Evolving a Creature with in-Built Projectiles [duplicate]

Question

The creature was in sight now. Jokval eased through the bushes, making no noise in his approach. The beast was feasting on the carcass of a deer. Jokval raised his bow, the only sounds were that of the monster's gluttony and his arrow drawing across the bow. He breathed in then out. Suddenly the beast whirled. Jokval maintained his position. There was no way a mere beast had spotted his position over 50 meters away. Suddenly a sharp pain in his side threw his aim away from the beast. He glanced down to see a sharp spine of bone. He could tell the shot was fatal. As the world faded to black, he realized why the deer had such great hearing.

Actual Question: What environment would a creature develop a natural kinetic projectile launcher? (No fireballs, venom, etc. the object should kill by going through the thing, rather than poisoning it.)

Bonus Points: Build a creature around the launcher!

Note: I understand this will sound something like this question. But I am asking about the what would make it develop, rather than the would it develop.

Answer 1

Animals already use "true" (untethered) projectiles:

• The Archerfish shoots down insects and other small animals by shooting water so it can eat them.
• Tarantulas kick off hairs, which are more like spines, to ward off predators.
• Antlions throw sand at prey to knock it back down
• California Ground Squirrels kick sand into predators' eyes to gain some more time to escape.

• Monkeys and other primates throw rocks, feces, and sticks for defense and war.

• There are many, many more.

The short answer is that these creatures have gained these adaptations for hunting and for defense. Why use projectiles instead of other options? It is simply what those animals had "on hand", and they conferred the behavior to their offspring, and those offspring to their offspring, until it became a behavior of that creature to throw things.

Throwing a physical projectile has a problem: ammunition and the energy requirements to produce that projectile. The item which is thrown needs to be cheaply available to the creature to be able to be used. That is likely why there appears to be more liquid projectiles used than solid ones. Liquids and poisons are often cheaper to produce than a barbed spines. Poisons also have the benefit of not requiring much effort to kill once delivered.