Primate duet display via arboreal locomotor predictability: emergent height and variety as selecting for more complex gibbon great calls

Disciplinary background A. Behavioral ecology is the science of modeling a species’ adaptive fit of their behavior to their environment (Fox and Westneat, 2010). Primates exhibit a vast array of behavioral modes and live in a wide variety of habitats across the world, but primarily in forested environs (Fleagle, 1999). Consequently, such a profusion of possible tree species exerting selection pressures on this array of primate positional and communicative modes challenges researchers with myriad habitat considerations. Presumably, such a wide variety of tree shapes, sizes, and statures should exert a corresponding diversifying selection on the behavior of its residents. This exceptional behavioral diversity of primates, incidentally makes them ideal models for testing evolutionary theories that interpenetrate the noesis of human behavior. Unlike our species, gibbons sleep in tall trees—that emerge through the forest canopies of southeast Asia—which also often serve as the primary setting for their exceptional vocal displays Alexander et al., 2018). Disciplinary background B. Many pair bonded primates participate in coinciding vocal behavior that often manifests in the form of calls with interacting male and female contributions. Socially monogamous gibbons (Geissmann, 1986), tarsiers (Clink, Tasirin and Klink, 2020), and callitrichids (Muller and Anzenberger, 2002) produce mutable vocal duets that feature such acoustic patterning. Gibbons, in particular, routinely exhibit duetting behaviors in all but a few species. The females’ “great call” forms the center-piece of such elaborate displays—often featuring a diversity of syllables which typically increase in frequency and accelerate into a rapid series of upward frequency sweeps (Raemaekers, Raemaekers and Haimoff, 1984) blurring repetition into both transposition and trill. Theories on the function of these calls range from resource spacing Mitani, 1985), to pair-bonding and mate attraction (Geissmann, 1986). But few studies to date have looked at various features, especially in combination, and how display structure could relate to ecology. Abstract Such structured patterning (e.g. rhythm)