Decomposition Dynamics of an Orangutan ( <i>Pongo pygmaeus morio</i> ) Carcass in a Tropical Forest: Implications for Conservation Practices

Over the past decade, more than 600 rehabilitated Bornean Orangutans (<i>Pongo pygmaeus morio</i>) have been released into protected forests in Borneo. Releasing rehabilitant Bornean Orangutans into the wild is a standard conservation practice, yet monitoring postrelease survival remains a challenge. Limited data exist on post release survival, with many individuals classified as "missing but presumed dead" due to the absence of a carcass for confirmation. Detecting carcasses in tropical forests is particularly difficult due to dense vegetation and the narrow time frame for observing remains before complete decomposition or scavenger removal. Here, we report the first documented observation of a wild adult female Bornean Orangutan carcass decomposing process in the Danum Valley Conservation Area, Malaysian Borneo, on 21 May 2023. The approximately 30 kg carcass was monitored using camera traps and field observations. Decomposition was assessed using Payne's (1965) decomposition framework, the Total Body Score (TBS) system, and Accumulated Degree Days (ADD) to evaluate the influence of ambient temperature on decay. Decomposition progressed to the dry-remains stage within 6 days, primarily driven by vertebrate scavengers such as the Asian water monitor lizard (<i>Varanus salvator</i>) and blow flies (<i>Calliphoridae</i>). This rapid decomposition rate challenges existing knowledge on the rate of decomposition of medium-sized carcasses (> 10 kg) and suggests that the common practice of weekly monitoring for post-release orangutans may be insufficient. Understanding decomposition processes and scavenger activity in tropical forests can improve carcass detection, refine mortality estimates for released Orangutans and other endangered species, and enhance conservation strategies for this critically endangered primate.