UNIVERSITY OF
•I l INOIS LIBRARY
ft' BIOLOGY
APR 91992
FIELD
Zool
NEW SERIES, NO. 50
Diet, Feeding Behavior, Growth, and Numbers
of a Population of Cerberus rynchops
(Serpentes: Homalopsinae) in Malaysia
Bruce C. Jayne
Harold K. Voris
Kiew Bong Heang
lift Mttfte. m
A Contribution in Celebration
of the Distinguished Scholarship of Robert F. Inger
on the Occasion of His Sixty-Fifth Birthday
September 30, 1988
Publication 1394
PUBLISHED BY FIELD MUSEUM OF NATURAL HISTORY
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Croat, T. B. 1978. Flora of Barro Colorado Island. Stanford University Press, Stanford, Calif, 943 pp.
Grubb, P. J., J. R. Lloyd, and T. D. Pennington. 1963. A comparison of montane and lowland rain forest
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and R. A. Schwarz, eds., Spirits, Shamans, and Stars. Mouton Publishers, The Hague, Netherlands.
Murra, J. 1946. The historic tribes of Ecuador, pp. 785-821. In Steward, J. H., ed., Handbook of South
American Indians. Vol. 2, The Andean Civilizations. Bulletin 1 43, Bureau of American Ethnology, Smithsonian
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FIELDIANA
Zoology
STEW SERIES, NO. 50
)iet, Feeding Behavior, Growth, and Numbers
rf a Population of Cerberus rynchops
^Serpentes: Homalopsinae) in Malaysia
Jruce C. Jayne Harold K. Voris Kiew Bong Heang
department of Developmental Department of Zoology Department of Biology
and Cell Biology Field Museum of Natural History University of Malaysia
Jniversity of California Chicago, Illinois 60605-2496 Kuala Lumpur, Malaysia
mine, California 92717
i Contribution in Celebration
)f the Distinguished Scholarship of Robert F. Inger
m the Occasion of His Sixty-Fifth Birthday
Accepted for publication February 23, 1987
September 30, 1988
Publication 1394
p
UBLISHED BY FIELD MUSEUM OF NATURAL HISTORY
© 1988 Field Museum of Natural History
ISSN 0015-0754
PRINTED IN THE UNITED STATES OF AMERICA
Table of Contents
;Abstract 1
Introduction 1
Materials and Methods 2
Results
j Diet 4
Predator/Prey Size Relationships 5
I Foraging 6
I Feeding Behavior 7
,! Growth 10
ii Population Structure 11
Population Size Estimates 11
(Discussion 11
Feeding Behavior 12
Growth 13
Reproduction and Population Numbers .13
Vcknowledgments 14
TERATURE ClTED 14
Jst of Illustrations
Schematic map of the study site in the
Muar estuary, Maylasia
2. Plot of log total mass of stomach con-
tents versus log mass of snake for the 181
Cerberus rynchops with prey items 7
3. Striking behavior of Cerberus rynchops
attacking a goldfish 8
List of Tables
Size distribution for samples of Cerberus
rynchops 5
Diet of Cerberus rynchops 6
Coefficients for multiple regression equa-
tions predicting prey handling times for
Cerberus rynchops consuming Perioph-
thalmus chrysospilos 9
Predicted handling times for Cerberus
rynchops eating Periophthalmus chrysos-
pilos 10
Snout- vent lengths and masses for 14 of
the 24 recaptured Cerberus rynchops with
the greatest percentage increase in mass . 10
4.
in
Diet, Feeding Behavior, Growth, and Numbers
of a Population of Cerberus rynchops
(Serpentes: Homalopsinae) in Malaysia
Abstract
Stomach contents were obtained from 181 of
6 1 1 Cerberus rynchops captured near the mouth
of the Muar River in Malaysia. Of the prey items,
69% were the goby Oxuderces dentatus; however,
as C rynchops become larger, ariid catfish, mullet,
and taenioid gobies are increasingly important
portions of the diet. These species of prey, com-
bined with direct observations, suggest that C. ryn-
chops usually forage on or near the bottom or in
very shallow water. Feeding behavior was ob-
served for 23 C rynchops which consumed 71
mudskippers. Initial seizure of the fish always in-
volved marked lateral flexion of the neck. Snakes
often held the fish before the initiation of swal-
lowing, and regression analysis revealed holding
was significantly longer with prey of larger size,
which struggled more. The venom apparatus of C.
rynchops is capable of immobilizing and killing
fish smaller than 3 g. Recapture of 24 tagged snakes
allowed estimation of average percentage of growth
rates in snout-vent length (x = 0.09%/day, range
0%-0.27%/day) and mass (x = 0.36%/day, range
-0.28%-1.43%/day). No evidence of a seasonal
reproductive pattern was found. These aspects of
the natural history of C. rynchops are compared
to those of the sympatric species of marine snakes.
Introduction
The diverse assemblage of southeast Asian ma-
rine snakes includes three distinct taxonomic
groups; homalopsines, acrochordids, and hydro-
phiids. Cerberus rynchops is one of several hom-
alopsine species that is abundant in a variety of
coastal habitats (Wall, 1918; Smith, 1943; Gyi,
1 970; Tweedie, 1 983). Dunson and Minton (1978)
found C. rynchops co-occurring with acrochordids
and hydrophiids in mangrove areas of the Phil-
ippines. Cerberus rynchops can excrete salt via a
salt gland (Dunson & Dunson, 1979) and can ac-
quire some oxygen through cutaneous uptake
(Heatwole & Seymour, 1978); however, these ca-
pacities apparently are not as well developed as
those of the hydrophiids. Smith (1943) reported
that C. rynchops is piscivorous, but the lack of
data on the species of prey prevents determination
of dietary overlap with sympatric species of ma-
rine snakes. Cerberus rynchops possesses opistho-
glyphous dentition, but the role of this dentition
in prey capture and manipulation is not known.
The biological role of opisthoglyphous dentition
is of considerable interest when discussing the evo-
lution of ophidian venom apparatus (Kardong,
1 980). Cerberus rynchops also has a relatively stout
body and fragmented head scalation (Gyi, 1970),
characters that Pough and Groves (1983) corre-
lated with the proficient handling of large prey by
snakes. However, the size of natural prey of C.
rynchops has not been documented.
Most information on the natural history of ma-
rine snakes pertains to the hydrophiids. Dunson
(1975) recently reviewed much of the literature
and provided most information for Australian
species. Much information about southeast Asian
species has come from studies of the hydrophiids
occurring in the main channel of the mouth of the
Muar River in Johore, Malaysia. These reports
have investigated diet (Voris & Voris, 1 983), feed-
ing behavior (Voris et al., 1978), reproduction,
growth, and population size (Voris & Jayne, 1 979;
Lemen & Voris, 1981; Voris, 1985).
