Diet of Juvenile Varanus niloticus (Sauria: Varanidae) on the Black Volta River in Ghana

Abstract

Published accounts of the diet of Varanus niloticus suggest that it is a generalized feeder, eating a wide range of invertebrates and vertebrates that are caught by hunting and scavenging (reviewed by Losos and Greene, 1988). Examination of the skull has suggested an ontogenetic change in diet from a generalized feeder to one specialized for molluscivory (Rieppel and Labhardt, 1979), but there is no direct evidence of a high degree of molluscivory in this species. Little is known about the diet of juvenile V. niloticus, nor of the of most other Varanus species. Schmidt (1919) examined nine juvenile V. niloticus collected in Zaire and reported that only three contained food (mantid, grasshopper, frog, four slugs, crickets and unidentified insects). Auffenberg (1994) examined the stomach contents of 92 juvenile V. bengalensis from various locations and found them to be entirely insectivorous, feeding mainly on hymenopterans and coleopterans. His study suggested animals from Pakistan accumulated large amounts of fat in the first few months of life preceding a period of fasting. Varanus niloticus is heavily exploited throughout Africa for its meat and skin (Luxmoore et al., 1988), but there have been few ecological studies of the animal on which to base conservation strategies (Lenz, 1995). This study investigated the feeding ecology of young V niloticus by examination of diets and fat bodies in a sample taken from a 12 km stretch of the Black Volta River. In this study juveniles are animals of the youngest age class (probably no more than four months old), and adults refers to all other age classes. Knowledge of the diet of juvenile monitor lizards is of particular interest because very little is known about the ecology of the of any Varanus species. I report on the alimentary contents of 43 juvenile [mean SVL = 143 + 2.7 mm (SE); mean mass 41 + 2.4 g] and five adult (mean SVL 390, + 32.7 mm; mean mass 1134 + 373.8g) V niloticus, caught on the Black Volta River between Batoo village (8?17'N, 2?15'W) and Bope camp (8?22'N, 2?17'W) between 929 July 1996, and deposited in the Cape Coast University Museum, Ghana. Animals were collected by canoe as they lay in riverbank vegetation. Alimentary tracts of preserved animals were opened and all items removed. Gut contents were rinsed into a strainer with mesh size of approximately 1 mm2 and examined under low-power magnification. Identifiable fragments were recorded and identified to order. Prey were classified according to residence in the stomach or alimentary tract (referred to in this work as lower gut), depending on whether the bulk of the item lay proximal or distal to the pylorus, respectively. Recognizable fragments other than single limbs were recorded as prey items. Intact prey items were TABLE 1. Diet of juvenile Varanus niloticus (N = 43), expressed as percentage of total number of prey items.