Contrasting insect activity and decomposition of pigs and humans in an Australian environment: A preliminary study

A review of multi-disciplinary decomposition research and key drivers of variation in decay

The decomposition of animal remains is a multifaceted process, involving ecological, biological, and chemical interactions. While the complexity is acknowledged through concepts like the necrobiome, it's unclear if this complexity is reflected in research. Appreciation of the complexity of decomposition is crucial for identifying sources of variation in estimations of time since death in medico-legal science, as well as building broader ecological knowledge of the decomposition process. To gain insights into the extent of multidisciplinary research in the field of decomposition science, we conducted an examination of peer-reviewed literature on four key drivers of variation: volatile organic compounds, microbes, drugs/toxins, and insects. Among 650 articles, we identified their scientific discipline, driver/s of variation investigated, and year of publication. We found that 19% explored relationships between two drivers, while only 4% investigated interactions between three. None considered all four drivers. Over the past three decades, there has been a steady increase in decomposition research publications, signifying its growing importance. Most research (79%) was linked to forensic science, highlighting opportunities for interdisciplinary collaboration in decomposition science. Overall, our review underscores the need to incorporate multidisciplinary approaches and theory into contemporary decomposition research.

Evaluation of porcine decomposition and total body score (TBS) in a central European temperate forest

The total body score (TBS) is a visual scoring method to scale the succession of decomposition stages. It compares decomposition between cadavers, to connect it with external taphonomic factors and estimate the post-mortem interval. To study decomposition in various climatic environments, pigs are often used as human proxies. Currently, there is one TBS system by Keough et al. (J Forensic Sci. 2017;62:986) for surface-deposited domestic pigs, coming from South Africa. Our study aims to evaluate this method and analyze porcine decomposition in Central Europe to inform forensic research and casework. We conducted an experiment studying six 50 kg pig carcasses in a temperate Swiss forest. Three observers documented decomposition patterns and rated the decomposition stages from photographs based on the porcine TBS model by Keough et al. (J Forensic Sci. 2017;62:986). We documented discrepancies between the carcass decomposition of our specimens and those in the South African study, especially related to the high insect activity in our experiment. Furthermore, we noted factors complicating TBS scoring, including rainfall and scavengers. The agreement between TBS observers from photographs was in the highest agreement category apart from one "substantial agreement" category. Our study is the first in Europe to systematically test the Keough et al. (J Forensic Sci. 2017;62:986) method. The results evidence that regional adaptations are required to be applicable for other environments. We present a modified approach based on experimental observations in a Swiss temperate forest. The identification of regional decomposition patterns and drivers will inform future taphonomy research as well as forensic casework in comparable contexts in Central Europe.

Bridging the gap between decomposition theory and forensic research on postmortem interval

Knowledge of the decomposition of vertebrate animals has advanced considerably in recent years and revealed complex interactions among biological and environmental factors that affect rates of decay. Yet this complexity remains to be fully incorporated into research or models of the postmortem interval (PMI). We suggest there is both opportunity and a need to use recent advances in decomposition theory to guide forensic research and its applications to understanding the PMI. Here we synthesise knowledge of the biological and environmental factors driving variation in decomposition and the acknowledged limitations among current models of the PMI. To guide improvement in this area, we introduce a conceptual framework that highlights the multiple interdependencies affecting decay rates throughout the decomposition process. Our framework reinforces the need for a multidisciplinary approach to PMI research, and calls for an adaptive research cycle that aims to reduce uncertainty in PMI estimates via experimentation, modelling, and validation.

