Too fresh is unattractive! The attraction of newly emerged Nicrophorus vespilloides females to odour bouquets of large cadavers at various stages of decomposition

The smell of death. State-of-the-art and future research directions

The decomposition of a body is inseparably associated with the release of several types of odors. This phenomenon has been used in the training of sniffer dogs for decades. The odor profile associated with decomposition consists of a range of volatile organic compounds (VOCs), chemical composition of which varies over time, temperature, environmental conditions, and the type of microorganisms, and insects colonizing the carcass. Mercaptans are responsible for the bad smell associated with corpses; however, there are no unified recommendations for conducting forensic analysis based on the detectable odor of revealed corpses and previous research on VOCs shows differing results. The aim of this review is to systematize the current knowledge on the type of volatile organic compounds related to the decomposition process, depending on a few variables. This knowledge will improve the methods of VOCs detection and analysis to be used in modern forensic diagnostics and improve the methods of training dogs for forensic applications.

Spatiotemporal variation in the competitive environment, with implications for how climate change may affect a species with parental care

Burying beetles of the genus Nicrophorus have become a model for studying the evolution of complex parental care in laboratory studies. Nicrophorus species depend on small vertebrate carcasses to breed, which they process and provision to their begging offspring. However, vertebrate carcasses are highly sought after by a wide variety of species and so competition is expected to be critical to the evolution of parental care. Despite this, the competitive environment for Nicrophorus is rarely characterized in the wild and remains a missing factor in laboratory studies. Here, we performed a systematic sampling of Nicrophorus orbicollis living near the southern extent of their range at Whitehall Forest in Clarke County, Georgia, USA. We determined the density of N. orbicollis and other necrophilous species that may affect the availability of this breeding resource through interference or exploitation competition. In addition, we characterize body size, a key trait involved in competitive ability, for all Nicrophorus species at Whitehall Forest throughout the season. Finally, we compare our findings to other published natural history data for Nicrophorines. We document a significantly longer active season than was observed 20 years previously at Whitehall Forest for both N. orbicollis and Nicrophorus tomentosus, potentially due to climate change. As expected, the adult body size of N. orbicollis was larger than N. tomentosus, the only other Nicrophorus species that was captured in 2022 at Whitehall Forest. The other most prevalent insects captured included species in the families Staphylinidae, Histeridae, Scarabaeidae, and Elateridae, which may act as competitors or predators of Nicrophorus young. Together, our results indicate significant variation in intra- and interspecific competition relative to populations within the N. orbicollis range. These findings suggest that the competitive environment shows extensive spatiotemporal variation, providing the basis to make predictions for how ecology may influence parenting in this species.

Succession of Dung-Inhabiting Beetles and Flies Reflects the Succession of Dung-Emitted Volatile Compounds

Chemical cues, such as volatile organic compounds (VOCs), are often essential for insects to locate food. Relative to the volume of studies on the role of VOCs in insect-plant relationships, the role of VOCs emitted by dung and carrion in mediating the behavior of insect decomposers is understudied. Such relationships may provide a mechanistic understanding of the temporal axis of community assembly processes in decomposing insect communities. We focused on the temporal succession of volatiles released by cow dung pats and the potential influence on dung-inhabiting insects. Using gas chromatography/mass spectrometry we identified and quantified VOCs released from dung 1-h, and 1, 2 3, 5, and 7 d-old. We then related changes in VOCs to successional patterns of dung-inhabiting beetles and flies. We detected 54 VOCs which could be assigned to two successional groups, with chemical turnover in dung changing around day 2. The early successional group consisted primarily of aliphatic alcohols and phenols, and the late one of aliphatic esters, nitrogen- and sulfur-bearing compounds. Flies were predominately associated with the early successional group, mainly with 1-butanol. Beetles were associated predominately with the late-successional group, mainly with dimethyl trisulfide. This association between insect and chemical successional patterns supports the idea that habitat filtering drives the community assembly of dung-inhabiting insects on an aging resource. Moreover, the affinity of both insect groups to specific VOC groups provides a mechanistic explanation for the predictability of successional patterns found in dung-inhabiting insect communities.

