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Reproduction in western ribbon snakes, Thamnophis proximus (Serpentes: Colubridae), from an east Texas bottomland.

Abstract.--Data sets on the reproductive biology of the western ribbon snake, Thamnophis proximus, in general, emphasize either characteristics related to the females or to the offspring but not both. In addition, records from multiple years at a given locality are rare. Gravid female T. proximus were collected for three consecutive years (1999-2001) from a bottomland hardwood site in northeastern Texas. These females were kept in captivity for a short time until they gave birth. Data recorded included maternal snout-vent length (SVL), pre-parturient mass, postparturient mass, clutch size, offspring SVL, offspring total length (TL) and offspring mass. In addition, mean clutch size and mean relative clutch mass were calculated for each year. Clutch mass and clutch size were both correlated with female SVL for the 10 clutches obtained in 2001. There were differences in parturition dates, clutch mass, female size and offspring size among the snakes in the three different years as well as differences between thes e records and previous reports. Thus both geographic variation and temporal differences are evident in the reproductive traits of this species of snake.


Reproductive data are available for many species of garter and ribbon snakes, genus Thamnophis (Rossman et al. 1996). However, very few reports include many of the important variables related to reproduction, e.g., female body size (snout vent length and mass), offspring size (including within clutch variation), relative clutch mass (RCM) and length of time females have been in captivity. In particular, female mass and offspring size are reported rarely, yet may be very important (Seigel & Ford 1988). In addition, literature records usually combine information obtained from several years and/or various localities. A number of recent studies have indicated that proximate environmental factors (e.g., prey availability) can have significant influence on the reproductive traits of snakes (Seigel & Ford 1992; Ford & Seigel 1994; Gregory & Skebo 1998). Since prey availability can vary annually and by locality, data collected from multiple years at each locality are preferable for examining the reproductive ecology of a species.

Reproduction in the western ribbon snake, T. proxirnus, was summarized in Rossman et al. (1996). Parturition was recorded from 25 June to 14 September. Percent of females gravid varied from 81 to 100%, and mean clutch size ranged from 8.4-12.9. The only offspring SVL data were for four clutches from Smith County, Texas where clutch means ranged from 13.0 - 17.4 cm (Ford et al. 1990). Offspring mass was available from three reports and ranged from 0.97 g to 2.5 g (Conant 1965; Clark 1974; Ford et al. 1990). The most recent and complete report (Ford et al. 1990) does not include percent females gravid and relative clutch mass. In addition, this report is only for four clutches and the females were collected over several years from unspecific localities within the county.

The ribbon snake is the dominant species of snake of the Texas Parks and Wildlife Old Sabine Wildlife Management Area (OSBWMA), a bottomland hardwood forest along a stretch of the Sabine River in northeastern Texas (Doles 2000). The abundance of this snake offered an opportunity to collect valuable reproductive data on the species. The purpose of this paper is to present a more complete picture of the reproductive biology of T proxirnus from one locality, over multiple years.


Adult female western ribbon snakes from the OSBWMA, Smith County, Texas were collected in May through July, 1999-2001. Each year females were taken to the laboratory where they were weighed to the nearest gram and then housed separately until parturition. They were maintained under a 14:10 light/dark cycle at 28[degrees]C, and offered tadpoles or fish once a week. They were checked daily for the presence of young. At parturition, the number of live and dead full term offspring was recorded as clutch size. Each offspring was measured for snout vent length (SVL) and total length (TL), and then weighed to the nearest 0.01 gram and its sex determined by eversion of the hemipenes in males (Gregory & Larsen 1996). Dates of parturition were recorded as well as the female's postparturient weight and SVL. Clutch mass was calculated by multiplying mean clutch offspring mass by clutch size and relative clutch mass (RCM) was calculated by dividing clutch mass by female postparturient mass (Shine 1980; Ford & Seigel 1989b ). After data collection, females and offspring were released in the same location where they were collected.

