Reproduction in Female Copperhead Snakes (Agkistrodon contortrix): Plasma
Steroid Profiles during Gestation and Post-Birth Periods
Author(s) :Charles F. Smith, Gordon W. Schuett and Shannon K. Hoss
Source: Zoological Science, 29(4):273-279. 2012.
Published By: Zoological Society of Japan
DOI: http://dx.doi.org/10.2108/zsj.29.273
URL: http://www.bioone.org/doi/full/10.2108/zsj.29.273
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¤ 2012 Zoological Society of Japan
ZOOLOGICAL SCIENCE 29: 273–279 (2012)
Reproduction in Female Copperhead Snakes (Agkistrodon
contortrix): Plasma Steroid Profiles during Gestation
and Post-birth Periods
Charles F. Smith1,4*, Gordon W. Schuett2,
and Shannon K. Hoss3
1
Department of Ecology and Evolutionary Biology, 75 N Eagle Road Unit 3043,
The University of Connecticut, Storrs, Connecticut 06269-3043, USA
2
Department of Biology and Center for Behavioral Neuroscience, Georgia State University,
33 Gilmer Street, SE, Unit 8, Atlanta, Georgia 30303-3088, USA
3
Department of Biology, San Diego State University, 5500 Campanile Drive, San Diego,
California 92182-4614, USA
4
Department of Biology, Wofford College, 429 North Church Street,
Spartanburg, South Carolina 29303, USA
We investigated levels of plasma progesterone (P4), 17β
β-estradiol (E2), testosterone (T), and corticosterone (CORT) during gestation and post-birth periods in wild-collected female copperhead
snakes (Viperidae; Agkistrodon contortrix). We also sought to determine whether CORT levels at
(or near) birth dramatically increase and were correlated with duration of labor and litter size. Specifically, pregnant subjects (N = 14) were collected during early- to mid-gestation, held in the laboratory,
and repeatedly bled to obtain plasma for steroid analyses. Progesterone showed significant changes
during gestation, with the highest levels at the onset of sampling (circa 50 days prior to birth); P4 progressively declined up to parturition, and basal levels were observed thereafter. At the onset of sampling, E2 was at peak levels and fell sharply at circa 30 days prior to birth, a trend observed throughout
the post-birth sampling period. Throughout the entire sampling period, T was undetectable. Although
CORT showed no significant changes during gestation and several days following parturition, there
was a highly significant peak at the time of birth. Our findings mirror the results of previous studies
on pregnancy and steroid hormones of other live-bearing snakes, lizards, and mammals. As expected,
there was a significant relationship between duration of labor and litter size; however, although levels
of CORT did not achieve significance, there was a positive trend with litter size. We suggest that elevation of CORT at birth is involved in the mobilization and regulation of energy stores necessary for
the physiological process of parturition and as a possible mechanism to trigger birth.
Key words: pregnancy, reproduction, reptiles, snakes, steroid hormones, viviparity
INTRODUCTION
Live-birth (viviparity) in reptiles is an ancient reproductive strategy (Qiang et al., 2010; Wang and Evans, 2011),
and among extant taxa persists only in lizards, amphisbaenians, and snakes (Blackburn, 1982, 1985; Shine, 1985;
Guillette, 1987; Murphy and Thompson, 2011). The endocrinology of gestation and birth has been studied rather extensively in lizards (Guillette, 1979; Jones and Guillette, 1982;
Cree and Guillette, 1991; Guillette et al., 1991; Rooney et
al., 1992; Cree et al., 2003; Martínez-Torres et al., 2003;
Atkins et al., 2006) but less so in snakes (Mead et al., 1981;
Kleis-San Francisco and Callard, 1986; Whittier and Tokarz,
1992; Robert et al., 2009; Taylor and DeNardo, 2011). Con* Corresponding author. Tel. : +1-864-597-4679;
Fax : +1-864-597-4629;
E-mail : [email protected]
doi:10.2108/zsj.29.273
centrations and temporal occurrences of steroid hormones
(e.g., androgens, estrogens, gluccocorticoids, progestins)
during gestation in lizards and snakes are similar and
resemble the patterns described for some fishes (elasmobranchs) and mammals (Norris and Jones, 1987). Accordingly, these convergent similarities are hallmarks for common function and adaptive evolution of pregnancy.