The purpose of this study is to expand the
JAYNE ET AL.: CERBERUS RYNCHOPS
knowledge of the marine snake fauna of the Muar
River estuary by investigating several aspects of
the natural history of Cerberus rynchops which
occurs on the adjacent intertidal mud flat. First,
the species composition and size of prey items are
determined. Second, feeding behavior is analyzed,
particularly to determine the role of opisthoglyph-
ous venom apparatus and to quantify the effect of
prey size on handling time. Third, growth rates
and population size are estimated. Finally, com-
parisons are made with the sympatric species of
marine snakes.
Materials and Methods
The fieldwork for this project extended from 1 4
January-5 March 1984 and 20 November 1985—
8 February 1986. The primary site for this work
was 400 m of shoreline along the south side of the
mouth of the Muar River, Johore, Malaysia (fig.
1). Habitats at the shoreline include mud, sand,
and gravel beach, man-made stone walls, and dense
mangrove. From these varied shore habitats, the
intertidal zone extends seaward as one continuous
mud flat about 50 m wide at the mouth of the
river to over 200 m wide at the mangrove area.
A second site was 1 5 km southeast at the coastal
fishing wharf of the town of Parit Jawa. This site
was used for field observations of foraging behav-
ior and as a supply of animals for the laboratory
observations. Although three species of homal-
opsine snakes were encountered in the study areas
(Cerberus rynchops, Bitia hydroides, and Fordonia
leucobalia), C. rynchops was the most common
species and is the subject of this report.
Initially, sampling along the shoreline at the pri-
mary site was conducted at all tidal stages and at
sunrise, mid-morning, noon, mid-afternoon, sun-
set, and at night. Individuals of C. rynchops were
observed during all tidal stages and times of day,
but the snakes were most active on the surface of
the mud flat, on the beach, and entering and leav-
ing the stone wall from 1900-2200. During this
time, snakes also were commonly on the leading
edge of the incoming tide, and most of our col-
lecting concentrated on this period. Notes on the
time of capture, habitat, tidal stage, and water depth
were recorded at the time of capture.
Following two to three hours of collecting, snakes
were taken to our laboratory and palpated for
stomach contents. If something in the stomach was
detected by palpation but was not regurgitated,
then the snake was preserved and the contents
were removed by dissection. The remaining snakes
which failed to regurgitate any food were assumed
to have empty stomachs (this assumption is sup-
ported by the dissection of a series of 24 snakes
early in this work). Snout- vent length (sv) and tail
length were then measured to the nearest 0.5 cm,
and the snake was tagged (Floy Tag & Mfg. Inc.,
#FD-67C). Mass of the snake (Ms) was determined
to the nearest gram by confining the snake with a
plastic bag and placing it on a digital top-loading
scale. The snakes were released within an hour of
measurement at the shoreline in the center of the
study area.
Prey items were immediately preserved using
10% formalin. After fixation, the maximum di-
ameter and various lengths of the fish were mea-
sured to the nearest 0. 1 mm, and after excess fluid
was blotted off, the mass of the fish (Mf) was de-
termined to the nearest 0. 1 g. Based on preserved
series of fish, linear least squares regressions es-
timated log Mf from the log of various measures
of length from partially digested fish.
Three methods were used to estimate the pro-
portion of diet comprised by each fish species. The
occurrence of fish species was estimated as the
percentage of stomachs in which they occurred as
well as the percentage of the total number of items
they comprised. The original wet biomass of each
species was also estimated and then expressed as
a percentage of the estimated total biomass of all
prey consumed by one sample.
Observations were made on the feeding of fresh-
ly captured C. rynchops from 3 1 January-5 March
1984 and 10 December 1985-31 January 1986.
Captive snakes were kept and observed feeding in
1.5 cm of fresh water inside white Styrofoam con-
tainers with an inside height, length, and width of
35, 40, and 60 cm, respectively. The snakes were
observed feeding at times ranging from 1 1 00-2300,
with air temperatures ranging from 26°-32°C.
Snakes that failed to feed at three consecutive trials
were released. A total of 23 snakes (sv 29.5-60.0
cm) was observed feeding for 34 trials, during which
7 1 mudskippers (Periophthalmus chrysospilos) were
consumed. The times between trials ranged from
two to four days. For 1 9 trials involving 1 2 snakes,
a single fish was offered to the snake. For the re-
maining trials, two to five fish were offered in rapid
succession. No more than five feedings were ob-
served per snake. Videotape facilitated the docu-
mentation of the 40 feedings during 1985-1986.
The total length (tl) of each fish was measured
to the nearest millimeter immediately before of-
FIELDIANA: ZOOLOGY
LEGEND:
WATER
if']!?'- MUD FLATS
MANGROVES
SUMATRA
(INDONESIA)
SINGAPORE/
104*
Fig. 1 . Schematic map of the study site in the Muar estuary. Areas indicating mud flat estimate the extent of
exposed mud during the spring low tides.
JAYNE ET AL.: CERBERUS RYNCHOPS
fering it to the snake. The tl ranged from 32-88
mm for the fish used in the experiments. The Mf
were estimated to the nearest 0. 1 g by a linear least
squares regression (log[MfJ - 3.543[log(TL)] —
4.999, r2 = .983) which was calculated from the
measurements of 24 fish that had been preserved
in 10% formalin. For each trial, the initial position
(ip) where the snake seized the fish was assigned a
value from 1 to 4 as follows: 1 = the mouth of the
fish was somewhere within the mouth of the snake,
2 = the snake bit the head or gill region while the
mouth of the fish remained outside of the snake's
mouth, 3 = the fish was seized between the pos-
terior margin of the operculum and the posterior
end of the first dorsal fin, and 4 = the fish was
seized posterior to the first dorsal fin. The amount
of struggle (s) by the fish after being seized was
rated subjectively from 1 to 3 as follows: 1 = the
fish only displayed slight movement when it was
initially seized, 2 = the fish struggled slightly while
it was being held, and 3 = the fish struggled more
than twice and violently enough to substantially
move the head and neck of the snake.
A digital stopwatch was used to determine (to
the nearest second) the total prey handling time
(Tt). Three phases were timed: holding time (Th),
the initial seizure of the fish until the start of lateral
jaw walking by the snake; jaw-walking time (Tjw),
the start of lateral jaw walking until the snout of
the fish was in the mouth of the snake; and swal-
lowing time (Ts), the entrance of the snout of the
fish into the mouth of the snake until the end of
swallowing as indicated by the disappearance of
the tail of the fish.
