O’SHEA, CRYAN & BOGAN: UNITED STATES BAT SPECIES OF CONCERN 
139 
they are inconsequential because they merely represent some small portion of the total population 
of M. lucifugus. Our results suggest that declines in M. occultus might jeopardize a unique south¬ 
western species.” 
Discovery and Early Findings: Hollister (1909) described M. occultus as a new species based 
on two specimens shot in flight on 14 May 1905 among cottonwood trees along the west side of 
the Colorado River, 16 kilometers upstream from Needles, California. Morphologically M. occul¬ 
tus was considered distinctive compared to other Myotis known at the time in that the skull had a 
low flat braincase and the rostrum was wide and flat; the specimens also showed reduction or loss 
of the upper third premolar (Hollister, 1909). J. Grinnell (1914) obtained an additional six speci¬ 
mens shot along the lower Colorado River in California (eight kilometers northeast of Yuma, Ari¬ 
zona) during May 1910, and noted that these also had a broad, flat-topped rostrum and braincase, 
and that three specimens lacked the upper third premolar, which was also much reduced in the other 
three specimens. The distinctiveness of these characters was affirmed by H. Grinnell (1918). An 
additional specimen was obtained in 1920 along the Rio Grande near Las Cruces, New Mexico; 
this specimen had one minute third premolar on one side of the upper jaw (Allen, 1922). In a sub¬ 
sequent systematic review of all North and South American myotis, Miller and Allen (1928) exam¬ 
ined a total of 27 specimens from California, Arizona, New Mexico, and Sinaloa, Mexico, and ver¬ 
ified the validity of the species based on the cranial characteristics noted by the previous authors. 
However, they also presaged later investigations with the comment (Miller and Allen, 1928:24) that 
“This name is applicable to a peculiar and imperfectly known species inhabiting the southwestern 
United States and adjoining parts of Mexico.” Stager (1943b) examined a further 91 skulls from 
southern California and confirmed the tendency for loss of the second upper premolar in 62 indi¬ 
viduals. 
Re-Classifying as a Subspecies and Subsequent Debate: The Arizona myotis was consid¬ 
ered a valid species for the next four decades after the work of Miller and Allen (1928), until Find¬ 
ley and Jones (1967) examined a larger sample of M. occultus and M. lucifugus from the south¬ 
western U.S. and Mexico. They suggested that differences in rostral area between more northern 
specimens assigned to M. lucifugus carissima (65 specimens) and southern specimens assigned to 
M. occultus (260 specimens) were a function of a clinal gradient in overall skull size (length of 
maxillary tooth row was a correlate of measures of skull size), with northern forms having overall 
smaller skulls and M. occultus showing more variability in skull size (Findley and Jones, 1967). 
However, they also noted that few specimens were available from geographically intermediate 
areas to verify this pattern. They remarked that most samples of putative M. occultus had “rela¬ 
tively very large teeth” compared to teeth of M. lucifugus from northern populations, and that the 
occultus forms have a more prominent sagittal crest and show a reduction in numbers of upper pre¬ 
molars (Findley and Jones, 1967:437), consistent with findings of past authors (Hollister, 1909; 
Grinnell, 1914; Miller and Allen, 1928). However, based on their overall geographic comparisons 
of cranial size and rostral area, the authors felt that geographic intergradation was present (but with 
just four intergrade specimens examined) and stated (Findley and Jones, 1967:438), “We tenta¬ 
tively conclude that M. occultus is a large-skulled, large-toothed southwestern race of M. lucifu¬ 
gus, ” On this basis they assigned 260 of the specimens examined from 38 localities as the sub¬ 
species M. lucifugus occultus rather than as M. occultus (Findley and Jones, 1967). 
Findley and Jones (1967) [and later Findley et al. (1975)] suggested that populations of the 
putative subspecies M. lucifugus occultus consisted of larger individuals where there was compe¬ 
tition with greater numbers of other species of Myotis in areas of sympatry. Subsequently, Barbour 
and Davis (1969) suggested that hybridization rather than intergradation may occur between the 
two species and suggested that more evidence to support the change from species to subspecies sta- 
