Elisabetta Vannoni, Alan G. McElligott. 2008. Low Frequency Groans Indicate Larger and More Dominant Fall Deer (Dama dama) Males. PLoS ONE 3(9):e3113.

This paper on fallow deer (Dama dama) investigated the relationship between various frequency parameters of groans and male quality. Male fallow deer groan during the rut or mating season most likely to attract mates and in male-male assessment and competition. These vocalizations are expected to be honest signals of male quality (here measured as body size, dominance rank or mating success), because they are largely constrained by physiological traits of the caller. The authors expected that fundamental frequency for example would be a poor predictor of body size because it is reliant on the rate of vibration of vocal folds of the larynx open and close which is independent of overall body size. Fundamental frequency of calls however could be a predictor of other indirect indicators of male quality. Resonance frequencies in mammals are closely related to body size as they are dependent on the length of the vocal tract and larger males with longer vocal tracts are expected to have lower resonance frequencies and therefore not be honest indicators of male quality traits that are not directly dependent on body size.

The authors took measurements of the hind legs and recordings of groans of 17 males over five breeding seasons as well as observations of various dominance relationships to asses body size, dominance rank mating success, fundamental groan frequencies, resonance frequencies and resonance dispersion (ie the interval between resonance frequencies). It was found that in fallow deer body size was not related to fundamental frequency but there was a tendency for body size to have a negative relationship with the fifth resonance frequency and there was a significant negative correlation between body size and the sixth resonance frequency and resonance dispersion. This finding supports the authors prediction that fundamental frequency would not be a good predictor of body size and that the resonance characteristics of the call would. The authors also found that higher ranking males had significantly lower minimum fundamental frequencies. Dominance rank was found to relate positively with mating success as measured by observed copulations, but was not found to correlate with body size.

These findings lead the authors to conclude that lower frequency groans indicated indirect measures of male quality independent of male body size. They also postulated that these differences in call frequency could be used by females to choose the most successful mates and also in male male competition to establish and hold dominance rank. One shortcoming admitted by the authors was that they had insufficient sample size to show an effect of body size on dominance rank. In other ungulates there is a clear relationship between male body size and rank, and lack of this effect was somewhat confusing. In many other ungulates, larger males tend to have higher dominance ranks and have more mating success. Because of this it seems one would expect, contrary to the authors predictions, that any acoustic feature that is a good indicator of body size should also be a good indicator of dominance rank and other characteristics of male quality. The authors did not seem to fully explain this apparent discrepancy in logic.

This study highlights the importance of not only fundamental frequency in sound, but also resonance, which was just recently discussed in lecture. It also attempts to address how size differences in individuals can alter the fundamental or resonance frequencies made by certain animals, and how those sounds can have sexually selected functions.