1. Size and Growth in Nerodia rhombifer (Serpentes: Colubridae) from the Lower Rio Grande Delta
Ruben D. Zamora
Department of Biology
University of Texas-Pan American
Edinburg, Texas 78541

2. Description
A polytypic species: Nerodia rhombifera rhombifera, N.r. blanchardi, N.r. werleri
Non-venomous, semi-aquatic
Scale coloration
Eye with round pupil, iris red to orange
Strongly keeled scales, anal plate divided
(Clay, 1938; Conant, 1969; Conant and Collins, 1991)

3. Range
U.S
Mexico (Conant, 1969; Smith and Smith, 1976; Conant and Collins, 1991; and Lee, 1996)

4. Study Site

5. Sampling Protocol
Two trapping grids at Willow Lake on Santa Ana National Wildlife Refuge.

6. Methods Capture-recapture PIT-tags (Gibbons and Andrews, 2004)
For two days at two week intervals
Effort per trapping period: 102 trap-days
Between August 1995 and December 1998
60 trapping periods.
PIT-tags (Gibbons and Andrews, 2004)
Date, total length (OAL), snout-vent length (SVL), mass, and water temperature were recorded.

7. Gravid females were collected off the refuge and housed in lab until parturition.
Data Analyses
Data tested for normality
Assumption violated, nonparametric equivalents used.
SVL was controlled for in cases where dependent variable covaried.
Significance was taken at P  0.05

8. Descriptive Statistics

9. Comparison of Upper Decile SVL
(Method for resolving SSD suggested by Case 1976)

10. Comparison of Neonate SVL
t36 = 1.009, P = 0.320

11. Mass vs. SVL for Field Data

12. Mass vs. SVL for Neonates

13. TL vs. SVL for Field Captures

14. Comparison of Relative Tail Length
U = 782.0, P < 0.001

15. TL vs. SVL for Neonates

16. Growth Models
Cross-sectional data, used to estimate average growth trajectories (Marvin 2001).
Combination cross-sectional and longitudinal data were used to fit growth models using methods described by Van Devender (1978) and Kaufmann (1981).
Age of smallest individuals was estimated to year using size data from Scudder-Davis and Burghardt (1996).

17. Growth Models (Van Devender 1978, Kaufmann 1981)

20. Discussion Size was comparable Body size reported to range from:
SVL 18.0 cm – cm
Mass 3.2 g – 2100 g (Keck 2004)
This study:
SVL 18.8 cm – cm
Mass 5.9 g – 1793 g

21. Discussion Female biased sexual dimorphism for SVL and possibly mass.
Pattern exhibited by NA watersnakes (Gibbons and Dorcas 2004, Keck 2004) and other natricines (Winne et al. 2005)
Neonates were similar to an Arkansas Population (Plummer 1992).
No difference in SVL between sexes (P = 0.320)
Female biased for mass (P = 0.021)

22. Discussion
As with other water snakes (Gibbons and Dorcas 2004) males had proportionally larger tails (P < 0.001).
Keck’s (2004, p. 165) data suggests that this proportionality is maintained throughout growth.
“Stub-tails” in this study?

23. Discussion
Growth
As with other reptiles (Andrews 1982, Gibbons and Dorcas 2004), GR was a function of size.
Unlike Preston’s data (1970), the data suggest that females grow significantly faster than males throughout life.
Different growth rates between sexes and early maturation in males explain disparity in adult sizes (Andrews 1982).

24. Discussion Problems with growth curves
Longitudinal data = pseudo-replication
Ideal time intervals not realized
Too short, errors increase
Too long, parameter estimates become less reliable.
Despite problems, curves compare reasonably well with estimated age at maturation (Betz 1963, Preston 1970, Keck 2004).

25. Discussion
Growth rates under proximate (Andrews 1982) and ultimate control (e.g. Scudder-Davis and Burghardt 1996, Bronikowski 2000).
What keeps males smaller in non-combative species?
Lower cost of maintenance? (reviewed by Weatherhead et al. 1995)
Lower cost of mobility? (reviewed by Weatherhead et al. 1995)
Cost of producing quality sperm? (Weatherhead et al. 1995)
Differential mortality? (Brito and Rebelo 2003)
Male tactile recognition and female choice? (Rivas and Burghardt 2001)