Forensic experiments on animal scavenging: A systematic literature review on what we have and what we need

Vertebrate scavengers frequently affect forensic casework by feeding on human remains or by scattering body parts and bones. Therefore, animal activity can influence complete recovery of bodies, trauma analysis, and the estimation of the postmortem interval (PMI), potentially hampering identification of the deceased and elucidation of the perimortem circumstances. Experimental research is well suited to investigate scavengers and their impact on carcasses over time, generating knowledge on the forensic relevance of certain scavenger species or communities. However, there are currently no systematised standards to conduct these investigations with a forensic focus, impeding comparison and synthesis of the studies. In our work, we performed a systematic literature review and found 79 publications featuring terrestrial experiments on vertebrate scavenging and/or scattering within a forensic context. We extracted 21 variables describing the study environment, experimental design and the specimens. The results show that there is considerable inconsistency in the study designs and that some of the variables are insufficiently reported. We point out research questions and areas that require attention in future studies, stressing the importance of infrequently mentioned or applied variables. Furthermore, we recommend guidelines to include and report a list of variables in forensic scavenging and scattering experiments. These guidelines will help standardising future research in the field, facilitating inter-study consolidation of results and conclusions, and consequently, inform forensic casework.

Retrospective analysis of factors affecting rate of skeletonization within a tropical climate

Estimating the post mortem interval (PMI) in skeletonized cases is an extremely challenging exercise. Sri Lanka lacks adequate taphonomic research which is a serious limitation when assessing PMI in forensic death investigations. Methods that have been proposed to estimate PMI using the total body score (TBS) and accumulated degree days (ADD) are mainly based on data from continental and temperate climates and have shown less reliability in tropical climates. With the intention of developing a region-specific, evidence-based guide which would be applicable to tropical climates like Sri Lanka, we selected thirteen skeletonized remains with known PMIs from forensic case records and analysed their taphonomy in relation to selected weather data. We also compared the ADD values within our dataset with reference ranges calculated using published formula. All except one were found from outdoor locations. The TBS ranged from 24 to 32 and had a weak positive correlation with the PMI. The earliest appearance of skeletonization was 15 days in a body found indoors. The highest rate of skeletonization was seen in a body with a TBS of 32 and a PMI of 23 days. The average daily temperature and relative humidity were similar across all the cases however, the amount of rainfall varied. Bodies exposed to monsoon rains (n = 6) had a lower mean rate of skeletonization compared to those that were not exposed (n = 4) suggesting lower rates of decomposition during periods of heavy rainfall. No correlation was found between ADD and TBS. In 9 (69.2%) cases, the actual ADD was much lower than reference ADD ranges for TBS values, indicating poor applicability of TBS and ADD based formulae in estimating PMI within the Sri Lankan climate. Our study shows a strong need for taphonomic and entomological research in tropical climates to further explore the impact of monsoons on biotic and abiotic factors affecting skeletonization.

Subaerial Decomposition of Small-Sized Remains in The Netherlands: Important Findings Regarding the PMI of a Four-Year Taphonomic Study

Studying post-mortem changes based on signs of decomposition (e.g., using scoring models) is one of the methods used in scientific studies to relate observable changes to the post-mortem interval (PMI). The majority of the studies on cadaver decomposition are based on large cadavers. There is limited literature on the decomposition pattern and rate of small cadavers, even though it is at least as important to be able to estimate the PMI for infants and subadults. Therefore, it is crucial to acquire knowledge of the decomposition process of child-sized remains. To fill this knowledge gap, a season-based subaerial outdoor decomposition study was conducted with small pig cadavers at the Forensic Anthropological Outdoor Research Facility located in Den Ham, The Netherlands, over a period of 4 years. Den Ham is located in the eastern part of the Netherlands, close to the German border, and has a temperate maritime climate, with a Cfb classification according to the Köppen-Geiger system. Salient findings were acquired during the decomposition study, specifically regarding a deviating decomposition rate during winter and the subsequent spring, reproducibility, the effect of body weight, post-mortem movement, the effect of heavy rainfall on insect activity, delayed bloating, the interaction of different insect species, and invertebrate activity. This article includes a systemic review of the results obtained during this four-year decomposition study and discusses the impact of the findings on the estimation of the PMI.

A synthesis of carcass decomposition studies conducted at a tropical (Aw) taphonomy facility: 2013-2022

Decomposition studies have been conducted in several regions of the world, but relatively few have investigated taphonomy in tropical environments. Even fewer have explored carcass decomposition during multiple tropical seasons, leaving the relationships between season and decomposition in tropical environments poorly understood. Ten decomposition studies using 30 carcasses were conducted in Honolulu, Hawaii, USA to start addressing this knowledge gap. These studies show that some postmortem processes were observed regardless of season. Carcass temperature and chemistry were spatiotemporally variable. Fly larval masses were consistently observed within 3 days (∼75 ADD) postmortem and carcasses lost 60%-90% of mass by 10 days (∼250 ADD) postmortem (Total Body Score ∼26). Season had a significant effect on decomposition, yet the warmest and most humid seasons did not always result in the most rapid and extensive decomposition. Seasonal variation appears to be less pronounced than at other tropical decomposition sites.

