Parent-offspring communication in the Nile crocodile Crocodylus niloticus: do newborns' calls show an individual signature?

Young Nile crocodiles Crocodylus niloticus start to produce calls inside the egg and carry on emitting sounds after hatching. These vocalizations elicit maternal care and influence the behaviour of other juveniles. In order to investigate the acoustic structure of these calls, focusing on a possible individual signature, we have performed acoustic analyses on 400 calls from ten young crocodiles during the first 4 days after hatching. Calls have a complex acoustic structure and are strongly frequency modulated. We assessed the differences between the calls of the individuals. We found a weak individual signature. An individual call-based recognition of young by the mother is thus unlikely. In other respects, the call acoustic structure changes from the first to the fourth day after hatching: fundamental frequency progressively decreases. These modifications might provide important information to the mother about her offspring--age and size--allowing her to customize her protective care to best suit the needs of each individual.