The saguaro has suffered major trauma. The trunk and arms are pitted and scarred after succumbing to the damage caused by packrats. Packrats seek water from the cactus during periods of extreme drought. Packrats can create spiral pathways by tunneling through the outer stem tissues. It appears as if the packrats used this staircase to reach the lower arm of the neightboring, otherwise unscathed, cactus.
Such water foraging may be rare since the cactus sap is very acrid and rich in acids and alkaloids. Presumably, packrats can process the sap but humans cannot and should not try.
This damage probably occurred many years ago and the cactus pictured was at least 100 years old.