The effect of postmortem penetrative trauma on oviposition behavior of Calliphora vicina

Flies in the family Calliphoridae are purported to demonstrate a strong attraction to and preferential colonization of wounds when present on human corpses or carrion. This well-circulated concept in Forensic Entomology is based on surprisingly few empirical studies that have examined the oviposition behavior of necrophagous Diptera toward wounds of vertebrate animals. In the present study, the oviposition behavior of Calliphora vicina toward piglets inflicted with postmortem sharp force trauma was examined during a 10-h test period under controlled laboratory conditions and in an outdoor urban environment. Three species of flies (C. vicina, C. livida and Cynomya cadaverina) deposited eggs on wounded and non-wound piglets during the field studies, although more than 80% of all eggs laid were by C. vicina regardless of wound status. For all species, oviposition occurred predominately on the head, including in eyes, ears, nose and mouth, and less frequently in other locations on the body. Eggs were never found in or near wounds. Similar oviposition patterns were displayed by C. vicina under laboratory conditions, with the exception for limited egg deposition in wounds under overcrowded conditions. In this latter scenario, egg deposition represented only 0.14% of the total reproductive output of adult flies. The findings do not support the contention that calliphorids preferentially oviposit in wounds resulting from sharp force penetrating trauma.

Developing a new scoring method to evaluate human decomposition in a humid, continental (Dfb) climate in Quebec

The published literature shows a lack of methods to evaluate the patterns and extent of decomposition of human remains and to estimate the post-mortem interval (PMI) in humid, continental (Dfb) climates such as Quebec. The aim of this study was to address this gap in the current knowledge base by providing the first observations from human corpses studied under controlled conditions in Quebec. A 12-month study was conducted at the site for Research in Experimental and Social Thanatology; the first human taphonomy facility in Canada. Six human donors with known time of death were deposited across spring (n = 1), summer (n = 3), and autumn (n = 2) 2021. The lack of suitability of the total body score method to evaluate the extent of decomposition at the facility prompted the development of a new scoring system based on the macromorphoscopic changes observed. The scoring system was applied to the donors to evaluate decomposition throughout seasons. All donors followed comparable decomposition trajectories, regardless of the season of deposition. Eighty-five percent of taphonomic patterns appeared in the first 25 experimental days or 5000 Kelvin accumulated degree days (350 ADD). Extensive desiccation of tissues was observed at a median of 21 experimental days across donors, resulting in a plateau within decomposition with no extensive skeletonization. To the authors' knowledge, this is the first published report of experimentally observed desiccation in such a form in a Dfb climate. This study provides new data on the types of decomposition patterns to expect in forensic investigations in southern Quebec and comparable climates.

Decomposition and insect succession on human cadavers in a humid, continental (Dfb) climate (Quebec, Canada)

Forensic entomology has been developing globally for decades. Despite this discipline being used in criminal investigations around the world, only a few controlled studies have been performed on human cadavers in human decomposition facilities, with the majority of these being conducted in warm and often dry climates. Therefore, the purpose of our research was to catalogue the first published data on insects associated with decomposed human bodies in a humid, continental (Dfb) climate. Specifically, our objective was to document the diversity and succession of the entomofauna associated with human cadavers throughout the decomposition process, in the Quebec province of Canada, during the summer season. Two human cadavers were studied in 2020 at the site for Research in Experimental and Social Thanatology, REST, located in Bécancour (Quebec, Canada). Insects (and other arthropods) were regularly sampled by visual observations, collection from the cadavers, and by using an entomological net and pitfall traps. Our results highlight that the decomposition process is a heterogeneous and complex process in Quebec, with cadavers showing signs of precocious desiccation/mummification. In addition, our observations confirm that the presence of superficial skin lesions accelerates the colonization of blow flies (Diptera: Calliphoridae) and, consequently, the process of decomposition. Finally, we were able to discriminate between "early colonizers" (e.g., Calliphoridae Lucilia sp. or Calliphora livida), "late colonizers" (e.g., larvae of Piophilidae or Heleomyzidae), and "non-specific colonizers." We also officially report the first observation of Cochliomyia macellaria (Diptera: Calliphoridae) in Quebec. These findings will provide new information to help medico-legal death investigations by determining the minimum time elapsed since death and the circumstances surrounding death.