Here, we report the profiles of several circulating (plasma)
steroid hormones (progesterone [P4], 17β-estradiol [E2], testosterone [T], and corticosterone [CORT]) during gestation
and post-birth periods in wild-collected copperheads
(Agkistrodon contortrix) from the northeastern extreme of
their range (Gloyd and Conant, 1990; Campbell and Lamar,
2004; Douglas et al., 2009). In this population, there is a single mating season restricted to late summer; females show
obligate long-term sperm storage until ovulation and fertilization in spring, and births occur from mid August to mid
September (Smith et al., 2009, 2010). In addition to documenting profiles of P4, E2, T and CORT during gestation,
274
C. F. Smith et al.
we attempted to determine whether CORT is elevated during the period at or near birth, which in reptiles has been
reported in the rattlesnake Crotalus atrox (Schuett et al.,
2004a; Taylor and DeNardo, 2005, 2011). Elevation of
CORT (and cortisol) at birth has been reported in mammals,
including humans (Bell et al., 1997; Smith, 1999, 2007;
Weiss, 2000; Beshay et al., 2007; McLean et al., 2007;
Wada, 2008). Furthermore, to determine whether duration of
parturition is influenced by litter size, and whether this relationship is related to levels of maternal CORT, duration of
birth and litter size were obtained from most mothers.
MATERIALS AND METHODS
Study site
The study site was located in a 485 ha parcel of basalt trap
rock ridge ecosystem situated 4.75 km NW of Meriden, Connecticut.
Topography is consistent with trap rock systems found throughout
the Central Connecticut River Valley. North and south oriented
ridges, ≥ 200 m in elevation, are bordered to the west by steep cliff
faces and extensive talus slides, and to the east by gently sloping
woodlands. Two prominent basalt ridges are located at this site.
Details of the topography and climate of this area are presented
elsewhere (Smith, 2007; Smith et al., 2009).
Research subjects
Females were located in the field by periodically visiting known
gestation (rookery) sites (Smith, 2007). Females suspected of being
pregnant (N = 14) were gently captured, brought to the UCONN laboratory, processed, and provided private enclosures, which consisted of plastic cages (61 cm L × 40 cm W × 12 cm H), supplied
with newsprint as a floor covering and substrate heating by heat
tape (8 cm wide) situated beneath and across the front end of the
cage (35°C). Collection dates for females brought into the laboratory ranged from 5 June to 28 July. Measurements for each snake
included body mass (BM: ± 0.5 g using a triple beam balance), and
snout-vent length and tail length (SVL: ± 0.2 cm; TL: ± 0.2 cm, using
a non-stretchable cloth measuring tape while the snake was
restrained in a clear acrylic tube). Artificial lighting (eight 40 W fluorescent tubes) positioned 3 m above the cage was timer-controlled
to simulate natural (Connecticut time) photoperiod. Laboratory
rodents (mice) were offered as food every 10 days until parturition.
However, none were consumed until after parturition. Water was
available in glass bowls ad libitum.
Collection of blood and plasma
Sampling was conducted from early- to mid gestation (mean
start of sampling pre-parturition 37.28 ± 1.25 SE; range 50–32
days) to 20 days post-parturition to document profiles of the steroid
hormones described above. To obtain a blood sample, each subject
was gently removed from their private enclosure using grabber
tongs and secured in a clear plastic tube of appropriate size. This
procedure was done quickly (1–3 min) to minimize handling stress
and possible effects on steroid levels (Schuett et al., 2004b). Once
secured, 1.0 ml of blood was harvested via cardiocentesis using 1.0
ml sterile tuberculin syringes (26-G5/8”) treated with porcinederived heparin sodium (1,000 units/ml). Subjects were returned
immediately to their enclosures following this procedure. Blood
samples were placed in disposable microcentrifuge tubes (1 mL)
and centrifuged at 2.3 g. at room temperature (~21–23°C) for 5 min.
