Fundamental frequency is key to response of female deer to juvenile distress calls.

Considerable attention is currently devoted to understanding acoustic mechanisms underlying animal responses to heterospecific vocalizations. A further complication ensues when the response of two species is asymmetrical. For example, white-tailed deer females approach a speaker only when it plays distress calls of conspecific fawns. Mule deer females approach when hearing distress calls of either white-tailed deer or mule deer. We hypothesized that selective species such as white-tailed deer respond to traits distinctive of their species and less-discriminating species such as mule deer respond to traits shared across species. Through an acoustic analysis of neonatal distress calls of six ungulate species, we found that mean and maximum fundamental frequency (F0) enabled the greatest statistical discrimination, and the pattern of frequency modulation (FM) was shared across species. Contrary to our initial hypothesis, playback experiments revealed that females of the two species respond similarly to manipulation of F0 and FM. F0 was critical to the response of females from both species, which tolerated the same relative F0 variation (approx. 0.6-1.4× the mean F0 for conspecific fawns). This discovery suggests that mule deer females only appear less discriminating because they are tuned to the higher F0 of mule deer distress calls (964 Hz vs. 546 Hz), resulting in a larger absolute response range that encompasses the F0 produced by white-tailed deer fawns. We propose that animals will have larger absolute response ranges, and therefore appear to be less discriminating, when they belong to a species that produces higher F0 calls.