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Evidence, generally speaking, is defined as anything that can prove or disprove a fact in contention [9]. Such facts may be anticipated based on the applicable laws as well as prior experiences. More specifically, evidence is used to prove guilt or innocence, identify victims, and identify suspects. The body presented for a forensic necropsy is evidence, as is anything removed from the body. Additionally, evidence may be present on the scene(s) as well as on the perpetrator(s). The evidence linkage diagram is used to illustrate how each piece of evidence is a means of linking the scene, physical evidence, the victim, and the suspect (Fig. 2.1) [9, 10].

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It should be understood that the desire to collect an item does not equate to permission to collect the item. For an item to be considered legally obtainable as a piece of evidence, it must be included in the search warrant obtained by law enforcement. This is often another area in which forensic veterinarians can be of assistance to the lead agency. For example, there have been many cases in which bodies excavated from the ground were determined to be inadmissible in court because the search warrant did not include areas “below ground.” In these situations, any evidence gleaned from the body would also be considered inadmissible as the result of a legal doctrine called “fruit of the poisonous tree.” This doctrine holds that evidence gathered from illegally obtained information will also be excluded from trial [3].

In addition to legal collection, items must also fall within other criteria in order to be accepted as evidence into a court of law. In 1975, the US Congress created the Federal Rules of Evidence. The use of these rules is only mandatory for federal courts; however, many states have also adopted them in an effort to establish consistency with the federal courts. There are two basic criteria the forensic veterinarian and pathologist should be aware of: first, an item must be relevant to the case (Rule 402). This is generally covered by the search warrant as a judge will not typically sign a warrant including irrelevant items. Second, an item must meet conditions of authentication (Rule 901) to be accepted as evidence in a court of law. This simply means that some form of proof must be given to show that the item in question is in fact what it is claimed to be. This is typically obtained through testimony of a witness with knowledge. Therefore anyone who comes into contact with an item of evidence, whether it is a body submitted for a forensic necropsy or a fecal sample collected from said body, may be called to testify regarding their knowledge of and interaction with the evidence. A chain of custody (discussed later in more detail) is the documentation that must be maintained to record all who have come into contact with the item [3].

All evidence, from the crime scene or necropsy, can be divided into two basic categories: testimonial and physical. Testimonial evidence is based on the statements made by witnesses and suspects. Because it is provided by humans, who are open to error, testimonial evidence cannot always be trusted. Stories can be altered through intentional deceit or forgetfulness, and people often interpret the same information differently. Testimonial evidence should not be ignored, but it should be supported by tangible facts rather than left to stand on its own [9]. It is not uncommon for information collected from a postmortem examination to be used in proving or disproving testimonial evidence, rather than the testimonial evidence being used in proving or disproving other facts.

Physical evidence encompasses any object, or observation of an object, that can be used to establish whether or not a crime has been committed or can provide or disprove a link between a crime and its victim or a crime and a suspect. Physical evidence can be found in many forms, some of which are tangible and capable of being collected and others which are not collectible, but rather need to be documented photographically. For example, if paint flecks are found adhering to fur near a patterned contusion on the exposed skin, then both the fur and contusion are of evidentiary value. It is possible to physically collect the paint flecks and a sample of the contused tissue, but the pattern of the contusion can only be “collected” through documentation.

There are two characteristics of physical evidence that should be understood by the forensic veterinarian and pathologist: transience and transference. When present, these characteristics can be very important in the analysis of physical evidence. Transient evidence includes elements of the scene or findings on the body that are temporary and can change. The wetness of a bloodstain, odors, condensation on a glass, and temperature are all examples of transient scene evidence. Rigor mortis, algor mortis, and a wet hair coat are likewise all examples of transient evidence on the body. Transient evidence will offer situational information particular to the moment in time at which the scene or body is first encountered, but such evidence will change with time. Transient evidence, because of this characteristic, must be documented expeditiously. Documentation should, ideally, occur as soon as possible following arrival on scene or after the acceptance of the body into your custody. Failure to promptly document transient evidence may result in its loss. This type of evidence will likely never solve a crime; however, it may help to clarify details. For example, consider the case in which an individual calls law enforcement because he found his dog deceased in the house with its throat incised. The owner states he was only gone from the home for 30 min and found the dog upon his return, but the dog is in full rigor mortis. Documenting the stage of rigor mortis as well as the ambient temperature of the house is very important. Waiting to document the stage of rigor mortis until the body is ready for necropsy could result in the rigor passing and a misinterpretation of the timeline. This information is important not necessarily to prove the owner guilty, but to indicate that the evidence does not match his story.

