Acoustic communication: body size versus frequency tuning as a balancing act

Acoustic communication signals, both their production and reception, are intertwined with all aspects of an animal’s biology. An intuitive example is the inverse relationship between maximum signaling frequency and body size; which is well supported across a wide range of animal taxa. But this same relationship, between maximum frequency and body size, also occurs for acoustic sense organs, however the relationship is a bit more complex. The maximum and resonant frequencies of the tympanic membrane are affected by its size, shape, and thickness; which in combination can help to compensate for a wider range of receiver body sizes and frequency ranges as required by life history. In the bush cricket species complex Ephippiger a large variety of body sizes can be found within and across multiple populations throughout its range. Despite body size differences, frequency components of male mating calls are quite similar across its range and thus, the hearing organs should also be similarly tuned. Individuals from four different populations in France were exposed to broadband acoustic chirps and the motion of their tympanic membranes measured using a 3D laser Doppler vibrometer. While signal frequency components are not as important for mate choice they do play significant roles in species recognition. Our initial results show that the response of the tympanic membrane is sometimes in agreement with the inverse relationship between frequency and body size; suggesting that at some body sizes a frequency shift of the tympanic membrane’s response may be a balancing act too difficult to prevent.