Plasma was collected using a micropipette fitted with a sterile disposable tip and transferred to another microcentrifuge tube that was
permanently labeled with the specimen identification code and date.
Plasma samples were placed in an ultra-low freezer (−80°C) until
radioimmunoassays (RIAs) could be performed.
Radioimmunoassays (RIAs) of plasma for steroids
The general procedures we followed for conducting radioimmunoassay (RIAs) for measurements of plasma P4, E2, T, and CORT
are published (Schuett et al., 1996, 1997, 2002, 2004a, 2004b,
2005, 2006; Schuett and Grober, 2000; Taylor and Schuett, 2004).
Briefly, all RIAs of total P4, E2, T and CORT included validation
(quantitative recovery and parallelism). Total hormone levels were
measured to permit comparison to our earlier studies (i.e. Schuett
and Grober, 2000; Schuett et al., 1996, 2004b) and to those of others. A single RIA was performed for P4 and assay samples were
run in duplicate (n = 394). The intra-assay coefficient of variation
(CV) was 8.7%. Two RIAs were performed for E2; the intra-assay
CVs were 7.9% and 12.5%, and the inter-assay CV was 11.9%.
Two RIAs were performed for T; the intra-assay coefficients of variation (CV) were 9.1% and 11.1%. One RIA was performed for
CORT, and the intra-assay CV was 2.4%. Values for all steroids are
presented as arithmetic means ± 1 SE (ng/ml).
Duration of parturition
Duration of parturition was investigated to determine whether
litter size influences maternal CORT levels. Females were observed
about every 3–4 hr from 0700 to 0200 h through the study period.
Imminent parturition was recognized by the onset of body contractions, and duration of parturition was recorded as the beginning of
body contractions (Time 0) until the last neonate was completely
expelled (Time 1). Parturition was observed for nine of 14 (64.3%)
females, and all births occurred between 1300 and 1900 h. The 14
mothers produced a total of 67 neonates. After birth, blood was
obtained from the mothers and subjected to abovementioned procedures for RIA analysis of steroids.
Statistical analysis
All statistical analyses were conducted using SAS 9.1 (SAS
Institute Inc., 2004). Prior to analysis, we ensured that the assumptions of normality and homogeneity of variances were met, and
whether outliers could be detected (Zar, 1999). To determine
whether plasma P4 and CORT levels varied among the sampling
dates (i.e., from 40 days pre- to 12 days post-parturition), we conducted separate repeated measures analyses of variance on logtransformed hormone values using PROC MIXED, with snake ID as
the repeated unit and sampling date as the main effect. We used
the variance components covariance structure in our models, as
this provided the best fit to our data out of 11 possible covariance
structures, as assessed by Akaike and Bayesian information criterion (Wolfinger and Chang, 1995). Post hoc tests were conducted
to determine least-squares mean differences in hormone levels
between all possible pairs of sampling dates; p-values were
adjusted for multiple comparisons using the Tukey-Kramer method.
Owing to the fact that levels of E2 were undetectable on the majority
of sampling dates, repeated measures analysis was not performed;
nonetheless, we do include these data in Fig. 1. Similarly, levels of
T were not detectable in any of the sampling dates, but were not
depicted in Fig. 1.
We used Pearson correlation to examine the relationship
between duration of parturition and litter size (i.e., number of neonates per litter) for nine of 14 females (Table 1). We were unable
to witness births for five females. We conducted an additional correlation analysis to determine whether length of parturition was
related to CORT levels measured at Day 1. Day 1 was selected
over Day 0 owing to larger sample size.
