Baby crocodiles start chatting to one another and to their mothers just before they hatch, researchers say.

The little reptiles make an 'umph! umph! umph!' noise and scientists believe they are signalling they are ready to be born.

"Crocodile mothers react strongly to playback of pre-hatching calls, most of them by digging the sand," Amelie Vergne and Nicolas Mathevon wrote in the journal Current Biology.

Crocodiles signal to each other just before they hatch

The researchers tested 10 crocodiles and their eggs, recording the sounds the babies made and then playing these, as well as random sounds, to the mothers.

The sounds seemed to spur siblings to start breaking out of their eggs, Vergne and Mathevon wrote.

"In the zoo where we did the experiments, eggs are removed within a few days after laying. In spite of this, females continue to guard the nest."

Eight of the mother crocodiles who were played recordings of the correct "umph" calls tried to dig up their clutches, while mothers who heard random sounds did not.

Mathevon said many baby reptiles are eaten right after birth, so it may be important for them all to hatch together and for the mother to be there when they do.

"In this sense, it is important for all embryos in the nest to be ready for hatching at the same time so that they all receive adult care and protection," he said.