Stress Physiology of Free-ranging Asian Elephants (Elephas maximus) : Influence of Ecological and Anthropogenic Stressors
POkharel, Sanjeeta Sharma
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Various ecological and human-induced disturbances play an important role in defining the health of an animal. To cope up with such threats or challenges to its homeostasis, an animal responds by secreting stress hormones (glucocorticoids) to mobilize the energy. In general, glucocorticoids help animals to cope up with the ‘stressors’. Prolonged exposure to any ‘stressors’ may lead to an elevation in the levels of glucocorticoids, thereby leading to reproductive inhibition, immune system suppression, neuronal death and impaired cognitive function and hence, threatening the survival and fitness of an animal. However, the long-term effects of environmental or anthropogenic disturbances on the fitness and survival are difficult to analyze in a slow reproducing, long-lived species such as elephants. Spread over 3% of India’s geographical area, India shelters around 28,000 to 30,000 Asian elephants (Elephas maximus; Bist, 2002; Santiapillai and Sukumar, 2006). Protected areas in India are estimated to form 22% of elephant habitat; while the rest of the elephant habitats extend outside the protected areas (Lenin and Sukumar, 2011) making them vulnerable to human-induced threats. Consequently, there are increased incidences of human-elephant conflicts leading to crop and property depredation and human-elephant mortality. Most of the studies, in context of such conflicts, highlight the temporal, spatial scales of conflicts majorly focusing on mitigation measures. None of the studies so far have elucidated how escalating human-elephant conflicts would influence the physiological health of Asian elephants. This study, hence, focused on addressing the proximate causation of the stress-response in free-ranging Asian elephants of the Bandipur National Park, the Nagarahole National Park and Hassan district of Karnataka, using a non-invasive technique for measuring faecal glucocorticoid metabolites (fGCM). There are several factors which could act as possible stressors to the physiological state of an elephant, but depending upon the feasibility and time limitations of conducting the study, we selected those factors which we thought are essential to address the proximate causation of stress-response in Asian elephants. Being the first detailed study to explore the stress-response in free-ranging Asian elephants, this study attempted to understand the influence of some of the fundamental ecological (seasonality, group size, and body condition) and human-induced stressors. The main body of the dissertation is divided into six chapters. The first and the last chapters discuss the general introduction and conclusion while the rest four chapters highlight the main four objectives of the study. The main objectives of the thesis were: 1. The first objective of this thesis was to validate and standardize some of the crucial parts of protocols to avoid any technique-based bias while interpreting the levels of faecal glucocorticoid metabolites. Under the first objective, the influences of some of the fundamental intrinsic and extrinsic factors on the fGCM levels were also assessed. 2. The second objective was to assess the association between body condition, seasonality and stress response. 3. Having assessed the relationship between body condition and stress-status, the third objective focused on the influence of socio-ecological correlates such as group size, lactational status and the presence of adult females in a herd on the stress status of female adult elephants. 4. With above three objectives majorly highlighting the influence of ecological factors, the fourth objective was to assess the influence of anthropogenic factors on stress-response of free-ranging elephants under which we compared the stress-response between crop-raiding in human-dominated and nonraiding elephants in forested habitats. The study was conducted during dry season (February to May) and wet season (August to December) of 2013 and 2015. The dissertation is organized in the following way: Chapter 1: General Introduction The first chapter provides a general introduction about stress response and its physiological pathway. It also discusses the importance of non-invasive technique used in our study. It highlights the significance of this study based on the available literature on both African and Asian elephants. Chapter 2: General validation and standardization of field techniques and assays; influence of intrinsic and extrinsic factors on faecal glucocorticoid metabolites This chapter is divided into two parts. The first part deals with general validation and standardization of field techniques and the second part assesses the influence of fundamental intrinsic and extrinsic factors. Being the first study on stress-response of free-ranging Asian elephants, influence of various field-based techniques and variations were studied to facilitate the reliable interpretation of fGCM levels. We first conducted the experiments to standardize the field protocol and the laboratory protocol. Under the field protocol, we found that there was no within sample variation (as observed in many other species) in fGCM levels from thoroughly mixed faecal samples and samples from the center of the same dung bolus. The time of collection of samples had a significant influence, suggesting that the concentration of fGCM remains stable until 6-9 hours. Influence of storage of lyophilized samples suggested that the concentrations of fGCM levels decline if exposed to moisture. Under the laboratory protocol, A group specific 11-oxoetiocholanolone EIA, used for analyzing fGCM levels (μg/g), was modified to enhance the repeatability, accuracy and precision. EIA was analytically validated in every hormonal analysis by calculating the inter- and intra-assay coefficients of variation, specificity and parallelism. In the second part, we assessed the influence of age, sex and season on fGCM levels and found there was no age-wise variation and sex-wise variation. Season-wise variation was significant only in female elephants. We discuss the potential physiological reasons behind such variation. These findings suggest that aspects such as time of collection, proper storage of samples and the influence of extrinsic stresses (seasonality) should be considered for reliable and unbiased interpretation of fGCM