Some Observations on the Natural History of Two Oregon Garter Snakes (Genus Thamnophis)

Thamnophis sirtalis concinnus (Hallowell) and Thamno- phis ordinoides (Baird and Girard)were studied primarily in the Willa- mette Valley. T. s. concinnus is most often found in wet lowlands and is uncommon in habitats lacking water. T. ordinoides is a secretive, terrestrial species usually inhabiting areas of dense vegetation in the lowlands and foothills. Maleconcinnusfrequently emerge to bask on mild winter days, but ordinoides are rarely seen during the winter. The breeding season of concinnus begins in mid-February, reaches a peak in mid-March, and ends in early April; that of ordinoides probably begins upon emergence in mid-March and ends in April. Some fall mating probably occurs in both species, but it is less common than spring mating. Most females of both species ovulate in May and bear their young by mid-August. New records for maximum size have been established for both species: T. s. concinnus female, total length 1130 mm, snout-vent length 890 mm (old record, total length 914 mm); T. ordinoides female (live measurements), total length 850 mm, snout-vent length 695 mm (old record, total length 745 mm)o Data on minumum adult size and newborn size are also presented. Newborn males of both species tend to be longer than females and to have proportionately longer tails. On- togenetic changes in these characteristics occur. Both species show positive maternal size-newborn size and maternal size-litter size cor- relations. The sex ratio in newborn young of both species is biased in favor of males. Some specimens of both species were cursorily examined for in- ternal and external parasites. Metacercarial cysts, lung flukes, and tapeworms were found in concinnus. No parasites were found in ordi- noides. Neither species harbored ectoparasites. * * *