Experimental validation of forensic evidence: a study of the decomposition of buried pigs in a heavy clay soil

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1 Forensic Science International 101 (1999) Experimental validation of forensic evidence: a study of the decomposition of buried pigs in a heavy clay soil Bryan Turner *, Patricia Wiltshire a, b a Division of Life Sciences, King s College London, Kensington, London W8 7AH, UK b Institute of Archaeology, University College London, London, UK Received 30 November 1998; accepted 1 February 1999 Abstract In a murder investigation, where the victim had been strangled and buried in a shallow grave, there were discrepancies between the post mortem interval (PMI) as estimated from entomological studies and estimations determined from other evidence. This inconsistency provided the impetus for examining the decay process using pig carcasses as analogues for the human cadaver. The pigs were buried in the immediate vicinity of the original burial site in December 1996, which was the month when the victim was purported to have been interred in the previous year. The buried pigs were then monitored for 5 months which, based on the evidence other than the entomological, was the period over which the corpse was thought to have lain in the ground. The pig corpses were disturbed by scavengers in mid April: this was the same time that the human corpse was discovered in the previous year by scavengers. Insects played no role in the decomposition process until the pig carcasses had been exposed by animals. Blowflies, notably Calliphora vomitoria, were attracted to the exposed tissues and laid eggs from which larvae developed. Calliphora vomitoria is a species often used to estimate PMI. This investigation has shown that soil conditions and low seasonal temperatures had preserved the pig carcasses for longer than might be expected. Using the blowfly larvae to estimate PMI would have produced erroneous results had not the burial environment and exhumation history been investigated Elsevier Science Ireland Ltd. All rights reserved. Keywords: Calliphora vomitoria; Scavengers; Fox; Shallow grave; Stagnogley; Winter *Corresponding author. address: bryan.d.turner@kcl.ac.uk (B. Turner) / 99/ $ see front matter 1999 Elsevier Science Ireland Ltd. All rights reserved. PII: S (99)

2 114 B. Turner, P. Wiltshire / Forensic Science International 101 (1999) Introduction and case background This study was initiated to establish some baseline data on the decomposition rates of buried corpses in an acidic, stagnogley soil, and in particular to demonstrate whether the post mortem interval (PMI) could be estimated from entomological evidence when burial occurred in mid-winter. It relates to a case, code named Operation Bandsman, investigated by the Hertfordshire Constabulary, UK. The corpse of the male murder victim, who had been strangled and buried in a shallow grave at Essendon wood in Hertfordshire, was discovered on April 15th The body had been partially uncovered by large scavengers, probably foxes, which had eaten some of the extremities. When exhumed, the corpse appeared to be very well preserved. Original entomological evidence taken from the human cadaver was interpreted as suggesting that the PMI was 6 7 weeks, putting the time of death as the end of February/beginning of March However, this was at variance with other evidence in the possession of the police which indicated a PMI of 17 weeks, i.e. the third week of December Entomological evidence can be highly accurate in estimating PMI [1,2]. In order to calculate the post mortem interval, it is necessary to know the stage of decay that a particular insect species is attracted to, as well as its rate of development at the temperatures in the vicinity of the larvae. Frequently, estimates are based on the stage of larval development of blowfly species (Diptera, Calliphoridae) that are attracted to a corpse at a very early stage of decay. Where there are factors that affect the course of the decay process (e.g. burial or low temperatures), the calculation of the post mortem interval from insect evidence becomes considerably more difficult [3]. In Operation Bandsman, a number of insects were collected from the scene of crime and sent to a forensic entomologist. Most were judged to be of no forensic relevance but a PMI estimate was made based on the finding of final instar Tephroclamys rufiventris larvae, a heleomyzid fly, on the corpse in the mortuary. Unfortunately, there is little published information available on the biology of this species, particularly with regard to details of its temperature thresholds for flight and egg laying, developmental rates of its larvae, or its diet. This heloemyzid has been recorded as adult throughout the winter and is often found indoors on windows [4]. It is one of the commonest heleomyzids and has been reared from a wide range of decaying animal and plant material [4,5]. There is little information in general on the biology of the Heleomyzidae in relation to corpses. Smith [6] notes that Noleria inscripta, is most frequently associated with carrion and Lundt [7] found that Morpholeria kerteszi laid eggs on the soil surface and the young larvae then burrowed down to reach carrion buried below. Concerns over the accuracy of this entomologically-derived PMI estimate for the murder victim, provided the impetus for the pig corpse decomposition study in the precise area (Essendon Wood) of the original investigation. 2. The site The soils in Essendon Wood consist of compacted stagnogleys derived from clayey drift and Tertiary clays of the Essendon soil association [8]. These are slowly permeable