These various times of prey handling were ana-
lyzed as the dependent variable in three multiple
regression models that were calculated using a hy-
brid stepwise procedure with anF> 4.0 (P < .05)
as the criterion for addition or deletion from the
model. Model 1 estimated Tt by summing sepa-
rate regression estimates of Th, Tjw, and Ts. Oc-
casionally, if a negative value was predicted for
one of these three times, this estimate was changed
to zero before adding it to the other times con-
tributing to Tt. Models 2 and 3 each used a single
multiple regression equation to predict Tt. Models
1 and 2 used a partial F > 4.0 (P < .05), whereas
model 3 used partial F = 2.9 (.05 < P < .10) to
determine the independent variables to be includ-
ed in the multiple regression model. For all han-
dling times sv, Ms, tl, Mf, ip, s, and ordinal num-
ber of fish within a trial were used as the
independent variables. For Tjw and Ts, Th was
also used as an independent variable.
Some Cerberus rynchops from Muar and Parit
Jawa were brought back to the United States for
additional experiments. These snakes were main-
tained on a diet of live goldfish and were usually
fed weekly in 1.5 cm of fresh water inside of a
Plexiglas aquarium (75 cm long x 50 cm wide x
30 cm high). A 2-cm grid on white paper under-
neath the aquarium provided fixed points of ref-
erence.
Two experiments were conducted to clarify what
stimulus might facilitate prey capture for C. ryn-
chops. For both experiments the water tempera-
ture was from 28°-29°C, and the snakes were given
five minutes to acclimate to the aquarium before
being tested. In the first experiment, nine goldfish
were placed in the aquarium for 30 minutes and
then removed immediately prior to the introduc-
tion of a snake. At intervals of about 30 seconds,
0.5-ml samples of aquarium water were dropped
into the aquarium from a hypodermic syringe held
30 cm above the water. The grid was used to es-
timate the distance between the head of the snake
and landing point of the drops. The water was
dropped only when all of the lower jaw of the snake
was below the water's surface, allowing the drops
to land laterally and anteriorly to the head of the
snake. Four snakes were subjected to this stimulus
ten times in succession, with two of the snakes
receiving drops within 4-8 cm the first five times
and 8-12 cm the second five times, and the other
two snakes receiving drops in the reverse order.
In the 2nd experiment, the same four snakes were
subjected to a similar procedure with clean water
in the aquarium. The aquarium was rinsed out ten
times between trials. Results were only kept for
snakes which ate a goldfish within five minutes of
the conclusion of an experiment. Videotape re-
corded the orientation of the strike during some
other regular feeding sessions with goldfish as prey.
Results
Diet
Of the 262 Cerberus rynchops collected and pal-
pated in 1984, 97 had one or more prey items. In
nine cases, however, the stomach contents were
fed back to the snakes that had been captured for
a second time; consequently, only the prey items
of the remaining 88 snakes were analyzed. Of the
349 C. rynchops collected in 1985-1986, stomach
contents were obtained from 93, and all of these
items were retained for analysis.
A total of 3 1 3 items was removed from the 1 8 1
FIELDIANA: ZOOLOGY
Table 1 . Size distribution for samples of Cerberus rynchops.
Snake size
classes by snout-vent length (cm)
25-
30-
35-
40-
45-
50-
55-
60-
65-
Sample
n
29.5
34.5
39.5
44.5
49.5
54.5
59.5
64.5
69.5
1984 diet
88
4.5
27.3
48.9
14.8
4.5
1985-1986 diet
93
1.1
15.0
22.5
18.3
17.2
11.8
5.4
5.4
3.2
15 Jan.-16 Feb. 1984
181
7.7
28.2
43.6
11.6
5.5
2.2
0.5
0.5
1-5 March 1984
67
3.0
20.9
41.8
23.9
5.9
4.5
20 Nov.- 17 Dec. 1985
237
5.5
21.9
29.5
13.9
10.5
3.4
2.5
2.1
0.4
18 Jan.-8 Feb. 1986
112
5.4
17.0
19.6
16.1
12.5
17.0
4.5
7.1
0.9
Frequencies of occurrence are all given in percentages for each 5-cm size class within a sample.
The first two rows in the table indicate the snakes with prey items used for analysis of diet. The remaining rows
indicate the size distribution of all snakes collected between the dates indicated at left,
n = Sample size.
C. rynchops with stomach contents. These snakes
ranged from 26-67 cm in sv and from 19-208 g
in Ms. The size distribution of snakes with stom-
ach contents is summarized in Table 1. The stom-
achs of all these snakes mostly contained four
species of oxydercine gobies, including 2 1 5 Oxu-
derces dentatus, 27 Scartelaos pectinirostris, 13
Periophthalmus chrysospilos, and 2 Boleophthal-
mus boddarti. Other gobiidae found in C. rynchops
included six large, elongate fish (5 Taeniodes cir-
ratus and 1 Odentamblyopus rubicundus) and two
small fish of the genus Acentrogobius. Two species
of catfish were represented by 1 9 specimens of
Ariidae (Arius sp.) and one Plotosidae (Plotosus
sp.). Twelve mullet (Mugilidae) were consumed,
of which four were identifiable as Liza sp. and two
as Valamugil sp. Two tongue fish (Cynoglossidae,
Cynoglossus sp.) and two Sillaginidae (Sillago sp.)
were also eaten. Only a single eel (Synbranchidae,
Macrotema sp.) was removed from C. rynchops.
One specimen each of Eleotrididae (Butis sp.) and
Polynemidae (Eleutheronema tetradactylum) was
also found. The remaining nine fish recovered from
C. rynchops were not identifiable.
Of the 181 C. rynchops, slightly more than half
(109) had only one item in the stomach. Of the
remaining 72 snakes with multiple prey items, 38
had 2 items, 2 1 had 3 items, 5 had 4 items, 5 had
5 items, and 1 snake each had 6, 7, and 8 items;
25 of these snakes had taken only Oxuderces den-
tatus.
The size distribution of snakes with stomach
contents collected in 1984 was significantly dif-
ferent from that of the 1985-1986 sample (x2 =
43.44, df=S,P< .001). Compared to snakes
collected in 1985-1986, those sampled in 1984
had proportionately fewer individuals with sv >
45 cm (4.5% vs. 43%). To facilitate comparisons
between snakes of the two study periods, an sv of
45 cm was used to subdivide samples. Table 2
summarizes the percentage of diet comprised by
the major groups of prey species for small and large
snakes. The 1984 and 1985-1 986 samples of small
snakes (sv < 45 cm) are very similar, with Oxu-
derces dentatus comprising the largest portion of
diet using any of the three measures of importance.
