||Temporary modified epidermis
||To assist in hatching, splitting the inner
membrane and cracking the outer membrane of the egg. It is
not a true tooth, and it is resorbed a few weeks after hatching.
||Epidermis at the tip of the upper jaw is modified
into a paired, horny point.
|The inside of a crocodile
egg is a great place to develop, but after more than two months
trapped in this small, ovoid shell the fully developed embryo needs
to get out fast before its oxygen demands become too great. Hatching
is actually a much more difficult task than you might imagine, but
the baby crocodile has developed ways to make the process easier.
The most obvious of these is the presence of the egg tooth to help
break out of the egg.
The photograph on the right
shows clearly the position of the egg tooth (circled in red) on
the crocodile's upper jaw. Despite its name, the "egg tooth"
is not actually a true tooth made from bone. Instead, it is a modified
piece of skin - a toughened, horny piece of epidermis which forms
during development of the embryo. The "tooth" is actually
paired. It's not as sharp as a bony tooth, but it is normally very
effective at cutting through egg membrane.
Crocodile eggs, unlike other reptile eggs, consist of a soft, inner
membrane and a hard, calcified outer membrane - very similar to
a bird's egg. So how does the crocodile use its egg tooth to break
out? When the time comes to emerge, the crocodile normally rubs
the tip of its snout up and down against the inner membrane of the
egg. The sharp egg tooth slices apart the inner membrane, and the
hatchling can then push its nose forwards forcefully to crack the
event, where the baby crocodile pierces the eggshell membrane and
sticks its snout out into the air, is called "pipping"
(see right). The hatchling may remain in this position for several
hours, although normally when the adult female opens the nest in
response to hatching calls from the eggs, her vibrations stimulate
the eggs to hatch rapidly. Some eggs do not always hatch immediately,
even when the female is present, and so she picks them up within
her jaws to gently squash the shell between her tongue and the roof
of the mouth, encouraging the hatchling to emerge. Without her presence,
some of these eggs may not hatch at all. If the nesting conditions
are slightly dry, for example, the inner membrane becomes dehydrated
and leathery. In this case, the crocodile trapped inside the egg
may not be able to split open the toughened membrane using its egg
tooth. Without assistance, the small crocodile will suffocate and
die within its prison.
the crocodile has successfully hatched, the egg tooth becomes completely
redundant - it is only ever used for a brief but important moment
in the crocodile's life. The photograph on the left shows a small
saltwater crocodile four weeks after hatching - the egg tooth has
been almost completely resorbed, and in a few more weeks there will
be no trace of it.