RESULTS
Data on mothers (N = 14) and offspring (N = 67) are provided in Table 1. Offspring were removed from their mothers
immediately after birth. Steroid profiles are provided in Fig.
1. In the repeated measures analysis of P4, the main effect
Agkistrodon Pregnancy Hormones
of sampling date was significant (F14, 158 = 99.34; P < 0.001),
indicating that mean levels of P4 varied throughout pregnancy and following parturition. Post hoc comparisons
revealed that all pre-parturient levels of P4 were significantly
higher than all post-parturient levels (P < 0.05). In the
repeated measures analysis of CORT, we found a significant main effect of sampling date (F14,158 = 18.28; P <
0.001), and mean levels of CORT on the day of parturition
(Day 0) and the day after parturition (Day 1) were significantly greater than on all other sampling dates (P < 0.05)
but were not significantly different. Between 30 and 40 days
pre-parturition, mean E2 levels were elevated but quickly
dropped to mostly undetectable levels throughout the
remainder of pregnancy to 12 days post-parturition. T levels
were not detected throughout the entire sampling period,
and thus not depicted in Fig. 1.
275
We found a highly significant correlation between duration of parturition (Time 0 to Time 1) and litter size (r = 0.88,
P = 0.009, N = 9), which indicated that duration of labor was
longer in females with larger litters. Although a significant
correlation was not found between duration of parturition
and CORT levels at Day 0 (P > 0.05, N = 7), a positive trend
was detected at Day 1 (r = 0.67, P = 0.09, N = 9).
DISCUSSION
Over the past several decades, sources and functional
roles of circulating steroid hormones (e.g., androgens,
estrogens, glucocorticoids, progestins) in females, particularly in viviparous taxa, are better understood (Murphy and
Thompson, 2011). For example, various reproductive
tissues of females, such as gonads, follicles, yolk, adrenals,
and corpus luteum—even the placenta and extra-embryonic
membranes—can synthesize and/or
store steroids (e.g., Schwabl, 1993;
Albergotti et al., 2009). Moreover,
androgens, estrogens, and progestins can have relatively straightforward effects on behaviors associated with sexual receptivity (McNicol
and Crews, 1979; Wu et al., 1985;
Whittier et al., 1987; Whittier and
Tokarz, 1992), vitellogenesis and
yolk synthesis (Ho et al., 1982;
Garstka et al., 1985; Ho, 1987;
Wilson and Wingfield, 1992), and
maintenance of pregnancy (Chan et
al., 1973; Highfill and Mead, 1975a,
b; Xavier, 1987; Bonnet et al., 1994,
2001; Edwards and Jones, 2001;
Tsai and Tu, 2001; Custodia-Lora
and Callard, 2002). Also, a growing
number of studies provide information on patterns of circulating sex
steroids related to gestation in livebearing snakes (Whittier and
Fig. 1. Mean (± 1 SEM) plasma sex steroid profiles during and following pregnancy of female
copperheads (Agkistrodon contortrix) based on 14 wild-collected individuals held in the laboraTokarz, 1992; Schuett et al., 2004a;
tory. CORT = corticosterone; E2 = 17β-estradiol; P4 = progesterone. Testosterone (T) levels were
Taylor et al., 2004; Lind et al., 2010;
undetectable and thus not depicted (see text). Significance is designated by different letters.
reviewed by Taylor and DeNardo,
Table 1.
Data on reproduction of female copperheads (Agkistrodon contortrix) in this study.
