26 JUNE 2008
Crooning crocs = unscrambled eggs?
If you are reckless enough to hang out at a crocodile rookery, and your timing is just right, you may hear a strange, subterranean "oom-pah" noise. If the four-meter momma croc on the sand does not scare you across the river, you'll notice that she begins digging lickety-split once she hears those murmurs.
The sound of the Crockettes, ready to hatch and high-kick their way across the beach. Says noted music critic Bel Canto Betty, "The warm yet staccato bursts of the mezzo-soprano denote excellent breath support from the diaphragm; however, the phrasing remains choppy."
Oom-pah, maybe, but we are not talking reptilian polka...
If you dared poke a camera into the nest, which can contain up to 30 eggs, you'd notice the eggshells cracking pretty much in unison, spewing out a croc apiece, just in time for mom's frantic digging.
Is this communication, or coincidence?
All graphics and sound courtesy Amélie Vergne
How do you scare off a momma croc long enough to bury a speaker where she thinks her nest is located? Quickly, using a shovel with a really long handle.
A new study comes down firmly on the side of communication, showing that Nile crocs -- a common and dangerous native of Africa -- uses vocal signals to choreograph the delicate dance that delivers the hatchling from a buried egg to mom's loving embrace.
The study, from the lab of Nicolas Mathevon, in the sensory ecology and neuro-ethology lab at Jean Monnet University (Saint-Etienne, France) had two phases. First, researchers exposed mature croc eggs, aged about three months, to silence, random noise, or a recording of a croc-embryo aria. Eighty percent of the eggs that heard the aria began their own vocalizations, compared to 22 percent of the noise group, and 7 percent of the silence group. Fifty-five percent of the croc-song group began to move within a minute, compared to 12 percent in the noise group and none in the silence group.
Mommy, dearest, are you listening?
The second phase of the experiment showed that mom is equally responsive to recorded vocalizations coming from a speaker buried in the sand. All the females moved in response to the croc-sound, compared to only 40 percent among those that heard noise. The moms were also much more prone to start digging upon hearing a recorded croc-song, rather than noise or silence.
A mother digs the sand as she hears pre-hatching calls. Gotta dig that toothy grin!
People have known for years that about-to-hatch crocs murmur from below the sand, but until now nobody had documented the sound's effect on behavior, says Amelie Vergne, a doctoral researcher at Jean Monnet. "We made the experiment to see what was the cause for the behavior. Many people who met crocodiles in the wild heard them calling, but nobody really knew what was the function, why the baby was calling."
Crooning is powerful, she found: all four of the seven eggs that audited the real croc-leider broke through the shell within 10 minutes. Only one of the six eggs that hatched in the other two groups (each containing five eggs) hatched in that time period.
Coordinating the below- and above-ground movements of mom and her mini-crocs has obvious evolutionary value, Vergne says. Although she cannot prove that the animals would die without the communication, it certainly seems to boost their odds of surviving the vulnerable hatching phase, and many birds also use their voices to signal the onset of hatching, Vergne notes.
The acoustic signals of crocodilians play a major role in the first stages of life, particularly during interactions between juveniles and adults. This baby croc is shown before a graph of its "let's get hatching" sound.
As a bunch of self-centered mammals, we had to ask if the study says anything about critters that do not hatch from eggs. "The only thing we can say is that this is a very common point with birds as well," Vergne says. "And since birds and crocodiles have a common ancestor, the archosaur, this can bring interesting information about birds, crocodiles, and the dinosaur that is their common ancestor."
Neither recordings nor sheet music of the archosaur's song has been located, perhaps because the critter dates back about 200 million years -- long before mammals began cooing and crooning. But now we know: Oom-pah! Long live the song!
- David J. Tenenbaum
Related Why Files
• Snail Sex
• Monster Crocs
• Tracking Hatchlings
Bibliography
• Amelie Vergne and Nicolas Mathevon, Crocodile egg sounds signal hatching time, Current Biology, Vol. 18, No. 12, June 24, 2008.
• Nile croc info
