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Herpetologists' League
Demography of Green Snakes (Opheodrys aestivus)
Author(s): Michael V. Plummer
Vol. 41, No. 4 (Dec., 1985), pp. 373-381
Published by: Herpetologists' League
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VOL. 41 DECEMBER 1985 NO. 4
Herpetologica, 41(4), 1985, 373-381
? 1985 by The Herpetologists' League, Inc.
ABSTRACT: Sex ratio, population size, age structure,
and survivorship
of an Opheodrys
were studied by mark-recapture
in Arkansas.
Sex ratio varied monthly, favoring males
in early season (60-80%) and females in midseason
(60-80%). Females predominated in one year
but the sex ratio was 1:1 in two other years. Population density (approximately 430/ha) was
statistically constant over two years. Age structure of females varied little between years, but age
of younger males changed from one year to the next. Age-specific
to be constant in adults, was greater in females (49%) than in males (39%). Survivorship
the first year was low (21%).
A life table yielded a net reproductive rate (R0
= 0.84) insufficient to
sustain the population. The calculated survival rate of first-year snakes, and possibly of adults,
underestimated actual survival.
Key words: Reptilia; Colubridae;
Survivorship; Life history
REPTILIAN demographic studies have
been concerned primarily with lizards.
The fact that demographic and other eco-
logical studies have provided much in-
sight into ecological processes
led Huey et
al. (1983) to state:
for many types
of ecological studies lizards are model or-
ganisms-moreover, they now challenge
birds as the paradigmatic organism of
ecology." Demographic studies of their
close relatives, the snakes, have not been
as instructive. Several attributes of snakes
generally contrast those of lizards and
contribute to snakes being less tractable
study subjects. These attributes
include in-
conspicuousness, nocturnality, unpredict-
able and potentially extensive move-
ments, long periods of inactivity, and
apparent low population densities. Recent
evaluations of the suitability of snakes for
demographic studies (Lillywhite, 1982;
Turner, 1977) have not been favorable.
The rough green snake, Opheodrys aes-
tivus, is an arboreal
species which prefers
the brushy vegetation of fence rows and
forest edges. In contrast to many snake
species, it has attributes that would appear
to enhance demographic analysis. Al-
though diurnally cryptic, green snakes are
abundant, have low vagility, and have a
conspicuous sleeping posture at night (on
the distal ends of branches)
which renders
them easily and predictably collected
(Plummer, 1981). Some demographic at-
such as reproduction and growth,
have been reported (Plummer, 1984,
1985). The purpose of this paper is to ana-
lyze sex ratio, population size, age struc-
ture, and survivorship. Field observations
from August 1977 to September 1984 in-
dicated that my study population was re-
stable. If these observations
correct, the analysis of the interaction of
reproduction and mortality should yield a
net reproductive rate that would sustain
the population over the long term.
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374 HERPETOLOGICA [Vol. 41, No. 4
80 .
Q 40 //
Ql- 1978 * (n=390) /
20 1979 o (n=353)
1981 a (n=169)
1984 A (n=399)
FIG. 1.-Sex ratio variation in monthly samples of
The study area was the forest bordering
a narrow
50 m) 700 m long
channel where Overflow Creek entered
Bald Knob Lake in White County, Arkan-
sas. The habitat has been described pre-
viously (Plummer, 1981). Green snakes
habited the dense forest edge vegetation
near the shoreline and were collected by
hand after searching the vegetation at
night with a spotlight while slowly cruis-
ing the shoreline in a boat. Collecting oc-
curred 2-3 times per week from late April
through early October in 1978 and 1979
and irregularly
in 1977 and after 1979. In
1984, snakes were sampled biweekly from
Bald Knob Lake and nearby localities for
related studies. These data were included
in the sex ratio analysis. On initial capture
in 1977-1979, snakes
were given a unique
mark by clipping ventral scutes (Brown
and Parker, 1976). Date, snake number,
sex, and snout-vent length (SVL) were re-
corded at each capture. Snakes were pro-
cessed in the field and were released with-
in 10 min at the point of capture.
procedures follow SPSS Inc. (1983).
At this locality, green snakes grow pri-
marily from May through September.
Oviposition occurs in June-July and
hatching in August-early September.
Snakes enter into hibernation by October
and ernerge in late April-early May
TABLE 1.-Observed frequencies of single and mul-
tiple captures
of Opheodrys
aestivus compared to a
Poisson distribution
of expected fre-
quencies assuming
was constant.
No. of
samples 1978 1979
in which
an individ- Observed Expected Observed Expected
ual was no. of no. of no. of no. of
captured snakes snakes snakes snakes
1 126 131.88 114 114.89
2 65 57.24 44 51.53
23 19 20.88* 28 19.58*
X2 = 1.48 X2= 4.73
P > 0.10 P < 0.05
* Cells with expected frequencies c5 were pooled.
