Eumops perotis
western bonneted bat

The distribution of the western mastiff bat is patchy. It can be found from the coast of the southwestern United States into central Mexico and southeast to Cuba. The northern limit of its range is the southern half of California. In the United States it extends southeast into western Texas through southern Nevada and southwestern Arizona. The southern limit of its range is in Argentina. This species is non-migratory (Hall, 1981, Allen, 1987, Cockrum, 1960).

Suitable habitat for the western mastiff bat consists of extensive open areas with potential roost locations having vertical faces to drop off from and take flight, such as crevices in rock outcropings and cliff faces, tunnels and tall buildings. This species inhabits various types of open, semi-arid to arid habitats. These include coastal and desert scrublands, annual and perennial grasslands, conifer and deciduous woodlands, as well as palm oases (Ahlborn, 2000; Cockrum, 1960; Allen, 1987).

Eumops perotis is easily identified by large ears united across the top of its skull and projecting about 10 mm beyond its snout. It is the largest molossid in North America. Characteristic to the family Molossidae, its wings are distinctively long but rather narrow. Their flight membranes are tough and leathery. This is a free-tailed bat whith relatively large feet. Its pelage is short, velvety, and whitish at the roots. Coloration is dark to greyish brown dorsally and more pale ventrally. The dental formula is I 1/2, C 1/1, P 2/2, M 3/3 X 2 = 30. Both sexes of the western mastiff bat possess a peculiar dermal gland on the throat which looks like a pouch and produces an odoriferous secretion, athough this gland is much more developed in the males (Ahlborn, 2000; Texas Tech, 1997; Allen, 1987).

Males attract females with secretions from their enlarged dermal gland during the mating season.

Males and females of this species remain together throughout the year, including the period when young are produced. Mating occurs in early spring when the dermal gland of adult males is most functional and the testes enlarge and descend. Normally only one young is produced per pregnancy, with twins being very rare. Eumops perotis is a eutherian with a gestation period of approximately 80 to 90 days. The offspring are dull black in color at birth and are naked, except for tactile hairs on the feet and face. The period of parturition usually extends from June into July, varying more than for any other bat in the United States. A nursery colony of these bats may contain young ranging from newborn individuals to ones already several weeks old. Nursery roosts are located in tight rock crevices or holes in buildings at least 90 cm deep and 5 cm wide (Texas Tech, 1997; Ahlborn, 2000).

Males remain with females during the period when young are produced but it is uncertain what degree of assistance in care they actually provide.

This species is non-migratory and may move among alternate

daytime roosts. Some roost sites are occupied throughout the year although different seasons are usually spent at different sites. Roost entrances are typically horizontally oriented with moderately large openings and face downward so that they can be entered from below. Western mastiff bats seeks out roosts below which there is an unobstructed drop of several meters allowing for sufficient momentum to become airborne to be achieved when they take off. Eumops perotis exhibits yearlong nocturnal activity and generally goes into daily torpor from December through February, maintaining foraging activity at night except when temperatures drop below 5 degrees C.

Western mastiff bats are capable of fast and prolonged flight but can not get airborne from the ground. They will scramble up a post or tree in order to achieve a minimum height of some 5 m necessary for launching into flight. Their colonies are small, usually numbering fewer than 100 members, and adult males can be found in maternity colonies (Cockrum, 1960; Texas Tech, 1997; Ahlborn, 2000).

Western mastiff bats feed primarily on insects which they catch in flight. These bats rarely utilize night roosts and feed at night, with foraging ranges exceeding 24 km from roost sites, and a long foraging period of 6 to 7 hours. Prey includes relatively small, low-flying, and weak-flying insects. They usually feed from ground to tree-level but may soar to heights of some 60 m in rugged terrain. It is interesting to note that flightless insects, including ants and crickets, comprise part of their diet even though these bats are unable to take off from the ground, requiring that the prey be snatched up as the bat flies by. These prey items are likely to be taken from surfaces such as canyon walls (Cockrum 1960; Texas Tech, 1997; Ahlborn, 2000).

Some insect prey include: moths, crickets, grasshoppers, bees, dragonflies, leafbugs, beetles, true bugs, ants and wasps.

Eumops perotis is an insectivore feeding primarily on flying insects.

Western mastiff bats feed on various insects and may play a role in controlling their populations, hence decreasing losses to agricultural products upon which these insects feed.

This species sometimes roosts in high buildings or tunnels where it can be an unsightly nuisance.

Apparently, litte data is available for the current status of this bat species. Bat Conservation International lists Eumops perotis on its Threatened and Endangered Bats List due to the the fact that it uses only select drinking sites and is severely limited by the availability of drinking water. Because its wing structure is adapted for fast and straight-line flight, it is unable to drink from water sources less than 30 m long. As a consequence, western mastiff bats are no longer found in many previously occupied areas and populations may be in decline (Acker, 2001).

Temperate North American bats are now threatened by a fungal disease called “white-nose syndrome.” This disease has devastated eastern North American bat populations at hibernation sites since 2007. The fungus, Geomyces destructans, grows best in cold, humid conditions that are typical of many bat hibernacula. The fungus grows on, and in some cases invades, the bodies of hibernating bats and seems to result in disturbance from hibernation, causing a debilitating loss of important metabolic resources and mass deaths. Mortality rates at some hibernation sites have been as high as 90%. While there are currently no reports of Eumops perotis mortalities as a result of white-nose syndrome, the disease continues to expand its range in North America. (Cryan, 2010; National Park Service, Wildlife Health Center, 2010)