Historical Assumptions about the Predation Patterns of Yellow Anacondas (Eunectes notaeus): Are They Infrequent Feeders?

Size and function of digestive organs in snakes are modulated by feeding frequency and by the energetic demands of reproduction. This allows snakes to minimize costs and maximize the energetic gains from predation. Examination of Yellow Anacondas (Eunectes notaneus) acquired from sustainable management activities provided an opportunity to study the predation in this species. Field evidence poses doubts on the historical assumption that they are infrequent feeders. We probed this question by analyzing data of postprandial modulation of stomach, liver, kidneys, and heart, as well as fat bodies in the light of foraging theory. This allowed us to evaluate intersexual differences in the allometry of such structures. We analyzed 95 snakes, and we show that both sexes have similar organ masses. However, livers in females were 55% larger than in males. We also show, for the first time, the postprandial hypertrophy of a digestion-related organ in a wild snake. In specimens with prey in their intestines, postprandial response was significant for only the liver (57% of increase). No other organ presented postprandial hypertrophy. The biggest prey represented 23% of the snake's body mass, and the prey mass had no significant effect on organ mass. The meager or absent postprandial hypertrophy observed here is similar to frequent foraging snakes. The abundance and levels of consumption of small prey are high in habitats occupied by Yellow Anacondas at our study site. We believe that field data and physiological postprandial responses allow us to regard Yellow Anacondas as active foraging snakes that feed frequently.