(Plummer, 1981, 1984, 1985). I assigned
snakes to age classes
according to the size-
age criteria
of Plummer (1985). Age classes
were: age class E = egg (from oviposition
to hatching), H = hatchling (from hatch-
ing to first hibernation), 0 = first-year
snake (from May following the first hi-
bernation through September of the same
year), age class 1 = 1-yr-old (from May
following the second hibernation through
September of the same year), age class
2 = 2-yr-old (from May following the third
hibernation through September of the
same year), and so forth.
Sex Ratio
From April 1978-October 1979, 743
captures were made on 333 snakes in the
study area. In 1978, the overall sex ratio
of 0.94 male: 1 female (210 snakes) was
not significantly different from 1:1 (X2
0.30; P > 0.50). In 1979, the overall sex
ratio of 0.72 male: 1 female (186 snakes)
was significantly different from 1:1 (X2
4.83; P < 0.05). Removal collecting of 169
snakes in 1981 at the opposite end of the
lake yielded a sex ratio of 1.01 male: 1
female which was not significantly differ-
ent from 1:1 (X2
= 0.01; P > 0.90). Sex
ratio varied monthly in all years (Fig. 1).
Males tended to predominate in early sea-
son samples and again, but to a lesser ex-
tent, in late season samples. Females pre-
dominated in mid-season (late June-July)
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December 1985] HERPETOLOGICA 375
TABLE 2.-Statistics of population estimates
of Opheodrys aestivus by the Schumacher-Eschmeyer method.
Confidence limits were calculated according to Caughley (1977). Correlation coefficients and significance
levels of regression equations are indicated. See text for an explanation of the meaning of the last four
popula- No.
tion size 95 a confidence snakes
Year (N) limits of N captured Regression
of m,/n, on M, 1/N r P
1978 303 275-337 210 y = 0.001 + 0.0033x 0.0033 0.99 <0.001
1979 256 219-307 186 y = 0.007 + 0.0038x 0.0039 0.98 <0.001
samples when oviposition
occurred (Plum-
mer, 1984).
Population Size
An assumption
of population estimation
methods by mark-recapture is the equal
probability of capture of all individuals.
To test this assumption on Opheodrys, I
fitted the observed number of snakes cap-
tured 1, 2, and 3 or more times in monthly
(May-September) samples to a zero-trun-
cated Poisson distribution of frequencies
to be expected if catchability was constant
(Caughley, 1977). The null hypothesis of
equal catchability can be rejected for
samples in 1979 but not in 1978 (Table 1).
Population sizes in each year were es-
timated by the method of Schumacher and
Eschmeyer (1943) from monthly (May-
September) samples. The equation is
N = -
2; Mimi
where N is the estimated population size,
M, is the number of individuals marked
prior to the ith sampling period, and n, is
the number of individuals cantured in the
ith sample of which m, had been marked
previously (Caughley, 1977). The method
assumes no birth, mortality, emigration,
or immigration during the census. Young
of the year were not included in the cen-
sus. No estimate of emigration or immi-
gration was available; the mean distance
between recaptures
was only 26 m (Plum-
mer, 1981). Two advantages of the Schu-
macher-Eschmeyer method are allowing
a standard
error of N to be calculated and
providing an additional check on the as-
sumption of equal catchability. Unless the
of equal catchability is violat-
ed, regression of m,/n, on M, is lin-
ear through the origin with slope 1/N
(Caughley, 1977). Correlation
for both regressions were highly signifi-
cant and, corroborating
the checks of equal
catchability (Table 1), the intercept was
closer to the origin and there was closer
agreement between 1/N and slope in 1978
than in 1979 (Table 2). For these reasons,
I consider
the population
estimate for 1978
to be closer to the actual number of that
year. Since the population estimate for
1978 is contained within the confidence
limits for 1979, it is likely that the esti-
mates for the 2 yr are not significantly
Based on a population size of 300 (Ta-
ble 2), there were approximately 0.21 0.
aestivus per linear meter of shoreline.
density is similar to those reported (no./
m) for other shoreline snakes (Regina sep-
temvittata 0.18-0.26, Branson
and Baker,
1974; three spp. Nerodia 0.03-0.18, He-
brard and Mushinsky, 1978). Plummer
(1981) found that 0. aestivus activity was
concentrated near the shoreline with 86-
88% of all captures made within 3 m of
the shoreline and 96-97% made within 5
m. Using the habitat boundaries of 3 m
and 5 m, respectively, I calculated popu-
lation densities of 714 snakes/ha and 429/
ha. While the exact extent of habitat uti-
lization away from the shoreline may be
argued, at a conservative estimate of 429
snakes/ha, this population of 0. aestivus
was, with few exceptions, among the most
dense of reported snake populations (An-
dren and Nilsen, 1983; Blaesing, 1979;
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376 HERPETOLOGICA [Vol. 41, No. 4
(78) _>5 (109)
0 1979
(101) >5 (109)
0 ~~~~~~978
40 3020 10 10 20 3040
FIG. 2.-Age structure of an Opheodrys
population in 2 yr. Data include only those snakes
actually captured. Open bars indicate mature indi-
viduals, closed bars represent immatures. Numbers
in parentheses are sample sizes.
