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Is a biological tourniquet plausible and in what scenario would a creature evolve this?
I got an idea for an organism that evolved biological tourniquets.
The way it works is that they are basically ring muscles around blood veins that prevent blood loss; it is a mechanism that only activates when the organism loses its limb or limbs.
It's definitely plausible, because both of the things you're asking for already exist. All you need is a prey animal who evolved limb or skin loss as an emergency escape mechanism when attacked by predators.
For the biological tourniquet: vasoconstriction. Vasoconstriction is the narrowing of blood vessels through muscle constrictions, and is an important processes for stopping blood loss (hemostasis).
For why having this tourniquet is useful: autotomy. Autotomy is a self-amputation behavior designed to escape or distract predators by sacrificing part of an animal's body. Several species of lizards are able to detach their tails when stressed, and in some cases the tail continues to wriggle to distract a predator (a few can even regrow their tails after detaching them). One mammal, the spiny mouse, has autotomy, and is able to discard and regrow chunks of skin to wriggle out of predator's grasps.
Regrowing an arm or a leg is a bit more unlikely to develop than regrowing skin or a tail, mostly because animals who can (slowly) regrow limbs wouldn't survive long enough without them to pass on their genes. However, it's definitely possible, and if such autotomy is common in a species then strong vasoconstriction would also be a very useful trait along with it.
Lizards have muscles that cut off blood flow after they lose their tail.
The only reason a creature would evolve this is if they expect to lose the extremity, as a means of defense against predators. For creatures which can't regenerate lost limbs and who would be permanently rendered less viable in a reproductive sense by the loss of the limb (eg. early humans), there'd be no reason to evolve a mechanism to survive the loss.
This already exists. Its the primary way for the body to distribute blood around to where it needs to by constricting and dilating bloodvessles. Most of the larger bloodvessles wont close completely for most people, but it is possible. I spoke to a man who had his hands cut off because he refused to kill other members of his village in a grisly recruitment drive for a local warlord. He stuck his stumps in the mud to stem the bleeding and after a while the bloodvessles had constricted enough for him to remove his arms and seek help (the blood had barely clotted, the wound was too big). The people of his village were murdered anyway so he had to seek a village without members of a warlord for help. Point is: given enough time your body is able to restrict even the (almost) largest bloodvessles in your body.
Why isnt this a more present evolutionary trait? Because similar to brain damage you were too unlikely to survive anyway and pass on your genes to your children. You have to survive the attack that ripped the bloodvessles open, survive the bleeding and lastly survive the infection. It exists for small bloodvessles for control of the amount of blood, but not in a very viable capacity for the biggest bloodvessles like the Aorta as they kinda need to be pumping constantly for you to survive. That said, you could call it an accidental evolutionary trait (arent they all?). For example some species have evolved to be able to drop their tail, which would include the closing off of the bloodvessles. If the tail has evolved away but the genes for the tail's bloodvessle constriction mutate to be active around other major bloodvessles you could have your tourniquet evolve naturally I suppose.
