Fur seal mothers memorize subsequent versions of developing pups' calls: adaptation to long-term recognition or evolutionary by-product?

In pinnipeds and especially in otariids, mothers and pups develop the capacity to recognize each other's voices. Pups become able to discriminate their mother's voice a few days after birth. For females, this discrimination seems to occur earlier, probably during the few hours after parturition. However, during lactation, mothers are confronted with a major problem: the change of the characteristics of their pup's calls. To investigate this problem, we first performed an acoustic analysis of pups' calls from birth to weaning to identity the successive different versions of these calls. Secondly, we performed playback experiments just before weaning to test if females retain these different versions over a long time period. The acoustic analysis of pups' calls reveals that several characteristics of their vocalizations change with age. Playback experiments demonstrate that females still recognize all the successive immature and mature versions of their pup's calls. In our opinion, this long-term memorization seems to be a by-product of the permanent pups' voice learning from birth to weaning since no apparent adaptive benefit seems to arise from this capacity (C) 2003 The Linnean Society of London.