Adaptive Value

Rearing offspring is an incredibly demanding task, requiring the investment of valuable time, resources, and energy on the part of the parent(s). This important investment is therefore best directed towards an individual’s own young, not the young of competing individuals. The parent’s ability to recognize their own offspring, and even differentiate them from unrelated offspring, therefore, is an essential aspect of the maximization of fitness for an individual (Hepper et al., 1986; Clutton-Brock et al., 1991; Trimble et al., 2009). For colonial animals, such as sea lions, individuals live in large groups with breeding seasons yielding hundreds of pups that can be easily lost in the crowd. With sea lions only giving birth to one pup (rarely two) each season, early development of a mother-pup recognition system is crucial for mothers to be sure they are taking care of their own pup. In sea lions, acoustic vocalization is the primary method of mother-pup recognition, with scent recognition also playing a partial role (Weckerly et al., 1998).

The vocal recognition process during the breeding period can be considered as an adaptation to the severe ecological constraints imposed on this species (Charrier et al., 2001). Species showing high selective pressures for individual recognition (i.e. high-density colonies, high degree of polygyny, long maternal absences, rare or absent allo-suckling) tend to present a strong individuality in their vocalizations, as is the case in most otariid species, and contrary to phocids (Trimble et al., 2009). Mothers will leave their pup ashore while they forage for food at sea, and vocalize immediately up their return to the breeding site. The higher the individuality of the vocalization, the easier it is for the mother to recognize her pup, and vice versa. For parents, offspring recognition prevents misdirected parental care, limits expenditure of energy, and helps to ensure reproductive success; natural selection, therefore, has likely led to adaptations for improved mother-pup recognition (Charrier et al., 2002).

Sea lion rookery

In addition, it has been observed that in the northern fur seal, another otariid species with a similar pattern of maternal attendance, mothers and pups are able to mutually retain their calls for at least four years. This result is somewhat surprising from an evolutionary point of view since there is no apparent adaptive benefit for this long-term memory. Nevertheless, some hypotheses have been proposed to explain this long-term recognition ability suggesting that fitness benefits may arise from cooperation or mate choice.  This information suggests that the vocal recognition after weaning or the mother’s ability to retain each version of their pup’s developing call are by-products of the strong permanent and unalterable learning experienced by mothers when rearing their young.  (Charreir et al., 2001)