Why do dead bugs and mice ‘stiffen up’ in specific positions where as other dead animals simply go limp?
In: Biology
I am not sure what you mean with insects stiffening when they die but I can explain why they curl up on their backs. It is not 100% proven but it is thought that when an insect is dead or dying, it cannot keep it’s limbs under enough tension so it goes into a “state of relaxation”. That is why the legs curl up.
Another possibility is that without blood flow to the limbs, the muscles contract and fold up.
So insects are an entirely different matter and i do not know enough about them to help. However you are mistaken, dead animals do not simply go limp. The animal dies and is indeed limp but then the body goes into rigor mortis. All animals do this it is a stiffening of the muscles that happens a couple hours after death and can last for 12 hours. Then the body will relax again.
I believe that the larger the animal the slower rigor mortis sets in, so it is possible you see it more with mice simply because they move more quickly through the death stages.
I don’t know about bugs, but animals don’t go limp, I mean immediately after death they do, but within hours rigor mortis sets in. That’s because the SERCA pump required to sequester calcium in storage (terminal cisternae, sarcoplasmic reticulum) needs energy (ATP) to operate. No oxygen and metabolism upon death means no more ATP generated, so calcium starts to diffuse out of storage, go to the sarcomeres (force generating units), bind and shift proteins causing many cross bridge formations (tiny flexes) which slowly contracts the muscle. In order for the proteins in the sarcomeres to let go (myosin let go of actin) it also needs ATP, so you slowly flex more and more but you can’t relax, because that requires energy. So you get rigor mortis.
Something that hasn’t been mentioned here yet is that spiders and insects “muscles” are basically hydraulic in nature. When they die, the pressure that kept them extended goes away, and thus the curl-up
All animals will stiffen up, or go through rigor mortis (I think I spelled it right), because the muscles will contract. Its simply that different animals go through the process at different speeds. You can also slow down rigor mortis, as done in the meat industry. I’m not versed in how it’s done so I won’t go into detail, but that’s the basics.
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