If you listen to the egg of a crocodile, you can tell when it's going to hatch by the small squeaks coming out of it. The squeaks come from the unborn babies and sound like "umph! umph! umph!". These calls are common to all crocodilians and while zoologists have always suspected that they serve a specific purpose, until now, no one had ever tested this theory with an experiment.
Amelie Vergne and Nicoals Mathevon at the Universite Jean Monnet are the first to do so and they show that the youngsters call to tell their siblings that it's time to hatch. And given that Nile crocodiles bury their eggs in sand, the calls also tell the mother that it's time to dig her babies out.
Vergne and Mathevon collected 17 eggs from the clutches of captive Nile crocodiles, all of which were just 10 days away from hatching. Some of the eggs were kept in total silence, others listed to recordings of random noise and a third group listed to recordings of pre-hatch calls.
They found that the unborn crocodiles that heard pre-hatch calls were most likely to respond themselves, answering back 80% of the time and moving about 55% of the time. About half of them hatched within ten minutes of hearing the recordings. In contrast, those that heard noise or nothing were far quieter and less active, and only one hatched in response to the playback.
To see how the mother crocs responded to the calls, Vergne and Mathevon first drove them away before hiding small speakers in the sand near their nests. As expected, the pre-hatching calls grabbed the attention of the mothers more often and more quickly than random noise did. Eight of the ten females responded to playbacks of baby calls by digging, whereas only one did the same when she heard the noise.
Vergne and Mathevon suggest that egg calls could be vitally important if the babies are to survive their first experiences outside the safety of the nest. Baby crocodiles take a while to develop into their fearsome adult forms, and in their early years, they are very vulnerable to predators. So it pays a young crocodile to recruit the assistance of its mother, and it pays the siblings to synchronise their birth, to make sure that the entire clutch is ready when mum makes her appearance.
Reference: Current Biology Vol 18 No 12 R513