IFebruary 1992

Asiatic Herpetological Research

Vol.4, pp. 132-136|

Preliminary Research on the Function of the Eggshell in the Chinese
Alligator {Alligator sinensis)*bihui Chen

1and baodong Liang 2'Department of Biology, Anhui Normal University, Wuhu, Anhui, China^Anhui Research Center of Chinese Alligator Reproduction, Xuancheng, Anhui, China

Abstract. -There is a layer of mucous material on the eggshell of freshly laid Chinese Alligator eggs.
Alligator eggshells have different functions in different periods of embryonic development. Over 95% of
the eggshell consists of calcite and calcium carbonate. The freshly laid egg is strong and rigid to withstandthe weight of the adult female alligator crawling over the closed and compacted nest. As incubation
proceeds, erosion craters and cracks appear on the surface of an eggshell. This change of the eggshell isadapted to more efficient oxygen requirements and prevents the egg from dehydration or from too muchwater flowing into the egg, as the embryo rapidly develops. The eggshell does not provide calcium tosupply embryonic developmental demands.

The eggshell membrane plays an important role in theantimicrobial defence of the egg.

Key words: Reptilia, Crocodilia, Alligatoridae, Alligator sinensis, China, eggshell function

Introduction
Ecological examination in the field andlong term artificial culture has revealed thata series of changes appear in the Chinese
Alligator's (Alligator sinensis) eggshell as
the egg develops. These changes seem torelate closely with the alligators' embryonicdevelopment and climatic conditions. In
order to study this relationship, in 1976 we
made some observations and experimentson the change of the Chinese Alligator's
eggshell in the course of incubation. In
1987 and 1988, we made some additionalexperiments.

Methods
Experiments concerning the mucous
covering freshly laid eggs
investigate its influence on egg incubation.
Four clutches laid by a field alligator weremarked as A, B, C, and D respectively.
There were 21 eggs in clutch A (with 1broken and 1 infertile), 17 eggs in clutch B,20 eggs in clutch C and 18 eggs in clutch D
material(withinfertile). The eggs of clutches Aand B were laid in the same day. Eleven
1+This publication was previously published inChinese by Chen and Liang (1990).

© 1992 by Asiatic Herpetological Research

eggs from clutch A were selected randomly
for the experiment, with 10 eggs for thecontrol.
Eight eggs of clutch B wereselected randomly for the experiment, with9 eggs for the control. The experimental
eggs of clutches A and B were put together
into group I.
Control eggs were puttogether into group II. The eggs were laid
one day earlier in clutch C than in clutch D.
Each half of the eggs of clutch C and D
used were put together in group III, theother half of the eggs were used for acontrol and were put together into groupIV.
Each egg of group I and III waswashed slightly with gauze in 26°C distilledwater, had its mucous material cleaned andwas wiped dry. The eggs of group III andIV were submerged in oxygenated distilledwater (27-28°C) for 8 hours. Then the fourgroups were set in an environmental
chamber in the same condition andincubated at 30-32°C.

The next experiment investigated theinfluence of humidity on egg incubation.
Eighty-seven eggs, which were laid by anAnhui
Research Center of Chinese Alligator
Reproduction (ARCCAR), were selected
randomly and divided into three groups. Ingroup 1, humidity was maintained at 95%
fromto 21 days after laying, at 80-85%
artificially cultured alligator in theFebruary 1992

TABLE

1 .

Vol.4, p. 133

The influence of the mucous material surrounding Chinese Alligator (A. sinensis) eggshells

during incubation.

Groups

Asiatic Herpetological ResearchVol. 4, p. 134

Asiatic Herpetological Researchevaporates easily leading todehydration.
Conversely, duringcontinuous cloudy and rainy days, the rain
rapidly

February 1992

stage of incubation, due to nest material

cavity and influences normal egg
development. A small opaque white patch
was observed on the top surface of theeggshell of a freshly laid Chinese Alligatordecay, the resulting acidic effect produces
erosion craters on the eggshell surface.
The eggshell's strength decreases, but atthat time, the frequency of the female
alligator crawling over nest tops decreases,
so the eggshell isn't damaged. Moreover,there is better air circulation to allow more

As incubation progressed, the patch

efficient oxygen requirements during rapid

expanded in width around the shell centerand in length towards the ends of the shell.
About one day after egg laying, it expands
approximately 2/5 around the shell, 3/4
around in two days, and completely aroundthe shell after three days. This band slowly

embryonic development. In addition, there
is an increase of exchange between interior
and exterior water of the egg. At that time,

easily permeates loose nests into the egg

egg.

extends in length, completely reaching theends of the shell after about one month.

