation, but the limits 

 of variation which they can endure have not been fully ascertained. 

 Before entering hibernation they become very fat, storing up inside 

 of their skins a rich supply of oily food material sufficient to support 

 the greatly reduced vital processes during their torpor of winter. 



Before they are fully torpid their stomachs and alimentary canals 

 become entirely empty, their temperature gradually falls to approxi- 

 mately that of the surrounding air. and their circulation and respira- 

 tion become greatly reduced. When fully in the embrace of the 

 winter sleep their bodies are cold and motionless, and they are ap- 

 parently dead. Thus they hang often four or five months until the 

 warmer air of spring penetrates to their chambers and stirs their 

 circulation to renewed activity. 



In the Tropics all species appear to be active throughout the year. 



FOOD HABITS 



So far as known the food of the free-tailed bats consists wholly 

 of insects, almost entirely of night-flying species captured on the 

 wing. Moths and beetles seem generally to form the great bulk of 

 the food, but many other insects also are eaten, and in case of an 

 unusual abundance of any nocturnal species, these might be expected 

 to figure largely in the food. 



These bats are gluttonous feeders, and in some species 20 minutes 

 after their appearance in the evening the stomachs have been found 

 distended with food, the contents averaging one-quarter the weight 

 of the animal. This would imply a capacity for at least half their 

 weight in insects every night, and even a possibility of their actually 

 eating their weight in them every 24 hours. Such estimates are 

 merely suggestive and must not be used in any conclusive sense until 

 more careful tests can be made of the food actually consumed by each 

 species under varying conditions. 



Many other bats not colonial in habits probably have a similar 

 capacity, and over parts of the country where they exist in sufficient 

 numbers may well have an economic value comparable to that of in- 

 sectivorous birds. 



If there w T ere more caves attractive to bats and a greater food 

 supply there would undoubtedly be more bats, whereas any consider- 

 able diminution of either factor would tend to reduce their num- 

 bers. If, however, the bats should become so numerous as to destroy 

 most of the insect life, there would necessarily result a correspond- 

 ing decrease in the bat population. The fact that bats of this species 

 each produce only one young a year suggests a long-established and 

 conservative balance between the food supply and the increase of 

 the species. 



GUANO DEPOSITS 



Bai droppings composed entirely of insect remains and well 

 moistened with hat urine accumulate under the roosting places, 

 often in such large quantity as to be of value as fertilizer. When 

 neither too wet nor too dry it is rich in nitrogen, phosphoric acid, 