The diet of the large C rynchops (sv > 45 cm)
differs markedly from that of the smaller snakes.
Although O. dentatus comprised the greatest por-
tion of items in the larger snakes, it accounted for
less than 10% of the biomass consumed. Equal
percentages of large snakes contained O. dentatus
and sea catfish, but the sea catfish had almost twice
the biomass of the O. dentatus. Together, mullet
and elongate gobies comprised less than 20% of
the prey items; however, they accounted for nearly
two-thirds of the prey biomass of the large snakes.
Predator/Prey Size Relationships
Total mass of the stomach contents per snake
significantly increased with the Ms (fig. 2). These
data were log transformed to equalize variance of
the dependent variable. For the 181 snakes, log
total mass of contents consumed per snake =
-1.198 + 0.875 log Ms; r2 = .24. For example,
this least squares regression predicts a 50-g snake
would consume 1 .94 g, about 4% of the Ms. These
predicted masses of meals are much less than the
maximum consumed by snakes. For example, one
Cerberus rynchops (sv = 64 cm, Ms = 124 g) con-
sumed a single mullet (Liza sp.; maximum height
x width = 37 x 26 mm; Mf = 66 g) that was 53%
of the Ms. However, meals of such large relative
size were uncommon for the snakes sampled in
this study. In fact, the second largest meal was
another Liza sp., and it comprised only 28.8% of
the Ms. Only 25 of the 181 snakes with contents
JAYNE ET AL.: CERBERUS RYNCHOPS
Table 2. Diet of Cerberus rynchops.
n
Percentage
occurrence
of prey species
Sample
Od
Sp
Pc
Ar
M
EG
Other
1984 <45 cm
% Snakes
84
77.4
16.7
6.0
1.2
3.6
1.2
9.5
% Total items
155
79.4
9.0
3.2
0.6
1.2
0.6
1.2
% Prey biomass
(total 131.9 g)
64.6
13.0
2.0
1.7
7.0
6.4
5.6
1985-1986 <45cm
% Snakes
53
67.9
17.0
9.4
9.4
3.8
0
15.1
% Total items
93
65.6
9.7
5.4
5.4
2.2
0
11.8
% Prey biomass
(total 103.9 g)
52.9
8.1
7.4
8.9
1.4
0
21.2
1985-1986 >45cm
% Snakes
40
32.5
7.5
5.0
32.5
17.5
12.5
2.5
% Total items
58
43.1
5.2
5.2
22.4
12.1
8.6
3.4
% Prey biomass
(total 271.5 g)
8.8
1.2
1.5
15.6
48.8
22.6
1.5
Od = Oxuderces dentatus; Sp = Scortelaos pectinirostris; Pc = Periophthalmus chrysospilos; Ar = ariid catfish;
M = mullet; and EG = elongate gobies. See text for complete explanation of prey categories.
Percentage of snakes with prey species does not sum to 100 for a sample because of stomachs containing more
than one species.
had relative mass of the total contents > 10%. The
snakes with the six largest relative masses of stom-
ach contents each had consumed single fish, none
of which were oxydercine gobies. The seventh larg-
est set of contents consisted of three Oxuderces
dentatus which were 18.4% of the mass of a 25-
cm snake. Snake sv did not significantly affect the
number offish consumed (F — .40, df= 1,179; P
> .50).
A detailed comparison of the size of the prey
relative to the morphological limits of gape is be-
yond the scope of this study, but some evidence
suggests that C. rynchops tends to take relatively
small prey. Although the shape of fish may vary
radically among different taxa, the maximum di-
ameter of a fish approximates the difficulty a snake
may have swallowing it. In addition to the mullet
mentioned previously, some of the largest maxi-
mum diameters of fish consumed by C. rynchops
were 13.0, 19.7, 19.9, and 31.7 mm for snakes
with sv of 27, 38, 46, and 62 cm, respectively. In
contrast to these large fish, 7.9 mm was the largest
maximum diameter measured for any of the 2 1 6
O. dentatus consumed by C. rynchops.
Foraging
Water conditions at the Parit Jawa site often
permitted observation of Cerberus rynchops for-
aging in water as deep as 1.3 m. Whether in water
or on mud fiat, snakes were rarely sedentary for
more than a minute. Swimming C. rynchops con-
sistently moved along the bottom in contrast to
the surface swimming that is commonly used by
colubrid snakes such as Nerodia (Jayne, 1985).
Snakes usually performed sidewinding locomo-
tion on mud that was firm enough to support their
weight. If snakes sank in mud past the first few
dorsal scale rows, then lateral undulation was used
for surface locomotion as well as swimming through
the mud slightly below its surface. Snakes usually
explored burrows and irregularities of the sub-
strate regardless of whether they were under water.
Occasionally, snakes swam with their mouths open
slightly, and the lateral movements of the head
were exaggerated compared to that during normal
swimming. On two of these occasions, individuals
of C. rynchops were observed capturing very small,
schooling fish, and two other snakes used this be-
havior to capture an Oxuderces dentatus and a
mullet that had just escaped after the snake at-
tempted to swallow it. In two other instances,
snakes swimming in muddy water were observed
with this open-mouthed posture, but no fish could
be seen. Another snake remained stationary, as it
was in the midst of a school of fish, and it re-
peatedly used similar alternating lateral move-
ments of the head and neck until the school offish
dissipated. Two other strikes at fish observed in
the field also seemed to have a distinct lateral com-
ponent.
FIELDIANA: ZOOLOGY
2.0
1.5
E
3 1.0
-i
<
2
rrt 0.5
to
<0
<
<
i-
O 0
I-
o
o
-0.5
•1.0
-1.5
1.0
1.5 2.0
LOQ SNAKE MASS (gm)
2.5
Fig. 2. Plot of log total mass of stomach contents versus log mass of snake for the 181 Cerberus rynchops with
prey items. Both masses were originally in grams. The line indicates the least squares regression, where log mass of
stomach contents = - 1.198 + 0.875 log snake mass, r2 = .24.
Analysis of video tapes of 65 strikes of captive-
fed C. rynchops confirmed that there was always
a lateral movement involved in aquatic prey sei-
zure (fig. 3). The initial phase of the strike could
be directed in nearly any direction; however, a
subsequent rapid lateral flexion of the neck mo-
mentarily caused a posture with the anterior region
of the snake forming an arc of about 270° (fig. 3).