Brown, 1970;
Clark and Fleet,
1976; Fitch, 1982; Freedman and Catling,
1978; Godley, 1980; Madsen and Oster-
kamp, 1982; Spellerberg
and Phelps, 1977;
Stickel et al., 1980; Turner, 1977). Perhaps
the greater extent of the vertical compo-
nent of the habitat of this arboreal snake
permitted greater density than in non-ar-
boreal species.
Population Structure and Survivorship
The resulting age structure (Fig. 2)
showed a year-to-year constancy in fe-
males which suggested that a stable age
distribution had been achieved. The age
distribution of males was more variable
especially in the 0-2-yr-olds.
Reproduction in this population was of
the birth-pulse type. Because of the small
size of hatchlings and, consequently, the
difficulty of sampling them, and the re-
stricted period in which to sample them
before winter mortality occurred, I esti-
mated reproductive
output indirectly
fecundity of the 1978 population (Fig. 2).
Using the function, number of eggs = -4.1
+ 0.023 SVL (Plummer, 1984), the esti-
mated 1978 egg production was 482 eggs
of which 241 were males and 241 females
since the sex ratio at hatching, like most
snakes (Shine and Bull, 1977), was 1:1
(Plummer, 1984).
Only 10 male and 11 female first-year
snakes were captured in 1979. Based on
egg production
in 1978, these capture data
provide survivorship
of 4.1%
4.6%, respectively.
For age classes -0, I determined sex-
specific survivorship by the proportion of
individuals in each 1978 age class n pres-
ent in the 1979 age class n + 1. Survivor-
ship from age class 0 to age class 1 was
38.5% for males and 30.8% for females.
For age classes - 1, I regressed
the natural
logarithm of the number of survivors (In
S) in an initial cohort of 100 on age x (in
years). The regression equations were, for
males, In S = 5.43 - 0.947x (r = -0.995,
P < 0.001) and, for females, In S = 5.48 -
0.722x (r = -0.995, P < 0.001). As with
snakes in general (Brown and Parker,
1982; Parker and Brown, 1980; Turner,
1977), the correlation coefficients indicat-
ed only slight deviations from the hypoth-
esis of linearity (age-constant mortality)
(Turner, 1977).
The slopes for males and females were
significantly different (ANCOVA, P <
0.01). Annual adult (-1 yr of age) surviv-
al (S = e", where b is the slope from the
above regressions
and e is 2.718) for males
was 38.8% and for females 48.6%. The
greater adult survivorship of females ap-
parently influenced the sex ratio from
younger to older snakes (Fig. 2). The x2
values for the 0, 1, and 2-yr-olds were not
significantly different from a 1:1 sex ratio
in 1978 (X2
= 3.46, P > 0.05) or in 1979
(x2 = 0.01, P > 0.90). In contrast, the x2
values for the 3, 4, and >5-yr-olds signif-
icantly departed from 1:1 in favor of fe-
males in both 1978 (X2= 12.86, P < 0.001)
and 1979 (X2
= 11.58, P < 0.001).
A life table for 0. aestivus (Table 3)
was based on these assumptions: (1) age E
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December 1985] HERPETOLOGICA 377
began at oviposition;
(2) fecundity was de-
termined from median-sized snakes in
each age class by the previous function
and adjusted by assuming 50%
of females
reproduced at age 1 and 100% thereafter
(Plummer, 1985); (3) sex ratio at birth was
1:1; (4) survivorship
in age classes - 1 was
0.486; (5) survivorship
from the egg to age
1 (= "first-year")
was calculated indirect-
ly by dividing the mean number of 2-yr-
old females in 1978-1979 (25) by survi-
vorship in age class 1 (0.486) to determine
the mean number of snakes in age class 1
that should have been present in 1978-
1979 (number of 1-yr-olds = mean no.
in age class 1 = 25/
0.486 = 51). Survivorship
in the first year
(0.212) was calculated as the number in
age class 1/number in age class E = 51/
241 = 0.212).
Male preponderance in early spring
samples has been found in other snakes
and Fleet, 1976; Fitch, 1949, 1975;
Gregory, 1977; Turner, 1977; Voris and
Jayne, 1979), and probably is related to
greater male activity when searching for
mates. Whether the same explanation
holds true for the much less pronounced
preponderance of male 0. aestivus in the
fall is uncertain. Richmond (1956) ob-
served fall mating of 0. aestivus in Vir-
ginia, but Plummer (1984) found no evi-
dence of fall mating in the Bald Knob
Lake population. The preponderance of
females in midseason may be related to
the greater conspicuousness of the already
larger females being swollen with eggs.