Succession patterns of sarcosaprophagous insects on pig carcasses in different months in Yangtze River Delta, China

The Yangtze River Delta is one of the most economically developed regions in China, and many cases in the area require an estimation of the postmortem interval each year. This article reports forensic entomological research conducted in this region in different months. A total of 18 domestic pigs were used to study carcass decomposition, species composition, development events, and succession patterns of insects in different months from 2015 to 2021. A total of 53 arthropod species were identified, of which 38, 40, 41, and 25 were observed in April, June, September, and November, respectively. We demonstrated that insects vary in number and species from month to month, and the same species vary in number among the different months. Larvae of Calliphoridae mainly drove the decomposition of carcasses, and the decomposition rate of carcasses varied over the months. Particularly, the decomposition rate was highest in June and September, slower in April, and slowest in November. The predominant species on the carcasses also varied from month to month; Calliphora grahami Aldrich, Chrysomya pinguis (Walker), Lucilia sericata (Meigen) (Diptera: Calliphoridae), and Hydrotaea spinigera Stein (Diptera: Muscidae) were most predominant in April; Chrysomya megacephala (F.) and Chrysomya rufifacies (Macquart) (Diptera: Calliphoridae) were most predominant in June and September; and Ca. grahami was most predominant in November. In addition, the seven dominant insect species displayed four developmental events, which could be used to determine minimum postmortem interval, including adult arrival (T1), larvae wandering (T2), pupation (T3), and eclosion (T4) time on the carcasses. Insect succession patterns on carcasses also varied across months. For example, Ch. megacephala appeared later in the composition stages and was not numerically dominant in April but appeared earlier and was dominant in June and September. The results of this study guide the use of insect evidence in different months in the Yangtze River Delta region of China for estimating the postmortem interval of carcasses.

The ephemeral resource patch concept

Ephemeral resource patches (ERPs) - short lived resources including dung, carrion, temporary pools, rotting vegetation, decaying wood, and fungi - are found throughout every ecosystem. Their short-lived dynamics greatly enhance ecosystem heterogeneity and have shaped the evolutionary trajectories of a wide range of organisms - from bacteria to insects and amphibians. Despite this, there has been no attempt to distinguish ERPs clearly from other resource types, to identify their shared spatiotemporal characteristics, or to articulate their broad ecological and evolutionary influences on biotic communities. Here, we define ERPs as any distinct consumable resources which (i) are homogeneous (genetically, chemically, or structurally) relative to the surrounding matrix, (ii) host a discrete multitrophic community consisting of species that cannot replicate solely in any of the surrounding matrix, and (iii) cannot maintain a balance between depletion and renewal, which in turn, prevents multiple generations of consumers/users or reaching a community equilibrium. We outline the wide range of ERPs that fit these criteria, propose 12 spatiotemporal characteristics along which ERPs can vary, and synthesise a large body of literature that relates ERP dynamics to ecological and evolutionary theory. We draw this knowledge together and present a new unifying conceptual framework that incorporates how ERPs have shaped the adaptive trajectories of organisms, the structure of ecosystems, and how they can be integrated into biodiversity management and conservation. Future research should focus on how inter- and intra-resource variation occurs in nature - with a particular focus on resource × environment × genotype interactions. This will likely reveal novel adaptive strategies, aid the development of new eco-evolutionary theory, and greatly improve our understanding of the form and function of organisms and ecosystems.

