Movements, Home Range, Dispersal and the Separation of Size Classes in Nile Crocodiles

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Abstract
The movements of 100 crocodiles (0.3–4.3 m) were followed over 3 years by mark-recapture, spotlight survey and radiotelemetry at Ngezi, Zimbabwe. Home ranges were based on frequency of occurrence of animals in 100 m grid squares. Most crocodiles were nocturnal, but adults occasionally moved in daylight. Animals 0.4–2.2 m had home ranges of similar sizes, but dispersed at 1.2 m. Smaller crocodiles were restricted to the river (the only nesting area) whereas all larger animals occupied the lake into which the river flowed. From 2.2 m the range of movement increased until large subadult females (2.7 m) travelled widely without distinct home ranges. Subadults were still largely excluded from the river and the smallest nesting female migrated out of the river after each breeding season. Large breeding females (>2.8 m) had small home ranges near prime nest sites. There appeared to be no difference in the behavior of the sexes up to 2.2 m. There are few data on larger males, but four mature animals (>3.2 m) had distinct home ranges. Home ranges of small juveniles increased in size in the hot season, but for adults the effects of climate and breeding were confused. The different behavior of the various categories of crocodiles resulted in the marked separation of breeding females with their last few season's offspring from immature animals 1.2–2.2 m. Dispersal appears to occur at the same stage in other crocodilians with similar scaling parameters because adults become increasingly intolerant of intermediate-sized animals. It is suggested that size-class separation is part of a general density-dependent regulating mechanism in crocodilian populations.

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