Although difficult to compare statistically (because of seasons, size classes, and types of analyses used), the ontogenetic trends of crocodilian diet are similar for the different species. Young crocodilians primarily consume insects and arachnids (Platt et al., 2002); these decrease in importance as crocodilians increase in size (Tucker et al., 1996). The intermediate size classes of crocodilians have the most diverse diet and feed on amphibians, small mammals, birds, reptiles, crustaceans, gastropods, and
arachnids in varying proportions, as well as insects and fish (Tucker et al., 1996; Barr, 1997). Then, fish become the most important prey for the sub-adult and adult crocodilian with the occasional large mammal (Blomberg, 1976; Games, 1990). An increase in the percentage of empty or nearly empty stomachs correlates to an increase in total length (Games, 1990). These are broad generalizations, and the proportions vary with the crocodilian species, geographical location, and season; this has been documented for several species (Webb et al., 1991; Barr, 1997; Platt et al., 2006).
Bias in crocodilian dietary studies resulting from differential digestion rates of prey items, in particular slow digestion of chitinous material, was first suggested by Neill (1971) and most recently by Janes and Gutzke (2002). A few studies have also used regression formulae to predict the original mass of freshly ingested prey from recovered fragments, as well as relating the prey to its gastric residence time, thereby reducing bias as only freshly ingested prey was used in the primary analysis (Webb et al., 1991; Barr, 1997; Hirai and Matsui, 2001).
A fundamental concept of understanding the ecology of an animal is its dietary habits (Rosenburg and Cooper, 1990), which within
crocodilians affects growth, behavior, and reproduction (Lang, 1987). This study examined the diet of the Nile Crocodile with reference to size class, seasons, and differential digestion rates by analyzing prey that was eaten within 24 h of capture.
MATERIALS AND METHODS
The Okavango River originates in the central highlands of Angola. The Cubango and Cuito tributaries form the Okavango River
which eventually enters northwestern Botswana. The upper part of the delta, the panhandle, is a 15-km broad fault-bounded, flatbottomed valley, with a well-defined meandering channel. Two hundred kilometers later the Okavango River branches out to form the Okavango Delta, a tectonically forced alluvial fan, 22,000 km2 in size, subject to annual flooding and composed of a mosaic of floodplains and islands (Andersson et al., 2003). The Okavango Delta is a flood-pulsed wetland (Wolski and Murray-Hudson, 2005) and is a recognized RAMSAR site (a wetland of international importance according to the Ramsar Convention on Wetlands, 1971).