Means and standard deviations (SD) for SVL, post-parturient mass, clutch size, clutch mass and RCM were calculated for females for each year. Means, standard deviations, and coefficients of variation (CV) for offspring SVL, TL and mass of each clutch also were calculated. The overall mean for each of these factors was calculated for each year. In 2000 and 2001 these data were separated by sex of the offspring. Linear regressions of clutch size, offspring SVL, offspring mass and clutch mass against female SVL and a correlation matrix of the log of these factors were generated using STATVIEW (SAS Institute Inc. 1999).


Twenty-six adult female T. proxirnus were collected in the OSBWMA. Ten (67%) of 15 snakes produced clutches in 2001, six (75%) of eight in 2000 and all of three in 1999 (Table 1). Parturition dates ranged from June 23 to August 3. In 1999 and 2001, the average maternal SVL was approximately 60 cm, however in 2000 females were larger (Table 1) which may be related to the fact that female postparturient mass, clutch size and clutch mass were also larger in that year (Table 1). The clutch mass for 1999 was lower than for 2001 even though the female mean sizes were the same (Table 1). This lower clutch mass in 1999 is associated with the smaller mean offspring mass in that year (Table 1 & Table 2). For 2001, the year with the largest sample size, significant correlations occurred among clutch size and clutch mass, and female SVL (Figures 1 & 2). RCM appears to exhibit some year-to-year variation but this could be due to the small sample sizes in some years (Table 1).

A total of 202 offspring T. proxirnus were measured (Table 2). The sex ratio was approximately 1:1 for both 2000 and 2001 (Table 2). Overall, the smallest clutch mean offspring SVL was 15.8 cm and the largest mean was 19.1 cm. The offspring mean total lengths varied from 21.9 to 28.2 cm (Table 2). The variation within a clutch was relatively low (maximum CVs of usually less than 6%, Table 2). Offspring mass showed more variation from a clutch mean of 1.1 to 3.1 g (maximum CVs of generally over 10% up to one of 25.3%, Table 2). For the 10 clutches in 2001, offspring size factors were not correlated with female SVL ([r.sup.2] = 0.23 for log offspring SVL; [r.sup.2] = 0.27 for log offspring mass).


The ribbon snakes at the OSBWMA tend to have earlier parturition dates than other populations of T. proximus that have been studied elsewhere (Rossman et al. 1996). Indeed, the first date of parturition (June 23) is two days earlier than any published date for T. proximus (Table 1). The percentage of gravid females is slightly lower than recorded elsewhere (Rossman et al. 1996). For example, Clark (1974) in a study in southeastern Texas reported 100% of eight adult females reproduced. The OSBWMA experiences periodic, but often large-scale perturbations (flooding) several times a year which result in pulses of amphibian and fish numbers (Doles 2000). This stochastic resource availability may be effecting frequency of reproduction in these ribbon snakes.

The correlation among clutch size and clutch mass, and female SVL seen in this study (Figures 1 & 2) is common for members of the Thamnophiine (Seigel & Ford 1987; Rossman et al. 1996). Even so, clutch size is a little higher in this population. This may be due to larger average female sizes. Alternatively, it could be the result of higher prey availability (Ford & Seigel 1989a; Gregory & Skebo 1998). Offspring size data are lacking for most populations of T. proximus but Ford et al. (1990) had mean sizes for four clutches that were smaller than those in this study even though the females were from the same county. Offspring size in Thamnophis tends to be less influenced by food intake than by clutch size or total clutch mass in laboratory experiments (Ford & Seigel 1989a; Gregory & Skebo 1998). This suggests that the larger offspring size in ribbon snakes of the OSBWMA may be an adaptation to the unique, local conditions of this bottomland hardwood forest (i.e., larger specimens may be more successful in exp loiting the available prey). Geographic variation in reproductive traits is known to be widespread for snakes (Fitch 1985; Gregory & Larsen 1993; 1996; Seigel et al. 2000; Seigel & Ford 2001). However, temporal variation in RCM has not previously been demonstrated for snakes (Seigel & Fitch 1984; Seigel et al. 1986). Complete data sets are critical to further the understanding of this important life history component.

