One of the most famous residents of El Malpais are bats. The most commonly seen species during the summer is the Brazilian free-tailed bat (Tadarida brasiliensis), however this is only one of fourteen species that have been observed in the area or call El Malpais home.
These species are some of the more than 1,200 species of bat in the world. These range from the California myotis (Myotis californicus)--one of the smallest bat species in North America with a wingspan of 9 inches (22 cm)--to the fruit bats and “flying foxes” of Asia and Australia whose wings can span up to almost 6 feet (2 meters) . Bats make up one fifth, or twenty percent, of all mammals around the world.
Like all mammals, bats give birth to live young, produce milk, and have fur. However, bats are the only mammal capable of true, sustained flight. Bat wings consist of modified hand structures, which create lift and momentum using the same motion as Olympic swimmers doing the butterfly stroke.
There is a large variety within the 1,200 species of bats. Although most of the bats at El Malpais eat insects, other species consume pollen or nectar, serving as pollinators for plants used by humans, typically as food. Some of these source plants include bananas, mangoes, figs, cashews, and agave. In regions where fruit bats and “flying foxes” are more common, they also act as seed dispersers after eating overripe fruit and excreting the seeds.
Over time, humans have found bats’ habits to be beneficial to our way of life. Roughly 70 percent of bat species are insectivorous, and happen to eat the many of the same night-flying insects whose larvae eat agricultural crops. Diets vary according to bat species, but moths, mosquitoes, and beetles are common menu items for bats. Bats will typically eat half of their body weight in insects in a single night, and a nursing female may eat twice that in order to sustain the needs of herself and her pup. People have also used bat feces—more commonly known as “guano”—as an ingredient in plant fertilizer due to its high phosphorus and nitrate levels.
Human activity also impacts bats. Expanding development may change roosting sites for bats to use, or eliminate needed food sources to survive. Cavers may transport fungal spores that cause white-nose syndrome from one cave to another with contaminated gear without knowing.
Unique Adaptations for Unique Environments
Bats are one of the few types of mammals capable of echolocation, or the generation of high-frequency sounds similar to SONAR. Most bat species capable of echolocation eat insects, and use this ability to detect their food’s size, shape, speed, and movement. Depending on the bat species, some of these high-frequency calls are above the threshold of human hearing, reaching as high as 100 kHz. The size of the bat generally corresponds with frequency of the sound the bat makes: larger bats usually have a lower frequency sound, and the smaller bats have a higher frequency.
These high frequencies are typically released as a series of short pulses, and bats’ large ears pick up the echoes of these pulses. The rate and change of sound frequency help bats determine their surroundings, the type of prey that may be around them, and their prey’s location. Typically, the higher the frequency a bat emits, the smaller the insects it can detect.
Bats also use lower-frequency sounds for communication. Some of these are within the human range of hearing (20 to 20,000Hz). These vocalizations can be for claiming and protecting territory, attracting a mate, warning calls, recognition between roosting individuals, and helping a mother bat find her pup in the dark.
It is important to note that both vocal and echolocation calls are used in combination with bats’ regular eyesight. Contrary to the phrase, “blind as a bat,” no bat is completely blind. Echolocation is used at night and especially in caves for bats to better visualize their environment, but they still use their eyes to see areas beyond that range.
All bat species in the United States have dichromatic vision, or are red/green colorblind so see the world in shades of blue and yellow. Biologists have suggested that this trait helps bats better detect colors in low light conditions. Some species of bats have been known to see colors in the ultraviolet (UV) spectrum, while others have been observed using polarized light on the horizon to visually orient themselves.
A PDF document listing El Malpais bats is under construction for future download.
Hibernation and Migration
Many bats will either migrate to warmer regions or hibernate when less food is available during the winter. Good hibernation roosts, or hibernacula, are typically found in caves much like the lava tubes at El Malpais, abandoned mines, or other protective shelters within forests such as hollow tree cavities or large gaps within tree bark. Some species of bats may roost in large colonies, while others may be solitary.
Regardless of where or how closely together bats congregate, hibernating bats enter a state of torpor by late fall. This torpor means a bats’ body temperature drops to within a few degrees of their environment’s ambient temperature, their heart rate slows, and metabolic activities are greatly reduced. Hibernation typically lasts 5-6 months in which the bats must survive on a small amount of fat storage.
Although bats do periodically awaken on their own to drink, urinate, shift, and mate, excessive disturbances can be deadly. Every time a bat wakes up, it burns its fat stores quicker than necessary, and if woken often enough can burn through it all and starve before spring returns. Cave closures at El Malpais occur seasonally to protect migrating bats. White-nose syndrome, a devastating fungal disease, has been the cause for large winter die-offs of millions of hibernating bats in the United States.
Bat migrations vary not only between species, but individual populations. The most common long-distance migratory bat populations at El Malpais is the Brazilian free-tailed bat, which spends the summers in the lava tube caves and winters in the Sierra Madre mountain range in Mexico. On the contrary, free-tailed bats from western Arizona spend their winters in Baja, Mexico, and a population in eastern Texas does not migrate at all, but will shift roost sites throughout the season.
See Bats at El Malpais
Watching thousands of bats pour out of a cave on their nightly search for food is an experience many remember for years. Learn how to see the bats on a warm summer evening at El Calderon!