An alternative explanation is that males
decrease feeding and become relatively
inactive as occurs in Nerodia sipedon
(Feaver, 1977). Was the skewed sex ratio
in 1979 favoring females the result of
greater collecting effort in June-July? It
was not likely because 164 June-July cap-
tures of 390 total captures (42.1%) in 1978
compared favorably to the 151 June-July
captures of 353 total captures (42.8%) in
Parker and Brown (1980) and Brown
TABLE 3.-Life table for Opheodrys aestivus: x =
age (years); 1 = age-specific survival rate; m, = age-
specific fecundity rate; Ro = net reproductive rate.
See text for assumptions.
x m, l.m.
E 1.000 0 0
1 0.212 1.03 0.218
2 0.103 2.81 0.289
3 0.050 3.25 0.163
4 0.024 3.52 0.084
5 0.012 3.67 0.044
6 0.006 3.75 0.023
7 0.003 3.81 0.011
8 0.001 3.84 0.004
Ro = 0.836
and Parker (1984) found greater adult
survival rates in snakes than reported in
other studies. They attributed their results
to techniques of enclosing virtually entire
by fencing around communal
dens which resulted in capturing most in-
dividuals each year. In most studies, such
as the present, where such enclosure tech-
niques were not feasible, any estimate of
survivorship probably is lower than the
true rate because of less reliable recapture
and Brown, 1980). In
0. aestivus, several lines of evidence sug-
gested that actual survivorship,
in the first year, was much greater than
that estimated. Foremost was an unbeliev-
ably low net reproductive
rate (Ro
= 0.058)
which could be calculated using an esti-
mated first-year survivorship of 0.014
from egg to age 0 x survi-
vorship from age 0 to age 1 = 0.046 x
0.308 = 0.014), and assuming a constant
adult survivorship
of 0.486 and the above
fecundity schedule. This calculation in-
dicated that the population was rapidly
in 1978-1979. If continued,
would quickly lead to local extinction, a
conclusion inconsistent with field obser-
from August 1977-September 1984
which indicated that the population was
remarkably stable. As demographic attri-
butes in snakes are known to vary consid-
erably from year to year (Brown and Par-
ker, 1984; Feaver, 1977; Parker and
Brown, 1980), 1978 may have been an un-
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378 HERPETOLOGICA [Vol. 41, No. 4
usually low year for survival. However,
the constancy of the female age structure
in 1978-1979 (Fig. 2) points toward the
attainment of a stable age distribution
which can result only from a relatively
long-term constant mortality schedule and
rate of increase or decrease (Caughley,
Because of the discrepancy between
field observations
and estimated survivor-
ship, I estimated first-year
females by another independent method.
Age class 2 is the first
class to contain more
individuals than the next older age class
(Fig. 2), at least in 1978. In a population
with a stable age distribution,
this pattern
probably exists because proportionally
more 2-yr-old females were caught rela-
tive to the actual number in the class than
in any younger age class. Therefore, I as-
sumed that population size, mortality, and
fecundity had been constant for several
years, and I calculated a first-year survi-
vorship (0.212, above) based on the ob-
served number of 2-yr-olds. The life table
constructed from these assumptions re-
sulted in an Ro
of 0.836 (Table 3), a value
still less than the Ro
= 1.0 needed for each
female to replace herself each generation,
but probably much closer to reality than
the estimate based on actual recapture of
in classes 0 and 1.
My best estimate of first-year survivor-
ship in 0. aestivus (0.212) is similar to
those found for other colubrids such as
Coluber constrictor 0.156 (Fitch, 1963b);
Rhabdophis tigrinus 0.199 and Elaphe
quadrivirgata 0.177 (Fukada, 1969); Col-
uber constrictor 0.170, Masticophis tae-
niatus 0.145, and Pituophis melanoleucus
0.200 (Brown and Parker, 1982); and Ne-
rodia sipedon 0.235 (Feaver, 1977). If in
fact actual adult survivorship was under-
estimated (cf. Brown and Parker, 1984;
and Brown, 1980), any increase in
this value also would increase Ro.
annual adult (snakes -1-yr-old) survival
estimates in 0. aestivus (males 0.388, fe-
males 0.486) are toward the low end of
published values of approximately 0.30-
0.86 (x = 0.57) for 12 species of colubrids
(Brown and Parker, 1982; Feaver, 1977;
Turner, 1977).
Implicit in demographic analysis by
is the assumption
of equal
probability of capture of all individuals.
This assumption held true because no
Opheodrys ever escaped capture once
sighted. However, if the behavior of any
snake had decreased the probability of its
being seen, the assumption would be vio-
lated. Unequal catchability could not be
demonstrated in 1978 but was
likely in 1979 (Table 1). Possible sources
of unequal catchability included the ten-
dency of males for greater movement
(Plummer, 1981), and seasonal
in activity between the sexes (Fig. 1).