Date of
collection
SVL (cm)
Mass (g)
pre-parturition
Mass (g)
post-parturition
Range of
sampling dates
Number
of times
sampled
Date of
parturition
Duration of
parturition
Number of
neonates
Mass (g)
(range, mean)
Total mass
(g)
622B
Unk 1
Unk 2
0423
040C
2001/7/11
2001/6/5
2001/6/6
2001/7/18
2001/7/24
61.5
64.5
57.5
59.7
60.3
143.8
155
137.5
172.2
136.1
102
104.6
99.02
103.7
99.3
20 Jul–14 sep
14 Jul–26 Aug
7 Jul–6 Sep
19 Jul–3 Sep
1 Aug–16 Sep
15
13
16
14
13
31-Aug
16-Aug
26-Aug
23-Aug
5-Sep
105 mins
135 mins
*
175 mins
*
7.61–8.32 (7.94)
8.01–9.20 (8.60)
7.23–8.36 (7.49)
8.69–9.48 (9.08)
7.6–8.21 (7.93)
23.83
34.42
22.48
54.49
23.8
795D
2001/6/20
63
191.8
127.7
8 Jul–20 Aug
12
11-Aug
140 mins
7.32–8.99 (8.24)
49.44
Unk 3
Unk 4
Unk 5
Unk 6
Unk 7
Unk 8
Unk 9
Unk 10
2001/6/12
2001/6/14
2001/6/18
2001/7/20
2001/6/23
2001/7/28
2001/6/6
2001/6/19
60.5
61.4
60.5
51
52.8
53.5
60
53.5
152
156.2
215.5
154.9
169.5
176.1
212.1
170
106.7
100.7
140
103.1
119.4
105.7
142.7
111
22 Jul–4 Sep
28 Jul–17 Sep
10 Jul–28 Aug
4 Aug–22 Sep
24 Jul–14 Sep
7 Aug–26 Sep
9 Jul–31 Aug
18 Jul–7 Sep
14
15
14
15
14
14
14
14
24-Aug
3-Sep
17-Aug
9-Sep
1-Sep
13-Sep
18-Aug
25-Aug
120 mins
145 mins
160 mins
*
115 mins
*
185 mins
*
3 live
4 live
3 live
6 live
3 live
5 live,
1 stillborn*
4 live
5 live
6 live
4 live
4 live
6 live
8 live
5 live
6.98–7.76 (7.31)
8.08–8.64 (8.23)
9.21–9.77 (9.54)
8.35–9.08 (8.74)
7.58–9.85 (8.86)
8.06–9.57 (8.73)
5.98–7.55 (6.88)
8.39–8.73 (8.58)
29.24
41.48
57.5
34.99
35.43
52.36
55.02
42.88
ID
* The still born individual was fully developed.
276
C. F. Smith et al.
2011). However, these studies represent less than threetenths of one percent of the approximately 3,000 extant species of snakes. Accordingly, studies of reproduction in
female snakes are in their infancy (Taylor and DeNardo,
2011). Thus, conclusions of endocrine control are limited
and preliminary, due to the fact that experimental research is
largely limited to a single genus (Thamnophis), and predominately the taxon Thamnophis sirtalis parietalis. Expanding
research to a range of taxa from various major lineages is
needed for a robust synthesis (Shine, 2003; Taylor and
DeNardo, 2011).
Here, we documented profiles of several plasma sex
steroids (P4, E2, T, and CORT) during pregnancy, at or near
birth, and post-birth periods in a population of copperheads
(Agkistrodon contortrix) from the extreme northeastern area
of their distribution (Smith et al., 2009, 2010). Our study is
among the few that provides individual-based steroid profiles in a live-bearing snake (see Murphy and Thompson,
2011). Based on extensive field observations of this population (Smith et al., 2009, 2010), the females described in the
present study likely were mated in late summer, showed
long-term sperm storage during winter, and ovulated in mid
spring (late May or early June). Thus, based on the dates of
collection (range: 5 June–28 July), our study documented
steroid profiles of pregnancy from early- to mid gestation to
birth. The timing of ovulation and parturition in captive-held
females were indistinguishable from females we observed in
the wild (Smith et al., 2009).
