The evolution of acoustic size exaggeration in terrestrial mammals
Benjamin D. Charlton1 & David Reby2
1School of Biology and Environmental Science, Science Centre West, University College Dublin (UCD), Belfield, Dublin 4, Ireland.
2Mammal Vocal Communication and Cognition Research Group, School of Psychology, University of Sussex, Sussex BN1 9QH, UK.
Correspondence and requests for materials should be addressed to B.C. (email: ).
Recent studies have revealed that some mammals possess adaptations that enable them to produce vocal signals with much lower fundamental frequency (FO) and formant frequency spacing (ΔF) than expected for their size. Although these adaptations are assumed to reflect selection pressures for males to lower frequency components and exaggerate body size in reproductive contexts, this hypothesis has not been tested across a broad range of species. Here we show that male terrestrial mammals produce vocal signals with lower ΔF (but not FO) than expected for their size in mating systems with greater sexual size dimorphism. We also reveal that males produce calls with higher than expected FO and ΔF in species with increased sperm competition. This investigation confirms that sexual selection favours the use of ΔF as an acoustic size exaggerator and supports the notion of an evolutionary trade-off between pre-copulatory signalling displays and sperm production.