DOI: 10.5061/dryad.hmgqnk9h7
title: Data to support playback experiments to female northern elephant seals

description: 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.
description: 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>i.e.,</i> from a distant (&gt;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.