How do mother sea lions find their own pup on a croweded shore?

Individual vocal recognition between mothers and pups has been widely observed in pinnipeds, especially sea lions. Sea lions are colonial breeders, with mothers suckling only their own offspring, even aggressively attacking nonrelated young. Mothers alternate foraging trips to find food with periods ashore suckling their pup. The first separation occurs very early, some 2-8 days after parturition (Charrier et al., 2002). At each return to the colony mothers and pups must find each other amongst the hundreds of other sea lions ashore (Charrier et al., 2006). When mothers return they are reunited in less than 7 minutes (Charrier et al., 2002). Acoustic mother-pup recognition appears to be the primary means by which female sea lions achieve this essential task (Insley et al., 1989).

Sea lions (also known as fur seals) are classified as otariids or eared seals, and comprise 16 species in 7 genera. Otariids belong to the pinniped superfamily, which includes walruses (odobenids) and true seals (phocids) (Brunner et al., 2003). Sea lions are social animals, forming groups of several hundred individuals, often congregating in caves just offshore. Being polygynous, sea lions will establish breeding territories with a single male and multiple females. Breeding season lasts from May to August, and sexual maturity is reached at about 5 years old. Three weeks after giving birth, females are ready to mate again; pups are weaned at 10 months old (Weckerly et al., 1998).

Mother-pup acoustic vocal recognition in sea lions is best examined using Tinbergen's four questions:

1. Ontogeny (or development) explains changes over the lifespan of a sea lion pup, in relation to learned vocal recognition behavior in particular.

2. Phylogeny (or evolution) refers to the history of a vocal recognition, including its origin and subsequent inheritance in sea lions.

3. Adaptive value (or function) describes how acoustic vocal recognition in sea lions can increase their lifetime reproductive success.

4. Mechanism (or causation) usually refers to physical morphology or molecular mechanisms to explain how acoustic vocal recognition behavior in sea lions is constructed.


Tinbergen's Four Questions

Tinbergen further divided these four categories into proximate (how) and ultimate (why), with mechanism and ontogeny falling under proximate and adaptive value and phylogeny falling under ultimate. In addition, the timeframe of the study is specified, with mechanism and adaptive value falling under immediate, and ontogeny and phylogeny falling under chronicle.