Another possible source, with probably
greater impact, concerned juvenile snakes.
have commented on the
in sampling
juveniles (Brown
Parker, 1984; Clark,
1970; Clark
and Fleet,
1976; Fitch, 1960, 1963a,b, 1965; Grego-
ry, 1977; Hirth and King, 1968; Jackson
and Franz, 1981; Lillywhite, 1982; Parker
and Brown, 1973, 1980; Platt, 1969; Sem-
litsch et al., 1981; Stickel et al., 1980). The
possibility of juveniles having behavior
patterns different from adults such as rel-
ative inactivity or different microhabitat
preferences (Fitch, 1960, 1963a; Gibbons
et al., 1977; Jackson
and Franz, 1981; Lil-
lywhite, 1982; Semlitsch
et al., 1981; Voris
and Jayne, 1979) would contribute to bias
if the behavioral differences caused visible
access of juveniles to be lower than that
of adults. Despite the lack of correlation
between perch height and SVL in 0. aes-
tivus (Plummer, 1981), the possibility ex-
isted that juveniles spent relatively more
time on the ground or in the lowermost
vegetation stratum where visible access
was minimal. Combined with their small
size, this behavior would render them less
susceptible to capture and would result in
an unrealistically high mortality estimate
and an underrepresentation
of the group
in the age structure (cf. Fig. 2).
Maximum longevity also suggested that
survivorship was greater than measured.
Two 3-yr-old females first captured in
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December 1985] HERPETOLOGICA 379
September 1977 were recaptured in July-
August 1981, and thus survived to their
seventh year. Another female (505 mm
SVL) was first captured in April 1978 and
was recaptured in April 1981. Although
estimating age at this size was not reliable
(Plummer, 1985), conservatively (if it were
a fast grower), an estimate of 5 yr would
be realistic and would indicate that this
snake survived to at least its eighth year.
If my inferred survivorship in the popu-
lation was correct, the chances of surviv-
ing to an age of 7 and 8 yr, would be three
in 1000 and one in 1000, respectively (Ta-
ble 3). In fact, these three snakes repre-
sented an approximately
one in 100 chance
(three survivors in a population of ap-
proximately 300). An even greater num-
ber of snakes actually survived to at least
their fifth or sixth year despite the calcu-
lated probability being less than 12 in
Feaver (1977) identified two general life
history tactics among 17 species of colu-
brid and viperid snakes. One, in which 0.
aestivus better fits, is characterized by
early maturity, small female size at ma-
turity, large clutches, steep increase of
clutch size with female size, annual
clutches, females the larger sex, and low
adult survivorship. Various measures of
reproductive output were relatively in-
variable both within (Plummer, 1983; Sei-
gel and Fitch, 1984) and between Opheo-
drys populations (summary in Plummer,
1984). In contrast, lipid storage in the Ar-
population was highly variable an-
nually and presumably was affected by
food availability (Plummer, 1983). A re-
productive tactic of a relatively invariant
reproductive output compared to stored
lipids has been predicted to develop in
relatively long-lived species in environ-
ments where juvenile survivorship
is high-
ly variable and unpredictable (Congdon
and Tinkle, 1982).
Demographic studies of snakes in which
the interaction of mortality and reproduc-
tion has been evaluated have yielded net
rates below that necessary for
long-term stability (Nerodia sipedon,
Feaver, 1977; Xenochrophis vittata, Col-
uber constrictor, and Carphophis vermis,
Turner, 1977 (from data of Bergman,
1950; Clark, 1970; Fitch, 1963b); 0. aes-
tivus, present study). Exceptions include
only the viperid Agkistrodon contortrix
(Vial et al., 1977) and those few colubrid
populations which could be completely
enclosed by fencing around communal
dens (Pituophis melanoleucus, Coluber
constrictor, Masticophis taeniatus, Brown
and Parker, 1984; Parker and Brown,
1980). Despite the attributes of 0. aesti-
vus (above), I was unable accurately to
especially in juveniles.
This lack of success resulted in an Ro in-
sufficient to sustain the population over
the long term and was in direct contrast
to field observations.
Because of these re-
sults, I agree with Turner (1977) and Lil-
lywhite (1982) that snakes probably are
poor candidates for studies of pure de-
mography. Why they are poor candidates,
however, is an interesting question in it-
self. While population
studies in snakes are
proliferating, the life history stages that
we know least about, the egg, hatchling,
and first-year stages, are receiving little
attention. Few authors have ever found
snake eggs in the field, and the literature
is replete with statements regarding the
unavailability of juveniles (above). Al-
though juvenile snakes
may be "miniature
the chances seem
quite good that their ecology is funda-
mentally different from adults. Many be-
havioral and ecological characteristics of
reptiles have underlying physiological
bases, some of which change profoundly
through ontogeny (Pough, 1983). For ex-
ample, juveniles of several species of
snakes have limited endurance capacities
compared to adults (Pough, 1983). Such
differences could fundamentally affect
foraging and other activity patterns.
