Ontogeny of conspecific and heterospecific alarm call recognition in wild Verreaux's sifakas (Propithecus verreauxi verreauxi)

The production of vocalizations in nonhuman primates is predominantly innate, whereas learning influences the usage and comprehension of vocalizations. In this study, I examined the development of alarm call recognition in free-ranging infant Verreaux's sifakas. Specifically, I investigated their ability to recognize conspecific alarm calls as well as those of sympatric redfronted lemurs (Eulemur fulvus rufus) in Kirindy forest, western Madagascar. Both species have functionally referential alarm calls for aerial. predators and give general alarm calls for both aerial and general predators and also other kinds of threats, such as intergroup encounters with conspecifics. I conducted playback experiments with members of two birth cohorts (nine and ten individuals) to determine the age at which infant Verreaux's sifakas discriminate between conspecific alarm calls, heterospecific alarm calls, and non-alarm vocalizations (parrot song). Most 3-4 months old infants fled toward adults after hearing any playback stimuli, whereas 4-5-month-old infants did so only after presentation of alarm calls. Moreover, all infants of these age classes showed a longer latency to flee after the parrot song indicating their emerging ability to discriminate between alarm calls and non-alarm stimuli. At an age of about 6 months, infants switched from fleeing toward adults to performing adult-like escape responses after presentation of conspecific and heterospecific alarm calls. Thus, the ability to discriminate between alarm from non-alarm stimuli precedes the appearance of adult-like responses. The transition to adult-like escape behavior was coincident with the physical independence of infants from their mothers.