Olfactory Choice for Decomposition Stage in the Burying Beetle Nicrophorus vespilloides: Preference or Aversion?

Simple Summary

In the burying beetle Nicrophorus vespilloides, mating pairs raise their offspring together on a small carrion resource. We tested whether carcass age affected brood quantity and quality and found that pairs had significantly more offspring on fresher carcasses. To determine whether this reproductive benefit translates to an olfactory preference or aversion for carcass age, we conducted a series of olfactory trials testing adult mated female preferences for carcasses differing in age. Mated females spent more time associating with fresh carcass odors relative to those of an aged one, but also spent more time in empty chambers than those with an aged carcass, suggesting that mated females have a general aversion to those odors. Lastly, we characterized the odor profiles of fresh and aged carcasses to determine which compounds might be driving this olfactory aversion in mated female burying beetles.

Abstract

Sensory cues predicting resource quality are drivers of key animal behaviors such as preference or aversion. Despite the abundance of behavioral choice studies across the animal kingdom, relatively few studies have tested whether these decisions are driven by preference for one choice or aversion to another. In the burying beetle Nicrophorus vespilloides, adult pairs exhibit parental care to raise their offspring on a small carrion resource. We tested whether carrion decomposition stage affected brood quantity and quality and found that mating pairs had significantly more offspring on fresher carcasses. To determine whether this observed reproductive benefit correlates with maternal preference behavior, we conducted a series of olfactory trials testing mated female preferences for mouse carcasses of differing decomposition stages. When given the option between fresh and older carcasses, females associated significantly more with fresher, 1-day old carcasses. However, this behavior may be driven by aversion, as females that were given a choice between the 7-day old carcass and a blank control spent significantly more time in the control chamber. We characterized volatile organic compound profiles of both carcass types, highlighting unique compounds that may serve as public information (sensu lato) conveying resource quality information to gravid beetles.

The Attraction of the Dung Beetle Anoplotrupes stercorosus (Coleoptera: Geotrupidae) to Volatiles from Vertebrate Cadavers

During decomposition, vertebrate carrion emits volatile organic compounds to which insects and other scavengers are attracted. We have previously found that the dung beetle, Anoplotrupes stercorosus, is the most common dung beetle found on vertebrate cadavers. Our aim in this study was to identify volatile key compounds emitted from carrion and used by A. stercorosus to locate this nutritive resource. By collecting cadaveric volatiles and performing electroantennographic detection, we tested which compounds A. stercorosus perceived in the post-bloating decomposition stage. Receptors in the antennae of A. stercorosus responded to 24 volatiles in odor bouquets from post-bloating decay. Subsequently, we produced a synthetic cadaver odor bouquet consisting of six compounds (benzaldehyde, DMTS, 3-octanone, 6-methyl-5-hepten-2-ol, nonanal, dodecane) perceived by the beetles and used various blends to attract A. stercorosus in German forests. In field assays, these beetles were attracted to a blend of DMTS, 3-octanone, and benzaldehyde. Generalist feeding behavior might lead to the super-dominant occurrence of A. stercorosus in temperate European forests and have a potentially large impact on the exploitation and rapid turnover of temporally limited resources such as vertebrate cadavers.

Forest habitat parameters influence abundance and diversity of cadaver-visiting dung beetles in Central Europe

Dung beetles provide crucial ecosystem services and serve as model organisms for various behavioural, ecological and evolutionary studies. However, dung beetles have received little attention as consumers of large cadavers. In this study, we trapped copronecrophagous dung beetles on above-ground exposed piglet cadavers in 61 forest plots distributed over three geographically distinct regions in Germany, Central Europe. We examined the effects of land use intensity, forest stand, soil characteristics, vascular plant diversity and climatic conditions on dung beetle abundance, species richness and diversity. In all three regions, dung beetles, represented mainly by the geotrupid species Anoplotrupes stercorosus and Trypocopris vernalis, were attracted to the cadavers. High beetle abundance was associated with higher mean ambient temperature. Furthermore, A. stercorosus and T. vernalis were more abundant in areas where soil contained higher proportions of fine sand. Additionally, an increased proportion of forest understorey vegetation and vascular plant diversity positively affected the species richness and diversity of dung beetles. Thus, even in warm dry monocultured forest stands exploited for timber, we found thriving dung beetle populations when a diverse understorey was present. Therefore, forestry practices that preserve the understorey can sustain stable dung beetle populations and ensure their important contribution to nutrient cycles.