levels. Chapter 3: Assessment of season-dependent body condition scores in relation to faecal glucocorticoid metabolites in free-ranging Asian elephants. (Published in Conservation Physiology; Pokharel et al., 2017; doi:10.1093/conphys/cox039) We studied seasonal and annual changes in visual body condition scores (BCS), and assessed how these scores were related to levels of fGCM levels in free-ranging Asian elephants in the seasonally dry tropical forests of the Mysore and Nilgiri Elephant Reserves in southern India. We assessed the animals’ BCS visually on a scale of 1 to 5; where 1 represents a very thin and 5 represents a very fat elephant. To understand the influence of seasonality on BCS, we sampled the population during dry and wet seasons of 2013 and 2015 while, for annual changes in BCS, we sampled nine free-ranging adult females from different family groups that had been repeatedly sighted over seven years. To evaluate the influence of body condition on fGCM, we measured the fGCM levels from fresh fecal samples collected from the body-condition scored animals. Effect of age and season on BCS in relation to fGCM was also studied. We found that the BCS was related with season, i.e. individuals with low BCS were more frequent during the dry season as compared to the wet season and fGCM levels were negatively associated with BCS. To be precise, concentrations of fGCM were highest in individuals with the lowest BCS (BCS-1) and then significantly declined till BCS-3. fGCM levels were almost comparable for BCS 3, 4 and 5. This pattern was more conspicuous in female than in male elephants. We discussed the possible reasons behind such relationship between fGCM and BCS. The findings from this chapter suggest that season-dependent BCS, hence, reflect the stress status as measured by fGCM, especially in female Asian elephants and could be used as an important non-invasive approach to monitor the physiological health of free-ranging elephant populations. This is the first study to compare the body condition scores and stress-response in free-ranging elephants. Chapter 4: Influence of ecological and physiological correlates on stress physiology of free-ranging female Asian elephants. We investigated the influence of herd size (HS), lactational status and number of adult females present in a herd on adrenocortical function in free-ranging adult female Asian elephants by measuring their fecal glucocorticoid metabolites (fGCM) levels. We found that there was a negative association between herd size and fGCM levels when only HS and fGCM were compared. Lactating females showed higher fGCM than that observed in non-lactating females, which might reflect the nutritional stress on mother and anti-predatory challenges against the calves. Herds with only one adult female had higher fGCM levels than herds with more than one adult female which could be because of social challenges. The poor association between herd size and fGCM when all predictor variables were combined elucidate that group size may not necessarily influence the adreno-cortical function. However, the numbers of adult females present in a herd and their lactational status has a significant role in defining the stress-response in free-ranging adult female Asian elephants. This study is the first study to document the influence of lactational status with the stress-response in free-ranging elephants. Chapter 5: How physiologically costlier it is to be a crop-raider in a human-dominated landscape? Diet quality as a possible ‘pacifier’ against stress. We studied the stress-response in crop-raiding elephants in a human-dominated landscape and non-raiding elephants in the protected forested areas. While raiding agricultural crops, elephants face various associated threats such as retaliation by humans, human-induced disturbances and stress of raiding which could enhance the energetic costs, ultimately elevating their stress levels. We hypothesized that crop-raiders (in human-dominated landscape; Hassan district of Karnataka, India) will exhibit higher faecal glucocorticoid metabolite (a proxy of stress-response; fGCM) levels as compared to the nonraiders (in forested areas; Bandipur and Nagarahole National Parks). Contrary to our hypothesis, fGCM levels were found to be higher in both nonraiding female and male elephants than female and male crop-raiders. To assess the influence of benefits obtained from crop-raiding as one of the possible factors for the lower fGCM in crop-raiders, the difference in vegetation between human-dominated landscape and forested areas were further analyzed by using a remotely sensed NDVI (Normalized Differential Vegetation Index) and was ground-truthed by measuring the quality of diet (C:N ratio through fecal samples). Interestingly, the NDVI values were higher at the human-dominated and the C:N ratio was lower (higher Nitrogen content) in faecal samples of crop-raiding elephants (both in females and in males) than the nonraiders in the forested areas. Positive correlation was observed between C:N ratio and fGCM levels. These findings suggest that crop-raiding comes with the benefits of easy access to good quality of diet which may help in reducing the stress-response in elephants while being in the human-dominated landscapes, provided there is a low intensity of human disturbance. This is the first study to document and compare the stress-status of crop-raiding Asian elephants with elephants in their natural habitats using NDVI and C:N ratio. Chapter 6: Conclusion In conclusion, the work presented in this dissertation provides an overview of how ecological and anthropogenic factors could influence the physiological health of free-ranging Asian elephants. This dissertation focused on providing detailed insights about the stress-response in free-ranging Asian elephants. The objectives have been achieved by assessing the relationship between ecological stressors such as seasonality, body condition, herd size, lactational status, a presence of individuals in a herd and anthropogenic stressors such as human-induced threats or crop-raiding using well-validated and standardized laboratory and field protocols. This study provides valuable insights into the physiological health or stress-response being synergistically influenced by various ecological, social and anthropogenic factors in free-ranging Asian elephants. Findings obtained from this study could help in addressing the issues related to the management of free-ranging elephant populations.