by Lewis Young
The Yuma Myotis (Myotis yumanensis) is another of the small, mostly brown colored bats that are found in Montana. In fact, it is so similar in looks to the more common Little Brown Bat that these 2 species are very difficult to tell apart in the hand. More about that later. The species was first described in 1864 from specimens taken near present day Yuma Arizona thus its name. Yuma Myotis have relatively short ears and dark membranes. Fur on the back is typically nonglossy, and color ranges from black through reddish brown to light brown with underfur that is light brown to yellowish white. Wingspan is approximately 9 inches, and weight is 5-6 grams (0.18-0.21 ounces). Life span has not been documented but likely is similar to other myotis species that can live 20-30 years.
In Montana, they have only been found in the northwestern portion of the state as we appear to be on the northeastern edge of their range. Yuma Myotis range from central British Columbia down to central Mexico and from the Pacific Ocean inland through Oregon, Washington, Idaho, California, Arizona, New Mexico, and parts of Nevada, Texas, Utah, Colorado, and Montana.
This species is more closely associated with water than most other North American bats. It is found in a wide variety of upland and lowland habitats, including riparian, desert scrub, moist woodlands, and forest, usually near open water. Yuma myotis are nocturnal, and forage for insects above the surface of standing or slow moving water or in vegetation close to the water’s edge. Foraging begins at dusk and finishes a few hours after sunset. They feed on a variety of insects and are opportunistic hunters with no preference for particular prey. Instead, they feed on whatever is most common in their area. They either catch the insects in their mouths or use their tail membranes as a pouch to snag larger insect prey. It is likely that they eat insects at a high rate similar to other bats—sometimes more than 1000 per hour.
As with all other bats in Montana, echolocation is used to navigate in darkness and find food. Ultra-high frequency sounds are emitted from the mouth then the ears detect the sound waves reflected off prey and inanimate objects. Yuma Myotis echolocation calls are in the 47-55 kilohertz range, well above human hearing capability. They also have social calls at much lower frequencies that are audible to humans.
Frequency is one of the ways to separate Yuma Myotis (47-55 kilohertz characteristic frequency) from Little Brown Bats (37-43 kilohertz characteristic frequency). Another way is to use DNA from a guano or skin sample. If the forearm length is less than 36.5 mm (1.4 inches), it may be either a Yuma Myotis or Little Brown Bat and then it is necessary to use DNA or call frequency to separate the two species. A forearm longer than 36.5 mm can safely be called a Little Brown Bat.
Roosts in spring and summer are typically in buildings, bridges, caves, mines, and hollow trees. Suitable locations may have thousands of individuals. Maternity roosts are located in warm sites that are favorable for growth of the young. Males tend to roost singly in the summer.
Winter roosts are cooler environments such as caves and mines. Hibernation takes place from October to April and involves an extreme reduction in metabolic rate, heart rate, and respiratory rate that allows them to survive long periods of time without food. The heart rate drops from 200-300 beats per minute to 10 beats per minute, and they may go minutes without taking a breath. The body temperature can also drop to near freezing, depending on the temperature of the bat’s surroundings. Other bodily functions also slow down, which reduces energy costs by about 98%. In this state of “torpor,” bats are experts in high energy efficiency! During hibernation, bats cycle through periods of torpor interrupted by brief periods of arousal when their body temperatures return to normal for a few hours.
Yuma Myotis are considered yearlong residents in Montana although no hibernation sites are known. It is possible that some short distance migration occurs between summer and winter.
Mating occurs in the fall before hibernation, but the females retain the sperm for several months. Ovulation and fertilization do not occur until spring then gestation is around 60 days. Females typically pairs with several males, each of which also pairs with several different females. One young is born between late May and late June and weighs around 1.4 g (0.05 oz) at birth. Initially blind and hairless, the eyes open around the fifth day, and the pups are completely furred by day nine. The young can fly in 3-4 weeks and are weaned in 5-6 weeks. Males and females are usually sexually mature at 1 year.
Their global conservation status is considered secure due to the wide distribution in western North America, use of both natural and human-made structures, and being locally common. In Montana, the Yuma Myotis is listed as a Species of Concern. Although populations of this species are believed to be stable, the threat of catastrophic decline from White-Nose Syndrome (WNS), a fungal disease of bats responsible for the deaths of millions of individuals of closely related species in other areas, presents a threat of substantial declines within the state. Recent observations from Washington have confirmed the susceptibility of this species to WNS infection.
Yuma Myotis are not easily observed because they are active at night and roost out of sight during the day, but they are a valuable component of our wildlife diversity and contribute to the amazingly large amount of insect control that bats provide at night.