Growth and Feeding Ecology of the Nile Crocodile Crocodylus niloticus at Ngezi, Zimbabwe

Abstract

(1) In a study of the population ecology of the Nile crocodile at Ngezi, Zimbabwe, animals were measured, and thermal and feeding data collected during a 3-year mark-recapture experiment. (2) Juvenile growth was confined to the hot season and physical condition declined in the cool season. Large animals grew in irregular spurts. (3) Low cool-season growth was attributed to the physiological affects of temperature on feeding. Juvenile body temperatures equilibrated with the water to which they returned at night and were therefore as high as 30 ?C in the hot season, but fell to 15 0C in the cool. Animals of all sizes basked in the sun, but only for short periods in the cool season did body temperatures exceed 25 ?C and therefore approach temperatures efficient for digestion. Juveniles were more often seen feeding and were more replete in the hot season. Hatchlings were able to grow at lower temperatures than older animals. (4) On the basis of size, it was impossible to assign juveniles to accurate age-classes after 3 years, and growth curves resulting from the estimation of individual ages by skeletochronology were more accurate than those constructed from growth data. (5) Because growth was slow, females took approximately 30 years to reach sexual maturity. The growth rate of males declined more slowly than that of females and thus they attained larger sizes. (6) At a size of 600 mm snout-vent length, the diet of juveniles changed from a predominance of insects to fish and birds and a concurrent reversal of several important allometric relationships of the head was believed to be an adaptation for the efficient capture of prey of an increasing size. A fundamental change of home range behaviour also occurred in these animals and the abruptness of the dietary transition may partly reflect a change in prey availability.