The steroid profiles of female A. contortrix in this study
were similar to those reported in other live-bearing vipers
(Bonnet et al., 2001, 2002; Tsai and Tu, 2001; Schuett et al.,
2004a; Taylor et al., 2004), as well as live-bearing lizards
(Xavier, 1987; Edwards and Jones, 2001; Martínez-Torres
et al., 2003). Furthermore, patterns of P4 and CORT were
similar to those reported in mammals, including humans
(Smith, 1999, 2007; Weiss, 2000; Zhang et al., 2008).
Therefore, we believe that the hormone profiles documented
here are a result of pregnancy, and not an artifact of captivity
or capture stress.
In snakes, most reports show elevated E2 levels during
vitellogenesis, yolk synthesis/deposition, and follicular maturation, with subsequent declines at ovulation and throughout
gestation (Bonnet et al., 2001; Edwards and Jones, 2001;
Taylor et al., 2004; Moore and Johnston, 2008). Recently,
studies by Van Dyke and Beaupre (2011) on female A.
contortrix and other live-bearing snakes have documented
that metabolic costs associated with vitellogenesis and follicular development exceed the cost of pregnancy. Consequently,
the dynamics of steroid hormones (e.g. E2, T) and metabolism of female reproduction is a rich topic for future exploration. We detected that levels of E2 fell sharply at about −30
Day, and levels remained basal throughout sampling. Several authors have suggested that high post-ovulatory levels
of P4 suppress levels of E2 during pregnancy (Bonnet et al.,
1994; Edwards and Jones, 2001), and this association was
detected in the present study; however, this relationship
remains to be tested experimentally.
The role of P4 in the maintenance of pregnancy in squamates (snakes and lizards) has been reviewed (Edwards
and Jones, 2001; Custodia-Lora and Callard, 2002;
Martínez-Torres et al., 2003), but its involvement in labor is
less well understood. Nonetheless, we detected the characteristic rapid decline in P4 levels just preceding birth, and
assume that this change was due to luteolysis, regression of
corpora lutea (Highfill and Mead, 1975a, b; Jones and
Guillette, 1982; Xavier, 1987; Martínez-Torres et al., 2003).
Relatively few studies report on T and other androgen
levels in female snakes (Saint Girons et al., 1993; Taylor
and DeNardo, 2011), but these levels tend to be high only
during vitellogenesis and follicular growth, and similar to E2,
and are generally low (basal) following ovulation and during
gestation. Because we initiated sampling at early- to mid
gestation, the fact that T levels were not detected in our
samples was not unexpected.
The function of CORT and corticotropin-releasing
hormone (CRH) during labor has been studied rather extensively in animal (e.g., sheep) models and humans (Norwitz
et al., 1999; Beshay et al., 2007). The characteristic sharp
spike in plasma CORT at parturition is possibly derived from
multiple sources (Bell et al., 1997; Petershack et al., 1999;
Wada, 2008). The source of plasma CORT in pregnant
snakes has not been experimentally investigated, but the
involvement of the adrenals and other sources (e.g., fetuses
and placenta) are implicated (Smith, 1999; Weiss, 2000;
Girling and Jones, 2006; Wada, 2008).
The significance of plasma CORT to reproductive
processes in live-bearing reptiles is not well understood,
especially in snakes, and results vary (Wilson and Wingfield,
1992; Girling and Cree, 1995; Schuett et al., 2004a; Taylor
et al., 2004; Taylor and DeNardo, 2011). Several studies
show that elevated levels are associated with the occurrence of sexual behavior (Schuett et al., 2004a; Taylor et al.,
2004), as well as vitellogenesis and yolk deposition (Wilson
and Wingfield, 1992; Schuett et al., 2004a; Taylor et al.,
2004). Levels of CORT in this study were largely unchanged
before birth (see Girling and Cree, 1995), a possible maternal response that might protect developing embryos from
high levels of CORT and stress (Cree et al., 2003; Cartledge
and Jones, 2007; Robert et al., 2009; Itonaga et al., 2011),
and in the post-parturient period of sampling. There was,
however, a highly significant surge at the time of birth.