This quick lateral flexion usually occurred just as
the snake's mouth contacted the fish. During this
stage of prey seizure, the fish would often not be
grasped securely in the snake's jaws, and the ori-
entation of the snake frequently trapped the fish
between the snake's mouth and body. This en-
abled some snakes to quickly reposition their jaws
or to recapture fish that had momentarily escaped.
Feeding Behavior
The following descriptions are representative of
the variation in observed captive feeding behav-
ior. The figures in parentheses indicate the elapsed
time (in seconds) after the snake initially seized
the fish.
A Cerberus rynchops (sv = 32 cm, Ms = 22 g)
seized a Periophthalmus chrysospilos (tl =75 mm,
Mf = 4.7 g) just posterior to the operculum as the
fish was moving near the snake. Immediately after
striking the fish, the snake rapidly moved the fish
back to the corners of its mouth and held the fish
perpendicular to its neck. During this initial sei-
zure, the fish moved only slightly. As the snake
continued to hold the fish, there were occasional
JAYNE ET AL.: CERBERUS RYNCHOPS
Fig. 3. Striking behavior of Cerberus rynchops attacking a goldfish. The illustration is based on tracings made
from videotape. Pairs of successive images are superimposed, with the dotted outline indicating the earlier position
in each pair. A, Position at time = 0 and 1/15 second; B, position at time =1/15 and 2/15 second.
FIELDIANA: ZOOLOGY
Table 3. Coefficients for multiple regression equations predicting prey handling times for Cerberus rynchops
consuming Periophthalmus chrysospilos.
Dependent
Coefficients of independent variables
Constant
(sec)
variable
(sec)
Mf
(sec/g)
sv
(sec/cm)
s
(sec)
ip
(sec)
Multiple
r2
Model 1
Th
Tjw
Ts
62.1 (.37)
12.4 (.52)
11.3 (.45)
-7.79 (-.28)
-1.13 (-.29)
-1.78 (-.43)
105 (.31)
NS
NS
NS
9.29 (.31)
NS
117
18
79
.43
.50
.35
Model 2
Tt
84.1 (.43)
-10.1 (-.32)
125 (.32)
NS
185
.53
Model 3
Tt
83.0 (.43)
-10.7 (-.34)
94 (.24)
39* (.16)
166
.55
Figures in parentheses after coefficients are standardized regression coefficients.
n = 7 1 for all regressions; ns = not significant; sv = snout-vent length of snake; Mf = mass of fish; ip = initial
position where snake seized fish; and s = struggle of fish.
♦Partial F = 2.95 (.05 < P < .10).
biting-like movements of the snake's maxillae (20,
278, and 392). While the fish was being held by
the snake, some fin, gill, and mouth movements
were apparent. The snake then began to lateral jaw
walk toward the snout of the fish (400) while the
fish showed only very slight gill and mouth move-
ments. Immediately after reaching the snout of the
fish (507), swallowing began and continued until
the tail of the fish disappeared from view (590).
As the snake was swallowing, no fish movements
could be discerned.
The duration of this holding behavior by C.
rynchops varied considerably as illustrated by
another individual (sv = 36.5 cm, Ms = 35 g) that
ate a mudskipper (tl = 69 mm, Mf = 3.5 g). This
snake seized the fish on the gill region, and the fish
flopped violently as the snake briefly held it. The
snake started slow lateral jaw walking to the snout
of the fish (9) as the fish continued to make whole
body undulations. Upon reaching the snout of the
fish (63), the snake started swallowing, and the fish
continued to move slightly until its tail disap-
peared from view (94).
During another trial, a snake (sv = 34 cm) was
disturbed and released the fish (2.5-g mudskipper)
after holding it for 117 seconds. The snake was
then removed from the container, and the fish was
observed until it died 16.5 minutes after being
seized. During other feeding trials with mudskip-
pers, as the snake held the fish, there was some-
times a marked darkening of the fish that spread
from the site of the bite. Occasionally, there was
also a noticeable dilation of the pupils of the mud-
skippers while they were being held. In the field,
a C. rynchops was observed holding a mullet (tl
= 82 mm, Mf = 7.2 g) that was still moving slight-
ly. By the time the snake was captured, the fish
had been released and had died. In the laboratory,
several snakes (sv = 29-51 cm) were forced to
release goldfish (0.8-3.7 g) just as lateral jaw walk-
ing began. The Th varied from 0.2-6.9 minutes.
Of the 3 1 observed goldfish, 1 6 died after being
held from 1.1-6.9 minutes. The times of death
after initial seizure ranged from 6.0-44.0 minutes;
nine of these 16 goldfish died in less than 16.5
minutes after being seized by snakes. Hence, the
venom of C. rynchops appears capable of immo-
bilizing and killing selected prey.
Table 3 summarizes the coefficients of the sig-
nificant independent variables in the various mul-
tiple regression equations. As suggested by the
standardized regression coefficients, the Mf was
always the most significant factor affecting all prey
handling times. Increased sv of the snake always
significantly decreased handling times. Struggling
by the fish primarily increased Th. More posterior
ip increased predicted Tjw. Interestingly, Th (and
presumed envenomation) did not significantly af-
fect Tjw or Ts.
Table 4 lists select predicted values for the three
models of total handling time. The Ms can be
predicted from sv by the least squares regression
log Ms = 2.878(log sv) - 3.0 1 8, r = .969, n = 1 8 1 .
A 45-cm C. rynchops has about twice the mass of
a 35-cm snake (55 vs. 27 g). For a given size, s,
and ip of mudskipper, predicted Tt for the 45-cm
snake can be from '/j-% that predicted for the 35-
cm snake. For a given snake, handling a 2-g mud-
skipper may take from V-tr-xk the Tt predicted for
a 4-g fish. Increased struggle of the fish may cause
JAYNE ET AL.: CERBERUS RYNCHOPS
Table 4. Predicted handling times (in seconds) for Cerberus rynchops eating Periophthalmus chrysospilos (see text
for explanation of models).
Independent variable
Model 1
Model 2
Mf
sv
Model 3
(g)
(cm)
s
ip
Th
Tjw
Ts
Tt
Tt
Tt
2
35
1
3
74
31
39
144
124
169
4
35
1
3
198
54
62
314
208
252
2
35
1
2
74
21
39
134
208
213
4
35
1
2
198
47
62
307
292
379
4
45
1
3
119
44
44
198
191
247
4
45
1
2
119
35
44
189
191
208
2
45
1
3
0*
19
21
40
23
42
2
35
3
3
284
30
39
353
374
238
2
45
3
3
206
19
21
246
273
240
2
45
1
1
0*
1
21
22
23
-182
4
55
1
3
42
33
26
101
91
120
4
55
3
3
252
33
26
311
340
318
2
55
1
3
0*
9
4
13
-78
-36
* Negative value was changed to 0.
