Biomass loss during decomposition – A new tool in improving PMI estimation?

Determining the minimum post-mortem interval (minPMI) is a useful tool in the investigation of recovered human remains. Though typically based on temperature of remains in the early stages, and insect activity later in the decomposition process, attempts at concealment of remains can hinder these processes. Concealment can affect not only the immediate temperature remains are exposed to but may also result in an inability for insect colonisation to occur. This in turn will hinder the ability to estimate an accurate minPMI, and therefore timeline of events. As such, an understanding of the decomposition patterns where insect access is restricted through physical exclusion may assist in establishing a minPMI. This study utilised still born piglets (n = 48) over a 14-day period in November 2019 – 2022, in Geelong, Victoria, Australia. Piglets were either exposed, (controls), or had complete insect exclusion through a mechanical barrier which still allowed air flow and sampling to occur, with a total of 6 replicates of each condition per year. Piglets were observed daily, with temperature of remains, stage of decomposition, weight, and any insect activity present recorded. Physical barriers successfully excluded insects for the duration of the trial, however produced a greenhouse effect, producing a total of 4500 – 6000 accumulated degree hours (ADH) during the trial while controls only reached 3000–4500ADH in the same period. Bloat phase was severely prolonged in insect exclusion replicates, and subjects did not reach the dry phase by 2700ADH across all years, compared to control piglets which all had entered dry phase by this time. When comparing percentage biomass loss, consistency in overall biomass loss was noted in control piglets, as well as insect exclusion piglets in all trials. Control piglets showed a standard deviation of 5 % biomass loss at any given ADH. Exclusion of insects reduced overall biomass loss in comparison to ADH, however trends in the rate of loss could be established within the corresponding years. This study highlights that biomass loss can occur in the absence of insect exclusion through other decomposition processes, such as bacterial interactions, and may be a useful tool in the estimation of minPMI.