Assessment of juvenile survivorship
pears to be the single greatest problem in
snake demography. Approaches to the
study of such stages perhaps
could include
marking large numbers of hatchlings or
neonates obtained from laboratory fe-
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380 HERPETOLOGICA [Vol. 41, No. 4
males and releasing them in the field
(Blanchard and Finster, 1933; Fukada,
1969) or by constructing artificial nesting
sites in the field. Whatever the approach,
innovative work needs to be focused on
these little known and apparently vulner-
able stages in the life histories of snakes.
-The assistance
of several stu-
dents, notably T. M. Baker, M. W. Patterson,
D. E.
and M. White, is gratefully acknowledged.
Thanks are due to the Bald Knob City Government
for permitting
me gratis
use of Bald Knob Lake. This
paper was greatly improved by the constructive crit-
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Accepted: 10 April 1985
Associate Editor: Kentwood Wells
Department of Biology, Harding Uni-
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... Demographic vital rates can be difficult to estimate when studying cryptic species, as estimates may not accurately reflect actual values driving the trajectory of a population. Deviations of estimated demographic rates in some snake species are most likely influenced by low recapture rates (Plummer 1985). Most studies that have quantified critical life stages of snakes have focused on larger species such as Enhydrina (Voris and Jayne 1979), Acrochordus (Houston and Shine 1994), Nerodia (Brown and Weatherhead 1999) and Elaphe [Pantherophis] (Blouin-Demers 2002), with little work published on smaller, more fossorial species. ...
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Snakes are generally considered poor subjects for studies of population and community ecology because of low recapture rates and high variability in density and survival estimates. Some species, though, can be found seasonally in large numbers and thus may be good model organisms for life-history studies. I conducted a capture-mark-recapture study on an urban population of Ring-necked Snakes (Diadophis punctatus) in Jefferson City, Missouri, USA over three years to determine population structure and growth rates. I divided the population into three age classes: Year 1, Year 2, and Adult (≥ 3 y of age). Survivorship varied depending on age, but recapture probabilities were low across all age classes. Less than half of the population was predicted to survive to reach 3 y of age, although a λ value of 1.23 signified a growing population. Sensitivity of age classes suggests that population trajectories are reliant upon recruitment of year-class two individuals into their third year of life.
... D EMOGRAPHY, the statistical study of populations, especially with reference to age, size, density, and distribution, provides a key to understanding life history patterns, population dynamics, and community composition, all of which have obvious applications to conservation biology. Indeed, snake population demography has been the foundation for many studies of snake ecology and conservation (Plummer, 1985;Brown and Weatherhead, 1999;Blouin-Demers et al., 2002). Moreover, because snakes exhibit cryptic behavior and low or sporadic activity patterns, demography is often used as a proxy for direct monitoring of population parameters (Madsen and Shine, 2000;Lacki et al., 2005;Willson et al., 2006). ...
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Ecologists often rely on a suite of demographic parameters-such as age structure, body size distributions, population density, and sex ratios-to understand life history patterns, population dynamics, and community structure of snakes. Unfortunately, in many cases little consideration is given to how sampling techniques may influence the outcome of demographic studies. Herein, we use a combination of field capture techniques, an extensive database of field-captured snakes, and laboratory and field experiments to evaluate how capture methods may influence demographic assessments of several North American semi-aquatic snake species, including Agkistrodon piscivorus, Farancia abacura, Nerodia fasciata, N. floridana, N. rhombifer, N. taxispliota, Regina rigida, Seminatrix pygaea, and Thamnophis sauritus. We found that commercially available aquatic funnel traps (i.e., minnow traps) generally yielded biased assessments of population demography, but that the nature and magnitude of these biases varied predictably by species and trap type. Experimental manipulations of funnel opening diameter in aquatic funnel traps demonstrated that such modifications allowed for capture of larger snakes but that the size of funnel opening necessary to capture the largest individuals varied between species. Additionally, we found differences between snake species in their ability to escape from different types of traps at birth, suggesting that escape of neonates through trap mesh can lead to the lack of small snakes often observed in field samples. Overall, our results demonstrate that capture methods may bias assessments of snake population demography, but that careful design of sampling methodology, with consideration of potential biases, can yield meaningful data on snake biology.
