182 PROCEEDINGS OF THE NATIONAL MUSEUM vol. ho 



temperature of the bat and upon whether or not it was lethargic during 

 a portion of the period. Variation in environmental conditions in 

 hibernating quarters and nursery roosts thus may have a profound 

 effect on gestation. The gestation period varied from 56 to 100 days 

 in the California populations of P. townsendii studied by Pearson, 

 et al. 



Parturition. — The birth process was observed in a captive female. 

 She hung from the wire top of her cage by widespread hind feet and 

 thumbs. Her tail was curled upward so that it completed the "in- 

 verted umbrella" effect and made it quite unlikely that the young 

 could fall. In fact, not even a drop of blood or fluid reached the cage 

 floor. This position was maintained all during parturition and for 

 half an hour afterward. The bu-th was a breech presentation. With- 

 in minutes the fluids and afterbirth were consumed by the mother, and 

 while she licked her membranes and offspring, the young moved for- 

 ward, secured a nipple, and attached its feet to the top of the cage. 



A single young is the rule in this species, and 49 out of 50 embryos 

 were found in the right horn of the uterus. After parturition this 

 horn never returns to its original small size, but remains larger than 

 the left, about 1.5 to 2 mm. in diameter. This disparity in size may 

 lead to presumably erroneous reports of fall pregnancies as that of 

 Bailey (1936, p. 390). 



The average date of parturition of a colony varies from year to 

 year, and there seems to be a considerable variation in the spread of 

 birth dates in different colonies (19 to 35 days). It is probable that 

 birth dates extend over a long period in a colony whose females 

 roosted in diverse places with a variety of environmental temperatures 

 before they congregated at the nursery colony. There is evidence 

 also that young females tend to join the nursery colony later in the 

 spring than do adults, and that they bear their young later in the 

 season than do adults. 



Pearson, et al., estimated birth dates for several hundred western 

 American P. tovmsendii from specimens, field notes, and literature by 

 reference to graphs relating embryo size and forearm length to age 

 (figs. 20, 21). The earliest calculated birth date was 19 April in 

 CaUfornia, and the latest was 22 July in Arizona. Kecords for the 

 Appalachian population indicate a similar breeding season. 



Growth of young. — Pearson, et al., described the newborn P. tovm- 

 sendii as a grotesque creature, its large ears flopping over its unopened 

 eyes and the disproportionately large size of its thumbs and hind feet 

 giving it a spider-like appearance. The following chronology of 

 growth was set forth : 



The average weight of newborn young was 2.4 grams, and the 

 forearm length averaged 16.6 mm. (there is no significant size varia- 



