Rev
A short (<1 second="" tonal="" vocalization="" that="" is="" almost="" always="" followed="" by="" a="" rumble="" without="" an="" exhalation="" the="" rev="" has="" harmonic="" structure="" and="" appearance="" similar="" to="" short="" but="" its="" fundamental="" frequency="" 50-90="" hz="" significantly="" higher="" revs="" are="" very="" rarely="" heard="" in="" wild="" little="" known="" about="" their="" usage="" p="">
References: Leong et al 2003; Poole 2011. (Full reference list)
This behavior occurs in the following context(s): Affiliative, Calf Reassurance & Protection