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, 13 (7), e0200255

Ridge Number in Bat Ears Is Related to Both Guild Membership and Ear Length


Ridge Number in Bat Ears Is Related to Both Guild Membership and Ear Length

Brian W Keeley et al. PLoS One.


The ears of many mammals have a set of uniformly spaced horizontal ridges that form groove arrays. Contact of coherent waves (e.g. acoustic waves) with a series of slits or grooves causes diffraction, which produces constructive and destructive interference patterns. Increases in signal strength will occur but will depend on the frequencies involved, the groove number and their separations. Diffraction effects can happen for a wide range of frequencies and wavelengths, but no array can diffract wavelengths greater than twice the groove separation, and it is for those wavelengths comparable in size with the groove separation that the effects are greatest. For example, when ridges in bat ears are 1 mm apart, the strongest influence will occur for a 1 mm wavelength which corresponds to a frequency of 343 kHz. If bats could use these wavelengths, it would help them to resolve objects or surface textures of about 0.5 mm. Given how critical acoustics are for bat function, we asked whether bats may be taking advantage of diffraction effects generated by the grooves. We hypothesize that groove number varies with bat foraging strategy. Examining 120 species, we found that groove number is related to both guild and ear length. Bats in guilds that glean prey items from foliage or ground have on average more grooves than bats in other guilds. Harmonics generated by echolocation calls are the most likely source for the wavelengths that would correspond to the groove separations. We apply the physical principles of wave reflection, diffraction, and superposition to support the hypothesis that acoustic responses generated from grooves may be useful to bats. We offer an explanation why some bat species do not have grooves. We also discuss the presence of groove arrays in non-echolocating Chiropterans, and five additional mammalian orders.

Conflict of interest statement

The authors have declared that no competing interests exist.


Fig 1
Fig 1. Bat ear with grooves.
These six evenly spaced ridges in the ear of a big brown bats (Eptesicus fuscus) form an array containing five grooves.
Fig 2
Fig 2. Diffraction effects from optical transmission gratings occur when light waves pass through slits.
Each slit produces wavelets that create interference patterns which become more distinct as increasing groove numbers contribute to the effects. Multi-frequency waves such as depicted here with white light are broken into separate frequencies because the different wavelengths emerge at different angles. The same effect would occur with an acoustic frequency modulated (FM) sweep in bat ears deflecting from grooves (image courtesy of Pasco Scientific).
Fig 3
Fig 3. Constructive and destructive interference effects occur when two waves meet.
Constructive interference will retain the frequency and double the amplitude when two identical waves of the same period and amplitude that are completely in phase meet. Destructive interference occurs when identical waves meet that are completely out of phase because it cancels the signal frequency and the amplitude to zero.
Fig 4
Fig 4. Distribution of groove number for species in each bat guild.
1—open space, aerial; 2—edge space, aerial; 3—edge space, trawling; 4—narrow space, flutter detecting; 5—narrow space, passive gleaning; 6—narrow space, active/passive gleaning. The sample size is given above the boxes for a total of 119 species within the six guilds. Guild 7 was not included in the analysis because it only contains one species.
Fig 5
Fig 5. The phenomenon of sound waves of different frequencies creating a beat frequency is called superposition.
For example, a frequency of 100Hz integrates with a frequency of 110Hz to form a 10Hz beat frequency [13]. Image courtesy of SFU School of Communication.

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Grant support

The authors received no specific funding for this work.