Delayed onset of vocal recognition in Australian sea lion pups (Neophoca cinerea)

Early onset of postnatal individual vocal recognition in a highly colonial mammal species

Mother-young vocal recognition is widespread in mammals. The features of vocal recognition are known to be shaped by the ecological constraints faced by each species. In some species, a rapid establishment of mother-young vocal recognition is crucial for offspring's survival. However, knowledge of the precise features of this recognition system, especially the timing of the onset in the first hours after birth, is often lacking. Here we show that Cape fur seal females can recognize their pup's voice 2-4 h after parturition and that pups develop this aptitude 4-6 h after birth. This study is the first to investigate this mechanism in a wild and free-ranging mammal from only 2 h after birth. We report the fastest establishment of mother-young vocal recognition for any mammalian species, including humans, described to date. Such early vocal identification in pups suggests an in utero vocal imprinting. These findings highlight the synergistic role of environmental constraints and biological traits in optimizing the timing of individual vocal recognition onset in vertebrates.

Mother-young bond in non-human mammals: Neonatal communication pathways and neurobiological basis

Mother-young bonding is a process by which the young establish social preferences for their mother. It fosters reproductive success and the survival of offspring by providing food, heat, and maternal care. This process promotes the establishment of the mother-young bond through the interaction of olfactory, auditory, tactile, visual, and thermal stimuli. The neural integration of multimodal sensory stimuli and attachment is coordinated into motor responses. The sensory and neurobiological mechanisms involved in filial recognition in precocial and altricial mammals are summarized and analyzed in this review.

Mother-pup recognition mechanisms in Australia sea lion (Neophoca cinerea) using uni- and multi-modal approaches

Communication is the process by which one emitter conveys information to one or several receivers to induce a response (behavioral or physiological) by the receiver. Communication plays a major role in various biological functions and may involve signals and cues from different sensory modalities. Traditionally, investigations of animal communication focused on a single sensory modality, yet communication is often multimodal. As these different processes may be quite complex and therefore difficult to disentangle, one approach is to first study each sensorial modality separately. With this refined understanding of individual senses, revealing how they interact becomes possible as the characteristics and properties of each modality can be accounted for, making a multimodal approach feasible. Using this framework, researchers undertook systematic, experimental investigations on mother-pup recognition processes in a colonial pinniped species, the Australian sea lion Neophoca cinerea. The research first assessed the abilities of mothers and pups to identify each other by their voice using playback experiments. Second, they assessed whether visual cues are used by both mothers and pups to distinguish them from conspecifics, and/or whether females discriminate the odor of their filial pup from those from non-filial pups. Finally, to understand if the information transmitted by different sensory modalities is analyzed synergistically or if there is a hierarchy among the sensory modalities, experiments were performed involving different sensory cues simultaneously. These findings are discussed with regards to the active space of each sensory cue, and of the potential enhancements that may arise by assessing information from different modalities.

Mutual mother-pup vocal recognition in the highly colonial Cape fur seal: evidence of discrimination of calls with a high acoustic similarity

The Cape fur seal (Arctocephalus pusillus pusillus) is one of the most colonial mammal species in the world. Females exclusively nurse their pups for 9 to 11 months, during which they alternate frequent foraging trips at sea with suckling periods ashore. The survival of the pup thus depends on the ability of the mother-pup pair to relocate each other among thousands of individuals. Previous work has demonstrated identity information encoded in pup-attraction (PAC) and female-attraction (FAC) calls. Here, we investigated vocal recognition between mother and pup using playbacks of PAC and FAC performed during the breeding season at Pelican Point, Namibia. Both females and pups were able to specifically discriminate the voice of their pup or their mother from non-affiliated pup or mother. Females were able to memorize previous versions of their pup's calls (evidence of recognition up to 73 days after pup's calls recording). Vocal recognition was demonstrated in pups from 1- to 13-weeks old age. Females and pups did not respond differently to the non-filial or non-mother (for pups) stimulus even if it had a strong acoustic similarity with the filial or mother stimulus. This suggested that Cape fur seal mother-pup pairs have high perceptual and cognitive abilities, allowing individuals to identify kin's vocalizations in a very noisy and confusing environment.

Individual vocal recognition across taxa: a review of the literature and a look into the future

Individual vocal recognition (IVR) has been well studied in mammals and birds. These studies have primarily delved into understanding IVR in specific limited contexts (e.g. parent-offspring and mate recognition) where individuals discriminate one individual from all others. However, little research has examined IVR in more socially demanding circumstances, such as when an individual discriminates all individuals in their social or familial group apart. In this review, we describe what IVR is and suggest splitting studies of IVR into two general types based on what questions they answer (IVR-singular, and IVR-multiple). We explain how we currently test for IVR, and many of the benefits and drawbacks of different methods. We address why IVR is so prevalent in the animal kingdom, and the circumstances in which it is often found. Finally, we explain current weaknesses in IVR research including temporality, specificity, and taxonomic bias, and testing paradigms, and provide some solutions to address these weaknesses. This article is part of the theme issue 'Signal detection theory in recognition systems: from evolving models to experimental tests'.

