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Characterising Ontogenetic Niche Shifts In Nile Crocodile

Nile crocodiles undergo a three to five order of magnitude increase in body size during their lifespan. This shift coincides with a change in resource and habitat use which influences the strength, type and symmetry of interactions with other species. Identifying size-specific crocodile groups displaying similar traits is important for conservation planning. Here, we illustrate how stable carbon (δ13C) and nitrogen (δ15N) isotope analysis of scute keratin, together with breakpoint modelling analysis can be used to characterise
ontogenetic niche shifts. Using a sample set of 238 crocodiles from the Okavango Delta, Botswana (35–463 cm total length), we found prominent size-related changes in the scute keratin δ13C and δ15N profiles close to 40 and 119 cm snout-vent length. The first shift corroborated the findings of a traditional stomach-content study conducted on the same population at the same time, and the second conformed to known crocodile ecology. This approach can be used as a first approximation to identify size-specific groups within crocodile populations, and these can then be investigated further using isotopic or other methods.

INTRODUCTION
Body size has profound effects on an organism’s energetic requirements, habitat utilisation, abundance, nutrient turnover, predation risk and feeding ecology [1–3]. Consequently, variation in body size can have important effects on biological organisation across multiple scales from the individual to the ecosystem [3]. Many species that experience a one or more orders of magnitude change in body size during a lifetime undergo ontogenetic niche shifts that can result in complex changes in ecological interactions within the ecosystems they occupy [1,4,5].Within species, ontogenetic niche shifts are believed to reduce competition for resources, minimise predation risk through habitat changes and maximise growth through dietary shifts [1,5]. However, the implications of these ontogenetic shifts in body size distributions for the dynamics of entire food webs are still largely unknown [3].

Many crocodilian species display a three to five orders of magnitude increase in body size during their lifetime. This change in body size coincides with marked changes in diet and habitat use with intra- and inter-species variation depending on location [6–14]. Differences in resource use among size classes may be of such magnitude that the different size classes may even be considered separate ‘ecological species’ [4,13]. Dietary shifts experienced by crocodiles have important implications for ecosystem functioning and for their conservation and are a fundamental component of the broader changes in ecological niches.Movement of different-sized alligators (Alligator mississippiensis) has, for example, been shown to create functional connectivity between aquatic systems [15]. In another case, size-specific mortalities of Crocodylus johnstoni in Northern Australia were blamed on the ingestion of poisonous invasive cane toads (Bufo marinus) by intermediate-sized crocodiles [16]. This led to population size structural changes and can have serious short-term consequences for the crocodile population due to reduced recruitment into the adult breeding population. To successfully conserve crocodilians and the aquatic ecosystems they occupy, it is important to understand and predict the size-specific changes in diet and habitat use they experience. Only then can management plans be devised that encompass all facets important for crocodilians throughout their lifespan [17].

Changes in diet are arguably the most compelling and measureable indication of ontogenetic niche shifts within a species. In crocodilians, dietary shifts are usually described in relation to changes in overall body size (usually based on length measurements), but it has also been shown to correlate more subtly with changes in cranial morphology, especially in smaller size classes. In Crocodylus niloticus [8,18], A. mississippiensis [7], Crocodylus porosus [19], Crocodylus novaeguineae [20] and C. johnstoni [12], there is initially positive allometric growth in the preorbital part of the head followed later by a relative broadening of the skull. The widening of the cranium (reflected by changing ratios of dorsal cranial length/width) is believed to facilitate the
ability of individuals to handle bigger prey [20].

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