Heavy metal exposure and estrous cyclicity in semi-captive Asian elephants (Elephas maximus)

Abstract

Background: Pollutants cause significant harm to ecosystems and organisms and have become a major concern. While some toxic substances are natural, like heavy metals, and others are anthropogenic like PCBs and microplastics, human activities are the main contamination source of both. Even though pollutants affect all organisms, they often affect long-lived animals like eagles, whales and polar bears the most due to bioaccumulation, causing various negative effects. The effects on reproductivity are of special interest, as normal reproduction is vital for any species survival. The purpose of this thesis is to study the heavy metal concentrations in endangered Asian elephants (Elephas maximus) in Myanmar, using a semi-captive population where blood sampling is possible, and to perform a pilot study to measure estrous cyclicity in females. The aims are to map element concentrations in elephants, and to verify a method for estrous cyclicity measurements for future studies.

Methods and results: The heavy metal concentrations were measured from whole blood samples collected from 80 semi-captive Asian elephants and the effects of age, sex and geographical location on heavy metal concentrations were studied. Age had a significant negative effect on copper concentrations, and significant positive effect on the concentrations of chromium, nickel and lead. The concentrations of aluminium and nickel were significantly lower in females than in males, and the concentrations of vanadium and lead were significantly increased. The effect of camp was ambiguous. The estrous cyclicity was examined by measuring allopregnanolone levels from serum samples collected from 11 elephant females. The method allowed successfully characterising the estrous cycle profiles of 11 elephants that were classified into three categories based on their allopregnanolone levels: into cycling (n=5), non-cycling (=4) and indefinite (n=2). The effects of heavy metal concentrations on estrous cyclicity could not be confirmed due to small sample size (n=6), however, no abnormalities in estrous cycles in respect to heavy metal concentrations were observed.

Conclusion: In this thesis, the heavy metal concentrations and estrous cyclicity were measured for the first time in this study population. There were no signs of acute heavy metal poisoning, but there were some possible indicators of chronic exposure. Also, zinc levels were found to be lower than reference values, raising concerns of possible trace element deficiency. Estrous cycle profiles corresponded to the reproductive history or status of the elephants. Two elephants out of eleven had indefinite estrous cycles, however, their cycling profiles are likely to be affected by aging. This thesis facilitates future studies on element concentrations and allopregnanolone measurement in endangered Asian elephants, which can be used in e.g. conservational efforts.