[thedoc (OP)]

Steven Balliette asked the Naked Scientists:
   Skunks have such an effective defence I am wondering why evolution has not given it to more animals.  I am happy that squirrels and rabbits do not spray, but why not?
What do you think?

[alancalverd]

Every defensive system adds weight and consumes energy, which is why nobody uses battleships any more: protection against submarines and naval guns was just about feasible until the 1940s but adding effective antiaircraft defences meant that the ship spent more time defending itself than attacking the enemy.

Squirrels can run up trees to escape predators, and rabbits tend to feed in large groups with lookouts and exceptional directional hearing, but skunks are solitary and just the right size for a wolf's or bear's lunch, so they carry a weapon for repelling land-based attackers. However

Skunks are reluctant to use this weapon, as they carry just enough of the chemical for five or six uses – about 15 cc – and require some ten days to produce another supply

and it seems that, despite their top camouflage, they are vulnerable to airborne attack by owls.

[evan_au]

Some insects can spray toxic chemicals to repel predators. Others have toxic stings.
Other insects (especially brightly-colored ones) are toxic; while this won't protect the insect being eaten, it might protect his similarly bright siblings.
https://en.wikipedia.org/wiki/Defense_in_insects#Chemical_defenses

Many plants, fungi and bacteria produce toxic chemicals as defence, either before or after being eaten.
 

[chiralSPO]

Skunks are a mustelids, and many mustelids have musk, which they may use for defense, marking territory, or other forms of chemical communication. Wolverines use their musk to defend their food (a wolverine may kill a moose, bury it in the snow, spray it with musk, and know that it can eat the thing all winter long without worrying about other scavengers stealing that precious meat...)


Edit: apparently skunks are no longer considered to be in the same family as other mustelids (taxonomy has changed a lot since I was in school...). They are still considered closely related.