Final answer:
The rigidity of the dead deer is due to a postmortem change called rigor mortis.
Step-by-step explanation:
As a student in forensics, you would describe the rigidity of the dead deer as a postmortem change known as rigor mortis. Rigor mortis is the stiffening of muscles that occurs after death due to a lack of adenosine triphosphate (ATP), which is necessary for muscle relaxation. Rigor mortis usually begins within a few hours after death and peaks around 12-24 hours, gradually disappearing over the next 48-72 hours.