AMERICAN BATS — HANDLE Y 171 



line. The bats normally pack so tightly in the cluster that their 

 number is usually considerably underestimated in a casual observa- 

 tion. As shown by Twente (1955b, pp. 707, 714, 723) the function of 

 the cluster is seasonally variable. 



Contrary to the opinion of many authors, the winter cluster func- 

 tions to protect the bats from heat rather than from cold. The body 

 temperatures of the clustered bats approach the temperature of the 

 substrate upon which they are resting, whereas the body temperature 

 of the solitary bat is more affected by the surrounding air temperature. 

 Low body temperatures and consequent low metabolic rates are im- 

 perative to insure conservation of fat reserves through the hibernating 

 season. Beer and Richards (1956, p. 39) have demonstrated for 

 Eptesicus that in some circumstances there may be a delicate balance 

 between length of fasting (hibernating) season and quantity of stored 

 fat in the bat. Unusually, high metabolism in the bat or abnormally 

 protracted cold weather result in starvation of the bat. 



Since temperatures within spring and summer clusters may exceed 

 the environmental temperatures, heat conservation seems to be the 

 primary function of the cluster at these seasons. The higher tempera- 

 tures presumably would aid digestion and assimilation of food. 

 Twente found that Myotis moved to the cooler parts of caves and 

 assumed deeper torpidity, as in winter hibernation, when stormy 

 weather prevented their normal summer foraging flights. Warmer 

 sites were chosen and higher cluster temperatiu-es prevailed after the 

 bats had eaten. Pearson, et al. (1952, pp. 303, 311) suggested that 

 gestation, lactation, and other reproductive processes also might be 

 facilitated by the elevated temperatures maintained in clusters. 

 Clustered young, their metabolism possibly accelerated by the higher 

 temperatures, may grow more rapidly than solitary young. 



According to the observations of Pearson, et al. (1952, p. 275) in 

 central California and Twente (1955b, p. 724) in Oklahoma and 

 Kansas, nursery colonies of pregnant females begin to form early in 

 April and increase gradually well into May. The colonies may consist 

 of individuals from diverse hibernating sites. Varying from a few to 

 200 individuals, these colonies remain more or less intact until after 

 the young are weaned in late sunmier. The bats ordinarily return to 

 the same sites — buildings, mine tunnels, or caves — year after year. 



Population : By studying and banding nursery colonies, Pearson, 

 et al. (1952, p. 314) were able to calculate probable survival rates in 

 P. townsendii. Although differences in habits of males and females 

 render it difficult or impossible to determine the sex ratio in the adult 

 population, the ratio is about 50 : 50 in the young. Only 5.5 per cent 

 of 470 females examined in nursery colonies were neither pregnant nor 

 lactating (reproductive failure is more common in young females than 



