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size and growth in nerodia rhombifer serpentes colubridae from the lower rio grande delta n.
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Ruben D. Zamora Department of Biology University of Texas-Pan American Edinburg, Texas 78541 PowerPoint Presentation
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Ruben D. Zamora Department of Biology University of Texas-Pan American Edinburg, Texas 78541

Ruben D. Zamora Department of Biology University of Texas-Pan American Edinburg, Texas 78541

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Ruben D. Zamora Department of Biology University of Texas-Pan American Edinburg, Texas 78541

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

  2. Description (Clay, 1938; Conant, 1969; Conant and Collins, 1991) 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

  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 • 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 U = 3.00, P < 0.001 (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)

  18. Discussion • Size was comparable • Body size reported to range from: • SVL 18.0 cm – 126.0 cm • Mass 3.2 g – 2100 g (Keck 2004) • This study: • SVL 18.8 cm – 120.0 cm • Mass 5.9 g – 1793 g

  19. 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)

  20. 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?

  21. 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).

  22. 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).

  23. 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)