Ontogenetic dependence of Nile crocodile (Crocodylus niloticus) isotope diet‐to‐tissue discrimination factors

Abstract

Rationale The diet of wild Nile crocodiles (Crocodylus niloticus) is difficult to assess because they are cryptic and nocturnal predators that are extremely sensitive to disturbance by observers, and stomach content analysis is challenging, especially in large specimens. Stable light isotope analysis provides a means of assessing their diet, but diet‐to‐tissue discrimination factors have yet to be established for the species. Methods Isotope ratio (15N/14N and 13C/12C expressed as δ15N and δ13C) analyses of scutes, claws, and blood of farmed crocodiles of different sizes were compared with the isotope values of their lifelong diet, which comprises chickens from a single supplier. Results Systematic size dependence in the diet‐to‐tissue discrimination factors for scute collagen, scute keratin, and claw keratin is described in regression relationships against the snout to vent length. Fixed values are presented for erythrocytes and blood plasma because blood was not sampled from juveniles. Conclusions The diet‐to‐tissue discrimination factors help assess the diet of wild crocodiles. The diet of crocodiles from Lake Flag Boshielo shows a clear ontogenic shift, as has been seen in other studies, and the results strongly indicate a dependence on the terrestrial food web rather than a fish diet. That this population may exploit a terrestrial diet highlights potential conflicts for conserving Nile crocodiles outside protected areas.

Dietary shifts may underpin the recovery of a large carnivore population

“…Therefore, our SIA results present a broad picture of the isotopic landscape of the estuarine crocodile population over two different periods. We found that individuals became more 13 C-and 15 N-enriched as they grew, and this relationship was similar between 'historical' and 'contemporary' cohorts (figure 1 [27, [36][37][38][39] and other large carnivores that show indeterminate growth [40] and are suggested to reflect changes in diet and metabolism as the animal grows [38,41]. This evidence suggests that the δ 15 N of estuarine crocodiles scaled similarly with body size class under high (contemporary) and low (historic) population densities.…”

“…We found that individuals became more 13 C- and 15 N-enriched as they grew, and this relationship was similar between ‘historical’ and ‘contemporary’ cohorts (figure 1 and table 1). Ontogenetic changes in isotopic values are common in crocodylians [27,36–39] and other large carnivores that show indeterminate growth [40] and are suggested to reflect changes in diet and metabolism as the animal grows [38,41]. This evidence suggests that the δ 15 N of estuarine crocodiles scaled similarly with body size class under high (contemporary) and low (historic) population densities.…”

Trophic Ecology of African Dwarf Crocodiles (Osteolaemus spp.) in Perennial and Ephemeral Aquatic Habitats

“…Ours is one of a few studies of isotopes from crocodilians to report isotope signatures from both collagen and keratin (Adame et al, 2018; Woodborne et al, 2021). Although caudal scutes are commonly used for isotopic analyses of crocodilians, scutes have rarely been separated into keratin and collagen fractions (Wheatley et al, 2011; Radloff et al, 2012; Caut, 2013; Smolensky et al, 2015).…”

“…Previous studies indicated that keratin has a faster turnover rate than collagen and therefore should reflect material assimilated over the previous few months, whereas collagen could reflect the diet over as much as a year (Spearman, 1966; Radloff et al, 2012; Rosenblatt and Heithaus, 2013). Indeed, Woodborne et al (2021) analyzed collagen and keratin in detail in Crocodilus niloticus and concluded that diet-to-tissue determinations were dependent on the type of tissue analyzed.…”

The histology and growth rate of Nile crocodile (Crocodylus niloticus) claws

“…The methods described in Woodborne et al (2012) for time series dietary reconstruction from crocodilian claws could be applied on living animals to infer trophic dynamics of the aquatic/terrestrial interface at a decadal scale (Santos et al, 2018; Woodborne et al, 2021). This has larger implications for managing the Nile crocodile population of the Olifants River Gorge in the Kruger National Park.…”