PROCEEDINGS OF THE CALIFORNIA ACADEMY OF SCIENCES 
Series 4, Volume 65, Supplement I 
near equal adult sex ratios in some but with a preponderance of females in others (Schmidt, 1999). 
The largest warm season colony in the latter study was about 200-300 bats. Recent (2000-2013) 
maximum counts at the four largest known summer colonies in abandoned mines in the Lower Col¬ 
orado River area of southeastern California and southwestern Arizona ranged from about 100 to 
500 individuals, predominantly males, whereas counts in spring can be much higher and include 
females (Brown, 2013). 
Night Roosts: These bats night roost in a wider variety of shelters than are used as daytime 
retreats (but may use diurnal retreats for night roosting as well). California leaf-nosed bats tend to 
begin appearing in night roosts about two hours after emergence and often join other conspecifics, 
followed by additional foraging bouts prior to returning to the diurnal roost (Vaughan, 1959; Brad¬ 
shaw, 1961; Bell et al., 1986). They have occasionally been reported to night roost in buildings and 
bridges, where they hang up to manipulate prey and digest their early evening meals at tempera¬ 
tures warmer than ambient (for example, Grinnell, 1914; Hatfield, 1937; Vaughan, 1959; Brown, 
2013). 
Population Ecology. — Litter Size, Natality, and Female Reproduction: Stephens (1906) 
reported a high incidence of twinning in California leaf-nosed bats but gave no details. Subsequent 
observations show that twinning can occur but is very infrequent (Bleier, 1975a,b). “Nearly every 
one” of 61 adult females captured in a cave with a colony of 300 in Riverside County, California 
in April 1908 had a single embryo, and “none was found to contain more” (Grinnell, 1918:257). 
Cockrum (1955) summarized records of seven other females, six with one embryo and one with 
two. Bradshaw (1961) reported only single embryos in 175 cases from Arizona. Five females from 
Baja California, Mexico also had single embryos (Jones et al., 1965). Young are bom around June, 
following fertilization in autumn and a long period of delayed development with an embryonic dia¬ 
pause of over four months (Bradshaw, 1961, 1962; Bodley, 1974; Bleier, 1975a,b). Sex ratios of 
young are 1:1 (Bradshaw, 1961). 
Females mate in their first autumn, but males do not (Bradshaw, 1961; Krutzsch et al., 1976). 
Some of the above natural history observations on litter size suggest that natality is high, although 
all such observations stem from captures at maternity roosts. Huey (1925) reported all of 12 
females taken at a maternity colony in a mine during May 1924 were pregnant. One study found 
that 95% of 188 females taken in mist nets over water in southern Arizona during the maternity 
season were reproductive, although the great majority of these were lactating and thus had greater 
water needs (Schmidt, 1999), perhaps adding a positive bias. Nonetheless, this result was identical 
to the simultaneous finding that 95% of 268 females taken at maternity roosts in the nearby Agua 
Dulce Mountains also were reproductive (Schmidt, 1999). We are unaware of any published liter¬ 
ature with quantitative data concerning inter-birth intervals. 
Survival: We are unaware of any published literature with quantitative data on survival for 
this species. 
Mortality Factors: Mortality due to vandalism has been recorded, including the killing of 120 
California leaf-nosed bats by teen-aged boys in an old min e (later permanently sealed; O’Farrell, 
1970) near Las Vegas, Nevada during winter 1928 (Burt, 1934). Much more recent killing has been 
reported at mines monitored along the Lower Colorado River in California and Arizona (Brown, 
2013). 
This species may be more susceptible to accidental mortality (such as ensnarement on spines 
of desert plants; Stager, 1943a) than other species of bats because of their habit of foraging close 
to the ground. Possible predators include skunks and owls (Bradshaw and Hayward, 1960; Brad¬ 
shaw, 1961). Deaths due to rabies have been documented (Constantine, 1979). They have been 
sampled for persistent chemical contaminants, with some chemicals causing concern but without 
documentation of associated mortality or effects on reproduction (King et al., 2001, 2003; see 
