Rigor mortis is the stiffening of the body that occurs after death due to chemical changes in the muscles. When an animal dies, ATP production stops in the muscles, causing them to become rigid and unable to relax. This occurs because the actin and myosin filaments within muscle fibers lock together in a contracted state in the absence of ATP.
Rigor mortis causes the limbs and jaw of animals to become stiff or rigid. The onset and duration of rigor mortis can provide useful information for estimating time of death. Understanding how this process affects the muscles in various animals after death is important for fields like veterinary forensics and meat processing.
What is Rigor Mortis?
Rigor mortis refers to the stiffening of the body’s muscles and joints that occurs after death. It is caused by chemical changes in the muscles after circulation stops. Once the heart stops pumping blood, oxygen no longer reaches the muscles and they become unable to relax. This causes them to become stiff and fixed in place.
Rigor mortis begins within 2-6 hours after death. The muscles first begin to stiffen around the eyes, jaw, and neck before progressing downward to the arms, torso, and legs over the next 24-48 hours. Rigor mortis reaches maximum stiffness around 12-24 hours after death and then gradually dissipates until the body is limp again after 48 hours.
The onset of rigor mortis is one of the recognizable signs that death has occurred. Forensic investigators and medical examiners can estimate the time of death based on the presence and degree of rigor mortis in the body.
When Does Rigor Mortis Start in Dogs?
Rigor mortis usually sets in within 3 hours after a dog’s death, though it can begin as soon as 10-15 minutes after passing (Source). The onset of rigor mortis marks the second stage of death, and indicates that the dog’s muscles have depleted their energy stores. As ATP is used up in the muscles, actin and myosin filaments bind together rigidly, preventing muscles from relaxing. This causes the stiffening associated with rigor mortis.
Rigor mortis starts in the smaller muscles like those in the head, neck, and chest, and progresses to the larger muscles. The muscles first begin to stiffen, then become completely rigid overtime. Once full rigidity sets in, the dog’s joints will be fixed in the position they were in at death. As an early indicator of death, owners can check for stiffening of the jaw muscles that may make the mouth difficult to open.
Stages of Rigor Mortis
Rigor mortis progresses through several distinct stages over 24-84 hours after death.
The first stage is called primary flaccidity, which occurs immediately after death. During this stage, the dog’s muscles are completely relaxed and flexible.
The next stage, onset of rigor mortis, begins within 2-6 hours after death. During onset, the dog’s muscles begin to stiffen and become difficult to move or manipulate. According to this source, the smaller muscles like those in the face, ears and tail will stiffen first.
In the following stage, full rigor mortis, the rigidity spreads to all the muscles in the body. This stage is reached around 12-24 hours after death. The muscles are fully contracted and inflexible during this stage.
Between 24-84 hours after death, rigor mortis begins passing off. The muscles start relaxing, although they may still feel stiff. The dog’s limbs can be moved more freely again.
In the final stage, secondary flaccidity, rigor mortis ends and the muscles become completely relaxed again. This stage begins 2-3 days after death.
Factors that Affect Rigor Mortis
There are several key factors that influence the onset and progression of rigor mortis in dogs:
Temperature – Higher ambient temperatures accelerate rigor mortis, while colder temperatures delay the process. According to a 1963 study by Kolder, the onset of rigor mortis in dog hearts occurred within 10-20 minutes at 50°C, but was delayed beyond 6 hours at 1°C [1].
Cause of Death – The cause of death impacts rigor mortis. For example, violent exercise or convulsions before death hastens rigor mortis. Meanwhile, exhaustion, emaciation or disease states like sepsis can delay the onset [2].
Age/Health – Rigor mortis tends to occur faster in younger, robust dogs. Older dogs or dogs with poor health may exhibit delayed onset of rigor mortis [3].
Muscle Mass – Dogs with greater muscle mass tend to stiffen faster after death compared to smaller, leaner breeds [2].
Monitoring the Onset of Rigor Mortis
Rigor mortis refers to the stiffening of the muscles that occurs after death. Monitoring the onset and progression of rigor mortis can help estimate the time of death in dogs.
Here are some tips for monitoring rigor mortis in dogs:
– Check the major muscle groups like the legs, jaw, and neck for stiffness periodically after death. Gently try to bend and flex these areas to test for rigidity.
– Document the time when stiffness is first noticed. Typically, rigor mortis starts between 2-4 hours after death, beginning in the head, neck and chest before extending to the limbs (1).
– Continue checking and noting the increasing stiffness over several hours. Full rigidity where the whole body is stiff may take 8-12 hours after death (2).
– Also note when the rigor mortis peaks and starts dissipating. This usually occurs 24-84 hours after death as decomposition begins (3).
Carefully tracking rigor mortis onset can help narrow down the estimated time of death, especially in the first 24 hours. However, many factors like temperature, exercise before death, and cause of death can affect timing.