Transfer evidence is produced when physical contact between two objects results in the exchange of matter. This concept is based on the Locard exchange principle which, when stated simply, equates to “every contact leaves a trace.” [9] Transfer evidence is typically thought of as being small in size (e.g., hair or fibers) or “trace evidence” which is discussed later. However, it is entirely possible sometimes that transfer evidence is large enough to be clearly seen with the naked eye. If a suspect suffered a dog bite wound and then grabbed the dog’s collar before killing it, any blood deposited on the collar would be considered transfer evidence.

Evidence can also be labeled in terms of how reliable it is in proof of fact. The terms circumstantial evidence and direct evidence are commonly used in the court of law. However, whether an item of evidence is considered circumstantial or direct should be of no concern to the person collecting it, and in fact, this categorization will most likely be unknown at the time of collection. Circumstantial evidence (also known as indirect evidence) is that which requires inference to prove a fact [11]. In contrast, direct evidence “clearly and directly relates the proof of an existence of a fact” without any inference or presumption required [12]. If a suspect has had no known contact with an animal, yet fibers from his carpet are found on the animal, this could be considered direct evidence. In contrast, consider the case in which the owner of an animal is suspected of its injuries and the owner’s hair is found on the animal. This would be considered circumstantial evidence because the fact that the owner’s hair is on the animal could be a result of having caused the injury or by normal contact with the animal. For both situations, at the time of collection, the forensic veterinarian or pathologist would not know from whom the questioned hair originated. Therefore, it would be necessary to collect the hair, regardless of its later usability by the court.

Despite all of the information that can be gleaned from evidence, the best characteristic of physical evidence is that it cannot lie [9]. It may be misinterpreted or the analyst may be dishonest about test results, but the physical evidence will always tell the truth. Physical evidence can be large or minute and may require processing at the scene or packaging for processing later at a laboratory. Depending on the type of physical evidence, it may not always be necessary to collect an item. In some situations documentation with photographs and sketches may be sufficient [3, 9].

Most items of evidence will have characteristics that will define their class or individualization. Class characteristics are properties that can only be associated with a group and never a single source. While it may not be possible to individualize the item of evidence, class characteristics can be helpful in eliminating possible suspects. For an item to be compared to and identified to a specific source, individual characteristics are necessary. These characteristics are ones that can be attributed to a common source with an extremely high degree of certainty. One of the most familiar types of evidence with both class and individual characteristics would be blood. While the blood type is the class characteristic that could eliminate potential donors, the DNA would be the individual characteristic that allows the blood to be identified to a specific individual. Although it is not always possible for an item of evidence to be identified as having come from a single source, this does not diminish the importance of the information it can offer. It is important to recognize the needs of the case and how the information that is available can be utilized [3].

A forensic necropsy begins at the crime scene [1, 2, 3, 4]. Knowledge of the scene may be crucial to interpreting findings at necropsy and determining the cause of death, making it preferred for a forensic veterinarian or pathologist to examine the scene for every case in which they assist [8]. In many cases, however, this may not be possible. They may not be available or the investigating agency may not be aware that they should be present or is willing to travel to the crime scene. Although reviewing crime scene photographs and law enforcement reports is beneficial, the information that can be gleaned from examining the scene in person is irreplaceable [1, 2, 13]. Examining the body at the scene is analogous to taking a medical history, thereby requiring practice and skill [8]. Many law enforcement officers do not possess the specialized training required to investigate an animal abuse scene, making it possible for information to not be noted or simply overlooked. The same issues can occur at the scene of a human death. This is why, in many localities, the medical examiner (or coroner) or their investigators are routinely present on scene with the sole responsibility of examining the body and the conditions that may be related to the death. Unfortunately, due to lack of personnel or funding, the forensic pathologists will often only examine the scenes of deaths that appear to be complicated or unusual. Not examining the body at a death scene has been regarded as one of the classical mistakes in forensic pathology [1, 2, 3].