Distress vocalizations of <i>Aspidoscelis costatus costatus</i> (Squamata: Teiidae): when does this behavior occur in the ontogeny?

Abstract Vocal sound production has been reported in juveniles of mammalian, avian, and nonavian sauropsid lineages. In this last taxon, studies concerning the ontogenetic development of vocalizations have mainly been focused on Crocodylia and Testudines. Less is known about Squamata, though this behavior has been recognized in a few juvenile gecko species, albeit only characterized in the Mediterranean House Gecko (Hemidactylus turcicus). In the present study, we identified the vocalizations emitted by the juvenile Balsas Basin Whiptail lizard (Aspidoscelis costatus costatus), a species endemic to Mexico. We sought to characterize the spectro-temporal variables of the juveniles' calls and determine whether these vocalizations occur during early ontogeny or are associated with a minimum snout-vent length. The study included 74 individuals (41 hatchlings, 33 juveniles); only 21 juveniles vocalized (16 females and 5 males). The calls possessed simple and complex modulation patterns and were only emitted when the individuals were seized. Thus, we classified them as distress calls. Sound production occurs before the first year of life. However, hatchlings did not vocalize, and individuals required a minimum snout-vent length to emit sound. Three nonmutually exclusive hypotheses are proposed to interpret these findings: 1) the vocal structure responsible for sound production develops during ontogeny; 2) juveniles do not have enough energy to vocalize, as this energy is primarily allocated for growth and maturation; and 3) juveniles only vocalize once an adequate size has been obtained, which allows calls to reach an effective intensity and/or duration.