Published 1 March 2016
Monitoring using stable isotopes
Quantitative measurement of stable isotope ratios in metabolically inert tissue samples has been identified as a potential tool for wildlife crime investigations that can be applied to differentiate between wild and captive-sourced animals. Targeted collection for international trade is regarded as one of the greatest threats to the survival of many species, particularly with regards to the commercial trade of reptiles as relatively little information on the status of most wild populations exists.
Monitoring lizards: using stable isotope analysis to determine the origin of monitor lizard Varanus spp. skins
Vicki Crook, Louisa Musing, Stefan Ziegler
In an effort to meet demand and reduce pressure on wild populations, several countries have encouraged the establishment of captive breeding facilities, however there are reports of fraudulent claims of captive-breeding, which undermine the trade regulations in place, such as those under the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES). Concerns over high levels of exploitation and trade in an increasing number of monitor lizards, Varanus spp., have been reported in recent years.
An overview of trade in Varanus species over the past 25 years (1990-2014), based on CITES trade data, is presented in this report (Part I). Globally, the largest quantities of Varanus specimens are traded as skins and leather products for the leather industry. This trade is predominantly made up of wild sourced specimens, with no significant changes in reported sourcing to captive-breeding facilities in recent years.
The second most traded commodity of Varanus is live specimens, and for a number of Varanus species, trade data show a shift to reported captive-born or captive-bred sourcing in the last few decades. The majority of these shifts appear to have occurred for Varanus species native to and exported from Indonesia. The trade data analysis complements the pilot study carried out over six months in 2015-2016, testing the applicability of isotopic markers in the epidermis to discriminate between captive and wild individuals using various monitor lizard species and the Crocodile Lizard Shinisaurus crocodilurus (Part II).
For the pilot study three different isotopic datasets of lepidosaurian reptiles were generated and tested for their potential to predict specimen provenance. The results showed that:
- Wild specimens of species with vast distribution ranges, such as Varanus salvator, show considerable variation in isotope ratios, and therefore provenance is difficult to establish using isotope analysis.
- Under quasi-controlled feeding of specimens in captivity, isotopic ranges are smaller than those in wild populations. This result strongly supports the potential for development of a reference framework of breeding farms against which specimens of ambiguous origin can be crosschecked.
- An isotopic 15N marker (glycine) is still detectable in shed epidermal fragments of monitor lizards more than three months after the application and thus, has potential for specific forensic applications, such as trade chain analysis.
- Marking juvenile and sub-adult specimens with enriched 15N glycine as chemical imprint of legal captive-bred origin might not be effective due to the rapid epidermal renewal cycle.
The trade data and isotope analyses carried out as part of this pilot project both provide useful information in order to focus ongoing and future isotope research on species and regions of the world where sourcing claims may require further verification in the future. Based on the pilot study isotope analysis results, it is recommended that a follow on project looks into several breeding farms of reptiles in international trade for which an isotopic reference framework could be established. Of particular interest would be to assess whether the isotopic profiles of specimens kept in such facilities remain constant over time. Stable isotope analysis can then be used for geographic assignments within a probability context and exclude unlikely areas of provenance. Based on the trade data analysed, priority species for further testing could include live wild-taken (W) and captive-born (F) and/or captive-bred (C) specimens of a number of Varanus species for which high numbers of C and F specimens were identified in trade over the last decade, such as V. timorensis, V. prasinus, V. indicus, V. beccarii and V. rudicollis. Furthermore, it would be essential to compare isotopic signatures of W, F and C specimens that have been collected/bred in Indonesia and elsewhere (potentially from/in areas of close proximity or similar habitat conditions) in order to establish whether variations in isotope sources are significant enough to differentiate specimens in such cases. It would be beneficial to carry out some additional background research to establish the precise focus of such a follow on project, covering the following aspects:
- Review trade data with a specific aim of identifying CITES-listed reptile species (not only Varanus) with restricted ranges, traded as live specimens for the pet trade and for which a move to captive-bred sources has been reported; where many species are identified, those for which the EU appears to be an important end market, should be prioritised.
- Identify bona fide breeding farms, zoos and herpetological associations for potential future collaboration, and collate any readily available information on breeding practices and feeding regimes (such as procuring food from the wild) used for the species identified in the trade review, above.
- Review results of other recent/ongoing isotope research projects focusing on verifying the origin of reptile specimens (including skins), such as those on pythons and tortoises, and take into consideration any lessons learnt from these.
- Consider the feasibility of testing species which have slower growth/renewal rates, such as tortoises, and may therefore be more suitable for marking than Varanus.
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