3 B. Turner, P. Wiltshire / Forensic Science International 101 (1999) and poorly drained, often seasonally waterlogged. This means that levels of free oxygen are often low, resulting in a relatively low redox potential, and therefore low bioactivity. The southern end of Essendon Wood is dominated by old Carpinus betulus (Hornbeam) coppice which has not been cut for fifty years. There is virtually no ground flora. As is characteristic of hornbeam woods, several centimetres of mor humus constitute the O horizon of the soil profile. This mor humus at the surface overlies a humic, moderately friable clay which merges into a less humic, highly compacted clay subsoil with blocky, subangular peds. No earthworm activity was observed and no soil animals whatsoever were encountered during the digging of a pit to obtain a soil profile in August 1996 or when the graves were dug in December. The soil is highly acidic (ranging from ph ) throughout its profile and, therefore, probably base-poor with high levels of polyphenolic compounds such as tannins. Generally, plants have higher production of tannins and other polyphenolic substances when growing in acid soils [9]. Tannins have the property of combining with the proteins and carbohydrates in organic matter, rendering them resistant to microbial degradation [10]. Consequently, acidic conditions such as those found under hornbeam lead to deep mor humus profiles since they do not support the larger fauna such as woodlice, millipedes, earthworms, molluscs etc. which are important agents in the decomposition of plant material. Most bacteria are inhibited by low ph and high tannin levels [9,11]. Furthermore, the high clay fraction, with its variable chemistry may be expected to adsorb certain compounds. It is known, for example, that some enzymes and antibiotics are adsorbed by soil clays [12]. It is possible that microbial enzymes, or even breakdown products produced during decomposition, could become attached to clay molecules and rendered inactive. Again, this might impede decomposition and odour production. Additionally, by virtue of reduced porosity, the compacted nature of the clay soil may impede any odours released from a corpse. 3. Methods Three freshly-killed (by phenobarbitone overdose) 4-month-old pigs each approximately 120 cm long, 30 cm in diameter and weighing approximately 55 kg, were buried in Essendon Wood in shallow graves ( cm) such that the overburden was no deeper than 10 cm. The pig graves were between 7 and 15 m apart in the vicinity of the human grave site. The shallowness of the burials simulated the grave of the human victim. Each pig was laid on its left side in its grave which was then filled, compacted by trampling and camouflaged with scattered litter. The overburden was a random mix of the soil, humus and litter layers which had been upcast during the grave digging. Grave 1 was 3 m NE of the site of the human grave and had a similar soil profile. Grave 2, some 7.0 m to the E of the human grave site, was characterised by a soil profile containing a higher proportion of silt and many rounded pebbles. Grave 3 was 15 m to the NW of the human grave site. Here the soil was a heavy yellow clay with a few pebbles. Burial of all three pigs was completed within 5 h of death. The experimental burial was planned to cover the period from December 1996 to April 1997, i.e. the 17 weeks which encompass both the entomological estimate of PMI