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The ‘rate‐of‐living’ theory predicts that life expectancy is a negative function of the rates at which organisms metabolize. According to this theory, factors that accelerate metabolic rates, such as high body temperature and active foraging, lead to organismic ‘wear‐out’. This process reduces life span through an accumulation of biochemical errors and the build‐up of toxic metabolic by‐products. Although the rate‐of‐living theory is a keystone underlying our understanding of life‐history trade‐offs, its validity has been recently questioned. The rate‐of‐living theory has never been tested on a global scale in a phylogenetic framework, or across both endotherms and ectotherms. Here, we test several of its fundamental predictions across the tetrapod tree of life. Global. Present. Land vertebrates. Using a dataset spanning the life span data of 4,100 land vertebrate species (2,214 endotherms, 1,886 ectotherms), we performed the most comprehensive test to date of the fundamental predictions underlying the rate‐of‐living theory. We investigated how metabolic rates, and a range of factors generally perceived to be strongly associated with them, relate to longevity. Our findings did not support the predictions of the rate‐of‐living theory. Basal and field metabolic rates, seasonality, and activity times, as well as reptile body temperatures and foraging ecology, were found to be unrelated to longevity. In contrast, lower longevity across ectotherm species was associated with high environmental temperatures. We conclude that the rate‐of‐living theory does not hold true for terrestrial vertebrates, and suggest that life expectancy is driven by selection arising from extrinsic mortality factors. A simple link between metabolic rates, oxidative damage and life span is not supported. Importantly, our findings highlight the potential for rapid warming, resulting from the current increase in global temperatures, to drive accelerated rates of senescence in ectotherms.
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The 20-30 species of the genus Boiga (Colubridae, Boiginae) range from tropical Africa through southern Asia to Melanesia and Australia (Leviton, 1968). Collectively, they are known as catsnakes, mangrove snakes, or treesnakes (Obst et al., 1988; Greene, 1989). The common name "catsnakes" is sometimes used for snakes in the genus Telescopus as well (Obst et 1, 1988). Members of the genus Boiga are nocturnal, oviparous, opisthoglyphic, euryphagic, and slender; they have vertical elliptical pupils (thus “cat” snakes) set in large eyes, and short, blunt heads that are noticeably larger than their necks. With one exception they are arboreal or semiarboreal (Obst et al., 1988), but they are found on the ground more frequently than some other arboreal snakes. Most inhabit forested areas, although the one terrestrial species, Boiga trigonata, ranges into the steppes of central Asia (Obst et al., 1988). Little is known about the catsnakes. Not much has been published about most species other than basic descriptions (Leviton, 1968) and scattered natural history notes (Woodward, 1960; Jones, 196 1; Rosevear, 1965). Boiga dendrophila is sold commonly as a pet and zoo animal; its venom and habits have been studied, as have those of several other species (especially B. cyanea and B. blandingfi, mostly in captivity (Barach, 1952; F. Groves, 1973; J. D. Groves, 1974; Burger, 1975; Sakai et al., 1984; R. D. Howard, 1984; C. J. Howard, 1987; Minton and Dunson, 1978). The gut contents of Boiga museum specimens were tabulated by Greene (1989) and Shine (1991). Intensive field studies of snakes of the genus Boiga have been conducted only on B. irregularis (> 100 papers).
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The Miami Laboratory of the National Marine Fisheries Service (NMFS) conducted a study to determine the distribution and abundance of marine turtles in the Ten Thousand Islands of southwest Florida. A turtle originally identified as an immature hawksbill was, in fact, a hawksbill-loggerhead hybrid. Details given on the genetics and multiple recaptures of this hybrid marine turtle.
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A six-year mark-recapture study, consisting of 567 captures of 216 individually marked snakes, and the birth of 889 young in 53 litters born to 39 wild-caught females were used to estimate demographic and life-history parameters of a Northern Illinois population of Thamnophis radix. Using the von Bertalanffy growth model, males were found to differ from females in asymptotic size but not in the rate at which they approached this size. Size of known age individuals together with growth rate estimates were used to assign age at first capture. Age and sex were then used as grouping variables to obtain age- and sex-specific survival using program MARK. The results suggest that males and females have approximately equal survival in the 0 and 1 age classes but that females have higher survival than males as adults (0.45 vs 0.35). Population estimates calculated using Schumacher-Eschmeyer and Jolly-Seber methods indicate an adult population size of 64 and 172 adults, respectively, corresponding to densities of 40 and 107 adults per hectare. Average female fertility increased from 6.4 in one-year-old females to 21 among six-year-old females. Detailed demographic studies such as this have utility in development of management strategies and theories concerning life-history evolution.
The interaction between size at birth and early survivorship has received considerable theoretical attention in a variety of taxa. However, collecting empirial data to test these hypotheses has proved difficult, especially among ectotherms such as snakes. In this study, I examined winter survivorship of neonatal western rattlesnakes (Crotalus viridis) with respect to size at birth and site of hibernation. I collected 115 neonatal C. viridis in the fall of 1985, divided the sample into two groups, and released them to hibernate at two winter dens. Recaptures suggest that minimum survivorship during the winter of 1985 – 1986 was about 55% and that survivorship was independent of both weight and condition at birth. Young released at the two dens differed in both the pattern and amount of weight loss but not in survivorship. My results suggest that any survival advantage conferred by size at birth either varies annually or occurs during the first active season when food for small snakes may be difficult to obtain.
