Mechanism

The mechanism for mother-pup acoustic recognition depends on two dependent processes: accurate variation of acoustic parameters among individuals and the ability of receivers to detect and learn individual variations (Trimble et al., 2009).

Mother and pup vocalizations are highly individually stereotyped potentially providing many cues as to the identity of the caller. These potential cues include amplitude and frequency modulation, exact frequency value, and spectral energy distribution parameters. Experimental analysis of pup responses to their mother’s voice has revealed that pups use both amplitude and frequency modulations as well as the exact frequency values of the vocalization when identifying the mother (Park et al., 2006; Charrier et al., 2009).

Similar analysis of maternal responses to pup vocalizations has shown that mothers rely on the pattern of amplitude modulations and the exact frequencies contained within the call, but not frequency modulations. Mothers also need multiple frequency bands to perform discriminations, and it appears that the pups vocal signature is contained in the, potentially more stable, first half of their call (Pitcher et al., 2012). These conclusions are unsurprising, considering an analysis using frequency modulations proved to be the most effective for detecting a signal in a noisy environment, such as a breeding site containing hundreds of barking sea lions (Charrier et al., 2002).

Adapted from Insley et al., 2003

Barks specific to mother-pup recognition are similar for both mothers and pups, but exhibit longer sounds as compared to other vocalizations made by sea lions (e.g. aggression, territoriality, etc.). The complex acoustic structure of these calls permits messages to reach long distances without degradation or attenuation, proving effective when females attempt to locate pups immediately following periods of separation, thus enhancing the likelihood of mother-offspring recognition (Fernandez-Juricic et al., 1999).

In addition, studies have shown that in many pinniped species, spatial or geographical cues have been implicated in both reunion and recognition between mothers and pups. Studies of otariids have shown that mothers consistently return to the spot where they last suckled their pups and these locations are often defended from other females. Natal site fidelity increases the probability of mothers and pups encountering one another within a season while pups are still dependent, and also in future years when the pup is already grown. Future encounters provide the opportunity for cooperation through nepotism if there is long-term recognition (Insley, 2000, Insley et al., 2003).

This figure shows examples of variation in breeding densities among different pinniped species, a factor that likely plays a strong role in selection for recognition.