For baby eared seals (or otariids), being able to recognize mom in a huge crowd is vital for survival. Female sea lions and fur seals only nurse their own pups, and they’re very aggressive towards those who aren’t related to them. Researchers studying vocal recognition in Antarctic fur seals (Arctocephalus gazella) in the southern Indian Ocean have revealed that it’s a two-step process: pups identify their moms using different components of her vocal signature at longer distances than they do at closer range. The findings are published in PLOS ONE this week.

When Antarctic fur seals come ashore to breed, it’s in dense colonies of several hundred pairs of mother and pup. During the four-month lactation period, mothers alternate between suckling on land and foraging at sea. When a female returns, sometimes up to seven days later, she and her waiting pup call out to find each other, and then use smell as a final check. This vocal identification process needs to be incredibly efficient, otherwise females waste energy and pups risk injury.

A trio led by Thierry Aubin from Université Paris-Sud wanted to study the different components of vocal signatures that pups detect and then use them to identify their mothers on Kerguelen Archipelago at Cape Noir in the Courbet Peninsula in January and February 2000 during a lactation period. The team recorded pup attraction calls produced by 32 females who were locating their pups, and then carried out playback experiments with 29 wild pups using synthesized calls and playbacks at different distances.

Pups use modulations in both amplitude and frequency to identify their mother’s voice. Amplitude is the size of the sound vibrations (loudness), while frequency is the speed (pitch). Frequency modulations propagated reliably up to 64 meters (210 feet), and amplitude modulations degraded after eight meters (26 feet). 

The findings suggest that the mom-pup identification process has two steps: one over long distances, the next at closer range. This individual vocal recognition system developed by Antarctic fur seals, the authors write, is well adapted to face the main constraint of finding kin in a crowd.

^{Male Antarctic fur seal is calling. Pierre Jouventin}