Ultrasonographical and endocrinological characterization of the unique ovarian activity during the estrus cycle and the pregnancy in Asian elephants (Elephas maximus)

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For different mammal orders, established estrus cycle models describe ovarian activity and reproductive hormone profiles, with related species often following similar strategies. However, elephants deviate from these norms due to their isolated taxon position, complicating the application of established mechanisms from large ungulates. This results in unique and unexplained features in their estrus cycle and pregnancy, which is the longest of any mammal. Key questions arise, such as the presence of multiple corpora lutea despite being uniparous, the occurrence of two LH peaks during the follicular phase spaced 19-21 days apart, and the maintenance of their extended pregnancy. The study employed both ultrasound and endocrine techniques on over 30 Asian and two pregnant African elephants to address these observations. While ultrasonography and endocrinology are established non-invasive tools for characterizing reproductive events, their combination offers a comprehensive view of ovarian activity and reproductive organs. Ultrasonography allows for direct observation, while hormone measurements provide crucial functional insights, enhancing our understanding of the estrus cycle and pregnancy in elephants.