If you walked through an English deer park in July you would probably hear the peaceful sounds of songbirds. But in late September, the air is filled with the deep, rasping groans of male deer calling in the build up to the frenzied mating period known as the rut.
Fallow bucks (Dama dama) are among the most impressive vocal athletes of all deer, investing a large amount of time and energy in calling during the brief rutting period.
Why do fallow bucks groan?
Until recently we have known relatively little about why fallow bucks groan: who is listening to these calls? What information do they encode? What is the evolutionary purpose of these impressive but energetically expensive vocal displays?
By answering these questions we can learn more about how mammalian communication has evolved, and the steps that led to the evolution of complex communication such as human speech.
How are these sounds generated?
First, the ‘source’ sound is generated by vibrations of the vocal cords in the larynx. The pitch of the groan (also known as fundamental frequency) is determined by the rate at which the vocal cords vibrate.
In the second stage of the vocal production process the sound is ‘filtered’ through the air spaces in the vocal tract producing the sound we hear. These changes generate other frequencies in the sound that are known as formants.
Like blowing over the mouth of different sized bottles, larger air spaces result in lower frequency formants and one of the main contributors to size of the vocal tract is its length.
What’s known already?
Our previous work has shown that fallow bucks that have lower pitched groans are generally more dominant and mate with more females. In other mammals, including humans, low-pitched voices have been linked to higher levels of hormones such as testosterone, and to higher social dominance.
Larger and more dominant males also have groans with lower formant frequencies. Fallow bucks can also lengthen their vocal tracts while calling, making their groans sound deeper and potentially exaggerating the size of the caller.
These findings indicate tantalising links between the attributes of a caller and their mating success, but they do not show if it is males, females, or both, who are listening and responding to the information in groans.
What does our work show?
In our new study, published in BMC Evolutionary Biology, we demonstrate that fallow bucks listen to the calls of rival males and can use the information that they hear to ‘size up’ their competition.
We studied the fallow deer in Petworth Park, West Sussex, UK. The park has a large herd that is habituated to walkers, allowing us to approach close enough to observe the bucks throughout the rut.
It is also one of the few fallow deer populations in Europe in which bucks form a lek, a cluster of small territories where males compete for opportunities to mate with visiting females.
Bucks must fight regularly to assert their dominance and maintain their positions on the lek. Typically, only the most dominant bucks get to mate.
By broadcasting computer manipulated groans to males on the lek, we simulated the arrival of another buck. We compared the responses of target bucks to different manipulations of the same groans to explore how they perceived their groaning rivals. We measured how quickly bucks became alert after hearing the groans and if they approached or retreated from the sound.
We found that bucks could hear subtle differences in the pitch, formant frequencies and length of groans. Bucks treated low-pitched groans and groans with lower formant frequencies as more threatening. Shorter groans were treated as less threatening than longer groans.
Bucks are probably perceiving low-pitched groans as coming from a more dominant male, while groans with low formant frequencies sound like they come from a big buck. Both of these are potential threats in a fight.
Shorter groans are likely to indicate that the caller is becoming fatigued. Detecting subtle changes in the length of groans may allow bucks to decide when it is a good time to challenge a rival for a prized position on the lek.
This is the first time that a non-primate mammal has been shown to use the pitch of vocalizations to assess rivals.
This is the first time that a non-primate mammal has been shown to use the pitch of vocalizations to assess rivals. Like humans, fallow bucks can use the frequency of calls to make judgements about the level of threat or competition posed by other males.
Deep calls help to beat other males in the quest for mates, and over generations, competition between males for mates has driven the evolution of deeper, lower pitched and longer calls.
Traits such as low-pitched voices and a mobile larynx have been suggested to be important in the evolution of human speech. Our findings support the hypothesis that these may have originally evolved through sexual selection to signal fitness.