4 116 B. Turner, P. Wiltshire / Forensic Science International 101 (1999) and the period suggested by the alternative evidence. Burial took place on 23rd December 1996 which approximated closely to the date the previous year when the human victim was purported to have been buried. During burial, one of the pig corpses (Pig 1) was fitted with several needle thermistors to measure temperature variations of the corpse. One thermistor was placed at the base of the grave, directly at the skin soil interface. A second probe was inserted 6 cm into the abdominal cavity, and a third was inserted subcutaneously on the upper surface of the body. The thermistor cables were led away from the corpse and terminated in a plastic box which itself was buried in the soil under a log, but was easily accessible for taking readings on subsequent visits. 4. Results and discussion Initially a continuously operating datalogger (Grant Squirrel model SQ2) was used, but this failed after 4 days and readings were subsequently taken from the probes when the site was visited. The datalogger was started at 1430 h on 23rd of December, 1996 as soon as pig 1 s burial was completed. Although the logger stopped functioning after 4 days it was possible to extract the temperature measurements for that period from the datalogger s memory to provide information on the initial cooling of the corpse (Fig. 1). The air/soil surface temperature at the time of the burials was 28C. Fig. 1 shows the gradual cooling of the core temperature, measured by the probe inserted into the abdomen (mid), from 308C to48c over a period of 55 h. The recordings for the probes on the upper and lower sides of the corpse show slightly more erratic curves which become virtually identical at 2 38C after 3 days and probably reflect the surrounding soil temperature. Fig. 2 shows the temperature data for the entire burial period with the initial cooling of the corpse down to approximately 28C (detailed in Fig. 1) and then a very gradual increase in temperature over the experimental period. It is clear that the corpse remained below 58C for several months and even on the 15th April, temperature in the bottom of the grave did not exceed 6.68C. The measured air temperatures gradually rose from 28C in December to 15.58C in April but it should be noted that whilst these are spot measurements made at the same time as those from the corpse probes they would be expected to be far more variable than the grave measurements which are buffered by the soil. The changing condition of the buried corpse is expected to be influenced by the temperature, burial and soil conditions noted above. The cold, anoxic conditions at the burial site were strongly preservative of the pig tissues. During the experimental period brief partial exhumations were performed alternately on Pigs 2 and 3 in order to describe the corpse condition. Holes between 10 and 20 cm in diameter were carefully dug down to the corpse. Exposure was short, typically less than 30 min, before replacing and treading down the soil. On day 77 Pig 1 was totally exhumed to give a more complete assessment of the decay process and to confirm that the routine small excavations were giving an adequate picture of the corpse condition

5 B. Turner, P. Wiltshire / Forensic Science International 101 (1999) Fig. 1. The change in temperature at three locations in the grave of Pig 1 during the initial 4 days following burial. See text for details of the thermistor positions. through time In this instance the soil was removed in 5 cm layers and examined for insects. A Phaonia larva and two tipulid larvae were found in the top 5 cm layer. Neither are considered to have any relevance to the corpse decay process. No attraction to the corpse by any flying insects was observed during the time (30 min) it was exposed. The corpse was rapidly reburied to prevent any major changes to the temperature of the corpse and the soil. On reburial the soil layers were returned in their original sequence and trodden down. Later partial exhumations were performed by large scavengers and on these occasions parts of the corpses were consumed. On day 105 pig 3 was exhumed in the same manner as pig 1 on day 77. Table 1 summarises the changes in the condition of the corpses through time. It can be seen that the corpses remained in a relatively fresh state for several months (at least to the end of February) before beginning to show signs of decay. During our successive visits, digging small holes to examine the corpse, it was apparent that decay odours, noticeable to the human nose, were gradually rising through the soil to the surface. Odours reached the surface layer of the soil at the time when the scavengers took an interest. This suggests that those smells detectable by the human nose had similar soil permeability rates to those used by the scavengers to locate the graves. The graves first began to show signs of investigation by scavengers, in the form of small excavations, after 3 months (Table 2). Unlike insects which are cold blooded and dependent on relatively high air temperatures for activity, foxes, badgers and dogs would

