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PROCEEDINGS OF THE CALIFORNIA ACADEMY OF SCIENCES 
Series 4, Volume 65, Supplement I 
ment, and Brown (2013) noted they also use abandoned mines in southeastern California as night 
roosts. About 200-300 night roosted in buildings at the Imperial National Wildlife Refuge in south¬ 
western Arizona (Castner et al., 1995). A road tunnel in western Colorado was used as a night roost 
by 50-100 adult female and juvenile Yuma myotis, where they roosted together with male Brazil¬ 
ian free-tailed bats (Neubaum, 2017). 
Population Ecology. — Litter Size, Natality, and Female Reproduction: Litter size is 
usually one, although a female with three embryos has been documented (Finley et al., 1983). 
Dalquest (1947b) reported 63 females with one embryo each at multiple locations in California and 
at Wadsworth, Nevada. Single embryos were found in seven females from western Oklahoma 
(Glass and Ward, 1959) and in 12 females at Bosque Del Apache National Wildlife Refuge in New 
Mexico (Mumford, 1957; Commissaris, 1959). Hall (1946) reported 24 females each with single 
embryos in Nevada. One female from Imperial County, California, had a single embryo (Howell, 
1920a), as did a female sampled in southern Colorado (Davis and Barbour, 1970). One female from 
Sinaloa, Mexico, also had a single embryo (Jones et al., 1972). Four females each with single cling¬ 
ing young were reported by Dalquest (1947b) at a California roost. Natal sex ratios are 1:1 (Milli¬ 
gan and Brigham, 1993). 
Natality at maternity roosts has been suggested to be about 100% (Hall, 1946; Herd and Fen¬ 
ton, 1983), although Dalquest (1947b:245) noted that in California there are “a sizeable proportion 
of non-bearing females”. At Bosque Del Apache National Wildlife Refuge in New Mexico, all of 
41 females sampled at a maternity roost were reproductive in 1953 (Mumford, 1957) and 34 of 35 
(97%) were reproductive in 1957 (Commissaris, 1959). All of 16 females (100%) taken primarily 
at roosts in the Mogollon Mountains of southwestern New Mexico and adjacent Arizona during 
June and July in 1960 to 1962 were reproductive (C. Jones, 1964). Twenty-three of 25 females 
(92%) examined at a maternity roost in Wadsworth, Nevada, during 1945 were pregnant (Dalquest, 
1947b), and 40 of 45 females (89%) examined at about eight different locations in California dur¬ 
ing 1945 or earlier were pregnant (Dalquest, 1947b). It has been suggested that this species gives 
birth at age one year in British Columbia, but based only on indirect evidence (Herd and Fenton, 
1983). Frick et al. (2007; based on unpublished data from four colonies) noted only a 42% proba¬ 
bility of female Yuma myotis breeding in their first year. 
Although females measured at maternity roosts show high natality, somewhat fewer are repro¬ 
ductive when sampled away from maternity colonies. In British Columbia, female reproductive 
rates of Yuma myotis varied annually, with lowest rates during a summer with lengthy periods of 
cool, rainy weather (Grindal et al., 1992). Rates were 100% in 66 females captured over water in 
south-central British Columbia during 1979 (Fenton et al., 1980), about 90-95% in 89 females 
sampled over water in June and July during 1982 (Herd and Fenton, 1983), and about 18-30% in 
68 bats sampled over water in June and July of 1990, an unusually wet summer when it was sus¬ 
pected that rainfall affected both thermal energetics and the ability to forage, perhaps resulting in 
resorption of embryos (Grindal et al., 1992). Ten of 12 (83%) shot over water in Monterey Coun¬ 
ty, California were reproductive (Dalquest, 1947b). Easterla (1973) reported that 12 of 16 (86%) 
females captured in Big Bend National Park, Texas were reproductive during summers 1967-1971, 
as were six of eight (75%) females captured at net sites at Bosque del Apache National Wildlife 
Refuge in New Mexico during 1997 (Chung-MacCoubrey, 1999). 
Survival: Annual apparent survival estimates were calculated for Yuma myotis at two roosts 
unaffected by a contaminant spill (see “Mortality” below) in comparison with two roosts in the spill 
area (Frick et al., 2007). Apparent survival of adults was unaffected in the spill area and increased 
from 0.72 to 0.88 at all four roosts over the period 1992 to 1995, coinciding with increasing habi¬ 
tat recovery from a prolonged regional drought; juvenile survival in the area not subject to the spill 
also increased (from 0.60 to 0.80) over the same time period but was always lower than adult sur- 
