Morphometrics sex ratio sexual size dimorphism

Authors: Jonathan K Warner Xander Combrink Peter Calverley Garreth Champion Colleen T Downs

Publish Date: 2016/09/30

Volume: 135, Issue: 4, Pages: 511-521

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Abstract

Animal body size and sex are requisite data for understanding population structure and demography Little information exists regarding Nile crocodile Crocodylus niloticus morphometrics sex ratios of wild populations sexual size dimorphism and standing crop biomass We captured 322 C niloticus at Lake St Lucia and Ndumo Game Reserve South Africa the two largest crocodile populations at the southern extent of the species’ range in Africa and measured a suite of physical characteristics to create predictive models of body length from other morphological attributes and body mass Our sample included 118 hatchlings 91 subadults and 113 adults Strong positive allometric relationships were found between body length metrics total length and snoutvent length and other morphometrics All morphometric regressions were linear with the exception of the relationship of body length to body mass which was logarithmic Among relationships of cranial morphology and body length we found considerable individual variation among all size classes The mean head widthtohead length ratio was 19 ± 16 and mean head lengthtototal length ratio was 014 ± 0005 The sex ratios for nonhatchling individuals at both populations were essentially 11 but adult sex ratios were male biased We calculated a total standing crop biomass of 9686718 kg 16145 kg/km and 5264040 kg 150401 kg/km for C niloticus at Lake St Lucia and Ndumo Game Reserve respectively and an estimated 3650 nonhatchling individuals for the province of KwaZuluNatal The data presented here will help inform crocodile management and population surveys in South Africa where C niloticus is an important apex predator that partitions aquatic resources and occasionally comes into conflict with human beingsWe thank R Taylor J Myburgh and S Kyle for advisory support The success of this project was reliant on a great number of volunteers that assisted with fieldwork but we would like to thank F Myburgh and M Robertson in particular for their logistical support and provision of boats The Ford Wildlife Foundation and the South African Water Research Commission provided vehicle and financial support We thank for Ezemvelo KwaZuluNatal Wildlife and the iSimangaliso Wetland Park Authority for hosting this research

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