50 
PROCEEDINGS OF THE CALIFORNIA ACADEMY OF SCIENCES 
Series 4, Volume 65, Supplement I 
North America. One embryo was reported in each of three females taken in California (Grinnell 
and Swarth, 1913; Grinnell, 1918), Howell (1920a: 174) described “about a hundred females, each 
with a naked young” in a maternity colony in an old mine along the lower Colorado River in Cal¬ 
ifornia, and only single young were reported during dissection of about 260 reproductive females 
in an in-depth northern California study (Pearson et al., 1952). Single embryos were found in three 
females from the vicinity of Carlsbad Caverns National Park, New Mexico (Geluso and Geluso, 
2004), in one female from the Black Hills of South Dakota (Turner and Jones, 1968), in one female 
from Big Bend National Park, Texas (Easterla, 1973), in two females from the Chiricahua Moun¬ 
tains in southeastern Arizona (Cockrum and Ordway, 1959), and in two females from the Grand 
Canyon in northern Arizona (Ruffner et al., 1978). Cockrum (1955) summarized over 30 other 
cases from throughout the western states of females with single embryos, as well as additional 
cases of females with accompanying single young. However, Hall (1946) noted nine females with 
single embryos and one female with twins near Fallon, Nevada. 
The proportion of females in an area that are reproductive is variable, with estimates likely 
biased high by captures at maternity colonies. Easterla (1973) reported 99% of 76 females captured 
at maternity roosts and over water as reproductive in his studies at Big Bend National Park in 
Texas, and Fenton (1969) reported 100% reproductive in a sample of 37 bats captured at a pre¬ 
sumed maternity colony in a nearby region of Texas. Thirty-nine of 40 adult females (97%) sam¬ 
pled at maternity roosts in western Oklahoma in 1968 were reproductive (Humphrey and Kunz, 
1976), one of two females taken in Riverside Mountain in the lower Colorado Desert of California 
was reproductive (Grinnell, 1914), and 94% of 470 females examined at maternity roosts early in 
pregnancy in central California during 1947-1950 were reproductive (authors cautioned this pro¬ 
portion might decrease as embryos were resorbed later, and that some non-reproductive females are 
likely not present in maternity roosts; Pearson et al., 1952). All of eight females (100%) taken pri¬ 
marily at roosts in the Mogollon Mountains of southwest New Mexico and adjacent Arizona dur¬ 
ing June and July in 1960 to 1961 were reproductive (C. Jones, 1964). In South Dakota, 91% of 22 
females captured at maternity roosts were reproductive (Turner and Jones, 1968). In Colorado, all 
of 21 captured in a mine in Chafee County from mid-June to late July were reproductive (capture 
method unspecified), and four of 6 examined at a mine in La Plata County during mid-June were 
pregnant (Freeman and Adams, 1992 cited in Armstrong et al., 1994). Four of four females taken 
over water at Morefield Canyon at Mesa Verde National Park in southwestern Colorado during 
early August were lactating (Chung-MacCoubrey and Bogan, 2003). All of 19 females from mater¬ 
nity roosts near Pyramid Lake in Nevada were reproductive in 1924 (Hall, 1946), whereas 64% of 
14 females sampled over water in west-central Nevada in 1994 were found to be reproductive 
(Kuenzi et al., 1999). Biases towards higher assumed natality rates from sampling at maternity 
colonies is also indicated by the known roosting of non-reproductive females solitarily or in small 
groups at scattered locations other than maternity roost sites (for example, Pierson et al., 1999; 
Sherwin et al., 2000, 2003). 
Not all female Townsend’s big-eared bats breed in their first year of life, and young males 
probably do not mate at all during their first year (Pearson et al., 1952). Nine of 26 non-reproduc¬ 
tive females in an intensive California study were yearlings, and nine of 34 (26%) of the known- 
age one-year-old females in that study were non-reproductive (Pearson et al., 1952). We are 
unaware of any other published literature with quantitative data concerning age at first reproduc¬ 
tion or inter-birth intervals. Mating may occur in hibernation throughout the winter in multiple cop¬ 
ulations, with subsequent sperm storage until ovulation and fertilization take place in spring (Pear¬ 
son et al., 1952). Sex ratio at birth is 1:1 (Pearson et al., 1952). 
Survival: Pearson et al. (1952) indirectly estimated survival of females in California by 
