system
Given the following post, respond as if you're commenting on Reddit:
human
In animals like octopuses and cuttlefish that die shortly after mating, what is it that kills them?

In documentaries about cephalopods, sometimes footage is shown of octopuses and cuttlefish post-mating indicating that they die shortly afterwards. They usually look very disheveled, with their skin peeling off it looks as though they are literally disintegrating. What causes this, is it some sort of super fast aging process?
gpt
The trait you're looking for is called semelparity. A classic example is a marsupial mouse, which basically fucks its way into the grave. As it enters the mating season it starts producing a huge amount of the stress hormones adrenaline (epinephrine) and cortisol. These make fat stores more available for energy, reduce hunger, reduce pain, reduce fatigue, and kill the immune system in order to maximise the number of times it can mate in this one season. 

It will continue to have sex without eating, barely sleeping, until it is ridden with diseases (fur hanging off etc.), has no fat remaining and has been digesting its muscles for energy, and is completely exhausted.