6 118 B. Turner, P. Wiltshire / Forensic Science International 101 (1999) Fig. 2. The variations in temperature at three sites in the grave of Pig 1 during the experimental period of 5 months. See text for details of the thermistor positions. have been actively scavenging in the wood throughout the burial period, yet it is significant that the corpses remained undetected by them during January, February and most of March. This animal disturbance, first noted on day 91, was at the site of a pilot hole dug on the previous visit to examine the condition of the pig corpse. The infilled hole of disturbed soil was probably releasing some decay odours. The other graves were not touched at this time. The exposure of the pig corpses by scavengers provided access to the corpse for Diptera and other insects. Up until that time there had been no evidence of insect activity in the grave sites. Remarkably, weather conditions were warm and sunny on most visits to the site, and insect activity was seen in the wood generally from the January visit onwards but not specifically at the graves until they were disturbed by animals in April. During January, February and March there was no obvious entomological interest in the corpse during the brief partial exhumations. The only dipteran larva found during this period (Day 63) was a 3rd instar larva of the muscid Phaonia in the lower litter layer where an exploratory hole was being dug. This genus is carnivorous and often found in rotting trees and branches. Its presence in the vicinity of the grave is seen as coincidental. Several species, both as adults and larvae, were found on the corpses once they had

7 B. Turner, P. Wiltshire / Forensic Science International 101 (1999) Table 1 Changing condition of the buried pig corpses at Essendon Wood Date General Condition of corpse comments Skin Tissues Odour Start-day 0 Normal Normal Fresh 23/12/96 Day 32 Small area of Skin pink, hair intact No signs of decay to No odours above 24/1/97 upper surface and firmly anchored tissues. grave, faint sweet exposed. sickly smell (not putrescent) from exposed skin. Day 63 Small area of Skin pinkish with No odours above 27/2/97 upper surface blue grey blotches. grave, faint sweet exposed. Hair intact and firmly sickly smell present anchored. from exposed skin with underlying traces of sourness. Odour stronger than last time. Day 77 Total exposure of Mouth fresh Mouth bright pink. Strong odour 14/3/97 Pig 1. and bright pink. Skin detectable in the as before, pink and excavated soil. Corpse mottled. Hair loose, emitted a strong faecal easily detached. smell. Day 91 Small area of Soil skin interface Mouth bright pink. No odour detectable 28/3/97 upper surface distinctly grey above grave but exposed. (reduction of ferric excavated soil smelt to ferrous ions in strongly becoming more anaerobic conditions). faecal with depth. The skin under an appressed ear flap pink. Hair loose, easily detached. Day 105 All graves had Hair and epidermis Exposed tissues pink or Where graves had been 15/4/97 been opened by easily rubbed off. drying out (blackening). opened by foxes the scavengers. Tissues were softening exposed tissues Holes refilled. with some distinctly gave off a strong See Table 2 for mousse like. Fat layers putrescent smell with details. readily separated from butyric acid (cheesy) Pig 3 was underlying muscle. overtones. excavated. Trotters (hooves) Blowfly (C. readily detached. vomitoria) eggs Liquid contents of were present on abdominal cavity the exposed leaking out. Decay less muscles of the neck pronounced in tissues region. Scavenger positioned lower in the feeding and insect grave. eggs appeared to be very recent. Phorid flies (Conicera tibialis) were arriving on the exposed tissues.