Identification of decomposition volatile organic compounds from surface-deposited and submerged porcine remains

Cadaver dogs are routinely used internationally by police and civilian search organisations to locate human remains on land and in water, yet little is currently known about the volatile organic compounds (VOCs) that are released by a cadaver underwater; how this compares to those given off by a cadaver deposited on land; and ultimately, how this affects the detection of drowned victims by dogs. The aim of this study was to identify the VOCs released by whole porcine (Sus scrofa domesticus) cadavers deposited on the surface and submerged in water using solid phase microextraction gas chromatography mass spectrometry (SPME GC-MS) to ascertain if there are notable differences in decomposition odour depending on the deposition location. For the first time in the UK, the volatile organic compounds (VOCs) from the headspace of decomposing porcine cadavers deposited in both terrestrial and water environments have been detected and identified using SPME-GCMS, including thirteen new VOCs not previously detected from porcine cadavers. Distinct differences were found between the VOCs emitted by porcine cadavers in terrestrial and water environments. In total, seventy-four VOCs were identified from a variety of different chemical classes; carboxylic acids, alcohols, aromatics, aldehydes, ketones, hydrocarbons, esters, ethers, nitrogen compounds and sulphur compounds. Only forty-one VOCs were detected in the headspace of the submerged pigs with seventy detected in the headspace of the surface-deposited pigs. These deposition-dependent differences have important implications for the training of cadaver dogs in the UK. If dog training does not account for these depositional differences, there is potential for human remains to be missed. Whilst the specific odours that elicit a trained response from cadaver dogs remain unknown, this research means that recommendations can be made for the training of cadaver dogs to incorporate different depositions, to account for odour differences and mitigate the possibility of missed human remains operationally.

Mass mortality events and the role of necrophagous invertebrates

Scale is important in understanding and applying concepts in ecology. Historically, the mechanisms regulating necrophagous arthropod community structure have been well explored on a single vertebrate carcass. However, practically nothing is known of whether such findings can be extrapolated to cases where large numbers of carcasses have been introduced into an ecosystem at a single time point. With the increasing incidences of mass mortality events (MMEs), understanding how scale effects community assembly of necrophagous insects and the resulting bottom-up or top-down effects on the impacted ecosystem are of utmost importance. Unfortunately, MMEs are unpredictable, making their study nearly impossible within a robust experimental framework. The objectives of this paper are to provide a brief overview of what is known with regards to ecological responses to carrion, opine on the ramifications of MMEs on local communities, and provide a brief overview of knowledge gaps, avenues for future research, and a potential study systems for rigorous MME experiments.

Causes of Rapid Carrion Beetle (Coleoptera: Silphidae) Death in Flooded Pitfall Traps, Response to Soil Flooding, Immersion Tolerance, and Swimming Behavior

