Total body size for arboreal vipers is normally biased toward small males and larger females. We evaluated sexual dimorphism in adult and sub-adult Trimeresurus macrops, a small, arboreal green pit viper commonly found in Southeast Asia, but severely understudied. We evaluated morphological characters of 139 adult T. macrops obtained by active and opportunistic searches at Sakaerat Biosphere Reserve in northeast Thailand, from May 2012 to October 2014. We compared 7 external characters including residual index (Ri) and scaled mass index (SMI) between male and female T. macrops. Body length measurements and mass differed between males and females. Females had greater snout-vent length, body mass and head length and width, while males had greater tail lengths. A postocular stripe was always present on males, but never on females. Male head size was negatively correlated with SMI, which may reveal intersexual competition in T. macrops. Sexually dimorphic characters in T. macrops may have evolved through intraspecific resource partitioning.
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