RESUMO
Lizards, except geckos, are generally considered voiceless organisms, although some species emit oral sounds. For most of these "vocal lizards", however, there is almost no information on the characteristics of the sounds, precluding exploration of the functionality and evolution of the sounds. Pristidactylus are known as "grunter lizards" since individuals emit oral sounds under predation risk. We explored the characteristics of the sounds emitted by P. valeriae, recording 17 adults and 1 juvenile when they were threatened and captured by a predator. Only adults emitted sounds with open mouths and displayed aggressive postures, e.g., biting attempts. These sounds correspond to hisses, which lack amplitude or frequency modulation. The lizards emitted longer hisses when threatened than when captured by the predator, which may provide honest information on individuals' ability to escape. In addition, males may experience higher distress during threats since their hisses had higher aggregate entropy than those of the females. Finally, hissing has been documented in four of the five Leiosauridae genera, the family to which Pristidactylus belongs, suggesting that sound emission is ancestral to the family.