Bonding between mothers and their young is fundamental to mammalian reproductive behaviour and individual fitness. In social systems where the risk of confusing filial and non-filial offspring is high, mothers should demonstrate early, strong, and consistent responses to their kin throughout the period of offspring dependence, irrespective of maternal traits such as experience and temperament. We tested this hypothesis through playback experiments in the northern elephant seal Mirounga angustirostris, a phocid species that breeds in high-density colonies. We found that mothers recognised their offspring throughout lactation and as early as 1-2 days after parturition. Age and aggressiveness level of mothers did not predict their response strength to filial playback treatments, nor did pup age or sex. Some mothers showed great consistency in behavioural responses throughout the lactation period, while others were less predictable. The strength of a female’s response did not influence her pup’s weaning weight, however more consistent females weaned pups of higher mass. This is a rare demonstration of individual recognition among phocid mothers and their offspring, and suggests that consistency in maternal responsiveness may be an important social factor influencing the pup's growth and survival.

Playback experiments:

We tested 22 mothers during early, mid, and late lactation (weeks 1, 2, and 3) when filial pups were 7.5 ± 1.2 days, 14 ± 1.8 days, and 20.4 ± 3.2 days old. During the first season, playbacks were conducted with six untagged adult females. To avoid possible replication the following season, we selected 16 tagged individuals for participation in playback trials. Focal females were exposed to two successive treatments on each trial: one call series from her own pup that had been recorded 1 to 3 days prior, and one call series from a non-filial but similar-aged pup. The presentation order of filial and non-filial call treatments was changed between trials and balanced weekly and by individual. The non-filial pup on each trial was unfamiliar (i.e., from a distant (>85 m) harem) and selected from our call bank.

PCA scores to evaluate the response strength of each mother to each treatment:

Behavioural variables were pooled into a principal component analysis (PCA) with a composite score (PC1, the first principal component; [25]). We conducted separate PCAs for the longitudinal (weekly) playbacks, and the playbacks conducted 1-2 days postpartum, as these experiments involved different individuals. We used linear mixed models to determine which factors influenced behavioural responses. For the longitudinal playbacks, we used playback treatment, timing (week), and interaction between treatment and timing as fixed factors, while treatment presentation order and mother’s identity were considered random factors. No model selection took place and residuals were inspected to ensure normality of error. For the playbacks conducted 1-2 days postpartum, we used playback treatment as the fixed factor and treatment presentation order and mother’s identity as random factors.