Jakobsen, L., Ratcliffe, J. M. and Surlykke, A. (2013). Convergent acoustic field of view in echolocating bats.Nature. 493, 93-96.
Bats have surpassed all other some odd 6000 species of mammals by evolving the ability to fly. Bats use powered flight to streak through the night, unlike those wanna-be “flying” squirrels, sugar gliders and Brazilian daredevils. In addition to the unique ability to fly, microbats use sonar to “see” delicious insects whilst flapping about (versus the megabats aka flying foxes that presumably use vision to target juicy fruits). Microbats, although micro, come in a range of sizes (~4-16 cm, some would say). The size of a bat relates to the peak frequency of an emitted sonar beam, so that smaller bats use high frequency sonar beams and larger bats use lower frequency sonar beams. For years, bat fanatics have explained this frequency-size relationship under a dubious assumption. The story goes that, because small bats eat small insects, they need higher frequencies (shorter wavelengths) in their sonar beams to adequately reflect off the smaller prey, and vice versa for larger bats. As explained in this most exquisite batpape, this story is a damned lie!
First, even small insects will reflect echoes in the lower end of the bat’s call range. Second, by rudely removing hard-caught meals from bat stomachs, most bats are found to have eaten insects with lengths much shorter than the wavelengths in their sonar beams. To get to the bottom of this bat quandary, these ill authors used a microphone array to measure the shape of the sonar beams emitted by several different species, covering a range of bat sizes, while flying in the same flight arena. They found that each bat species could adjust the shape of the sonar beam dynamically to be wide (encompassing a large volume around the bat) or narrow (encompassing a smaller, more focused region in front of the bat). Narrow sonar beams concentrate energy, which counteracts atmospheric attenuation of high frequency sound, thereby increasing range. Wider beams are better for detecting peripheral targets at shorter ranges. In fact, each species studied used the same sonar beam shape in the fixed experimental arena, predicting that these bats were optimizing their calls to suit the environment that they were placed in.
But how does a gruesome bat change the shape of its sonar beam? There are some options: (1) the frequency of the call can be adjusted, with higher frequencies producing narrower beams than lower frequencies, and (2) the width of the mouth (gape) during a call can be adjusted, with larger gapes producing narrower beams than smaller gapes. Smaller bats have smaller mouths and cannot open as wide as their larger brethren. Smaller bats also cannot fly as fast as big bats in the open field. If you’re a fast flying bat, longer detection ranges are needed to avoid slamming into a tree or a windmill. For big bats, longer ranges are easily obtained by using lower frequency calls, which are more resilient to atmospheric attenuation, and their massive gapes can keep the beam narrow and focused. Small bats don’t have the big-mouth option; instead they must use higher frequencies to narrow the sonar beam, which explains the tendency for small bats to use higher frequency calls. In sum, this wicked batpape brings together an impressive amount of data to show that bats optimize their sonar beam shapes to suit their environments, and provides a vastly improved explanation for the relationship between call frequency and size.
Like the Joker, I am fascinated by this amazing bat technology. How do bats optimize their sonar beams on the fly? Presumably something interesting is going on in those bulging bat brains that allows them to converge on the appropriate beam for every occasion, no matter how big the bat might be.