O’SHEA, CRYAN & BOGAN: UNITED STATES BAT SPECIES OF CONCERN 
87 
lethargic within roosts (Bradshaw, 1961, 1962; Leitner and Ray, 1964). They also are capable of 
surviving somewhat lower body temperatures for short periods in laboratory experiments (Reeder 
and Cowles, 1951), but the thermoneutral zone (where increased metabolism is not required to 
maintain a stable body temperature) is limited to body temperatures of 33°C and above (Bell et al., 
1986). Using warm roosts in winter and minimizing energetic costs of echolocation appear to have 
allowed this species to extend its range farther north than any other bat in the mostly tropical Fam¬ 
ily Phyllostomidae (Bell et al., 1986). 
Shallow caves that are suitable summer retreats may be abandoned seasonally in winter (How¬ 
ell, 1920a), and in California these bats regularly use naturally geothermally warm abandoned 
mines during cold months (Bell et al., 1986; Brown et al., 1993a,b). Use of different min e tunnels 
during summer and winter also has been reported in northwestern Arizona (Cockrum et al., 1996). 
During winter in the California desert, where night-time temperatures can drop as low as 0°C, they 
are known to form colonies (about 200 bats or more in size) in just a few geothermally heated 
desert mines and will switch among these sites if disturbed (Bell et al., 1986). 
Fewer than 20 geothermally warm winter roosts were known in California, all in abandoned 
mines (Brown et al., 1993b). The largest currently known winter colony in the U.S. is in an aban¬ 
doned mine on Bureau of Land Management lands in southeastern California, where counts of up 
to 5,000 have been made since 2002 (Brown, 2013). Winter counts during emergence at another 
mine on Bureau of Land Management property in Arizona were as high as 3,500 in 2002, but fluc¬ 
tuated among years (Brown, 2013). Recent winter emergence counts of over 2,000 bats have been 
documented at another long-occupied and now gated mine in southeastern California (Brown, 
2013). In southwestern Arizona, one mine in the Trigo Mountains held about 1,500 to 2,000 Cali¬ 
fornia leaf-nosed bats in recent winters, with up to 3,500 estimated in 2002 (Brown, 2013). The 
largest winter colony size reported in mines in the Agua Dulce Mountains of extreme southern Ari¬ 
zona was about 500 bats (Schmidt, 1999). California leaf-nosed bats do not form dense clusters in 
winter (Brown, 2013). 
Warm Season Roosts: Vaughan (1959) described daytime roosts of California leaf-nosed bats 
in caves, deserted mine tunnels, and grottos in the Riverside Mountains of southeastern California, 
where these bats occurred in groups of from just a few to 100 or more. They were usually within 
9 to 24 meters of entrances and did not seem to require completely dark retreats. Most of the tun¬ 
nels observed to harbor bats were from 1.5 to 2.0 meters high and wide and five to over 30 meters 
deep. Bats were not observed roosting in tight clusters, but small groups of up to 20 individuals 
were observed with each bat slightly separated from adjoining individuals (Vaughan, 1959; see also 
Cockrum et al., 1996). 
Maternity colonies form during summer in mines or caves where temperatures reach 27-32°C 
(Brown and Berry, 1991). Banding studies suggest life-long fidelity to roosts but also show that 
movement to alternate sites may occur when the bats are disturbed (Brown et al., 1993a,b). Roosts 
in the Arizona portions of the range are in habitats that usually do not reach temperatures as low as 
in parts of California, and some of these caves and mi nes may be occupied year-round, whereas 
others may function principally as summer or winter roosts (Hoffmeister, 1986; Schmidt, 1999). At 
a well-studied roost in an abandoned mine near Silverbell in southern Arizona the population of up 
to 350 individuals consisted of about half males and half females during March and April, but in 
summer months females segregated into maternity colonies and males broke into small groups 
(Bradshaw, 1961). From August through October, the sexes mixed again at the roost and mating 
took place, with an influx of bats seen in November during a presumed local migration; numbers 
declined during winter and only males were present (Bradshaw, 1961). Seasonal changes in colony 
sizes have been reported in mines in the Agua Dulce Mountains of extreme southern Arizona, with 
