Western Mastiff Bat
Order Chiroptera : Family Molossidae : Eumops perotis (Schinz)

Description. A large free-tailed bat, similar to Tadarida and Nyctinomops in general appearance but nearly twice as large; foot large, ratio of foot to tibia about 60; ears large, united across the forehead and projecting about 10 mm beyond the snout; second joint of fourth finger about 6 mm; pelage short and velvety; upperparts brown or grayish brown, bases of hairs whitish; underparts paler. Males have a peculiar glandular pouch on the throat. Dental formula: I 1/2, C 1/1, Pm 2/2, M 3/3 X 2 = 30. External measurements average: total length, 167 mm; tail 57 mm; foot, 17 mm; ear, 40 mm; forearm, 76 mm.

Distribution in Texas. From southwestern United States (western Texas to California) southward to northern Argentina and southern Brazil, but not yet reported from Central America. In Texas, it has been taken at localities near the Rio Grande in Val Verde, Brewster, and Presidio counties.

Habits. Away from human habitations, this bat generally seeks diurnal refuge in crevices in rocks that form vertical or nearly vertical cliffs. The roost entrances typically are horizontally oriented, have moderately large openings, and face downward so they can be entered from below. In Capote Canyon in Presidio County, Texas, these bats were found utilizing a crevice formed by exfoliation of the nearly vertical rim rock. There are openings on both the lower and upper edges of the slab. At this site the canyon wall is about 38 m high and the rather steep slope below the cliff has no tall vegetation that might obstruct the takeoff and landing of the bats. Most authors agree that the bats choose a roost below which there is an unobstructed drop of several meters so that the emerging bats can drop and gain sufficient momentum to become airborne. Captive bats are unable to take off from the ground or from flat surfaces, and also are unable to maintain flight after launching themselves from the tops of tables. Bats tossed 4.5 m high in the air, however, are able to become airborne but those thrown half that distance cannot.

Colony size varies from two or three individuals to several dozen. Twenty individuals is a large colony of these bats although colonies of up to 70 are known. Harry Ohlendorf counted 71 individuals as they left the Capote Canyon roost about sunset on January 30. The first bats emerged about 6:45 p.m. and within 10 minutes 30 of them had taken wing. During the next 15 minutes 19 more emerged; 12 more took off during the next 10 minutes; four more in the next 15 minutes, and two more in the last 10 minutes. Thus, the exodus of the 71 bats was strung out over a period of 50 minutes. Just before launching themselves into flight, and during flight, the bats utter a series of loud, shrill, chattering calls that can be heard for a considerable distance.

These bats leave their day roosts late in the evening to forage. The stomachs of 18 bats collected in Big Bend National Park contained moths (79.9%), crickets, (16.5%), grasshoppers (2.8%), and unidentified insects (0.7%). Bees, dragonflies, leafbugs, beetles, and cicadas have also been reported in their diet. These bats are not believed to use night roosts, but instead soar at great altitudes all night long so that they can feed over wide areas. Insects carried aloft by thermal currents probably furnish an important portion of their diet. The presence of flightless insects, such as crickets, in their diet is interesting as these bats are unable to take off from the ground and therefore, cannot alight to capture such prey. These prey items could be picked from canyon walls as the bats forage.

Observations indicate that males and females of this species remain together throughout the year, even during the period when young are produced. Normally only one young is produced per pregnancy, but occasionally a female may give birth to twins. The period of parturition probably extends from June to early July and a nursery colony may contain young ranging from newborn individuals to ones that are several weeks old. At birth the young are dull black in color. The gestation period is approximately 80-90 days.

Photo credit: Merlin D. Tuttle, Bat Conservation International.