Terrestrial insects in water can often delay or escape drowning by floating and swimming. However, we observed that flooding of pitfall traps baited with rotting carrion results in high overnight mortality of captured beetles and reasoned that this risk may be enhanced by microbial respiration. By assessing carrion beetle (Coleoptera: Silphidae) response to flooding, tolerance to immersion, and swimming behavior, we offer insights to this cause of death and beetle behavioral physiology. Response of buried Nicrophorus orbicollis Say to soil flooding resulted in beetles moving to the soil surface. The lethal time to 50% mortality (LT50 (immersion); mean ± 95% CI) for Nicrophorus investigator Zetterstedt, Nicrophorus marginatus F., Necrodes surinamensis F., and Thanatophilus lapponicus Herbst was 14.8 ± 2.3, 9.0 ± 3.3, 3.2 ± 1.1, and 12.1 ± 2.5 h, respectively. Swimming behavior and survival time of N. investigator was tested using yeast:sucrose (Y:S) solutions to create a eutrophic, severely hypoxic aqueous environment. LT50 (swimming) for N. investigator was 7.5 ± 1.4, 6.0 ± 1.7, and 4.2 ± 1.2 h for the low, medium, and high Y:S solutions, respectively, and >24.0 h in control treatments. Nicrophorus investigator survived nearly twice as long when completely immersed in deoxygenated water, as might occur in flooded burrows, than when swimming on the surface. We document for the first time, the rapid induction of hypoxic coma and death for a terrestrial insect from enhanced microbial activity and CO2 production of an aqueous environment, as well as suggestions on trapping protocols related to the federally endangered Nicrophorus americanus Olivier.

Volatile Organic Compounds of Decaying Piglet Cadavers Perceived by Nicrophorus vespilloides

In the necrophagous burying beetle Nicrophorus vespilloides (Coleoptera: Silphidae), cadaver preference appears to depend on cadaver size and on the maturity of the beetle. We previously showed that newly emerged females with immature ovaries prefer later stages of decomposition of large cadavers. Our present aim is the determination of specific chemical compounds involved in the discrimination of cadaveric odor bouquets and the recognition of specific stages of decomposition. We used headspace samples of maggot-infested piglet cadavers at various decomposition stages and performed gas chromatography coupled with electroantennography (GC-EAD) to record 45 EAD-active compounds. Using GC coupled with mass spectrometry, we identified 13 of the EAD-active compounds. The headspace of the fresh decomposition stage was characterized mainly by high relative amounts of trimethylpyrazine. High relative amounts of dimethyl trisulfide were characteristic of bloated, post-bloating, and advanced decay stages. The advanced decay and dry remains stages were dominated by high relative amounts of phenol. Statistically, this compound had the highest impact on discrimination between the fresh decomposition stage, which is important for mature burying beetles for reproduction, and the advanced decay stage, which is nutritionally more attractive for newly emerged beetles. Phenol might, therefore, function as a key substance for newly emerged female burying beetles, so that they can locate suitable cadavers for feeding to maturation.

The colonisation of remains by the muscid flies Muscina stabulans (Fallén) and Muscina prolapsa (Harris) (Diptera: Muscidae)

In the field, the muscid flies Muscina stabulans (Fallén) and Muscina prolapsa (Harris) only colonised buried baits in June, July and August. The two-species co-occurred on baits buried at 5cm but only M. prolapsa colonised baits buried at 10cm. Other species of insect were seldom recovered from buried baits regardless of the presence or absence of Muscina larvae. In the laboratory, both M. stabulans and M. prolapsa preferentially colonised liver baits on the soil surface compared to those buried at 5cm. Baits buried in dry soil were not colonised by either species whilst waterlogged soil severely reduced colonisation but did not prevent it entirely. Dry liver presented on the soil surface was colonised and supported growth to adulthood but if there was no surrounding medium in which the larvae could burrow then they died within 24h. M. stabulans showed a consistent preference for ovipositing on decaying liver rather than fresh liver, even when it had decayed for 41 days. The results for M. prolapsa were more variable but it was also capable of developing on both fresh and very decayed remains. Blood-soaked soil and dead slugs and snails stimulated egg-laying by both species and supported larval growth to adulthood. Mushrooms, melon, and bananas also stimulated egg-laying although to a much lesser extent and very few larvae survived to adulthood. Horse faeces stimulated extensive egg-laying but the larvae invariably died during the first or second instar and none survived to pupariation. This information could be useful when determining the forensic significance of Muscina larvae recovered from dead bodies.

