Western Mastiff Bat: Eumops Perotis

Image of Western Mastiff Bat in arizona

The western mastiff bat is one of the largest bat species that call North America home. They can be found in the southern parts of the United States. They only eat insects and require a drop of 10 m to begin powered flight.

Learn More: United States Largest Bats

Description Of The Western Mastiff Bat

Western mastiff bats can be easily identified by their large ears that are united across their skull and protrude 10mm beyond the snout. In North America, it is the largest molossid.

Its wings are distinctly long but relatively narrow and are characteristic of the family Molossidae. This species has a rigid wing membrane and has relatively large feet. Its fur at the roots is short, velvety, and whitish.

The coloration is dorsally dark to greyish brown and ventral pale. A peculiar dermal gland on the throat looks like a pouch and produces an odoriferous secretion for both sexes of the western mastiff bat, although this gland is much more developed in males.

Western Mastiff Bat Behavior

This species is non-migratory and can move between alternative daytime roosting sites.

Although different seasons are generally spent at various sites, some roost sites are occupied throughout the year. Roost entrances with moderately large openings are typically horizontally oriented and face downwards so that they can be entered from below. Western mastiff bats are looking for roosts below which there is an unobstructed drop of several meters that allows for sufficient momentum to be achieved when they take off to become airborne.

Eumops perotis exhibits year-long nocturnal activity and from December to February generally goes into daily torpor, maintaining foraging activity at night except when temperatures drop below 5 degrees Celcius.

Western mastiff bats can fly quickly and for a long time but cannot get airborne from the ground. In order to achieve a minimum height of 5 m necessary for launching into flight, they will scramble up a post or tree. 1Go To Source animaldiversity.org -“Eumops perotis western bonneted bat”

Reproduction Of The Western Mastiff Bat

Copulation is most likely to occur in early spring when adult male testes (male reproductive cells) enlarge and descend. The duration of the gestation period is not known.

Parturition may occur in California from early April through August or September. Parturition dates for E. perotis differ more than any bat in the U.S.

A female of the Western mastiff bat species produces one offspring per year. Young are born naked (no fur), with their eyes closed, and have tactile hair on their feet and face.

Throughout the year, adults of both sexes can be found together. Both sexes have a dermal gland on the throat of western mastiff bats that produces an odoriferous secretion. This gland, which is highly developed in males, particularly in the spring, is likely to be associated with reproductive processes. 2Go To Source nrm.dfg.ca.gov -“WESTERN MASTIFF BAT Eumops perotis”

Western Mastiff Bat Diet

Moths (79.9 percent), crickets (16.5 percent), grasshoppers (2.8 percent), and unidentified insects were found in the stomachs of 18 bats collected at Big Bend National Park (0.7 percent ). Also reported in their diet are hawkmoths, bees, dragonflies, leaf bugs, beetles, and cicadas. 3Go To Source depts.ttu.edu -“WESTERN BONNETED BAT Eumops perotis (Schinz 1821)”

Habitats That Contain Western Mastiff Bats

With roost sites having vertical faces, this bat’s ideal habitat must have a large open area. In small colonies, they roost in rock fissures on high cliff faces.

This species requires 20 feet of vertical drop from their roosts to gain enough flight speed due to their large size. The bats will have to climb up a vertical surface if they end up on the ground in order to gain enough height to launch into flight. 4Go To Source desertmuseum.org -” Animal Fact Sheet: Greater Mastiff Bat”

Western Mastiff Bat Range

E. Perotis, with two subspecies restricted to South America, has a disjunct distribution. The subspecies californicus, which occurs in North America, ranges from central Mexico through the southwestern United States (parts of California, southern Nevada, Arizona, southern New Mexico, and western Texas). In both Arizona and California, recent surveys have extended the north’s previously known range (to within a few miles of the Oregon border). In southern Utah, the species has also been detected acoustically.

Published information suggests that the species occurs in California at only 1,230 feet and Texas at 3,600 feet. However, recent surveys in California have documented roosts of up to 1,300 feet and animals foraging at more than 8,800 feet. The allocation of E. Perotis is likely to be determined geomorphically, with the species only present where there are significant rock features that provide suitable roosting habitat. 5Go To Source tpwd.texas.gov -“Western Mastiff Bat (Eumops perotis)”



  1. Chebes, L. 2002. “Eumops perotis” (On-line), Animal Diversity Web. Accessed January 11, 2021 at https://animaldiversity.org/accounts/Eumops_perotis/
  2. Ahlborn, G. “WESTERN MASTIFF BAT Eumops Perotis.” California Department Of Fish And Wildlife, NRM, nrm.dfg.ca.gov/FileHandler.ashx?DocumentID=2357. Accessed 11 Jan. 2021.
  3. TTU. “A Species Account of the Western Bonneted Bat (Eumops Perotis) | Mammals of Texas | Natural Science Research Laboratory | TTU.” Texas Tech University, Texas Tech University Natural Science Research Laboratory, www.depts.ttu.edu/nsrl/mammals-of-texas-online-edition/Accounts_Chiroptera/Eumops_perotis.php. Accessed 11 Jan. 2021.
  4. “Greater Mastiff Bat Fact Sheet.” Arizona-Sonora Desert Museum, www.desertmuseum.org/kids/bats/greater_mastiff_bat.php. Accessed 11 Jan. 2021.
  5. “Western Mastiff Bat (Eumops Perotis).” Texas Parks And Wildlife, TPWD, tpwd.texas.gov/huntwild/wild/species/westmastiff. Accessed 11 Jan. 2021.