Rapid onset of maternal vocal recognition in a colonially breeding mammal, the Australian sea lion

Abstract

Background: In many gregarious mammals, mothers and offspring have developed the abilities to recognise each other using acoustic signals. Such capacity may develop at different rates after birth/parturition, varying between species and between the participants, i.e., mothers and young. Differences in selective pressures between species, and between mothers and offspring, are likely to drive the timing of the onset of mother-young recognition. We tested the ability of Australian sea lion mothers to identify their offspring by vocalisation, and examined the onset of this behaviour in these females. We hypothesise that a rapid onset of recognition may reflect an adaptation to a colonial lifestyle.

Principal findings: In a playback study maternal responses to own pup and non-filial vocalisations were compared at 12, 24 and every subsequent 24 hours until the females' first departure post-partum. Mothers showed a clear ability to recognise their pup's voice by 48 hours of age. At 24 hours mothers called more, at 48 hours they called sooner and at 72 hours they looked sooner in response to their own pup's vocalisations compared to those of non-filial pups.

Conclusions: We demonstrate that Australian sea lion females can vocally identify offspring within two days of birth and before mothers leave to forage post-partum. We suggest that this rapid onset is a result of selection pressures imposed by a colonial lifestyle and may be seen in other colonial vertebrates. This is the first demonstration of the timing of the onset of maternal vocal recognition in a pinniped species.

Figure 1. Experimental design.

A. Females received the first playback within 12 hours of the birth of their pup, followed by another at 24 hours and every 24 hours until they departed the colony at the end of the perinatal attendance period. B. Each playback trial consisted of two presentations separated by a minimum of 2 minutes. During each presentation the responses of the female were recorded for 1 minute. The presentation order of own and non-filial pup's female attraction calls (FAC) was randomised. C. Presentations consisted of a single FAC repeated three times separated by 3 seconds of silence. Spectrograms were made using Seewave .

Figure 2. Comparison of composite response scores using principle component 1 of the PCA at each playback.

From the first playback females show a trend to be more responsive to their filial pup's FAC. This difference becomes significant within 48 hours of birth. See table 1 for statistics, points = mean ± 1 S.E.M. * indicates significant difference.

Figure 3. Comparisons of female responses within behavioural measures.

A. Females call more in response to playback of their filial pup's and a non-filial pup's FACs within 24 hours of birth. See table 2, points = mean ± 1 S.E.M. * indicates significant difference. B. Within 48 hours of birth, females call sooner to the FAC of their filial pup than they do to the FAC of a non-filial pup. See table 2, points = mean ± 1 S.E.M. * indicates significant difference. C. By 72 hours after birth females look more rapidly to their filial pup's FAC than to a non-filial pup. See table 2, points = mean ± 1 S.E.M. * indicates significant difference. D. Females did not show a significant difference in their latencies to approach following playback of the FAC of their filial pup or a non-filial pup. See table 2, points = mean ± 1 S.E.M.