Lower levels of glucocorticoids in crop‐raiders: diet quality as a potential ‘pacifier’ against stress in free‐ranging Asian elephants in a human‐production habitat

Abstract

Overlapping habitats and sharing of resources between elephants and people has led to intense elephant-human conflicts, especially crop depredation by elephants, across elephant-range countries. While raiding agricultural crops, elephants face numerous threats from people through chase, injury and the risk of death which could enhance the associated energetic costs, ultimately elevating their stress levels. We hypothesized that crop-raiders (in the human-production habitat) would show higher faecal glucocorticoid metabolite (fGCM) levels, a proxy of stress-response, as compared to nonraiders (in protected forests). To study this, 208 faecal samples were collected from crop-raiding elephants in a human-production habitat and 394 samples from nonraiding elephants in protected forests during 2013 and 2015. Contrary to our expectation, fGCM levels were significantly higher in nonraiding than in crop-raiding elephants of both sexes. As one of the possible factors for lower fGCM in elephants inhabiting the human-production habitat, the influence of benefits obtained from foraging here was assessed. For this, the difference in vegetation greenness (standing biomass) between the human-production habitat and the protected forests was analysed from remotely-sensed Normalized Differential Vegetation Index (NDVI), and further confirmed by measuring dietary quality (faecal nitrogen (N) content and C:N ratio as proxies for crude protein). Interestingly, higher NDVI values (greater biomass availability), higher N content and lower faecal C:N ratio (indicating higher protein content in the diet) of elephants in the human-production habitat suggested enhanced nutritional levels here as compared to protected forests. Further, there were significant correlations between faecal C:N ratio (positive) or N content (negative) and fGCM levels. These findings suggest that crop-raiding comes with the benefits of a superior quality diet which may help in reducing human-induced stress-response in elephants inhabiting or foraging within human-production habitats.

Differing physiological and behavioral responses to anthropogenic factors between resident and non-resident African elephants at Mpala Ranch, Laikipia County, Kenya

“…For instance (Stabach et al, 2015) observed an inverse relationship between FGM concentrations and NDVI in wildebeest in Kenya. Pokharel et al (2019) also confirmed an inverse proportionality between FGM concentrations and NDVI in free-ranging Asian elephants in India, while Hunninck et al (2020) found an inverse relationship between FGM concentrations and NDVI in impalas in the Serengeti ecosystem. Our study was limited by having fewer samples collected during the dry season (end of April to mid-May); nevertheless, our findings confirm the influence that vegetation quality potentially has on the adrenal response of wildlife.…”

“…Differences in FGM concentrations of elephants at Mpala Ranch could be attributed to non-residents having access to higher quality diets due to greater foraging, resulting in lower FGM concentrations. In India Pokharel et al (2019) also observed lower FGM concentrations among crop raiding elephants, which was speculated to act as a mitigator against stress compared to elephants in protected forests with limited calorie-dense food sources.…”

Peer Review #2 of "Differing physiological and behavioral responses to anthropogenic factors between resident and non-resident African elephants at Mpala Ranch, Laikipia County, Kenya (v0.1)"

“…For instance, (Stabach et al, 2015) observed an inverse relationship between FGM concentrations and NDVI in wildebeest in Kenya. Pokharel et al (2019) also confirmed an inverse proportionality between FGM concentrations and NDVI in free-ranging Asian elephants in India, while Hunninck et al (2020) found an inverse relationship between FGM concentrations and NDVI in impalas in the Serengeti ecosystem. Our study was limited by having fewer samples collected during the dry season (end of April to mid-May);…”

“…Although elephant home ranges are restricted to both artificial and permanent water sources during the dry season (Chamaillé-Jammes et al, 2013;Purdon & Van Aarde, 2017), they are known to track peak forage quality (Loarie et al, 2009) to improve their energy balance which could have resulted in lower FGM concentrations in non-resident elephants at Mpala Ranch. Although the physiological demands of ranging over long distances exposes animals to stressful environments (Wilcove & Wikelski, 2008), access to better forage quality could act as a potential 'pacifier' against stress (Pokharel et al, 2019) which then leads to lower FGM concentrations among non-resident elephants. Shaw & Couzin (2013) Manuscript to be reviewed known to switch diets depending on seasonality (Kartzinel et al, 2019) and land use.…”

NDVI is not reliable as a surrogate of forage abundance for a large herbivore in tropical forest habitat

“…However, while NDVI has been found to be a good surrogate of total vegetation biomass/primary productivity in various habitats (examples: savannahs: Sjöström et al., ; Wu, De Pauw & Helldén, ; savannah–steppe mixed landscapes: Sannier, Taylor & du Plessis, ; shrublands: Wilson, Silander, Gelfand & Glenn, ; tropical forests: Roy & Ravan, ; Madugundu, Nizalapur & Jha, ; Das & Singh, ), tests of whether NDVI or other similar indices actually reflect the abundance or quality of vegetation relevant to the focal animal are unfortunately rare and largely restricted to open habitats such as grasslands and savannahs (e.g., Kawamura et al., ; Ryan et al., ; Zengeya, Mutanga & Murwira, ). We could find only one such study in temperate forest (Borowik, Pettorelli, Sönnichsen & Jędrzejewska, ) and one from a tropical forest habitat (Willems et al., ), despite the use of NDVI in studies of animal ecology in those habitats (Lakshminarayanan, Karanth, Goswami, Vaidyanathan & Karanth, ; Marasinghe, Dayawansa & de Silva, ; Pokharel, Singh, Seshagiri & Sukumar, ; Rahman et al., ; Rood et al., ; Srinivasaiah, Anand, Vaidyanathan & Sinha, ; Zinner et al., ). If remotely sensed indices do not reflect forage abundance, their use in studies of foraging and habitat use would lead to artifactual results.…”