Ruben D. ZamoraDepartment of Biology

University of Texas-Pan AmericanEdinburg, Texas 78541

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)

U.S

Mexico

(Conant, 1969; Smith and Smith, 1976; Conant and Collins, 1991; and Lee, 1996)

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

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.

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

U = 3.00, P < 0.001

(Method for resolving SSD suggested by

Case 1976)

t36 = 1.009, P = 0.320

U = 782.0, P < 0.001

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).

I n t e g r a t e d E q u a t i o n D i f f e r e n t i a l E q u a t i o n R e l a t i o n o f t h e P a r a m e t e r s

V o n B e r t a l a n f f y S V L = S V L + [ e x p a ( t t o ) ] a

1

G R = a S V L + b

a

bSVL

V o n B e r t a l a n f f y S = S [ 1 e x p b ( t + t o ) ]

bS

aG 1

a

b

S

1

L o g i s t i c S = S [ 1 + e x p b ( t + t o ) ]

1

G = a S + b

a

bS

G o m p e r t z S = S e x p [ e x p a ( t + t o ) ]

G = a l n S + b

a

bS ln

P o w e r

aottabS /1

bSaG lnlnln

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

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)

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?

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).

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).

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)