As in all otariids, Australian sea lion, Neophoca cinerea, females alternate foraging trips at sea with suckling periods ashore, and each time they return, mothers and pups have to find each other among individuals at the colony. The need for a finely tuned mechanism of individual recognition is exacerbated by their habit of changing the suckling location. Using playback experiments with modified calls, we investigated the acoustic parameters involved in the discrimination of the mother by pups. The signature efficiency was also tested in the three environments in which reunions at the study site occur by performing propagation tests. Pups paid great attention to both amplitude and frequency modulations, and to the exact frequency values of the call to identify their mother. When severe modifications of AM and FM patterns were made to mothers' calls pups no longer responded to them. This may have arisen because the calls fell outside the natural species range. The energy spectrum, even though highly individualized, was not important for individual recognition. Propagation tests revealed that one particular environment was extremely harsh and did not allow accurate transfer of the individual signature components whatever the distance. In two other environments, AM was only reliable to a distance of 16 m, but both FM and frequency spectrum could be transmitted with limited degradation over 32 m. The different environments greatly influenced the efficiency of the individual identification process. Accordingly, the area in which mother-pup reunions occur is an important factor in the success of individual recognition.