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From the egg, baby crocs call out

June 23, 2008
Courtesy Cell Press
and World Science staff

Ba­by Nile croc­o­diles’ pre-hatch­ing calls ac­tu­ally mean some­thing to their sib­lings and moth­ers, re­search­ers have found.

The call­s—which are per­fectly au­di­ble to hu­mans and sound like “umph! umph!”—tell the oth­er young­sters it’s time to hatch, and alert the moth­er to start dig­ging up the nest, ac­cord­ing to the group.

A hatching crocodile. (Im­age cour­tesy U.S. Geo­lo­gi­cal Sur­vey)


The find­ings, drawn from sound-play­back ex­pe­ri­ments, con­firm what some had sus­pected based on an­ec­dote, ac­cord­ing to the sci­en­tists, Amélie Vergne and Ni­co­las Math­evon of Un­iver­sité Jean Mon­net in France.

The call­ing be­hav­ior may be key to the rep­tiles’ early sur­viv­al, and their abil­ity to hatch to­geth­er of “vi­tal im­por­tance,” Ma­th­e­von spe­c­u­lat­ed. 

“Most mor­tal­ity oc­curs early in life and hatch­ing vo­cal­iz­a­tions might well at­tract pre­d­a­tors. There­fore, adult pres­ence at the nest and its re­s­ponse to ju­ve­nile vo­cal­iz­a­tions may of­fer pro­tec­tion…. 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.”

The re­search­ers di­vid­ed croc­o­dile eggs due to hatch with­in 10 days in­to three groups. One group was played record­ings of pre-hatch­ing calls; one was played record­ings of noise; and the last was left in si­lence. 

The eggs played the pre-hatch sounds more of­ten an­swered back, the ex­pe­ri­menters re­ported. Many of the eggs in that group al­so moved. Fi­nal­ly, the re­search­ers said, all of the eggs in the pre-hatch group hatched dur­ing the play­back or with­in 10 min­utes of it. Only once did the eggs hear­ing noise hatch, and the rest hatched at least five hours af­ter the last test.

The sci­en­tists then tested the moth­ers’ re­sponses to the calls. In the zoo where the ex­pe­ri­ments were car­ried out, eggs are re­moved from the nest with­in a few days of lay­ing, the re­search­ers ex­plained. In spite of this, fe­males con­tin­ue to guard the nest.

At the end of the in­cuba­t­ion per­i­od, the re­search­ers hid a loud­speak­er un­der­ground near the emp­ty nest. They then played pre-hatch­ing calls in­ter­spersed with noise to ten moth­ers. The adults more of­ten turned their heads or moved af­ter egg sounds than af­ter noise, they not­ed, and eight of the moth­ers re­sponded to the recorded calls by dig­ging. 

The find­ings ap­pear in the June 23 is­sue of the re­search jour­nal Cur­rent Bi­ol­o­gy.

The be­hav­ior may have a long his­to­ry, the re­search­ers said.

“As birds al­so pro­duce em­bry­on­ic vo­cal­iz­a­tions that in­duce pa­ren­tal care,” they wrote, “such acous­tic com­mu­nica­t­ion at an early stage of de­vel­op­ment may be a shared be­hav­ioral fea­ture of past and pre­s­ent ar­cho­saurs.” Ar­cho­saurs are an an­cient group of rep­tiles now repre­s­ented by mod­ern birds and croc­o­diles.

* * *

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Baby Nile crocodiles’ pre-hatching calls actually mean something to their siblings and mothers, researchers have found. The calls—perfectly audible to humans, and which sound like “umph! umph! umph!”—tell the other infants it’s time to hatch, and alert the mother to start digging up the nest, according to the group. The findings, drawn from sound-playback experiments, confirm what some had suspected based on anecdote, according to the scientists Amélie Vergne and Nicolas Mathevon of Université Jean Monnet in France. The calling behavior may be key to the reptiles’ early survival, and their ability to hatch together of “vital importance,” Mathevon speculated. “Most mortality occurs early in life and hatching vocalizations might well attract predators. Therefore, adult presence at the nest and its response to juvenile vocalizations may offer protection…. 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.” The researchers divided crocodile eggs due to hatch within 10 days into three groups. One group was played recordings of pre-hatching calls; one was played recordings of noise; and the last was left in silence. The eggs played the pre-hatch sounds more often answered back, the experimenters reported. Many of the eggs in that group also moved. Finally, the researchers said, all of the eggs in the pre-hatch group hatched during the playback or within 10 minutes of it. Only once did the eggs hearing noise hatch, and the rest hatched at least five hours after the last test. The scientists then tested the mothers’ responses to the calls. In the zoo where the experiments were carried out, eggs are removed from the nest within a few days following the laying date, the researchers explained. In spite of this, females continue to guard the nest. At the end of the incubation period, the researchers hid a loudspeaker underground near the empty nest. They then played pre-hatching calls interspersed with noise to ten mothers. The adults more often turned their heads or moved after egg sounds than after noise, they noted, and eight of the mothers responded to the recorded calls by digging. The behavior may have a long history, the researchers said. “As birds also produce embryonic vocalizations that induce parental care,” they wrote, “such acoustic communication at an early stage of development may be a shared behavioral feature of past and present Archosaurs.” Archosaurs are an ancient group of reptiles now represented by modern birds and crocodiles. The findings appear in the June 23 issue of the research journal Current Biology.