The Archosauria are represented by only two extant groups of animals: The crocodilians and the aves. Most people seem shocked to learn that, in extant species, birds are most closely related to crocodilians. These two groups diverged sometime in the mid to late Triassic, so it is of no surprise that upon initial glance these groups appear so drastically different. Birds are so highly modified and crocodilians seem frozen in evolutionary time. However, similarities especially arise when one considers strategies each group utilizes to ensure that their young are able to reach maturity and thus pass on their genetic lineage. Crocodiles, alligators and their relatives are in the paraphyletic clade the reptiles; most of whom do not perform parental care and lack vocalization. However, crocodilians, like birds, build and tend their nests, protecting them from predators and stay with their young after they hatch and can function on their own. Crocodilians also use vocalization as one of their main means of communication, which is unique among reptiles. Not only do these extant archosaurs use vocalization to communicate throughout their lifetime to survive, they communicate even before hatching from within the confines of their egg.
An article posted on Biologynews.net on June 23, 2008 reports on a study concerning pre-hatching calls in the Nile Crocodile, Crocodylus niloticus, preformed by researchers Amélie Vergne and Nicolas Mathevon of the Université Jean Monnet in France. This experiment is one of the first of its kind to collect data that supports that pre-hatching calls have meaning and play an important role in survival. The study was comprised of a series of “playback” experiments, which used recorded calls of juvenile crocodiles to analyze behavioral responses from both siblings and mothers. More often than not the groups of eggs who were played the recordings would respond, move and even hatch within ten minutes of the initial playback. The eggs in the mothers’ nests were removed and replaced with a speaker. They were also played the recorded calls and eight out of the ten mothers responded to the calls by digging at the nest.
The call of pre-hatched baby crocodiles is reminiscent of the familiar chick “peep” in which most people are familiar. Crocodilians have a small vocabulary in comparison to birds, but considering they lack vocal cords, they get the job done. To hear different crocodile calls, including the endearing hatchling call, a fun website to visit is Adam Britton’s “Crocodile Talk” webpage. There you can listen to short clips and even see some spectrograms of selected calls, which gives an idea of the selected call’s intensity.
Vocalization is an important means for survival for both birds and crocodilians. By being able to communicate to your siblings while still in the egg, it is possible to synchronize hatching and thus out number and overwhelm predators. When combined with parental care, vocalization can trigger the parent or parents to prepare to protect their offspring while hatching and through their juvenile period. Parental care greatly increases the chance that these offspring will reach adulthood and thus reproduce and continue the genetic legacy of their parents. Many factors play into the success of the archosaurs and their survival throughout their long life history. Perhaps the use of vocalization as a means of communication beginning pre-hatching has contributed in the success of the crocodilians and birds, a success that has preserved the group of animals most closely related to the dinosaurs.
posted by: "Jess Bouchard" (1).
This is a really interesting post. I had no idea that there were animals besides the chicks that had a hatching call. I would have never guessed that crocodiles would communicate in that way. You said that they lack vocal cords so how exactly do they make the calls? What other scenarios do they use vocalizations to communicate in? Is is mainly just between the mother and her young?
ReplyDeleteThanks for putting the link to the calls! The hatchling one is really endearing!
-Tara Quist
Response to Tara Quist's comment:
ReplyDeleteCrocodilians can produce all of their calls by manipulating air through the nostrils or throat, much like humans can to produce coughing sounds or grunts. So, most of their repertoire consists of hisses, grunts and coughs. They use this type of communication in a variety of situations. Some examples include distress calls by juveniles, threat calls to protect territory and courtship calls. Mothers even let out a call when they have laid eggs. I speculate that no one would want to mess with a croc mom!
Posted by: Jess Bouchard