Life on the Lowest Branch: Sexual Dimorphism, Diet, and Reproductive Biology of an African Twig Snake, Thelotornis capensis (Serpentes, Colubridae)

Abstract

Measurement and dissection of 144 twig snakes (Thelotornis capensis) from southern Africa provided data on morphology, sexual dimorphism, food habits, and reproductive biology of this species. Twig snakes are extremely elongate arboreal species that remain immobile for long periods and ambush passing vertebrates. Both sexes attain sexual maturity at about 60 cm snout-vent length, at around three years of age [based on growth rates from Jacobsen's (1980) markrecapture study]. Maximum size is about 80 cm SVL in both sexes, and this lack of size dimorphism is consistent with published reports of male-male combat in this taxon. Males have longer tails, smaller heads, and thinner bodies (and hence, weigh less) than do females at the same body length. Reproductive cycles are highly seasonal in both sexes. Testes in adult males are turgid in spring (the mating season) but flaccid over most of the rest of the year. Females undergo vitellogenesis in spring, ovulate in late spring, and oviposit in summer. Clutch sizes ranged from 4-7 eggs, with a mode of 6. The diet of T. c. capensis is diverse: of 56 prey items, most were lizards (63%), frogs (27%), and snakes (8%). Only a single bird was recorded in a dissected twig snake. Approximately half of all prey items were arboreal taxa (e.g., chamaeleons, dwarf day geckos), with the rest being terrestrial in habit (e.g., brevicepid frogs). Thus, we infer that T. capensis often forages from a relatively low perch, which enables it to detect and seize terrestrial as well as arboreal prey. An ontogenetic increase in the proportion of terrestrial prey was evident, but even the largest snakes took many arboreal prey items. Larger snakes took larger prey, and the relationship between prey size and snake body length differed between the sexes, in concert with the observed sexual dimorphism in relative head size. Our results generally agree with those of previous studies on the biology of twig snakes; the most surprising result from our work is that a snake with extreme morphological and behavioral modifications for arboreal life nonetheless feeds to a large degree on terrestrial prey.