8 120 B. Turner, P. Wiltshire / Forensic Science International 101 (1999) Table 1. Continued Date General Condition of corpse comments Skin Tissues Odour Day 120 No excavations Strong foetid odour in 30/4/97 carried out. Graves vicinity of pig 3 but not 2 and 3 heavily pig 2 grave which had disturbed by foxes. been refilled. Refilled. Day 135 Graves opened by Skin black, easily Fat layer mobile on Odour strong and 15/5/97 foxes. Refilled. detached difficult to underlying muscle. foetid. separate from soil. Freshly exposed tissue (at feeding site) pink and fresh looking. Tissues from deeper in grave more coherent little decomposition evident. Day 150 Graves still being Skin soil interface Small amounts of intact Odour strong and 30/5/97 excavated. Most indistinct. tissue present in the foetid. of the pig corpses lower levels of the Study 1 and 2 have been grave. Graves 2 and 3 terminated. consumed. contain brown liquid in Grave 3 which are many excavated. calliphorid larvae. Calliphora vomitoria still laying eggs on freshly exposed bone. been exposed by scavengers. The flies, Calliphora vomitoria, Lucilia sp. Bibio marci, and Conicera tibialis were all attracted to the exposed portions of the pig corpses in April. In May, when the phorids were quite abundant in the graves, a single specimen of the large burying beetle Nicrophorus investigator was seen. Of greatest interest was the finding that Calliphora vomitoria was ovipositing on the corpses that were 105 days old. Calliphora vomitoria was still being attracted to exposed portions of the corpse at the end of May 1996 when the study was terminated. This fly species is recognised as one of the earliest to find freshly dead corpses. Normally the corpse becomes unattractive to calliphorids as it ages and begins to decay. Thus this species is one of several that are typically used in the determination of the PMI. In this study the preservative qualities of the soil and climatic conditions have evidently retained those features that attract Calliphora until the corpse was exposed by scavengers. Adult flies visited the exposed tissues and not only fed, but also laid eggs. Subsequent dating, based on entomological techniques, could give no clue as to the original time of death in this situation, but only the time when the corpse was exposed to fly activity by the action of large scavengers. The combination of cold weather and specific soil conditions effectively inhibited the decomposition process and attendant odour production so that the corpses were undetectable by canids (foxes/ dogs) for a very considerable period of time despite the fact that there was only 10 cm of soil covering the pig corpses.

9 B. Turner, P. Wiltshire / Forensic Science International 101 (1999) Table 2 The time sequence of scavenger activity at the pig graves, Essendon Wood Date Scavenger activity Start-day 0 None 23/12/96 Day 32 None 24/1/97 Day 63 None 27/2/97 Day 77 None 14/3/97 Day 91 Graves 1 and 2 untouched. Grave 3 had small hole dug down to corpse at site of 28/ 3/ 97 previous exploratory hole. No damage to skin of corpse. Pad/ claw marks suggest large dog. Day 105 All three graves showed varying degrees of excavation (by foxes/ badgers). 15/4/97 Holes approx. 50 cm in diameter dug down to corpses. Muscle tissue eaten from neck/upper thorax/shoulder regions of pig 1 and 3. Bone chewed and moved out of graves. The abdomen of Pig 2 had been opened and intestine pulled out and eaten. Abdominal muscle eaten. Day 120 Slight excavation to pig 1, appeared to be abandoned. Pig 2 dug out in same 30/4/97 upper body region and back filled by police. Pig 3 with small area of skin/ tissue exposed mahogany red in colour. Day 135 Pig 1 no change, Pig 3 no disturbance, Pig 2 extensively exposed and eaten. 15/5/97 Day 150 Most of Pig 1 consumed grave slumped. Pig 2 consumed Hind part of Pig 3 still 30/ 5/ 97 intact. Exposed tissues moist and red indicating current and actively targetting by scavengers. 5. Conclusion This study has shown the value of using buried pigs as models to gain a clearer picture of the progress of decay in this specific situation. It has illustrated the importance of knowing the impact of the soil characteristics on the decay process and how they may confound entomological and other methods of dating the PMI in cases where the victim is buried. It has also indicated hitherto unrecorded aspects of calliphorid behaviour in relation to decomposing flesh as well as the importance of ambient climatic conditions with regard to insect activity and decomposition rates. Acknowledgements We would like to thank the Hertfordshire Constabulary for their support, encouragement and practical help; Peter Murphy (Univ of East Anglia) and Helen O Hare for help with field work; and Martin Hall for commenting on the manuscript.

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