Mf = Mass offish; sv = snout-vent length of snake; s = struggle offish; and ip = initial position where snake seized
fish.
up to a sixfold increase in Tt and elicit holding
behavior as well.
The data from the stimulus experiments were
tallied as strike or no response, combined for all
four of the snakes (n = 40), and arranged into two-
by-two contingency tables for chi-squared analysis
(x2 = 3.84, P < .05 used for decision-making). For
the experiment using water with fish odor, 10 of
the 16 strikes occurred during the first half of each
trial using clean water. Hence, for water with fish
odor (x2 = 1.67) and for clean water (x2 = 0),
response does not appear to be dependent on the
number of stimuli within each trial. In other words,
the snakes did not appear to be habituating to the
ten stimuli within each trial. When using the water
with fish odor, 15 strikes resulted from stimulus
within 4-8 cm of the head of the snake, and only
one strike occurred for the 8-12-cm distance;
therefore, response was dependent on the distance
from the stimulus (x2 = 20.67). For the experiment
with clean water, 1 2 strikes were within 4-8 cm,
and six strikes were within 8-12 cm. The x2 was
equal to 3.64, just slightly less than the critical
value. During all of the experiments and routine
feeding sessions, on only two occasions did snakes
attempt to strike at a handler or at moving objects
above the surface of the water. Thus, the response
to the waterdrop stimulus does not appear to be
defensive or visual in nature. Instead, this re-
sponse appears to be predatory and largely the
result of tactile stimulus.
Growth
As indicated by a high incidence of zero and
negative growth of 35 snakes recaptured in 1984
less than 20 days after marking, short-term growth
was probably obscured by measurement error and
handling stress. Consequently, the samples ana-
lyzed here are confined to 24 snakes recaptured
after 20 or more days. Table 5 lists relative growth
Table 5. Snout- vent lengths and masses for 14 of
the 24 recaptured Cerberus rynchops with the greatest
percentage increase in mass.
Snake
Elapsed
Initial
Initial
no.
days
sv (cm)
Ms(g)
628
20
32.5(1.5%)
19(15.8%)
630
20
38.0 (3.9%)
35 (28.6%)
3103
25
43.0(1.2%)
50(14.0%)
3090
26
30.5(1.6%)
16(18.8%)
3073
27
38.0(1.3%)
29 (20.7%)
3074
28
37.0 (2.7%)
28 (14.3%)
3094
28
42.0(1.2%)
42(21.4%)
3045
28
49.0 (2.0%)
70(17.1%)
630
29
38.0 (3.9%)
35(17.1%)
3023
32
40.0 (8.8%)
41 (17.1%)
1091*
42
40.5 (6.0%)
39 (25.6%)
1030*
45
50.0 (2.0%)
67 (23.9%)
1038*
65
44.5 (3.8%)
51 (13.7%)
922*
68
60.0 (5.0%)
124(8.1%)
Figures in parentheses indicate percentage increase be-
tween initial and final capture.
* Captured during 1986.
10
FIELDIANA: ZOOLOGY
for some of these snakes with the greatest increase
in Ms. At initial capture, the sv of the 1984 sample
of 15 snakes ranged from 30.5-49.0 cm (x = 38.5
cm, 5 = 5.04), and the Ms, from 16-70 g (x =
36.67 g, 5= 15.31). The average elapsed time be-
tween captures for this group was 26.7 days.
Growth varied considerably; on average, these
snakes gained mass at 0.50%/day (range -0. 19%-
0.76%/day, 5 = 0.40) and grew in sv at 0. 1 1%/day
(range 0%-0. 1 9%/day, s = 0.075). The nine snakes
recaptured in 1 986 initially ranged from 40.5-60.0
cm sv (x = 49.3 cm, s = 19.3) and from 32-124
g (x = 54.4 g, 5 = 28.1). Average time between
captures was 44.8 days (s = 6.5) for this group.
Average growth rates for the 1986 recaptures were
0. 1 2%/day (range -0.28%-0.6 1 %/day, s = 0. 1 5 1)
for Ms and 0.06%/day (range 0%-0.27%/day, 5 =
0.04) for sv. For both samples combined, average
growth rates were 0.36%/day (s = 0.40) for Ms and
0.09%/day (s = 0.07) for sv.
Population Structure
Table 4 lists the distributions of snake sv for
two subsamples each for 1984 and 1985-1986.
Using a chi-square test, no significant differences
were found between the two subsamples within
1984 (x2 = 1 1.47, df= 7, .1 < P < .2). Similarly,
no differences in size distribution were evident
when comparing the 1985 to the 1986 subsamples
(X2 - 13.30, df= 9, .1 < P < .2). This and the
fact that small snakes (sv < 30 cm) were contin-
uously encountered during this study suggest that
reproduction of this population is aseasonal. When
the total size distribution of 1984 was compared
to that of 1985-1986, a highly significant differ-
ence was found (x2 = 77.18, df=9,P<^ .001).
1978; Voris & Jayne, 1979; Voris, 1985), but rare-
ly trapped C. rynchops. These observations and
the high concentrations of subadults encountered
in the study area lead us to believe that we could
estimate the subadult population in the study area
within a limited period of time.
Three estimates were made. For the first esti-
mate, 108 snakes were marked and released be-
tween 15 January and 10 February 1984. Collect-
ing on 12-13 February produced 32 unmarked
snakes and 12 previously marked snakes. Using
Bailey's (1952) formula the population size esti-
mate is 374 (s = 84.3). For the second estimate,
the snakes collected on 12-13 February and two
other snakes collected earlier were marked and
released. The population was not disturbed by us
from 15 February-1 March. From 1-5 March, we
collected 44 unmarked snakes and 2 1 marked pre-
vious to 1 5 February. Bailey's estimate for these
data is 426 (s = 72.5). In 1985-1986 the third
estimate was made. From 20 November-17 De-
cember 1985, 210 snakes were marked. The snakes
were left undisturbed until 1 8 January-8 February
1986, whereupon 1 12 animals were collected. Of
the 16 recaptures during this period, seven had
Floy tags, and the rest had conspicuous scars where
the tags had pulled out. Bailey's estimate for this
period was 1,396 (s = 303).
