Introduction
The following acoustical assessment documents infrasonic vocalizations that are produced by giraffe and demonstrates these vocalizations occur in social contexts. It also proposes that Helmholtz resonance is used to produce such vocalizations. Giraffes are large ruminant browsers that live in sub-Saharan Africa. Giraffe females and young are usually sighted in small groups whose composition can change from day to day, while adult males live alone briefly joining groups of females to breed . Giraffes recognize each other "personally", maintain contact with one another over long distances, and hide their calves during the day and relocate them at night. In elephants and okapi (giraffes’ closest living relative), low-frequency vocalizations are used to coordinate these activities, possibly because low frequency sound shows little attenuation due to scattering in the environment. In fact, both elephants and okapi produce calls at similar frequency ranges and intensities (e.g., African elephants: 14-35Hz at up to 90dB).
In okapi infrasonic vocalizations are associated with two observable behaviors: a neck stretch (head and neck starts at about chest level, is thrown back over the body and curled upwards until the nose is straight up in the air) and a head throw (chin is lowered and quickly raised so that the nose is pointing straight up into the air). Giraffe also perform both of these behaviors, which coincide with infrasonic vocalizations. The distinctive anatomy of the giraffe may also allow it to use a unique mechanism to vocalize at low frequencies. Giraffe have a 2-3m long neck; when the head is extended in line with the neck their respiratory apparatus acts as a resonant chamber in which Helmholtz resonance is used to produce low-frequency sound. Helmholtz, an energy-efficient mechanism for sound production, relies on a simple resonator system that occurs when an enclosed volume of air, coupled to the outside free air by means of a single aperture, is subject to vibration. If giraffe use Helmholtz resonance to produce or amplify their vocalizations, then the frequency of these vocalizations should be consistent with calculations using the capacity and structure of the giraffe respiratory system as a resonator. To determine whether giraffe produce infrasonic vocalizations, whether the characteristics of these vocalizations are consistent with production using Helmholtz resonance, and whether vocalizations were produced in the context of social interactions, giraffe were recorded in two conditions. Controlled recordings allowed detailed analysis of vocalizations from a known source with limited acoustic interference, while naturalistic recordings were used to identify contexts in which these vocalizations might naturally occur.