Effect of combined source (F) and filter (formant) variation on red deer hind responses to male roars

Studying female response to variation in single acoustic components has provided important insights into how sexual selection operates on male acoustic signals. However, since vocal signals are typically composed of independent components, it is important to account for possible interactions between the studied parameter and other relevant acoustic features of vocal signals. Here, two key components of the male red deer roar, the fundamental frequency and the formant frequencies (an acoustic cue to body size), are independently manipulated in order to examine female response to calls characterized by different combinations of these acoustic components. The results revealed that red deer hinds showed greater overall attention and had lower response latencies to playbacks of roars where lower formants simulated larger males. Furthermore, female response to male roars simulating different size callers was unaffected by the fundamental frequency of the male roar when it was varied within the natural range. Finally, the fundamental frequency of the male roar had no significant separate effect on any of the female behavioral response categories. Taken together these findings indicate that directional intersexual selection pressures have contributed to the evolution of the highly mobile and descended larynx of red deer stags and suggest that the fundamental frequency of the male roar does not affect female perception of size-related formant information.