Insect abundance patterns on vertebrate remains reveal carrion resource quality variation

Resource quality is a key driver of species abundance and community structure. Carrion is unique among resources due to its high nutritional quality, rapidly changing nature, and the diverse community of organisms it supports. Yet the role resource quality plays in driving variation in abundance patterns of carrion-associated species remains poorly studied. Here we investigate how species abundances change with a measure of resource change, and interpret these findings to determine how species differ in their association with carrion that changes in quality over time. We conducted field succession experiments using pigs and humans over two winters and one summer. We quantified the effect of total body score, an objective measure of resource change, on adult insect abundance using generalised additive models. For each species, phases of increasing abundance likely indicated attraction to a high-quality resource, and length of abundance maxima indicated optimal oviposition and feeding time. Some species such as the beetle Necrobia rufipes had a rapid spike in abundance, suggesting a narrow window of opportunity for carrion resource exploitation, while species like the wasp Nasonia vitripennis had a gradual change in abundance, indicating a wide window of resource exploitation. Different abundance patterns were also observed between species occurring on pigs and humans, suggesting cadaver type is an important aspect of resource quality. Our findings show that species abundances, unlike species occurrences, can reveal additional detail about species exploitation of carrion and provide information about how resource quality may drive competition and variation in insect community succession.

How does mass loss compare with total body score when assessing decomposition of human and pig cadavers?

Providing accurate and reliable measures of decomposition is paramount for forensic research where decomposition progress is used to estimate time of death. Mass loss is routinely used as a direct measure of biomass decomposition in ecological studies, yet few studies have analysed mass loss in a forensic context on human cadavers to determine its usefulness for modelling the decomposition process. Mass loss was examined in decomposing human and pig cadavers, and compared with other common decomposition metrics, such as total body score (TBS). One summer and one winter field decomposition experiment was conducted using human and pig cadavers, as pigs are often used as proxies for human cadavers in forensic research. The two measures of decomposition revealed two contrasting patterns of decomposition on pigs and humans, particularly in winter where TBS stabilised at similar values, but mass loss differed greatly. Mass loss was found to be faster in pigs than humans during early decomposition. Pigs lost 75% of their mass in winter, while humans lost less than 50%; however, in summer, both lost around 80% of their mass. TBS displayed similar patterns in both experiments, with TBS increasing more rapidly in pigs compared with humans but both eventually reaching similar TBS values in late decomposition. Measuring mass loss can provide additional information about decomposition progress that is missed if using TBS only. Key differences in decomposition progress between cadaver types were also observed, suggesting caution when extrapolating data from pigs to humans for forensic research and decomposition modelling.

Is Resource Change a Useful Predictor of Carrion Insect Succession on Pigs and Humans?

Carrion is a dynamic and nutrient-rich resource that attracts numerous insect species that undergo succession due to the rapid change in the carrion resource. Despite this process being well-understood, few studies have examined resource change as a driver of carrion insect succession, and instead have focused on the effects of time per se, or on coarse, qualitative measures such as decay stage. Here we report on three field succession experiments using pig carcasses and human cadavers encompassing two winters and one summer. We quantified the effects of resource change (measured as total body score, TBS), carrion type, initial carrion mass, ambient temperature, and season on insect species richness and community composition. We found that all variables had an effect on different taxonomic or trophic components of the insect community composition, with the exception of initial carrion mass which had no effect. We found significant positive effects of TBS on beetle species richness and composition, while fly species richness was not significantly affected by TBS, but was by ambient temperature. TBS had a significant positive effect on all trophic groups, while ambient temperature also had a significant positive effect on the necrophages and predator/parasitoids. Our study indicates that resource change, as indicated by TBS, is an important driver of carrion insect species turnover and succession on carrion, and that TBS can provide information about insect ecological patterns on carrion that other temporal measures of change cannot.

