What causes that?

Photo by Ned Harris 1/8/2014

The splits in the saguaro 'skin' are the result of freezing. Water expands when it freezes, as you know. And when water freezes in a cactus, the skin can only expand so far before it splits. The saguaro can live a long time after this kind of damage, provided it's not too extensive.

Photo by Ned Harris 1/8/2014

Idiots caused this damage. You can see rocks embedded in this saguaro. (If I were queen, there would be a lot more beheadings.)

Photo by Ned Harris, hand by Lyn Hart 1/8/2014

The only animal who can make a cut like this is a human with a knife or a machete. See note above about beheadings.

And finally, my post "Prickly Pear Theories" caused a bit of confusion. Some people thought that damage was caused by (or actually was) cochineal. Nope. (And nope.) The damage might not have been intentional, but it was caused by rocks. You see prickly pears with pressure damage along the road and along trails that get a lot of traffic. Small rocks get kicked up (or picked up). Yes, the cochineal insect does feed on the prickly pear, but even a big infestation wouldn't cause that damage. More info on cochineal.