Ferguson (1982) has reported a similarchange for A. mississippiensis eggshells
and suggested a variety of explanations.
One of these explanations is thedevelopment of erosion craters thatrendered the calcite opaque, altering theoptical properties of the shell. Another is adrying out of the eggshell due topolarization of the watery albumin towards
the ends of the egg and an increase inporosity of the shell. We quite agree withhis explanations. Erosion craters on theChinese Alligator eggshell increased withthe advance of incubation and thenSome
progressively became cracks.
crackswereobservedonthelongitudinalaroundthethirdweekofeggshell
They became progressively
more extensive in number and diagonal
incubation.

cracks appeared around the fifth week.
Eventually these cracks became more andmore extensive in size, number, anddistribution up until hatching.

chemical analysis of the Chinese
Alligator's eggshell, calcium carbonate incalcite form reached above 95% (Gu et al.,This is similar to A
1987).
mississippiensis eggshells (Ferguson,1982). During examination of Chinese
Alligator nests in the field, broken eggs
were observed in nest cavities less than 1%In.

of the time.

Apparently, the freshly laidegg
strong and rigid to withstand theof
the adult female crawling over theweightclosed and compacted nest. In the initialisthe average atmospheric temperatureremains at approximately 29-30°C. It is
uncertain whether water loss from the egg
should be present or not. In order toexplore the relations between incubative
humidity and eggshell change, we madesome experiments on the hatching rate ofalligator eggs versus humidity.

As shown in table 2, group 1 had thebest efficiency, and the hatching rate was100%.

Group 2 was hatched under highhumidity throughout the incubative period.
After the inter-period of incubation, there
were 9 eggs which took in water, theeggshell swelling, and cracking. Four eggs
were particularly swollen, and the eggshells
membrane was cracked. Nine swollen
eggs were approximately 6.13±0.29 cm indiameter, and 4.46±0.21 cm in width.
Fifteen normal eggs, from the same
incubative period, were randomly selected

and measured 6.14±0.27 cm in diameter,and 3.48±0.12 cm in width. Both weresimilar in diameter, but the width of theformer is about one centimeter larger thanthe latter.

Packard et al. (1979) reported that thewater conductance of alligator eggs is fivetimes higher than that of birds eggs, whichis in keeping with the porous nature of thelate alligator eggshell. Our experimental
results agree with their report.
Thehumidityofgroup3was85-95%

throughout the incubation period, and only

on 10th day increased to nearly 100%.
There were no unusual phenomenon to beobserved. On the 30th and 40th day there
was one and three dead embryos
The above experiments
respectively.
indicate that high humidity in the inter-February 1992

Asiatic Herpetological Research

TABLE 2. The effect of humidity on A. sinensis egg development.
GroupsVol. 4, p. 135

Asiatic Herpetological Research

Vol.4, p. 136

embryos of A. mississippiensis obtains

much less calcium from the eggshell thaneither birds or turtles, indicating thatshould be possible to grow normalitusingalligatorsshell-lessFebruary 1992

assistance provided by the Chinese Science
also thank Dr. Theodore
Foundation.
J. Papenfuss and J. Robert Macey for theirWekind help in correcting this paper.

cultureLiterature Cited

techniques.

Gu et al. (1987) have analyzed thecalcium and magnesium contents ofChinese Alligator eggshells (Table 3). As
shown in table 3, calcium contents ineggshells after hatching are similar toinfertile eggs not incubated. This indicatedthat the calcium content in eggshells which
underwent incubation for about two months
did not decrease. This is greatly different
from bird embryos which obtain a large

amount of calcium from eggshells.

CHEN, B., AND B. LIANG.

1990. Preliminary
research on the function of the eggshell ofChinese Alligator. Pp. 242-245. In E. Zhao(ed.)

From water onto land.

Press, Beijing.

China Forestry(In Chinese).

FERGUSON, M. W. J.

1982.

The structure andcomposition of the eggshell and embryonicmembrane of Alligator mississippiensis.
Transactions of the Zoological Society ofLondon 36:99-152.

The

experiment proved that alligator eggshells
provide very little or no calcium for
Thus, theembryonic developmental.
A lot ofcalcium.
not
store
does
eggshellmade inincubation
artificialofexperiments
ARCCAR indicate that if Chinese Alligator
eggs are damaged due to various causes, so

GU, H., Q. XU.ANDGUG.

.

1987.

[Determination of the calcium and magnesium
contents of eggshell of Alligator sinensis andActa
discussion on relevant problems].

Herpetologica Sinica 1987-6(l):23-26.

(InChinese).

long as the eggshell membrane is notbreached, and temperature and humidity iswell controlled, the eggs will develop
normally; their hatching rate still reached

JENKINS, N. K. 1975. Chemical composition of

But if the membrane isover 80%.
microbes
easily invade and thepunctured,Thisstink.
androtsuggests that theeggs
animportant role ineggshell plays

JOANEN, T, AND L. MCNEASE.

eggs of the crocodile (Crocodylusnovae guineae). Comparative Biochemistry andPhysiology 51 A: 89 1-895.
the1977. Artificial

incubation of alligator eggs and post hatching
culture in controlled environmental chambers.

Proceedings 8th Annual Workshop World
Mariculture Society.

antimicrobial defense of the egg.

PACKARD G. C, T. L. TAIGEN, M. J. PACKARD,
AND R. D. SHUMAN. 1979. Water vapor

Acknowledgments

conductance of testudinian and crocodilian eggs.

We aregratefulforthefinancial

Respiratory Physiology 38:1-10.