Spatial ecology of the Trans-Pecos Rat Snake (Bogertophis subocularis) in the Chihuahuan Desert of Trans-Pecos Texas

The Trans-Pecos Rat Snake (Bogertophis subocularis ) is a medium to large rat snake that occurs from south-central New Mexico, south-central Texas, down into the north-central and northeastern states of Mexico. There is paucity in the ecology of B. subocularis to date. Some North American colubrid snakes, such as Thamnophis sirtalis, are the most represented species in thermal ecology and life history studies in all reptiles. There is nothing known on the winter ecological aspects of this species, and very little information regarding its overall ecology. The goal of this study is to determine home range, movement patterns, habitats and utilization of microhabitats, and the overwintering characteristics of the Trans-Pecos Rat Snake (Bogertophis subocularis ) in the Chihuahuan Desert on the Indio Mountains Research Station (IMRS) located in Hudspeth County, in far west Texas. From the summers of 2009 – 2011, six adult snakes were captured, monitored, and radiotracked in their active seasons (May – October) and winter seasons (November – March) (two males and three females). Average home ranges occupied by individuals was large (58.8 ha), and ranged from 20.9 to 123.6 ha. Snakes emerged from their overwintering sites in late April to early May, and returned to those sites in late October to early November. By sex, mean home ranges were 46.5 ha (n = 3) for females and 77.2 ha (n = 2) for males. Snakes occupied a small core areas (50 % kernel) (average 0.000005 ha), in which on average, males occupied slightly large core areas (0.000006 ha) than females (0.000004 ha). Daily movement for all snakes averaged 17.3 m/d, with females moving at a greater rate (18.4 m/d) than males (14 m/d). There was no significance between daily movement rates by month, however, daily movement rates between sexes was statistically significant. The greatest mean distance by month snakes traveled was May, followed by July, June August, September and October. Bogertophis subocularis was observed in four habitats on IMRS, and snakes were mostly associated rocky slope habitats (n = 74, 43 %), followed by alluvial flats (n= 43, 25 %), alluvial slopes (n= 32, 18%), and arroyo habitats (n = 24, 14 %). Snakes were found in five identified microhabitats, found under shrubs (36 %), followed by under rocks (20 %), and in burrows (20 %), with lesser occurrences under plant litter (14 %), and in crevices (10 %). The composition of microhabitats showed that snakes selected microhabitats in which vegetation was the most represented (41 %), followed by rocks (30 %), plant litter (20 %), and gravel (9 %). Bogertophis subocularis preferred microhabitats with specific compositional characteristics (vegetation) from a comparison of selected sites (n = 172) and random sites (n = 172). The mean body temperature of B. subocularis during the active season was 28.7° C, of which males (29° C) were generally higher than females (28.6° C). The months of July (31.8° C) and May (25.8° C) were the warmest and coolest (respectively) throughout the active seasons. The mean number of days that snakes resided in their overwintering sites during the winter periods was 172.5 days. Snakes had a mean, cold body temperature of 15.3° C throughout the winter periods, of which November (23.2° C) and January (12.2° C) were the warmest and coldest months, respectively. The range of body temperatures during the winter periods were from 2.3° C to 26.5° C. Snakes overwintered singly at different sites from late October or early November to late April/early May. All snakes overwintered at high slopes than normally found during surface activity throughout the active seasons