Because we had no samples at Day-1, we cannot be certain
whether the surge of CORT preceded birth; however, CORT
levels were not elevated in samples between Day-5 and -2.
Comparison of CORT levels at the time of birth was
essentially identical to levels reported in C. atrox (Schuett et
al., 2004a; Taylor et al., 2004), but comparison to other livebearing viperid species is not possible owing to lack of information. In many temperate pitvipers, neonates remain with
their mothers until they undergo their first ecdysis (Greene
et al., 2002; Reiserer et al., 2008), and the presence of neonates could potentially affect post-birth CORT patterns in
mothers. However, because we immediately removed neonates from their mothers after birth, mostly to make bleeding
mothers easier, we potentially confounded our ability to
interpret post-birth hormone levels (e.g., CORT) of mothers.
Nonetheless, this remains to be ascertained in vipers and
other snakes.
As expected, we showed that the duration of parturition
is positively correlated with litter size (number of progeny),
and that circulating levels of CORT derived from female
plasma showed a positive trend with litter size; however, sig-
Agkistrodon Pregnancy Hormones
nificance was not achieved. The high levels of maternal
CORT we detected at birth are unlikely a “flight or fight”
response (Greenberg and Wingfield, 1987; Romero, 2002;
McEwen and Wingfield, 2003). Rather, we suggest that elevated levels of CORT and duration of parturition are linked
to the mobilization and regulation of energy stores (Munck
et al., 1984; Guillette et al., 1995; Wada, 2008). Also, elevated levels of CORT might act as a mechanism to trigger
the birth process (Smith, 1999).
Despite the fact that proximate control of labor (parturition) has not been thoroughly studied in live-bearing snakes,
there is a growing body of information on live-bearing lizards, especially in phrynosomatids (Guillette, 1979) and the
scincid genera Niveoscincus and Tiliqua (Furgusson and
Bradshaw, 1992; Girling and Jones, 2003; Atkins et al.,
2006; Girling and Jones, 2006). In such species, a cascade
of endocrine events lead to labor and birth, which involves
neurohypophysial involvement (e.g., arginine vasotocin,
AVT) on local prostaglandin production (i.e., PGF2α), which,
in turn, appears to be modulated by the innervation of uterine tissues by the β-adrenergic system (Cree and Guillette,
1991; Atkins et al., 2006). Possibly, the action of AVT is glucocorticoid-mediated (Goodson and Bass, 2001), indicating
a dynamic role for CORT in the birth process. Furthermore,
in lizards, these mechanisms appear to be influenced by
environmental factors, such as temperature (Girling and
Jones, 2003; Atkins et al., 2006; Girling and Jones, 2006;
Rock, 2006). Clearly, further research will be required to
understand the role of CORT and other plasma sex steroids,
as well as environmental factors, in regulating female reproduction in copperheads and other live-bearing snakes.
ACKNOWLEDGMENTS
We thank J. Victoria and L. Fortin, Connecticut Department of
Environmental Protection Wildlife Division, for providing the necessary permits. E. A. Van Kirk and W. J. Murdoch conducted the
radioimmunoassays. S. Berube, H. Gruner, J. Mills and H. Quedenfeld
provided numerous favors. For valuable statistical advice, we thank
R. L. Earley. The American Wildlife Research Foundation, The University of Connecticut Department of Ecology and Evolutionary Biology
Wetzel Fund, the Connecticut Department of Environmental Protection Non-game Fund, Sigma Xi, Georgia State University (Biology
Department), Zoo Atlanta, and a National Science Foundation
Predoctoral Fellowship (CFS) provided funding. This research was
conducted under the supervision of The University of Connecticut
Institutional Animal Care and Use Committee (IACUC), protocol
number S211 1201.
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(Received October 13, 2011 / Accepted December 14, 2011)