(1) https://vetmed.iastate.edu/vdpam/about/production-animal-medicine/dairy/dairy-extension/humane-euthanasia/humane-euthanasia/confirmation-death
(2) https://journals.sagepub.com/doi/10.1177/0300985816629720
(3) https://veteriankey.com/estimation-of-time-since-death/
Uses for Measuring Rigor Mortis
Measuring rigor mortis in dogs can serve two primary purposes: estimating time of death and indicating potential toxins or poisons.
Rigor mortis progresses through distinct stages at consistent rates in most dogs. By assessing where rigor mortis is in the process, veterinarians can estimate approximately how much time has passed since the animal’s death. According to research, rectal temperature declines steadily for the first 10 hours after death in dogs and can help narrow down time of death along with rigor mortis assessments.[1]
The onset and progression of rigor mortis can also provide clues about potential toxins or poisons that may have caused or contributed to a dog’s death. Certain poisons like strychnine can accelerate rigor mortis, while others may delay its onset. If the timing or duration of rigor mortis is abnormal, it prompts vets to test for toxins as a potential cause of death.[2]
So by carefully monitoring rigor mortis in deceased dogs, veterinarians can gather crucial information about when death likely occurred and identify any suspicious toxins that could have been involved.
[1] https://veteriankey.com/estimation-of-time-since-death/
[2] https://journals.sagepub.com/doi/10.1177/0300985816629720
Rigor Mortis vs. Cadaveric Spasm
Rigor mortis and cadaveric spasm both cause stiffening of the muscles after death, but there are key differences between the two processes:
Rigor mortis refers to the gradual stiffening of the muscles over several hours after death as the body runs out of ATP energy. It begins in the smaller muscle groups and progresses to larger muscles over a period of 6-12 hours, reaching maximum stiffness around 12-24 hours after death. Rigor mortis then slowly dissipates over the next 24-84 hours as the muscles enter decomposition.
Cadaveric spasm, on the other hand, causes instantaneous contraction of muscles at the exact moment of death. It is thought to be caused by a massive release of calcium ions in neurons causing muscle fibers to contract. Cadaveric spasm occurs in more violent or instantaneous deaths and is seen in cases like drowning, electrocution, or gunshot wounds. The muscles appear fixed and contracted.
While rigor mortis takes time to spread through all muscle groups, cadaveric spasm instantly locks muscles in a contracted state at death. Rigor mortis can provide estimating time since death, while cadaveric spasm indicates the violence or speed of death but does not help establish a timeline. Understanding the differences can help investigators analyze crime scenes and autopsy findings.
Sources:
https://www.differencebetween.com/difference-between-rigor-mortis-and-cadaveric-spasm/
https://www.youtube.com/watch?v=Qbb-nzeXwHA
Other Postmortem Changes
In addition to rigor mortis, there are other postmortem changes that occur in a dog’s body after death:
Lividity refers to the settling of blood in the lower portions of the body, causing a purplish red discoloration of the skin. This begins shortly after death as the blood no longer circulates and is pulled down by gravity. Lividity becomes fixed 8-12 hours after death.
Algor mortis refers to the gradual lowering of the body’s temperature after death until it reaches ambient temperature. A dog’s normal body temperature is 101-102.5°F. After death, the body cools at a rate of about 1-3°F per hour. Factors like ambient temperature affect the rate of cooling.
Decomposition begins soon after death as cells break down and release enzymes that initiate autolysis and putrefaction. This process causes bloating of the abdomen and emits gases that produce noticeable odors. The rate of decomposition depends on temperature, access of insects, and whether the body is buried.
As gases build up internally, the abdomen, limbs and head can become bloated. The tongue may protrude and the eyes bulge. Skin discolors and pulls back. Decomposition progresses until only bones, cartilage and hair remain.
Conclusion
Rigor mortis is a natural process that occurs after death as the muscles stiffen due to chemical changes in the body. This article discussed what rigor mortis is, the stages of rigor mortis in dogs, factors that influence its onset and duration, how it can be used to estimate time of death, and other postmortem changes that occur. The key points are:
- Rigor mortis is the stiffening of the joints and muscles of the body following death.
- In dogs, rigor mortis usually begins within 2-6 hours of death and peaks around 12-24 hours after death.
- There are three recognized stages of rigor mortis: onset, maximum rigidity, and passing off.
- Many factors like breed, body condition, temperature, and activity can affect the onset and progression of rigor mortis.
- While not a precise science, tracking rigor mortis can provide useful information in estimating the postmortem interval.
- Rigor mortis is a normal part of the process following clinical death.
Understanding the usual timelines and patterns of rigor mortis in dogs can help veterinarians, researchers, and pet owners gauge approximately when death occurred. However, many variables exist that make an exact time of death difficult to ascertain based on rigor alone.