During the strongest tides, the area of the in-
tertidal zone within the study site is about 80,000
m2. The conspicuous concentration of snakes at
the edge of the water and the unknown extent to
which deeper water is utilized by the snakes, how-
ever, complicate calculation of the density per unit
area attained by C. rynchops at this site. Never-
theless, these estimates of population size suggest
there may be from one to three subadult snakes
per meter of shoreline within the primary study
site.
Population Size Estimates
Although most collecting was confined to hab-
itats within the primary study site, two adjacent
habitats were investigated. On the landward side
of the beach and stone wall, there was a mowed
soccer field and an unmowed grass field with a
large freshwater pond. No Cerberus rynchops were
observed in about six man-hours of exploring and
traversing this area. The portion of the river mouth
below the low tide level and about 100 r.i north
of the east end of the study site is serviced by two
stake nets. These nets have produced extensive
collections of sea snakes since 1975 (Voris et al.,
Discussion
For the communities of marine snakes that have
been previously studied, little or no overlap in diet
has been found. For a community of ten hydro-
phiids on the Ashmore reef in Australia, Mc-
Cosker ( 1 975) found practically no overlap in either
the diet or microhabitat preferences of the different
species. Similarly, for four different communities
of acrochordids and hydrophiids in Malaysia, Voris
and Voris (1983) found most species were dietary
specialists, and only modest overlap occurred
JAYNE ET AL.: CERBERUS RYNCHOPS
11
among the more dominant species of the com-
munity. Lapemis hardwickii is a notable exception
to this trend, as this hydrophiid has a very gen-
eralized diet (Voris & Voris, 1983).
In the Muar estuary, the homalopsine Fordonia
leucobalia feeds exclusively on crabs and has no
dietary overlap with other snakes. Preliminary
analysis of the diet of Bit ia hydroides suggests this
homalopsine feeds primarily on gobies and hence
has overlap with the diet of Cerberus rynchops.
Acrochordus granulatus captured from the Straits
of Malacca consume about 46% Eleotrididae and
54% Gobiodei with taenioid gobies comprising
7.7% of the prey items (Glodek & Voris, 1982).
The diets of juvenile and adult Enhydrina schis-
tosa are comparable, and this species, which is the
most abundant hydrophiid at Muar, consumes
76.7% ariid and 13.8% plotosid catfish (Voris et
al., 1978). The second most abundant hydrophiid
at Muar {Hydrophis melanosoma) eats exclusively
eels (Glodek & Voris, 1982). Various gobies com-
prise about 10% of the prey items of the third most
abundant hydrophiid {Hydrophis brookii) at Muar.
Hydrophis torquatus is the only other hydrophiid
at Muar for which dietary information is available,
and small samples suggest this species consumes
60% taenioid gobies (Glodek & Voris, 1982).
The extent of diet overlap can be calculated us-
ing the Schoener (1968) index, ex. For the 1985-
1986 sample of large C. rynchops compared with
Enhydrina schistosa, oc = .17, whereas overlap
between E. schistosa and 1985-1986 small C. ryn-
chops was only .05. Using the species level for
grouping prey items, no overlap occurred between
C rynchops and either H. melanosoma or H.
brooki. For large C. rynchops compared with H.
torquatus and A. granulatus, oc = .07. For the more
abundant snake species within a community, Glo-
dek and Voris (1 982) found oc rarely exceeded .10.
The extent to which dietary overlap is deter-
mined by predator choice versus microhabitat
preferences remains unclear. During all of the col-
lecting of homalopsines at Muar and Parit Jawa,
not a single hydrophiid was seen. The extensive
use of fishing nets has captured hundreds of hy-
drophiids in the main channel of the Muar River
(Voris et al., 1 978); however, these same nets have
yielded less than ten homalopsines. The relative
scarcity of adult C. rynchops collected from the
tidal edge and the occurrence of prey such as Tae-
niodes cirratus imply that large individuals of C.
rynchops are more likely to occur in deeper water
than small individuals. Unfortunately, it is diffi-
cult to collect snakes in this most probable region
of interspecific spatial overlap at water depths
ranging from 1-3 m. Yet it seems likely that the
greater dietary overlap of E. schistosa and large C.
rynchops is primarily the result of ontogenetic
changes in habitat preference which cause the rel-
atively opportunistic C. rynchops to overlap more
with the more specialized diet of E. schistosa.
Feeding Behavior
Aspects of the feeding behavior of Cerberus ryn-
chops, such as prey detection, capture, and han-
dling, resemble those of other aquatic snakes. Cer-
berus rynchops used a predominately lateral strike
to capture prey. Pelamis platurus is a surface feed-
ing hydrophiid, and it also uses a lateral strike to
capture fish (Pickwell, 1972; Kropach, 1975).
Another hydrophiid, Enhydrina schistosa, feeds
primarily along the bottom and it also uses a lateral
strike to capture fish. Both P. platurus and E. schis-
tosa hold and envenomate fish and wait until
struggling ceases before swallowing (Pickwell, 1 972;
Voris et al., 1978). As shown in this study, C.
rynchops were more likely to hold (and presum-
ably envenomate) fish that were relatively large or
struggled vigorously. However, initiation of swal-
lowing by C. rynchops may or may not occur be-
fore the fish has stopped struggling. Despite the
sharp spines present in the dorsal and pectoral fins
of ariid catfish, some individuals of C. rynchops
in the field were observed swallowing these catfish
while they were still moving. Catfish are always
consumed head first by E. schistosa (Voris et al.,
1978) and by individuals of C. rynchops observed
in this study. As one might expect for snakes that
inhabit muddy water and have nocturnal tenden-
cies, C. rynchops readily showed striking behavior
when exposed to mechanical stimulus. Feeding of
P. platurus also appears responsive to mechanical
stimulus (Kropach, 1975). As evidenced by the
ability of E. schistosa to feed in total darkness,
some combination of tactile and olfactory cues
appear sufficient for prey capture and consump-
tion (Voris et al., 1978).
In a series of carefully controlled experiments,
Drummond (1979, 1 985) has examined the effects
of visual and olfactory stimuli on predatory be-
havior of certain piscivorous natricine snakes.
Drummond (1979) found that individuals of Ner-
odia sipedon were not entirely dependent on chem-
ical cues to locate and capture fish. Moving models
offish were more effective than nonmoving models
for eliciting orientation, attacking, and searching
12
FIELDIANA: ZOOLOGY
behavior by N. sipedon. Among the predatory be-
haviors described for N. sipedon, Drummond
(1979) found that open-mouthed searching (i.e.,
lateral sweeps with open jaws usually while the
snake was moving) was used when N. sipedon were
under water, and this behavior did not require
visual stimulus, being more likely to occur after
an unsuccessful attack. These observations of open-
mouthed searching correspond closely with those
for a C. rynchops which was seen behaving in this
fashion at night, in muddy water, and after an
unsuccessful attack.
