AMERICAN BATS — HANDLEY 



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BIRTH 



Figure 20. — Average growth-rate of embryo in Plecotus townsendii pallescens. Measure- 

 ments in millimeters. (Modified from Pearson, et al., 1952, p. 302.) 



tion between sexes until late in development when smaller size of 

 male becomes apparent). Within a few hours the young one is able 

 to utter a characteristic sharp, metallic "chirp." The dried umbilical 

 cord may remain attached for a day or two. Before five days of age 

 the naked pink body becomes covered with short gray hair. At 

 seven days the young bat can make the "squaking" noise of the dis- 

 turbed adult, and the ears assume the erect position of the adult. A 

 few days later the eyes open. Up to the age of 16 days (forearm less 

 than 35 mm.), while the growth curve is approximately linear, the 

 forearm lengthens 1.2 mm. per day. Forearm length is a reliable 

 indicator of age up to about three weeks. At four weeks the forearm 

 is so close to adult length that it can no longer be relied upon. The 

 young fly at about two and a half to three weeks of age, and at six 

 weeks some young fly out at night as do the adults. The summer 

 colonies start to break up in August when the older young are just 

 over three months old. 



Lactation. — In Plecotus there is a single pair of pectoral mammae, 

 lateral in position. Tiny nipples (less than 0.6 mm. long in fresh 

 specimens) are characteristic of young and nonparous females, and 

 once a female has borne young the nipples do not return again to the 

 tiny category, although they frequently become small again. The 

 nipples of the young remain tiny through the summer, fall, and 

 winter of their first year, and begin to enlarge at about the time of 

 implantation of the embryo. The nipples reach full size ("up to about 

 3 mim. in length) at the end of pregnancy or shortly thereafter. 



