Why does a saguaro cactus get an hourglass ripple as it grows?

An adaptation to a water-poor environment is the reason for the hourglass ripple that characterizes the maturing saguaro cactus (Carnegeia gigantea).

Specifically, the phrase "hourglass ripple" is another phrase for cactus ribbing. The saguaro cactus has ribbing, to deal with the low moisture and rainfall world in which it is found. The ribbing lets the cactus decrease in size with less water reserves inside, and increase with more storage. It also helps the cactus minimize the amount of surface evaporation that otherwise takes place on a flat surface.

On a flat surface, the sun's rays equally reach all parts. So the most amount of moisture is lost. The sun's access is not equal on the cactus' ribbed, hourglass surface. So the least amount of moisture is lost.