Chemical Profiles of Integumentary and Glandular Substrates in Australian Sea Lion Pups (Neophoca cinerea)

Recognition of individuals or classes of individuals plays an important role in the communication systems of many mammals. The ability of otariid (i.e., fur seal and sea lion) females to locate and identify their offspring in colonies after returning from regular foraging trips is essential to successful pup rearing. It has been shown that olfaction is used to confirm the identity of the pup by the mother when they reunite, yet the processes by which this chemical recognition occurs remain unclear. Using gas chromatography-mass spectrometry, we examined chemical profiles of integumentary and glandular secretions/excretions from pre- and post-molt Australian sea lion pups (Neophoca cinerea) and compared fur and swab samples to assess data collection methods. Multivariate statistics were applied to assess differences in chemical composition between body regions and sexes. We found differences among secretions from various body regions, driven by the distinctiveness of the oral odor mixture. The fine-scale trends in pre- and post-molt pups seem to differ due to changes in the behavior of pups and consequent decrease in the transfer of compounds among adjacent body regions in older pups. Volatile compounds from exocrine substrates were not distinct for different sexes. We also show that swab samples provide better data for exploring social olfaction than fur samples for this species. Obtaining fundamental chemical information, in this case chemical profiles of animals, and discerning differences in chemical composition is an important step toward fully exploring the intricacies of mother-offspring olfactory recognition and its underlying processes.

Visual cues do not enhance sea lion pups' response to multimodal maternal cues

Mammals use multiple sensory cues for mother-offspring recognition. While the role of single sensory cues has been well studied, we lack information about how multiple cues produced by mothers are integrated by their offspring. Knowing that Australian sea lion (Neophoca cinerea) pups recognise their mother’s calls, we first tested whether visual cues are used by pups to discriminate between conspecifics of different age classes (adult female vs pup). We then examined if adding a visual stimulus to an acoustic cue enhances vocal responsiveness of Australian sea lion pups, by presenting wild individuals with either a visual cue (female 3D-model), an acoustic cue (mother’s call), or both simultaneously, and observing their reaction. We showed that visual cues can be used by pups to distinguish adult females from other individuals, however we found no enhancement effect of these cues on the response in a multimodal scenario. Audio-only cues prompted a similar reaction to audio-visual cues that was significantly stronger than pup response to visual-only cues. Our results suggest that visual cues are dominated by acoustic cues and that pups rely on the latter in mother recognition.

Mother Vocal Recognition in Antarctic Fur Seal Arctocephalus gazella Pups: A Two-Step Process

In otariids, mother’s recognition by pups is essential to their survival since females nurse exclusively their own young and can be very aggressive towards non-kin. Antarctic fur seal, Arctocephalus gazella, come ashore to breed and form dense colonies. During the 4-month lactation period, females alternate foraging trips at sea with suckling period ashore. On each return to the colony, females and pups first use vocalizations to find each other among several hundred conspecifics and olfaction is used as a final check. Such vocal identification has to be highly efficient. In this present study, we investigated the components of the individual vocal signature used by pups to identify their mothers by performing playback experiments on pups with synthetic signals. We thus tested the efficiency of this individual vocal signature by performing propagation tests and by testing pups at different playback distances. Pups use both amplitude and frequency modulations to identify their mother’s voice, as well as the energy spectrum. Propagation tests showed that frequency modulations propagated reliably up to 64m, whereas amplitude modulations and spectral content greatly were highly degraded for distances over 8m. Playback on pups at different distances suggested that the individual identification is a two-step process: at long range, pups identified first the frequency modulation pattern of their mother’s calls, and other components of the vocal signature at closer range. The individual vocal recognition system developed by Antarctic fur seals is well adapted to face the main constraint of finding kin in a crowd.

Social olfaction in marine mammals: wild female Australian sea lions can identify their pup's scent

Historically, anatomical evidence has suggested that marine mammals are anosmic or at best microsmatic, i.e. absent or reduced olfactory capabilities. However, these neuroanatomical considerations may not be appropriate predictors for the use of olfaction in social interactions. Observations suggest that pinnipeds may use olfaction in mother-pup interactions, accepting or rejecting pups after naso-nasal contact. Such maternal-offspring recognition is a favourable area for investigating the involvement of odours in social recognition and selectivity, as females are evolutionarily constrained to direct resources to filial young. However, there is no experimental, morphological or chemical evidence to date for the use of olfaction in social contexts and for individual odour recognition abilities in pinnipeds. Here, we report unequivocal evidence that Australian sea lion (Neophoca cinerea) females can differentiate between the odour of their own pup and that of another, in the absence of any other distinguishing cues. This study demonstrates individual olfactory recognition in a free-ranging wild mammal and is clear evidence of the social function of olfaction in a marine mammal.

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

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.

The memory remains: long-term vocal recognition in Australian sea lions

The ability to recognize other individuals plays an important role in mediating social interactions. As longitudinal studies are challenging, there is only limited evidence of long-term memory of individuals and concepts in mammals. We examined the ability of six wild Australian sea lions to discriminate between the voice of their mother and another adult female, both while they were dependent on their mother and when they were independent, 2 years after weaning. Here, we show that even after a long period of independence, juveniles retain the ability to identify their mother's voice. Both when dependent and independent, animals showed stronger responses to maternal calls than to the calls of another female. This demonstration of recognition provides rare evidence of the long-term memory capabilities of wild mammals.