Western Mastiff Bat (Eumops perotis)

Protection Status Notes

Since western mastiff bats produce piercingly loud calls that can easily be heard, experts can use these to assess status trends. They report an alarming loss of these bats' magical sounds over the Southwest over the past 30 years. While little specific information is available on population trends in Texas, severe declines have been documented in the Los Angeles basin in California, where it is considered a Species of Special Concern.

Description

Eumops perotis can be distinguished from all other North American molossid ( free-tail) species based on size. With a forearm of 73‑83 mm, it is North America's largest species.

Life History

Although maternity roosts for many bat species contain only adult females and their young, some E. perotis colonies contain adult males and females at all times of year. Unlike vespertilionids which mate in the fall, North American molossids, including E. perotis, appear to mate in the late winter/early spring and give birth to a single young in the early to mid‑summer. Available data suggest that, although most E. perotis young are born by early July parturition dates vary extensively and births are not synchronous, even within colonies. Unlike some molossid species (e.g. Tadarida brasiliensis) which undergo long distance seasonal migrations, E. perotis appears to move relatively short distances seasonally. It does not undergo prolonged hibernation, and appears to be periodically active all winter, and thus may seek winter refugia that are protected from prolonged freezing temperatures.

E. perotis is primarily a cliff‑dwelling species, where maternity colonies of 30 to several hundred (typically fewer than 100) roost generally under exfoliating rock slabs (e.g. granite, sandstone or columnar basalt). It has also been found in similarly crevices in large boulders and buildings. Roosts are generally high above the ground, usually allowing a clear vertical drop of at least 9.8 feet below the entrance for flight.

Due to its audible echolocation call, E. perotis can be readily detected in foraging areas. The diet consists primarily of moths (Lepidoptera), but also includes crickets and katydids.

Habitat

In California, the E. perotis is most frequently encountered in broad open areas. Generally, this bat is found in a variety of habitats, from dry desert washes, flood plains, chaparral, oak woodland, open ponderosa pine forest, grassland, montane meadows, and agricultural areas.

Distribution

E. perotis has a disjunct distribution, with two subspecies confined to South America. The subspecies that occurs in North America, E. p. californicus, ranges from central Mexico across the southwestern United States (parts of California, southern Nevada, Arizona, southern New Mexico and western Texas). Recent surveys have extended the previously known range to the north in both Arizona (several localities near the Utah border) and California (to within a few miles of the Oregon border). The species has also been detected acoustically in southern Utah. Published information suggests that the species occurs only to 1,230 feet in California, and 3,600 feet in Texas. Recent surveys in California, however, have documented roosts up to 1,300 feet, and foraging animals at > 8,800 feet. The distribution of E. perotis is likely geomorphically determined, with the species being present only where there are significant rock features offering suitable roosting habitat.

Threats and Reasons for Decline

Loss of large open-water drinking sites seems to pose a serious threat to mastiff bats in the Southwest. This species is also threatened by urban/suburban expansion, and by activities that disturb or destroy cliff habitat ( e.g. water impoundments, highway construction, quarry operations). Recreational climbing is another potential threat. Pest control operations have eliminated most known building colonies in the Los Angeles basin. Grazing and pesticide applications in agricultural areas may impact foraging habitat.

Ongoing Recovery

Loss of large open-water drinking sites seems to pose a serious threat to mastiff bats in the Southwest. Even where land has been set aside for national parks, open ponds are often lost, especially since when cattle are removed, wells are not maintained, and vegetation is allowed to overgrow the ponds. Prior to overgrazing, many of these places supported excellent springs and broad flowing streams.

Other

More surveys are needed, using acoustic techniques, to delineate the range of this species. More information is needed on distribution of breeding colonies, seasonal movements, roosting and foraging requirements. Methods need to be developed for assessment and on-going monitoring of population size.