As with many snakes, fecundity was correlated with female size in T. proxirnus of the OSBWMA. If samples examined during this study reflect an actual difference among the three years in female size, such differences could influence the dynamics of this population because of the importance of the temporal variation in clutch size. Offspring size showed some variation both within and among years. The role of environmental and genetic factors in these traits can not be evaluated with this current data. The implications of producing some robust and some thin offspring by a given female are worth exploring but are beyond the scope of this paper (see Seigel & Ford 1992 for an experimental approach to this problem). This does suggest that the stochastic and heterogeneous environment of some bottomland floodplains provide unique opportunities for better understanding variation in snake reproductive traits.


Table 1

Reproductive traits of western ribbon snakes, Thamnophis proximus, from
the Old Sabine Bottom Wildlife Management Are of northeastern Texas.
Means + one SD. RCM = Relative Clutch Mass, SVL = Snout Vent Length, PP
= Post parturient. See text for calculation of RCM.

Year # clutches Date born Material SVL

2001 10 July 7-Aug. 3 59.0 [+ or -] 4.72
2000 6 June 29-July 16 70.2 [+ or -] 8.77
1999 3 June 23-July 30 61.4 [+ or -] 9.34

Year PP mass Clutch size Clutch mass
 (g) (g)

2001 50.4 [+ or -] 15.09 9.5 [+ or -] 4.86 19.5 [+ or -] 10.48
2000 80.08 [+ or -] 32.03 14.2 [+ or -] 2.32 33.1 [+ or -] 4.80
1999 46.3 [+ or -] 11.37 8.3 [+ or -] 1.53 12.8 [+ or -] 2.10

Year RCM

2001 37.8 [+ or -] 13.40
2000 45.0 [+ or -] 11.20
1999 29.1 [+ or -] 9.19

Table 2

Offspring traits of western ribbon snakes, Thamnophis proximus, from the
Old Sabine Bottom Wildlife Management Area of northeastern Texas. Means
[+ or -] one SD (range in parentheses) of the means for all clutches in
a year is given. Second line is the range of coefficients of variation
(%) for all clutches in each year.

Year Sex # Offspring SVL (cm)

2001 M 51 17.5 [+ or -] 0.77 (16.5 - 18.4)
 1.0 - 3.5

 F 43 17.5 [+ or -] 0.58 (16.4 - 18.3)
 0.4 - 8.4

2000 M 37 18.0 [+ or -] 0.76 (16.9 - 19.1)
 2.0 - 5.4

 F 46 18.0 [+ or -] 0.72 (16.9 - 18.7)
 0.5 - 5.3

1999 Undeter- 25 16.7 [+ or -] 1.16 (15.8 - 18.0)
 mined 2.1 - 4.5

Year Offspring TL (cm) Offspring Mass (g)

2001 25.4 [+ or -] 1.18 (23.5 - 26.8) 2.0 [+ or -] 0.29 (1.64 - 2.45)
 1.8 - 6.4 0.5 - 11.0

 24.9 [+ or -] 0.90 (23.3 - 26.0) 2.1 [+ or -] 0.21 (1.81 - 2.50)
 1.5 - 8.9 0.8 - 25.3

2000 25.8 [+ or -] 1.48 (23.8 - 28.2) 2.4 [+ or -] 0.47 (1.8 - 3.1)
 0.9 - 5.2 2.3 - 12.3

 25.5 [+ or -] 1.16 (23.8 - 27.0) 2.4 [+ or -] 0.45 (1.8 - 2.9)
 0.8 - 5.2 3.1 - 14.6

1999 23.3 [+ or -] 2.00 (21.9 - 25.6) 1.5 [+ or -] 0.42 (1.1 - 2.0)
 1.7 - 5.3 2.8 - 9.7


We are grateful to the University of Texas at Tyler, the Columbus Zoo, and Whatcom Community College for providing funding for this research. We also extend our thanks to L. LeBeau and S. Lange of Texas Parks and Wildlife for their invaluable assistance. We thank D. Pogue, J. Ford and R. Seigel for helpful comments on an earlier draft of the manuscript and we thank two anonymous reviewers and B. Bliss for comments on later drafts. We are grateful for the lab and field assistance of R. Rodriquez, M. Wyatt, D. Freed, M. LeMere, K. Holland and C. Lancaster.


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Author:Lancaster, Debra L.; Ford, Neil B.
Publication:The Texas Journal of Science
Geographic Code:1U7TX
Date:Feb 1, 2003
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