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WALTER LOUIS HAHN. 



doors are favorable for their activities. The presence of bats in 

 the cave at night, when others are out searching for food, shows 

 that there is no definite time at which they all leave their roosts. 

 This fact has also been observed with regard to the European 

 pipistrelle {Pipistrellns pipistrelhis) by Whitaker ('07), who says 

 that only part of a large colony left their roost under a roof on 

 a certain night. If a large percentage of the animals does become 

 active at about the same time, it must be remembered that many 

 of them pass the summer in places where daylight reaches them 

 and the absence of light as night falls may be a direct stimulus 

 to activity. Falling temperature at sunset may also stimulate the 

 animals to activity when they are not in the caves. 



On rare occasions the hunger stimulus may be so strong as to 

 overcome the natural repugnance to light, and the animals come 

 out to search for food in daylight. I have witnessed this but 

 twice. In late autumn a Pipistrelhis was seen circling high above 

 the trees, and at another time. May 9, 1907, a Myotis lucifugus 

 was seen feeding in the bright noonday sun near the mouth of 

 Shawnee Cave . 



On the average, a bat certainly does not fly more than six 

 hours out of twenty- four, and that for not more than eight months 

 of the year. At least five sixths of its life is spent hanging head 

 downward in the dark. 



From the foregoing facts we may assume that a bat's life is 

 made up of a series of alternating periods of torpor and activity. 

 The relative and absolute length of these periods depends on the 

 state of bodily nutrition.' When the body is well nourished and 

 the quantity of reserve fat large, the periods of lethargy are long 

 and the time of activity short. As the stored fat is used up the 

 periods of lethargy become shorter and active states longer and 

 more frequent. During the season of greatest activity, from May 

 to July inclusive, the times may correspond to daylight and dark- 

 ness, and the condition of the animals to ordinary sleep and 

 activity. However, the longer periods have no direct relation to 



iThe physiology of hibernating bats has been studied by Rulot ('02) and Merz- 

 bacher ('03). According to the former, glycogen and albumen are consumed during 

 hibernation, especially toward the end of the period. It is evident that the hiber- 

 nating state in the bats studied by these authors is more profound than it is in the 

 bats which I obtained in the caves. 



