That's referred to as a "Required Secondary Superpower." In the superhero reconstruction web serial Worm, these are practically de facto and even have their ow

That's referred to as a "Required Secondary Superpower." In the superhero reconstruction web serial Worm, these are practically de facto and even have their own jargon assigned to them. Some characters find the secondary aspect useful all on its own! IRL, snakes tend to be immune to their own venom because of the frequency of bites from their own species. On the other hand, for some creatures, other defenses like tail-dropping or gut-expelling are expensive moves. But the first few members of a species developing a new characteristic, well, the self-defense might not be there yet. The first bombardier beetles might have been very unhappy, even at a fraction of what that defense might grow to.