The time of death in Dutch court; using the Daubert criteria to evaluate methods to estimate the PMI used in court

When a capital crime is committed the post-mortem interval (PMI) is of particular importance in investigating a suspect's alibi in court. A forensic expert can use different methods to estimate the PMI. This research focuses on who is considered an expert in court and whether the methods used to estimate the PMI are reliable. In this study, the methods used to estimate the PMI and the experts consulted, available in Dutch jurisprudence, in the period 2010-2019 were investigated. Ninety-four judicial cases were included and multiple experts and methods of estimating the PMI were found. As part of this study, the methods that were used to estimate the PMI in court were subjected to the Daubert criteria. Of these methods, only the Henssge nomogram and entomological methods met the Daubert criteria. However, the methods are only useful when applied by the right forensic expert and in the right manner. Unfortunately, this was not always the case.

Field succession studies and casework can help to identify forensically useful Diptera

Fly development rates, and to a lesser extent succession data, can be used to provide an estimate of a minimum postmortem interval (mPMI). Yet, these data are most useful when a full account of species' ecology, seasonality, and distribution is known. We conducted succession experiments on human cadavers over different seasons near Sydney, Australia, to document forensically useful information, including the pre-appearance interval for carrion flies. We also compiled a detailed record of flies identified in casework collected in 156 cases distributed across New South Wales, Australia. We then compared the occurrence of fly species from both field and casework datasets to identify any consistencies or gaps to determine how useful species might be for forensic investigations. In the field experiments, we found differences in species diversity and abundance between seasons, as well as yearly variation between two winter seasons. Most fly species we recorded ovipositing showed a 2- or 3-day delay between adult arrival and oviposition in summer, with a longer delay in winter. Species that were previously encountered in casework, such as Calliphora augur (Fabricius, 1775) and Calliphora ochracea Schiner, 1868, were confirmed as forensically useful, with their colonization behavior and seasonal preferences documented here. Although not encountered in casework, we confirmed Hemipyrellia fergusoni Patton, 1925 as a primary colonizer of human cadavers. Our study emphasizes the need to link field and casework data for a complete understanding of all aspects of a carrion fly's ecology to assist forensic investigators in mPMI estimations.

Temperature dynamics in different body regions of decomposing vertebrate remains

The decomposition of vertebrates is controlled largely by external temperature, yet internal temperatures can also play an important role but are generally poorly documented. In this study, we compared continuous hourly temperature recordings from the mouth, under the head, right chest and right abdomen, and in the rectum of one refrigerated human and one fresh pig cadaver during 29 days of decomposition. Each cadaver differed in its internal starting temperature, thus providing two contrasting case studies for examining temperature dynamics among body regions. We used time-series analysis methods common to hydrology to reveal key differences in internal temperature dynamics. Within both cadavers, the chest region experienced the highest average temperatures, and the mouth experienced the highest maximum hourly temperature. Temperatures exceeded 30 °C inside the pig for between 40% (rectum) and 75% (chest) of the duration of the study, but for only 20% (rectum) and 35% (chest) of the time in the human. Our study provides evidence of the different thermal trajectories occurring in different body regions, and some similarities between two cadavers despite their different starting thermal conditions. These results improve our understanding of why decomposition occurs at different rates within the same cadaver, and that the location of blowfly larvae collections should be noted to improve estimates of the post-mortem interval.

Post-Mortem Interval Estimation Based on Insect Evidence: Current Challenges

During death investigations insects are used mostly to estimate the post-mortem interval (PMI). These estimates are only as good as they are close to the true PMI. Therefore, the major challenge for forensic entomology is to reduce the estimation inaccuracy. Here, I review literature in this field to identify research areas that may contribute to the increase in the accuracy of PMI estimation. I conclude that research on the development and succession of carrion insects, thermogenesis in aggregations of their larvae and error rates of the PMI estimation protocols should be prioritized. Challenges of educational and promotional nature are discussed as well, particularly in relation to the collection of insect evidence.

The Pitfalls in the Path of Probabilistic Inference in Forensic Entomology: A Review

To bridge the gap between experimentation and the court of law, studies in forensic entomology and other forensic sciences have to comply with a set of experimental rules to generate probabilistic inference of quality. These rules are illustrated with successional studies of insects on a decomposing substrate as the main example. The approaches that have been used in the scientific literature to solve the issues associated with successional data are then reviewed. Lastly, some advice to scientific editors, reviewers and academic supervisors is provided to prevent the publication and eventual use in court of forensic studies using poor research methods and abusing statistical procedures.