of rabies, at least in the United States. They do, however, contain certain pesticides. White-nose syndrome (WNS) caused by infection by the fungus P. destructans has increased in prevalence since 2006, mostly affecting species of bats that roost underground such as the little brown bat. The fungus, now suspected to have spread from accidental transportation by human cave workers, is thought to cause frequent arousals during bat hibernation, causing an individual to use fat stores much more quickly and die of starvation before the end of winter. WNS can affect T. brasiliensis, but has yet to be greatly introduced to their habitat due to their preference for more arid caves. WNS has low prevalence in the subtropical and tropical regions where T. brasiliensis resides. Behavior Mexican free-tailed bats are nocturnal foragers and begin feeding after dusk. They travel 50 km in a quick, direct flight pattern to feed. This species flies the highest among bats, at altitudes around . Bats appear to be most active in late morning and afternoon between June and September. Free-tailed bats are more active in warm weather. The species has been measured at a ground speed of , measured by an aircraft tracking device. The measurement methodology did not simultaneously record wind speed and ground speed, so the observations could have been affected by strong local gusts, and the bat's maximum air speed remains uncertain. Guano and ammonia Among bats that roost in great, concentrated numbers, T. brasiliensis roosts produce large quantities of urine and guano; from 22 to 99 metric tons per cave and over 18,700 metric tons are produced annually. The concentrated waste generates high levels of toxic ammonia in the air of a cave. T. brasiliensis individuals have genetic adaptation for withstanding or countering these high levels of ammonia. The bats’ content of CO2 and protein in respiratory mucus and CO2 dissolved in blood plasma increase with increasing levels of dissolved ammonia, providing the bats with a buffer against pH change. This allows the bats to filter out a large majority of inhaled ammonia before it reaches toxic levels in the blood. T. brasiliensis bats are thought to swarm in spiraling motions within caves to ventilate ammonia and renew the air. Echolocation Mexican free-tailed bats use echolocation for navigation and detecting prey. Traveling calls are of a brief but constant frequency. However, they switch modulated frequency calls between 40 and 75 kHz