Bayesian tracer mixing models and isotope data elephant diet

Supplementary information and carbon and nitrogen isotope data analysed in publication 'Timing of dietary switching by savannah elephants in relation to crop consumption'.<br>Data were collected with permission of the Republic of Botswana Ministry of Environment, Wildlife and Tourism, research permit EWT8/36/4 XXXI (49). Farmers' reports of elephant crop consumption were collected by community officers of the NGO the Ecoexist project.<br><br><b>Supplementary information</b><br>In the supplementary information we explain why we did not use Bayesian tracer mixing models in the final paper.<br><b>Isotope data</b><br>We air-dried the faecal and vegetation samples in an air-drying cabinet. After transportation, we dried samples for a further 24 h at 70 °C and ground to pass through a 1 mm mesh at the Okavango Research Institute laboratory. Samples were then shipped to the stable isotope laboratory housed in the archaeology department at the University of Cape Town, South Africa. There they were weighed in to tin cups to an accuracy of 1 mg on a Sartorius M2P microbalance.Sample weights were 2.5 mg for samples with potentially low nitrogen content, 2.1–2.2 mg for legumes and agricultural samples and 2.3 mg for other sample types. Samples were combusted in a Flash 2000 elemental analyser interfaced to a Delta V Plus isotope ratio mass spectrometer (IRMS) via a Conflo IV gas control unit (Thermo Scientific,Bremen, Germany). The in-house standards used were: Sucrose(“Australian National University (ANU)”sucrose), MG (Merck Gel), Acacia (Acacia saligna, Glencairn). All the in-house standards were calibrated against IAEA (International Atomic Energy Agency) standards,either at UCT or by other labs. Nitrogen was expressed in terms of its value relative to atmospheric nitrogen, while carbon was expressed in terms of its value relative to Pee-Dee Belemnite (VPDB).In our analyses we use stable isotopic values from faeces and their association to isotopic values of most likely consumed browse and grass species as proxies for elephant diet. In reality, diet content can vary from isotopic content due to issues as fractionation of isotopes during digestion, this is why we correct the results with fractionation values (Codron and Codron, 2009).<br><br>We used a linear two-source mixing model (see Supplementary Information). Following Codron et al., 2011, we used the following linear two-source mixing model in order to calculate the percentage of C3 plants(trees and sedges) and C4 (other grasses):<br><b>% C4 in diet = (d13C C3 plants + efaeces-diet - d13C faeces) / (d13C C3 plants - d13C C4 plants) *100</b><br>In this formula the difference between faeces and diet isotope values (εfaeces−diet) is −0.9‰ (Codron et al., 2005), while for the other values we used the average values for faeces and C4 plants, and weighted average for C3 plants, due to availability of data on proportions of trees species in the elephants' diet (Vogel et al., 2019). Grasses and sedges have similarδ13C values, so we include both in C4 plants. <br><br><b><i>Legend</i></b><br><br><b>Code: </b>elephant dung ID<b>Group: </b>data collection round<br><b>Month: </b>month of data collection<br><b>dN:</b> δ15N value of dung sample<b>dC:</b> δ13C value of dung sample<br><b>dung_Nitro: </b>nitrogen concentration dung sample<b>Sex: </b>elephant sex (BH: Breeding Herd, M: Male bull)<b>Season: </b>rain season period (LD: Late Dry, EW: Early Wet, LW: Late Wet, ED: Early Dry)<b>Crop raids</b>: crop consumption reported by farmers<b>Rainfall: </b>occurrence of rainfall (based on average mm of rain &gt;0 mm reported by Botswana Environmental Statistics at the closest weather station in the region), not included in the current study<b>Crop_phenology: </b>recorded crop phenology based on random selection of five farms across the study site, detailed data available from the first author, not included in the current study<b>C4_nitro: </b>weighted average nitrogen concentration of C4 vegetation (mainly grasses) included in elephant diet, per season. <b>C3_nitro: </b>weighted average nitrogen concentration of C3 vegetation (mainly trees) included in elephant diet, per season. <i><br></i><i>For more information on the data collection, calculation and interpretation of the C4_nitro and C3_nitro values please see this preprint: https://www.biorxiv.org/content/10.1101/673392v1. Details on calculation and data available of these, or other measures are available from the first author.</i>