ECOLOGY OF THE WESTERN COTTONMOUTH (AGKISTRODON PISCIVORUS LEUCOSTOMA) IN NORTHEASTERN TEXAS

There are few long-term studies of North American pitvipers using mark and recapture methods. In particular, the ecology of the Cottonmouth (Agkistrodon piscivorus) has only been examined for relatively short periods. A total of 607 records from 247 individuals of the Western Cottonmouth (A. p. leucostoma) were collected from 1984 to 1997 at Sheff’s Wood, a 32.2 ha preserve in northeastern Texas. The site is a flat, open deciduous woodland with several small streams, standing pools of water, a sphagnum bog, and a shallow, permanent stream that leads to a pond outside of the preserve. The snakes were captured in drift fence traps, measured, sexed, and individually marked for future identification. Snakes were found in lowland areas (ca. 5 ha), and in the first five years of the study ca. 25 individuals were captured in the active season (5 snakes/line, March to November), but in the next six years ca. 85 were obtained each year (10 snakes/line). Recapture rates increased each year from 20% to 75% until the winter of 1995, when a drought occurred. The only A. p. leucostoma recorded in 1996 was unmarked, as were two captured in 1997. From marked neonates it was established that maturity could be reached in the fourth year, but growth was highly variable. Males continued to grow and reached larger sizes than females. Population estimates ranged from 130 to 170 for the 5 ha area. Although the adult female to male sex ratio was 52/47, females were more commonly captured in the spring and males in the late summer. Neonates and juveniles were predominately captured in the fall. Litter size averaged five, and parturition was in late August to early September. Less than 50% of the adult females were pregnant in a year, but some females reproduced in consecutive years. Department of Biology, University of Texas at Tyler, 3900 University Blvd., Tyler, Texas 75799, USA. E-mail: nford@mail.uttyl. edu mented. Much of the preserve is upland oak and hickory forest, but about 5 ha of flat bottomland hardwood forest is present, and A. p. leucostoma is restricted to this small floodplain (Ford et al., 1991). This area contains several small streams and oxbow pools, a sphagnum bog, and a shallow permanent stream (3 m wide) that leads to a pond (ca. 1 ha) located in the preserve boundaries. The trees in this area (Acer rubrum, Betula nigra, Liquidambar styraciflua, and Quercus nigra) are typical bottomland hardwoods. In general, the forests are mature and multiple trees fall each year creating logs and root cavities along with debris. The soil is loamy and sand rich with decaying vegetation. Because the main stream is spring-fed, the basic nature of the floodplain was similar each year. In wetter years, water flowed from upland sites through additional streams, but even during droughts water flowed in the main creek. During the midpoint of the study, oil exploration upstream caused silting in the main stream, lowering its depth and partially filling the pond. Data Acquisition Snakes were live-trapped using 45 cm high aluminum drift fences with box-shaped traps (L120 x W35 x H35 cm) as described in Ford et al. (1991). Traps were placed at the ends and in the middle of 15 m long drift fences. A series of three to eight lines were monitored every two to three days from 1984 to 1997, usually starting in March and running until the first cold period in November. The first three lines were established in fall of 1984, two were added in 1986 and 1988, and one in 1989 (Table 1). Initial lines were placed near the sphagnum bog and the permanent stream, and subsequent lines were placed near the pond. During surveys, A. p. leucostoma were occasionally captured outside the traps. For each snake captured the following was recorded: trap location, snout-vent length (SVL), mass, and sex. Snakes were marked with unique ventral scale clips until 1989, and thereafter with passive integrated transponder (PIT) tags that were injected into the body cavity (Avid Corp. Norco, California; Jemison et al., 1995). Animals were released near their capture location within two days, except in 1989 and 1990 when several pregnant females were transported to the laboratory and held until parturition occurred. The offspring of those females were measured, marked, and released in the area where their respective mother was collected. The number of A. p. leucostoma captured and recaptured was totaled each year. For each year from 1985 to 1995, population size (95% confidence intervals) was calculated by the Schnabel method 168 N. Ford Year N traplines Total N Total N N recaptures Population estimate captures individuals from previous years (95% confidence limits) 1984 (Oct-Nov only) 3 2 2 0 —