Recent studies have revealed that some mammals possess adaptations that enable them to produce vocal signals with much lower fundamental frequency ( F0) and formant frequency spacing (Δ F) 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 Δ F (but not F0) than expected for their size in mating systems with greater sexual size dimorphism. We also reveal that males produce calls with higher than expected F0 and Δ F in species with increased sperm competition. This investigation confirms that sexual selection favours the use of Δ F as an acoustic size exaggerator and supports the notion of an evolutionary trade-off between pre-copulatory signalling displays and sperm production.
The acoustic properties of vocal signals generally depend on body size, but in some species males have traits that exaggerate the size conveyed by their vocal signals. Here, Charlton and Reby show that among terrestrial mammals, species with sexual selection for large male body size also have more exaggerated vocal signals for their size.