Ecology of an Aquatic Snake (Thamnophis Marcianus) in a Desert Environment: Implications of Early Timing of Birth and Geographic Variation in Reproduction

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ABSTRACT.-We studied the ecology of the checkered garter snake, Thamnophis marcianus, in a desert grassland in southeastern Arizona. The adult sex ratio was 0.67 M : F and varied significantly among seasons. As with other species of Thamnophis, adult females were signif icantly larger than males in SVL and body mass, and females also had longer jaws. Activity occurred both during the day and nighttime, but was confined to aquatic areas or their immediate vicinity. Mating occurred in late March and females gave birth to a single brood of an average of 15 offspring in late May and early June. The timing of birth in this population was among the earliest on record for any live-bearing snake in North America and was much earlier than for other checkered garter snake populations. In comparison with populations of T. marcianus from northern and southern Texas, females from Arizona had larger maternal body sizes and their offspring were larger as well; conversely, we found no significant differences in brood size among localities once maternal body size was taken into account. The implications of early timing of birth and geographic variation in reproductive traits are discussed.

INTRODUCTION Although the garter snakes (Thamnophis) are arguably among the best studied snakes in the world, much of our knowledge of the 30 species in the genus is based on studies of north-temperate zone species, especially T. elegans, T. ordinoides, T. sauritus and T. sirtalis (see summary in Rossman et al., 1996). Information on species from tropical and southern parts of north temperate-zone areas (i.e., southern United States and northern Mexico) is relatively limited; exceptions include data on dietary and habitat use patterns of T. cyrtopsis, T. proximus and T. rufipunctatus in Texas, New Mexico and Arizona (Fouquette, 1954; Fleharty, 1967; Jones, 1990), reproduction, feeding habits and behaviors of T. eques and T. melanogaster in Mexico (Drummond, 1983; Macias Garcia and Drummond, 1988; Halloy and Burghardt, 1990; Manjarrez, 1998) and body temperatures of T. cyrtopsis, T. eques and T. rufipunctatus in New Mexico and Arizona (Fleharty, 1967; Rosen, 1991). However, longterm field studies on populations in the southern United States and Mexico are rare, especially in comparison to the numerous long-term studies on populations in the northern United States and Canada (e.g., Carpenter, 1952; Gregory, 1977; Kephart and Arnold, 1982; Peterson, 1987; Jayne and Bennett, 1990; Whittier and Crews, 1990; Fitch, 1998). Clearly, field data are biased towards northern forms, thus limiting our understanding of the evolution of ecological patterns in the genus (Rossman et al., 1996).

The checkered garter snake (Thamnophis marcianus) has a broad range in the southern United States and northern Mexico, with isolated populations in Central America (Rossman et al., 1996) which makes it an interesting comparison with garter snakes from more northern areas. In addition, although the majority of garter snakes are wetland species, most of the habitat within the range of T, marcianus is arid, ranging from grasslands to true deserts (Rossman et al., 1996). Little is known regarding the ecology of garter snakes in such arid environments.

Although experimental laboratory data are extensive for this species (Ford and Seigel, 1989, 1994; Perry-Richardson et al., 1990; Ford and Cobb, 1992; Seigel and Ford, 1992), few detailed field studies have been conducted on T. marcianus. Ecological data are confined to museum studies on reproduction (Karges, 1983; Ford and Karges, 1987) and field data on diet (Fouquette, 1954) and body temperatures (Rosen, 1991).

From 1988-1998 we conducted a field study of checkered garter snakes in a desert-grassland area in southeastern Arizona. Here we report information on sexual dimorphism, activity patterns, reproductive traits and feeding ecology of this population. We pose three questions; (1) How are the ecological traits of this species influenced by existence in an arid environment, (2) how do the traits of this species compare with garter snakes from more northern localities and (3) how much geographic variation in reproductive traits occurs among populations of Thamnophis marcianus? …