Drummond (1985) isolated visual and mechan-
ical stimuli for predatory behavior of natricines
and found that, in the presence of diffuse fish odor,
visual stimulus could elicit an attack. The role of
visual stimulus for predation by C. rynchops re-
mains unclear. Compared to N. sipedon, the eyes
of C. rynchops appear substantially smaller. The
C. rynchops that were fed Periophthalmus in Ma-
laysia only attacked fish that were moving, but
mechanical and chemical stimuli were also present
in these trials. Cerberus rynchops that were main-
tained in the United States for a longer duration
would attack nonmoving fish. During the daytime,
some attempts were made to capture C. rynchops
by reaching down from the seawall. The C. ryn-
chops were very adept at evading this method of
capture, and they usually dove below the surface
of the water even before the hand entered the water.
Hence, it is clear that C. rynchops can respond to
visual stimulus within about 1 m. Yet, the fact
that C. rynchops would attack vibrations caused
by waterdrops suggests visual stimulus may be
minimally important for the predatory behavior
of this species. Future, more controlled studies
comparing homalopsines, natricines, and hydro-
phiids should clarify different roles of various
stimuli on their predatory behavior.
Growth
The average growth rate of 0. 165 g/day for this
small sample of C. rynchops is about one-third the
estimated rate of 0.49 g/day for the sea snake En-
hydrina schistosa in this same estuary (Voris, 1 985).
The growth in sv of 0.42 mm/day for this sample
is also substantially less than the 1 .0 mm/day es-
timated for E. schistosa in the first year of life
(Voris & Jayne, 1979). One potential factor af-
fecting growth rate is the amount of prey con-
sumed. The total estimated biomass of prey taken
by C. rynchops was 514.2 g, which was 6.21% of
the total biomass (8,282 g) of the snakes that con-
sumed them. Only 29.6% of the C. rynchops ex-
amined had stomach contents. Assuming the sam-
ple of snakes with stomach contents was a random
subsample of all the snakes collected, one can es-
timate the biomass (in grams) of all the examined
snakes by the formula: 8,282 x (100/29.6) =
27,979. Hence, the corrected ratio of biomass of
prey consumed to biomass of predator equals
1.84%. Similar estimates of these ratios can be
calculated for the data set of 1 04 catfish (Voris &
Moffet, 1981) consumed by E. schistosa at Muar.
Enhydrina schistosa consumed an estimated 1 , 1 74
g of fish which was 9.27% of their total biomass
of 12,672 g. However, only 19.6% of the E. schis-
tosa had stomach contents. After correcting for
percentage of stomach contents, the ratio of total
prey biomass/predator biomass becomes 1.81%
for E. schistosa, and this figure is remarkably sim-
ilar to that of C rynchops.
These gross estimates of prey consumption ig-
nore the cost of capturing prey. Cerberus rynchops
was often sighted actively foraging, and on the
average it was taking relatively more and smaller
prey items than E. schistosa. Thus, C. rynchops
may be a more active forager than E. schistosa.
Reproduction and Population Numbers
The lack of a comprehensive collection pro-
hibits definitive conclusions about the reproduc-
tive cycle of Cerberus rynchops at Muar. Snakes
were only preserved sporadically when stomach
contents were not regurgitated. Two gravid fe-
males with barely visible embryos were collected
2-4 December 1985. One female (sv = 67 cm, Ms
without embryos = 208 g) contained 27 embryos,
and the combined mass of these eggs was 39 g.
The other snake (sv = 55 cm, Ms = 127 g) con-
tained 12 embryos which totaled 20 g. From 1-8
February 1986, three large females were pre-
served. Two of them (sv = 62.5, 64 cm) had neither
embryos nor enlarged follicles. The third female
(sv = 62 cm, Ms = 163 g) contained 1 8 very early
embryos weighing 29 g. Hence, the condition of
these reproductive tracts further supports a sup-
position of no strong seasonality of reproduction
for the C rynchops at Muar.
In contrast to the population at Muar, Saint
Girons ( 1 972) suggested that the reproductive cycle
of C. rynchops in Cambodia conformed to that of
other Cambodian homalopsine species. These
homalopsines generally start vitellogenesis in No-
JAYNE ET AL.: CERBERUS RYNCHOPS
13
vember, mating probably occurs in December to
early January, and parturition occurs in May (Saint
Girons, 1972). For C. rynchops in Java, Bergman
(1955) found females with eggs in the oviducts in
March, April, May, July, and October; however,
some months were not sampled. Smith ( 1 943) re-
ported sv of newborn snakes ranging from 17.5—
20.0 cm and brood size ranging from 8 to 26.
Considering this size of newborn snakes and the
continual occurrence of snakes between 25 and 30
cm, it is puzzling that no snakes shorter than 25
cm were collected. Perhaps births were occurring
in a different habitat, or there is some very weak
seasonality of reproduction.
Enhydrina schistosa, the most common hyro-
phiid occurring in the Muar estuary, shows marked
seasonality in reproduction. Voris and Jayne (1 979)
found that vitellogenesis in this species occurs dur-
ing November to December, ovulation probably
occurs in December, and young are born from
mid-February through March. Hydrophis melan-
osoma, H. brookii, and H. torquatus are the next
most common hydrophiids at Muar, and their re-
productive cycle is similar to that of E. schistosa
(Lemen & Voris, 1981). Limited data are available
for the reproductive cycle of Acrochordus granu-
latus at Muar. However, collections of A. granu-
latus from two sites on the west coast of Malaysia,
one within about 24 1 km of Muar and the other
80 km from Muar, suggest this species is aseason-
ally reproductive (Voris & Glodek, 1980).
Acknowledgments
We wish to thank the Department of Biology of
the University of Malaysia for its help. Dr. E. O.
Murdy kindly assisted in the field, as well as as-
sisting with identification of fish species. We also
thank Carole Jayne for her enthusiastic assistance
in the fieldwork and Helen Voris for her editorial
comments. Clara Richardson skillfully prepared
the figures. Financial support for this research came
from a gift from the Allen-Heath Memorial Foun-
dation and a grant (no. INT-8305817) from the
National Science Foundation.
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JAYNE ET AL.: CERBERUS RYNCHOPS
15
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