Phylogeny

Since sea lions live in dense colonies, the properties of sound render acoustic signals as a reliable method for mother-pup recognition (Halpin et al., 1991). Parent-offspring conflict theory emphasizes the fact that selection pressures differ for the offspring compared to the parent, in that the pup will not survive if a mother does not recognize it, whereas recognition error by the mother represents a relatively smaller cost for adult females. For pups, therefore, selection may favor probing and deception, and necessitates more liberal acceptance criterion (Insley et al., 1989).

Studies in the South American fur seal have shown that while calls of both mothers and pups display low variability within, and high variability among individuals, mothers’ calls were more individualistic than that of the pups (Stirling et al., 2000). This behavioral asymmetry may have different causes: Are the pup’s errors linked to a difference in motivation? Or are these errors merely a reflection of recognition ability? Current studies do not yet indicate a clear explanation (Charrier et al., 2002). At least on thing, however, is certain; When compared to other vertebrates this vocal signature of sea lions appears to be of moderate complexity and is likely to be the result of selection pressures imposed by the environment and the species' ecology (Pitcher et al. 2011).

Although there are still many gaps in the evidence concerning recognition in pinnipeds, some patterns are apparent. At the most basic level, there is sufficient evidence to show that individual recognition, at least that between mothers and offspring, is widespread in pinnipeds. Exceptions to this rule, such as the Hawaiian monk seal, which apparently is unable to exhibit maternal recognition behavior, provide clues as to the specific selective pressures driving recognition behavior in pinnipeds.

Adapted from Insley et al., 2003.

The phylogenetic tree above summarizes pinniped phylogenetic relationships along with the primary life history factors that are likely to affect mother-pup recognition.

1. Density of breeding aggregations (High Medium or Low)

2. Duration of offspring dependence in months

3. Maternal stragety (Income, Capital or Intermediate)

4. Frequency of mother pup separations (frequent, infrequent or intermediate)

Indirect and Direct evidence for recognition includes:

1. Prevalence of adoption and/or allo-suckling

2. Vocal recognition among pups

A clear split at the family level between phocids and otariids is evident. For example, otariids are “income” breeders, meaning that they regularly leave their pups to forage during lactation whereas phocids tend to be “capital” breeders, mostly fasting throughout lactation, and as a result normally remain close to their pups. In addition, otariids have substantially longer periods of dependence and breed in denser aggregations (on average) than phocids. The result is a predictably more difficult recognition task for otariids compared to phocid. Research also indicates that mother-pup recognition is generally better developed in oariids. Current supporting evidence is limited to the vocal/auditory sensory mode (Insley et al., 2003).

Current data support the statement that recognition is more refined in otariid pinnipeds. For example, playback experiments show a pattern of bi-directional (mutual) mother-pup recognition in most otariids. The fact that this trait clearly is present in northern fur seals suggests that it was acquired early in the otariid lineage. In contrast, results of experiments with phocids have demonstrated a unidirectional recognition, meaning that mothers recognize their pups but not vice-versa. In addition, otariid vocalizations appear to be more stereotyped (Insley et al., 2003).

Based on this information it is possible that phocids rely more heavily on chemical cues for recognition purposes than otariids. Nevertheless, even if this were true, the more frequent occurrences of adoption and allo-suckling in phocids supports a more highly-tuned discrimination ability in otariid pinnipeds (Insley et al., 2003). Thus, phocids and otariids most likely evolved differently and this could be due to the different selective pressures that drive recognition behavior in pinnipeds.