Fallow bucks attend to vocal cues of motivation and fatigue

Vocalizations encode a range of information about the caller, and variation in calling behavior and vocal structure may provide listeners with information about the motivation and condition of the caller. Fallow bucks only vocalize during the breeding season and can produce more than 3000 groans per hour. Males modulate their calling rates, calling faster when other calling males and/or females are nearby. Groans also reveal caller fatigue, becoming shorter and higher pitched toward the end of the rut. Thus, fallow deer groans vary both over very short (minute to minute) and longer timescales (the rut). However, no studies have investigated how intraindividual acoustic variation in fallow deer groans is perceived and how the information is utilized. Using playback experiments, we examined if fallow bucks can extract information about callers from groans and how groaning rate and fatigue affect the perceived competition posed by a caller. Males became attentive sooner and remained attentive for longer during high-rate playbacks than low-rate playbacks. Furthermore, males were attentive for longer during playback of early rut groans that are indicative of males in better condition. Over short timescales, fallow bucks gain information about the motivation of callers through groaning rates. While over longer timescales, males can detect declines in call quality, corresponding to the loss of condition in callers. Thus, over the course of the rut, fallow bucks can extract honest information from dynamic vocalizations to continually assess the current state of conspecifics.