Ultrasound presents two challenges for the animal trying to hear it. First, high frequency translates to short wavelengths; the hearing organ must be miniaturized to match the wavelength. Second, high frequency sounds tend to be supported by little energy. Not only do they dissipate rapidly as the sound travels, making them relatively faint even close to the source, they also are subject to absorption by the hearing organ without being transduced into a signal to the central nervous system.

In order to accommodate the lower energy of ultrasound, the hearing membrane, or tympanum, is typically thinner in animals which rely on ultrasound for communication or navigation. The outer ears (pinnae) of mammals which perceive high frequency sound may be quite complex; bat ears are characterized by grooves and channels which help to carry sounds to the typmpanum, as well as maintaining small differences in frequency (pitch) and amplitude (volume) which can be used to localize sound sources.

Ultrasonic signals are produced in two contexts. First, in echolocation, an animal (generally we think of bats doing this) produces high pitched sounds which are reflected off objects in the bats flight path. The use of high pitched sounds (ultrasound) has several advantages in echolocation. First, the short wavelength of these sounds makes them more likely to bounce back to the bat, rather than bend around the object. This, of course, is essential if echoes are to be used in orientation. Second, it takes relatively little energy to produce these sounds, and third, they dissiapate rapidly, reducing confusion from "old" sounds that could still be bouncing around an area.

Second, ultrasounds are used by several kinds of animals in social contexts. Bats use ultrasounds to communicate with mates, as do murid rodents (rats and mice) and various sorts of moths. In addition, rodent pups use ultrasound to call their mothers if they become isolated from her.

Because bats prey on insects, many insect species are attuned to bat echolocation calls and take evasive measures if they hear a bat call. Males produce a calling song to attract females in greater wax moths (Galleria mellonia) and lesser wax moths (Achroia grisella); they stop calling if a calling bat approaches; presumably the bat can orient to the mating call of the moth.

Blake B. H. 2002 Ultrasonic calling in isolated infant prairie voles (Microtus ochrogaster) and montane voles (M-montanus). JOURNAL OF MAMMALOGY 83 (2): 536-545
Greenfield MD, Baker M 2003 Bat avoidance in non-aerial insects: The silence response of signaling males in an acoustic moth. ETHOLOGY 109 (5): 427-442
Greenfield MD, Tourtellot MK, Tillberg C, Bell WJ, Prins N 2002. Acoustic orientation via sequential comparison in an ultrasonic moth. NATURWISSENSCHAFTEN 89 (8): 376-380
Jones G, Barabas A, Elliott W, Parsons S 2002Female greater wax moths reduce sexual display behavior in relation to the potential risk of predation by echolocating bats BEHAVIORAL ECOLOGY. 13 (3): 375-380
Moles A, D'Amato FR 2000. Ultrasonic vocalization by female mice in the presence of a conspecific carrying food cues. ANIMAL BEHAVIOUR 60: 689-694
Surlykke A, Yack JE, Spence AJ, Hasenfuss I 2003 Hearing in hooktip moths (Drepanidae : Lepidoptera). JOURNAL OF EXPERIMENTAL BIOLOGY. 206 (15): 2653-2663
Thornton LM, Hahn ME, Schanz N 2003 Genetic and developmental influences on infant mouse ultrasonic calling: Patterns of inheritance of call characteristics BEHAV GENET 33 (6): 721-721

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