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A population of the beaked sea snake, Enhydrina schistosa was sampled at Muar, Malaysia over a nine month period. Males and females have synchronous annual reproductive cycles. Gestation extends from November to February and March. The growth of young E. schistosa was rapid (about 0.12 cm per day) with no major seasonal decline. The rate of growth in snout-vent length slows after the first 18 months post-parturition. Males and females mature in about 18 months and females give birth to their first clutch of young at about 24 months. Clutch size increases with the size of the female and can reach thirty or more. Estimates of survival indicate that about 10 to 20% of the newborn snakes survive the first year of life and about 6% of the females survive to reproduce. Snakes four or more years old appear to make up a very tiny fraction of the population.
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Fisher's sex ratio theory predicts that parents should contribute the same effort to raise sons as daughters over the population as a whole. We studied sex ratios in the offspring of three live-bearing Australian snake species and reviewed published data on snake sex ratios at birth. Most species produce young of each sex in equal numbers, but one elapid species which we studied, Notechis scutatus, produced a statistically significant excess of males (1.5:1). This imbalance is not consistent with Fisher's theory. Two alternative models offer possible explanations, but the data are not sufficiently comprehensive to test either model.
Tantilla coronata were most active during July and August; number of captures was significantly associated with maximum and minimum air temperatures but not with temperature range or rainfall. Within habitats, xeric locations were more often utilized than mesic locations, but Carolina bays and pine plantations were equally used.-from Authors Savannah River Ecology Lab., P.O. Drawer E, Aikem SC 29801, USA.
Behavior, growth and age of black rat snakes under natural conditions were investigated by mark-recapture methods at the Patuxent Wildlife Research Center for 22 years (1942-1963), with limited observations for 13 more years (1964-1976). Over the 35-year period, 330 snakes were recorded a total of 704 times. Individual home ranges remained stable for many years; male ranges averaged at least 600 m in diam and female ranges at least 500 m, each including a diversity of habitats, evidenced also in records of foods. Population density was low, probably less than 0.5 snake/ha. Peak activity of both sexes was in May and June, with a secondary peak in September. Large trees in the midst of open areas appeared to serve a significant functional role in the behavioral life pattern of the snake population. Male combat was observed three times in the field. Male snakes grew more rapidly than females, attained larger sizes and lived longer. Some individuals of both sexes probably lived 20 years or more. Weight-length relationships changed as the snakes grew and developed heavier bodies in proportion to length. Growth apparently continued throughout life. Some individuals, however, both male and female, stopped growing for periods of 1 or 2 years and then resumed, a condition probably related to poor health, suggested by skin ailments.
Certain aspects of the ecology of a population of Thamnophis sirtalis sirtalis including population density, selection of cover objects, and sexual dimorphism and geographic variation in tail length proportions were studied on the campus of Western Illinois University in McDonough County, Illinois. The habitat of this area can be classified as semi-disturbed due to its heavy use by the university and students. Despite this degree of disturbance, a relatively successful population of garter snakes seems to exist based on population density estimates. Garter snakes showed a high association for cover objects, which may have been used for thermoregulation during warm and cool periods. Sexual dimorphism in tail length proportions (tail length/total length ratios) is present in this subspecies as reported previously in the literature. Geographic variation in tail length proportions exists for both males and females with three midwest and western races (T. s. parietalis, T. s. pickeringi, and T. s. concinnus) previously studied having longer tail length/total length mean ratios than T. s. sirtalis inhabiting west-central Illinois. One of these (T. s. concinnus) had a statistically highly significant difference in ratios from T. s. sirtalis for both males and females.
GENERAL CHARACTER Natrix vittata, the striped swimmer, is a harmless little snake, about half a meter long and as thin as a little finger, living in Java on coastal plains and in the hills up to 1200 m above see level. In a given area she will be very common but in adjacent regions, which to us may present exactly the same character of humidity, altitude, temperature, vegetation, etc., there will be only very occasionally a Natrix vittata among the catch (de Haas, 1941a). Even in the field she is easily recognisable, as she will lift her head high enough to show the curious design of her ventral shields. If there is an opportunity for examining the snake more closely, the twin small white spots on the head shields will identify her. The colour of the body is light bronze with a dark stripe on the spine and two stripes running on the sides. Although she moves gracefully and swims very well, she is easily caught but even then never tries to bite. There is some wriggling of the body in the hand and around the wrist but the head is erect and projected as far forward as possible swinging right and left in the attempt of seeking a way to escape. There is really only one disagreable feature about this natrix: when caught and especially when afraid or when too roughly gripped, she will open the cloaca and discharge the contents with a characteristic, most persistent and very unenjoyable odour. She is easy to maintain in captivity and unlike many other snakes, will take food readily. Once I saw three of these natrices in a cage, each gripping a part of the same small frog. Her inquisitiveness in the search for food makes her run big risks and her agility is only partly compensation for the lack of strong powers. This is well