Found dead in the snow – how microbes can help determine the time of death in forensic investigations under icy conditions

What happens to a body in an extremely cold environment? Does it decompose? How do these conditions affect how forensic scientists determine the person's time of death?

Estimating the time of death, also known as the postmortem period, is a complex task. It plays an important role in forensic investigations because it can provide important insights into the timeline of events leading up to a person's death. This information can narrow down potential scenarios and suspects, helping to solve crimes.

A variety of factors come into play at the scene of a death, from environmental conditions to the state of health of the person before death. In the past, scientists have estimated the time of death by observing postmortem physical and biological changes in the body, such as stiffening, fluid retention, and cooling.

However, these methods are limited by their variability and dependence on external factors. Calculating the postmortem interval became more precise with the advent of molecular biology. But it is still a challenging task, especially in extremely cold weather conditions. In the first few months after death, a frozen body often lacks obvious signs of decomposition.

We are forensic scientists who lead the forensic science programs at the University of North Dakota and the University of Central Lancashire. We use molecular biology and bioinformatics to develop tools that help researchers and investigators more accurately estimate the time after death. Our recent research published in Frontiers in Microbiology found that by studying the microbes involved in decomposition, we can predict the time elapsed since death under extreme cold conditions with high accuracy.

Decomposition in cold environments

Our study took place in Grand Forks, North Dakota, one of the coldest cities in the USA. In winter, temperatures there can drop to -40 degrees Celsius and strong winds with speeds of up to 50 kilometers per hour can occur.

In an extremely cold environment such as North Dakota winters, traditional methods may not be sufficient to understand decomposition and estimate time of death. For example, the body cools much more quickly in cold temperatures, which can distort estimates based on body temperature.

Likewise, cold environments can delay the onset and duration of rigor mortis, or stiffening of the body. The decomposition process, including the activity of insects and other scavengers that contribute to the decay of the body, can also be slowed or stopped by freezing temperatures.

Snow is another important factor in the study of decomposition. It can insulate a body by trapping residual heat and raising its temperature slightly above the ambient temperature. This insulating effect causes the body to decompose more slowly than bodies exposed to the open air.

Microbes and the time since death

In extreme cold, it is necessary to use additional tools to understand the decomposition process and estimate the time of death. Advanced molecular techniques such as analysis of the microbiome, gene expression and protein degradation can help provide valuable information about the crime scene.

Each organism has different microbial characteristics that act like a fingerprint. The necrobiome, a community of microbes associated with decaying remains, plays a crucial role in decomposition. Certain microbes are present at different stages of decomposition and help break down tissue and recycle nutrients. Forensic investigators can use the composition of the microbial population to sample which microbes live in a dead body to deduce how long ago a person died.

Our study focused on identifying common patterns in the microbial changes that occur during decomposition in extremely cold environments. Over a period of 23 weeks, we collected and analyzed 393 microbial samples from the inside and outside of the noses of dead pigs covered in snow. Pigs decompose in a similar way to humans and are commonly used in forensic research. We developed models to estimate the postmortem interval by combining microbial genetic data with environmental data such as snow depth and outside temperature.

Overall, we found that the bacterial species Psychrobacter, Pseudomonas And Carnobacterium can best predict the time after death under extreme winter conditions, namely up to six months after death, with an error margin of just over nine days.

We found that different types of bacteria are most prevalent at different times. For example, the concentrations of Psychrobacter increase five weeks after death and are most common after 10 weeks, while Pseudomonas increase between the fifth and ninth week and reach their peak after 18 weeks.

Improving forensics

Death is often an uncomfortable topic to talk about. However, from a forensic perspective, techniques and methods to determine a person's death can help bring justice and peace to loved ones.

Our study found that decomposition does not stop completely even in cold environments. Studying the microenvironment – the local conditions around the body, including temperature, humidity and microbial activity – can provide important information about the decomposition process. The key microbial species we identified served as biomarkers of death, allowing us to develop time-of-death models that help researchers overcome the limitations of simply visually examining the remains.

Microbes can become a crucial piece of the puzzle in solving a death, helping to establish more precise timelines even under extreme conditions.