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The animals in the previous post from the Galapagos, and the one before that, are both fond of the outer skin of young cacti.   Young, in this case, means plants that are several months to several years old.  In one of the photos above (with an iguana visible) you can see the youthful protective skin of the cactus, full of spines on the trunk.  As a cactus matures, it develops a bark-like skin.

The photos showing dark, mature skin close up are of a cactus whose trunk is 18-20 inches in diameter and several decades old.  Even with both of these protective strategies the tortoises and iguanas still find plenty of moments in the life-cycle of plenty of cacti to take bites of delicious, moist nutrition.  Most cacti continue to mature in spite of a bit of grazing, and the populations of land iguanas, tortoises and their favored food have historically (in an evolutionary time frame) grown well together.