Factors affecting quality of temperature models for the pre-appearance interval of forensically useful insects

In the case of many forensically important insects an interval preceding appearance of an insect stage on a corpse (called the pre-appearance interval or PAI) is strongly temperature-dependent. Accordingly, it was proposed to estimate PAI from temperature by using temperature models for PAI of particular insect species and temperature data specific for a given case. The quality of temperature models for PAI depends on the protocols for PAI field studies. In this article we analyze effects of sampling frequency and techniques, temperature data, as well as the size of a sample on the quality of PAI models. Models were created by using data from a largely replicated PAI field study, and their performance in estimation was tested with external body of PAI data. It was found that low frequency of insect sampling distinctly deteriorated temperature models for PAI. The effect of sampling techniques was clearly smaller. Temperature data from local weather station gave models of poor quality, however their retrospective correction clearly improved the models. Most importantly, current results demonstrate that sample size in PAI field studies may be substantially reduced, with no model deterioration. Samples consisting of 11-14 carcasses gave models of high quality, as long as the whole range of relevant temperatures was studied. Moreover, it was found that carcasses exposed in forests and carcasses exposed in early spring are particularly important, as they ensure that PAI data is collected at low temperatures. A preliminary best practice model for PAI field studies is given.

Decomposition odour profiling in the air and soil surrounding vertebrate carrion

Chemical profiling of decomposition odour is conducted in the environmental sciences to detect malodourous target sources in air, water or soil. More recently decomposition odour profiling has been employed in the forensic sciences to generate a profile of the volatile organic compounds (VOCs) produced by decomposed remains. The chemical profile of decomposition odour is still being debated with variations in the VOC profile attributed to the sample collection technique, method of chemical analysis, and environment in which decomposition occurred. To date, little consideration has been given to the partitioning of odour between different matrices and the impact this has on developing an accurate VOC profile. The purpose of this research was to investigate the decomposition odour profile surrounding vertebrate carrion to determine how VOCs partition between soil and air. Four pig carcasses (Sus scrofa domesticus L.) were placed on a soil surface to decompose naturally and their odour profile monitored over a period of two months. Corresponding control sites were also monitored to determine the VOC profile of the surrounding environment. Samples were collected from the soil below and the air (headspace) above the decomposed remains using sorbent tubes and analysed using gas chromatography-mass spectrometry. A total of 249 compounds were identified but only 58 compounds were common to both air and soil samples. This study has demonstrated that soil and air samples produce distinct subsets of VOCs that contribute to the overall decomposition odour. Sample collection from only one matrix will reduce the likelihood of detecting the complete spectrum of VOCs, which further confounds the issue of determining a complete and accurate decomposition odour profile. Confirmation of this profile will enhance the performance of cadaver-detection dogs that are tasked with detecting decomposition odour in both soil and air to locate victim remains.

Temperature-dependent appearance of forensically useful flies on carcasses

Flies are frequently used for postmortem interval (PMI) estimations. These estimates are usually based on the age of larval or pupal specimens. However, the age defines only the minimum PMI. In order to move forensic entomology further, a method useful for the estimation of an interval preceding insect appearance on a corpse called the pre-appearance interval (PAI) is needed. Recently, it was demonstrated that the PAI of several carrion beetles is closely related to the temperature prevailing throughout this interval. Hence, it was postulated to estimate PAI from temperature. In order to check premises for using this approach with flies, a test of the relationship between adult or oviposition PAI and temperature was made for nine species of European flies. Data on PAI originated from pig carcasses decomposing under various temperatures. Adult PAI of Hydrotaea dentipes, Hydrotaea ignava, Hydrotaea similis, Phormia regina, and Stearibia nigriceps and oviposition PAI of S. nigriceps were exponentially related to temperature. Only S. nigriceps revealed a close relationship, demonstrating solid premises for PAI estimation from temperature alone. Adult and oviposition PAI of Calliphora vomitoria and adult PAI of Hydrotaea pilipes were not related to temperature. Adult and oviposition PAI of Lucilia sericata and Lucilia caesar responded similarly, with an abrupt and large increase in a narrow range of low temperatures and no response in a broad range of high temperatures. Probably, different mechanisms form the basis for the response of PAI to temperature in flies colonizing carcasses shortly after death and flies colonizing carcasses later in the decomposition process.