Although similar glands appear in many other mammalian species, the skunk's must glands are highly developed. I remember reading that the only time a skunk sprays another skunk is when two males fight over a female, however other sources say no. Powerful muscles surround the skunk's musk glands which can squeeze the musk out of a pair of 'retractable nipples' located on each side of the anus. This spray can reach an enemy 10 to 15 feet away. Often little or no musk gets on the skunk's fur.
Skunks apparently try to avoid getting ANY musk on their fur.
This shows the 'retractable nipples' in action. A skunk cannot spray when held up by its tail. It appears to depend on the individual. If lifted after the nipples protrude, then a skunk can successfully spray. Skunk spray can cause permanent blindness.