Semi-aquatic Snake Communities of the Central Plain Region of Thailand

Abstract

The Central Plain is one of the major natural regions of Thailand. It is a wide alluvial plain with little topographic relief consisting of multiple river basins. The many natural and anthropogenic wetland habitats in the region make it an ideal habitat for many aquatic and semi-aquatic species. We document the semi-aquatic snake communities associated with aquatic habitats in the Central Plain of Thailand. We surveyed a diversity of localities in the Central Plain and documented the occurrence of 10 species based on a sample of 786 snakes. The snake communities of the region are dominated by homalopsid snakes (7 species). Enhydris enhydris, the Rainbow Water Snake, was the dominant species at all sites surveyed and accounted for 80.2% of the snakes collected. We collected data on sexual dimorphism, sex ratios, reproduction, and diet. We found that all species examined, except Homalopsis buccata, exhibited female-biased sexual size dimorphism and we document significant differences in length and mass among Central Plain populations of E. enhydris. Sex ratios varied among populations, from male to female dominated. A large proportion of the E. enhydris examined were in reproductive condition (67.5%, n = 160). For E. enhydris, we documented clutch size based on vitellogenic follicles (mean = 16.7 ± 1.07) and oviducal eggs (mean = 11.4 ± 2.00), a positive relationship between female body length and clutch size, and a mean relative clutch mass of 0.37 ± 0.07. Diet analysis indicated that homalopsid species are piscivorous and species of the Cyprinidae were the most common diet item. Our data suggest homalopsids eat numerous, relatively small prey items. The Central Plain snake assemblage is similar in species composition to other semi-aquatic snake communities in Indochina. The populations of E. enhydris from the Central Plain are large in body size and exhibit high reproductive effort compared to other populations of E. enhydris for which information is available. The robustness of the Central Plain population may be related to the high productivity of the wetlands of the Central Plain. We discuss historical, biogeographical, biotic, and abiotic factors that may contribute to the differences observed among semi-aquatic snake communities in the region and the differences documented among populations of E. enhydris.