Introduction

The ‘time since death’, or post-mortem interval, is the time that has elapsed between the death of an animal and the discovery of the body. Establishment of this interval aids investigation of the circumstances surrounding the death by narrowing the ‘window of time’ during which the death may have occurred. In so doing, specific people or events may be excluded, or included, in the investigations that follow.

In human forensic medicine, study of the post-mortem interval has been one of the most popular topics of research.¹ However, bedevilled as it is by the sheer variety of species involved, estimation of the post-mortem interval is much less developed in veterinary medicine. Indeed, there is a marked paucity of published information. Some attempts have been made with certain species of North American deer and other wildlife.^2–7 A more recent study that involved recording of post-mortem rectal temperatures and macroscopic and microscopic changes in a limited number of Beagles provides a practical approach to the development of a standardised method for the estimation of the time since death in dogs.⁸ However, this study was restricted to specific environmental conditions (10.9–16.8°C) and dogs weighing between 8.5 and 15 kg.

The veterinarian may become involved in widely differing incidents, including out-of-season shooting of game animals, poaching, death of livestock during transport, and cases of neglect or deliberate injury of companion animals. Additionally, estimation of the time since death may also be important when two or more animals are found at a single location. In such cases the veterinarian needs to determine, if possible, whether the deaths occurred as a result of a single event or as part of an ongoing, lengthier process. Determination of the time of death may have relevance to cases where the alleged offences are related to statutory time limits, e.g. checking of traps within defined periods, or disposal of carcasses.

The focus of an inquiry or intended prosecution will be influenced by the estimate of the post-mortem interval. This places a considerable responsibility on the veterinarian. If the estimate is inaccurate to a major degree, then time will be wasted on unproductive investigations and suspects may be charged unjustly.

In most cases it is impossible to establish the precise time of death. Consequently, the estimated time is best presented as a range based on minimum and maximum times.

The current state of knowledge should allow the experienced pathologist, in the normal course of events, to estimate the post-mortem period in blocks of time, such as less than 24 hours, 1–3 days, 3–7 days, 7–21 days, weeks, months or years. However, it is important that all parties are aware of the limited information available and that the Courts are not inadvertently persuaded that ‘time since death’ estimates are based on a firm and substantial data base.

Temperature-based methods

Countless crime novels and television murder dramas have reinforced the popular misconception that a single simple measurement of rectal temperature provides a reliable estimate of the post-mortem interval. In reality, cooling of the body is affected by many factors including insulation, ambient temperature, movement of air and immersion in water. Consequently, much investigation has been devoted to the effects of these variables on the cooling curve of the human cadaver. The results of this research have been incorporated into nomograms that, when used in conjunction with accurate measurement of core body temperature, provide reasonably tight maximum and minimum time limits.⁹

Similar nomograms have yet to be developed for domestic and free-living animals. As mentioned in the introduction to this chapter, some effort has been expended in attempting to create guidelines for cooling rates in some species of North American deer. Some of these methods are solely temperature based, whereas others combine temperature measurements and other changes such as pupil diameter or aqueous humour biochemistry.

Because many deer are field-dressed (i.e. the abdominal cavity is opened and the stomach and intestines are removed) shortly after death, rectal temperatures are inappropriate. In these cases, temperature measurement is made in the muscles of the thigh and intranasally. The rectum and cloaca are the usual sites for temperature recording in smaller mammals and birds.

Despite data being available on body surface area and body mass for different species, considerable care needs to be exercised during interpretation of post-mortem temperatures in wildlife. This caution is based on the inherent uncertainties that are commonplace in wildlife investigations. For example, there will be a lack of reliable information on ante-mortem body temperature, which may have been raised by physical exertion before death. Similarly, the actual influence of post-mortem handling procedures, including skinning, transportation on the roof of a vehicle or transportation within a closed vehicle with animals piled one on another, is as yet incompletely investigated.

On the domestic animal front, the cooling rates of carcasses of farm livestock slaughtered in abattoirs are well researched. However, there are no published accounts of studies on estimation of the post-mortem interval of similar animals found dead, but intact, in a farmyard or field.

The study by Erlandsson & Munro⁸ on beagles showed that over the first 10 hours after death rectal temperature could be helpful in establishing the approximate time of death within intervals, or time bands, of 2 hours. Further, under the conditions of the study (dogs lying undisturbed in cool dry air), body temperature declined to ambient temperature by 24–48 hours after death. Additional data are required on the effects on the cooling curve of higher ambient temperatures, different body weights, moving air, coverings and immersion in water.

Rigor mortis

Fully developed rigor mortis is an easily identifiable and reliable indicator that death has occurred. The time of onset is variable but it is usually considered to appear between 1 and 6 hours (average 2–4 hours) after death. Depending on the circumstances, rigor mortis may last for a few hours to several days.

The muscles of the face and neck are often the first to be affected and the rigidity spreads backwards over the trunk and limbs. Relaxation of the muscles occurs in roughly the same order. Contraction of the heart is an early and forceful change. In the healthy, non-hypoxic animal the left ventricle expels virtually all its contents of blood during this process, whilst contracture of the right ventricle is less intense, leaving a small quantity of clotted blood in this chamber.

The development and resolution of rigor mortis is complex, the rate of onset being greatly influenced by the glycogen content of muscle, the pH of muscle and the temperature. Adenosine triphosphate (ATP) is a necessary component in the relaxation of the myosin filaments of normal muscle. Rigor mortis commences when the rate of re-synthesis of ATP is less than its degradation. In the early hours after clinical death, muscle glycogen fuels the cycle of hydrolysis and re-synthesis of ATP. Consequently, ante-mortem events that reduce glycogen stores (e.g. hunting) can result in the rapid onset of rigor mortis. In contrast, 9–12 hours may elapse after slaughter of well fed, well rested cattle before the onset of rigor mortis.

Body temperature also affects the rapidity of onset. The most dramatic example is the onset of rigidity within minutes in cases of malignant hyperthermia.^10,11 Following fatal hyperthermia of dogs in vehicles, less than 1 hour may pass before rigor mortis is recognisable and it may spread rapidly throughout the muscle groups. Raised body temperature as a consequence of severe exercise may also significantly shorten the time before rigor mortis develops.

However, the effects of increased body temperature are not always so clearcut and the presence of systemic disease must be considered. Whilst pyrexia caused by acute fatal infectious disease may result in rapid onset of rigor mortis, fevered animals with high muscle pH may show delayed rigor mortis or may not develop rigidity at any stage.

In general, high environmental temperature will accelerate the onset, whereas low ambient temperatures have the opposite effect. The duration of rigidity is extended in dry cold conditions. Although the details of the relaxation process are not clear, it is generally accepted that the dissolution of rigidity is associated with early decomposition or denaturisation of muscle.

For the veterinarian, a further complication is the, as yet, largely undocumented variation in the times of onset and disappearance of rigidity that may exist among species and ages of the broad spectrum of animals presented for examination. As a result of these variables, it is prudent to consider rigor mortis as providing only a rough guide to the post-mortem interval.