Did you know that crocodiles call from inside their eggs with a 'cheep' sound shortly before they hatch?
Biologists in France studied crocodiles in zoos hoping to find out why. One hypothesis for the behavior is that the calls may coordinate hatching time for the nest. Essentially one baby starts to call, meaning "I'm ready to hatch!" and her siblings inside their eggs call back to say "OK! Me too!" so the whole brood hatches at the same time.
Another theory is that the calls signal the mother. Crocodiles lay their eggs in the ground and cover them with a mound of sand. Then she'll stick around and protect the nest until the eggs hatch. At hatching time, the mothers dig open the nest to help the young reach the surface. However, when females lay eggs in the zoo, zookeepers sneak in and remove the eggs to hatch them in an incubator. Since the mom crocodile doesn't know the eggs are gone, she continues to stand guard, and the biologists can test her responses to the egg calls.
They played recorded egg calls from a speaker buried in the sand mound. The mother crocodile immediately started to dig up the mound above the speaker. Then they played calls to the incubating eggs. When the eggs heard the recorded calls, they started to call back and began hatching out of their leathery shells. So both hypotheses may be true. The calls could signal both to the mother and to other eggs.