|Canyon Bat or Western Pipistrelle (Parastrellus hesperus)|
Wt=5-6g, L=2.4-3.4" (60-86mm) -- the smallest bat in the U.S., and
one of the smallest mammals in North America. Pale yellow or gray to reddish-brown
above, whitish below. Wings, "tail", and feet blackish, as is
the face, ears, nose, giving it a masked appearance.
NATURAL HISTORY: Insectivorous, using echolocation to find its aerial prey. Nocturnal, but seen frequently by people (despite its small size) because it is often one of the first bats out in the late afternoon and one of the last to roost in the early morning. Nonmigratory. Usually solitary, but may form small maternity colonies. Unlike most bats that have only one offspring per year, Western Pipistrelle's have two offspring per litter that usually are born in June. See Bat Facts for more.
BAT ECHOLOCATION: Some bats emit high-pitched (ultrasound) chirps from their mouth and/or nostrils, then listen for the echo to form a "picture" of their environment and to determine distance, direction and speed of movement, and texture of the objects (e.g., prey) around them. Object distance is determined by the time interval between emitting the chirp and hearing the echo. Some bats studied can detect moths within an 8' (2.4m) range (Alcock 1984). The object's direction of movement is determined by Doppler shift -- objects moving toward bat reflect an echo of higher frequency than chirp and vice versa. The object's size is determined by the fact that small objects reflect high frequency sound better. Object shape and texture also are determined by quality of echo. Most bat ultrasonic chirps are in the range of 30-140 kilohertz (the upper limit of human hearing is about 20 kilohertz). The "funny" shields and flaps on many bat faces direct the sound.
INSECT DETECTION AND REACTIONS: "The encounter between a bat and an insect is one that might rival the tactics of modern air-to-air combat, involving an efficient early-warning system, some clever aerodynamic engineering and the simple economics of making do with what is available" (May 1991). Some moths (and other insects including locusts, tiger beetles, katydids, praying mantids, crickets) can detect bat ultrasound using "ears" located on either side of their thorax (or elsewhere -- e.g., in the knees of crickets). Each moth ear contains a tympanic membrane and two sensory fibers, called A1 and A2. The A1 fiber is highly sensitive to low-intensity, pulsed, ultrasound and begins firing when the bat is about 100 feet away (out of range of the bat). Rate of firing of fiber is proportional to loudness so indicates distance to bat. Direction to bat is indicated by timing and loudness to each ear (the moth's body blocks some sound to opposite ear); up or down is indicated by whether wings block sound (bat up) or not. The A2 fiber responds only when ultrasound chirps are very loud. When only A1 fibers are firing, moth will fly away from bat (intensity to both ears equal). When A2 fibers fire, an immediated reaction is initiated. Depending on the insect species, the insect may dive straight down, may spiral down, or may just fly "out of control" so that not even the moth knows where it is going. The purpose of this evasive moves are to cause the bat to miss; indeed, the insects were 40% less likely to be captured compared to when they were "deafened."
Here is a likely scenario. Daytime predation selected for insects that
could fly at night. The availability of flying prey at night selected
for night flying predators (namely bats). Bats developed echolocation
as a means of navigating and finding and catching prey. This selected
for insects that could detect (or even "jam") bat ultrasound
and evade capture. This selected for bats that could "whisper"
and still detect their echos, allowing the bats to "sneak" up
on their prey. Other bats began using chirps at a lower frequency than
the insect "ears" are tuned to, thus avoiding detection altogether.
Certainly, this evolutionary arms race is not over yet.