Guanacos (Lama guanicoe) are highly social ungulates which exhibit subtle threat displays and aggressive vocalizations when feeding. Although studies have qualitatively described aggressive vocalizations, the causation and functional significance of aggressive vocalizations in feeding guanacos are poorly understood. In the General Introduction, I review game theory models for aggressive signals in animal conflicts. In Paper 1, I investigate the causation of aggressive vocalizations, namely, individual and contextual variations in a feeding group of guanacos. In Paper 2, I apply the optimal deceit model to the guanaco feeding group and discuss an alternative hypothesis to interpret the functional significance of aggressive vocalizations;Aggressive interactions were recorded in a captive group of guanacos (adult females, subadults, yearlings and juveniles) when feeding on hay from March to November 1989 at Ames, Iowa. Guanacos used 3 call types: squeak, grumble, and spit vocalization. Social rank had a significant effect on rates of aggression and rates of the 3 call types. Furthermore, analysis on sonagram measurements revealed positive correlations between social rank and durations of spit and grumble subtype 1 (G1). The 3 aggressive vocalizations in feeding guanacos could reflect the combinations of aggressive intensity, social rank, and individual recognition;The functional significance of these diverse calls in feeding guanacos was investigated by logistic curves (the probability of winning against difference of fighting ability rank). Logistic curves of grumble and spit vocalizations were the flattest and steepest, respectively. According to the optimal deceit model, grumble was the most deceitful call, and spit vocalization was the most truthful. However, based on social living style and grumble sonagrams, an alternative hypothesis, optimal level of aggression, was suggested. The optimal level of aggression is a balance between effectiveness in thwarting receivers and tolerance by receivers. The higher usage of the grumble in signallers, when ranked higher than receivers, may have resulted from guanacos contesting the low valued resource (hay) and settling contests with cheaper cues. In the most subordinate and youngest guanacos, developmental limitation in learning and a lower risk in provoking challenge might have resulted in the higher usage of the spit vocalization.