Feeding Characteristics of a Japanese Pitviper, Ovophis okinavensis, on Okinawa Island: Seasonally Biased but Ontogenetically Stable Exploitation on Small Frogs

Abstract

Ovophis okinavensis is an endemic pitviper of the subtropical region of Japan. Based on information from various localities, the species has been considered a dietary generalist that exploits various terrestrial vertebrates. We analyzed stomach contents of O. okinavensis in a northern mountain area on Okinawa Island, based on data of a 10-years-study, to examine its trophic features at the population level. We recovered a total of 461 prey items, which were comprised of six, three, five, and three species of frogs, reptiles, birds, and mammals, respectively. Approximately 93% of dietary items consisted of frogs, the majority of which belonged to two explosively breeding species aggregating to restricted spots along the stream only during their reproductive season (winter). On the other hand, from April to November, the snake mainly depended on vertebrates other than frogs. Neither an ontogenetic dietary shift nor intersexual dietary divergence was evident although males tended to exploit one of the two explosively breeding frog species more frequently than females. There were weak positive correlations between snake size and prey size, but large snakes did not drop small prey items from their diet. Average body mass of each prey item was only 5.2% of snake body mass. Characteristics of trophic features of O. okinavensis of this population, such as extremely high dependency on frogs in winter, the absence of ontogenetic dietary shift, and utilization of small sized prey, are discussed in comparison with other viperids.