Human vocalizations impact skink risk assessment depending on human exposure

Sounds, like screams, which contain acoustic nonlinearities can elicit fearful, emotional responses in animals. In humans, screams activate the amygdala and are perceived as less positive than spoken words. We conducted a playback experiment to see if two skink species (one of which is known to negatively respond to nonlinearities) responded differently to human spoken words and screams. We broadcast short phrases of humans screaming, humans speaking, or a control, a novel bird song. We studied the immediate response by quantifying the change in rates of vigilance and locomotion. We also studied a slightly delayed response in risk assessments by measuring skink flight initiation distance after the playback. Furthermore, we conducted experiments in nonresidential and residential areas to understand how human exposure may influence human discrimination and potentially valence perception (i.e., how sound is emotionally perceived). Blue-tailed skinks (Emoia impar) altered both immediate behavior and their delayed risk assessment when human screaming was broadcast, but only in residential areas. By contrast, white-bellied copper-striped skinks (Emoia cyanura) only altered their delayed risk assessment when human stimuli were broadcast in residential areas. These results indicate that skinks respond differently to human vocalizations depending on their exposure to humans and based on species-specific behaviors. Our results suggest the importance of considering many variables when studying cognition in nature. The amount of human exposure and the acoustic cues contained in auditory stimuli may affect an individual's risk assessment and behavioral responses.