Why do cholla cacti use torture?

Ken and I were in Arizona and thereabouts last week visiting friends.  It was lovely -- warm, sunny, lots of good food and good conversation.  We saw some excellent shows -- the Charles Harbutt exhibit at the Center for Creative Photography in Tucson, the Chihuly exhibit at the Desert Botanic Gardens in Phoenix, Penn and Teller in Las Vegas.

Chihuly glass sculpture, Desert Botanic Gardens, Phoenix

And we had some fine desert hikes.

The variety of cacti is astonishing, at least to a north easterner.  So many ways to live in the same harsh environment.

Now, everyone warned us not to touch the cholla cactus.  Or the jumping cholla cactus, or the teddybear cholla cactus, all close relatives.  They all have sharp spines, and whole arms of the plant seem to want to leap off the plant and embed themselves in your skin if you get too close.  Is it too much of a stretch to say it's almost as if the plant were striving to do so, like yesterday's bean plants?

Do not touch!

The arms do fall off easily and around the base of many cholla cacti you can see where pieces have fallen, taken root and started a new plant.

So, at the foot of the Superstition Moutains outside of Phoenix I was walking along, minding my own business when I lightly brushed up against one of these guys without even realizing it, and suddenly I had a 6 inch piece of cactus biting into the skin just above my elbow.  I only wish I'd had the presence of mind to take a picture, never mind video the whole experience like this guy did.



But I didn't because it hurt and I just wanted it -- them -- out.  It hurt particularly because each spine has tiny barbs all along the shaft that make coming back out really hard.

Wikipedia

I'll spare you the details and just say that, happily, unlike in the video, the piece that attacked me had a stem that we could hold onto and pull.  But you have to really pull.  We have since learned that we should take a comb into the desert -- apparently it's easier to disentangle yourself from one of these things using a comb.  Next time.

But it did make us think.  Why would a plant do this?  This brings us to one of our common topics to write about: the problem of the adaptationist assumption, that everything in nature has to be here because of natural selection and that we can infer the reason for that selection.  At least the second part often seems to be assumed when 'the' explanation is offered.

Here conventional wisdom would probably say the thorns are a defense mechanism.  Once poked, twice shy: animals would shun the cholla like the plague.  But why would plants 'want' to be left alone?  Plants, including cacti, can afford to lose a lot of themselves and still survive.  Why spend energy on growing all these spines?  And many plants build in attractors, not repellers -- flowers, aromas, colors, even hallucinogens or flavors. Being eaten, shaken, browsed, and so on is great for them and their potential to bear offspring.  So maybe that's not the answer.

A second explanation also seems obvious:  so many desert plants have spines that they must have something to do with water retention.  Otherwise, if they're just for defense, why aren't all temperate or tropic forest plants spiny?  The preponderance of spininess in the desert almost shouts 'succulence!' at you.  And maybe there aren't even as many animals browsing around in the desert as in rainier forests.

Or maybe it's a self-dispersal mechanism -- stick to a bear or wooly mammoth's coat and fall where you may.  Cholla blobs that land on the ground take root.   But this doesn't ring automatically true because  the mechanism is such over-kill that it's hard to imagine how these spiney blobs could fall out on their own.  So pity the poor javelina who gets one of these in its nose, and then tries to paw it out.  Near certain death -- though nice fertilizer for the plant.  Maybe it was planning ahead.  And if dispersal is the selected trick, why are most desert plants so short-spined?  Are those ones just for protection against animals?

The problem is that there may be no single reason, nor even any single kind of history involved here.  Maybe all these, and perhaps many other, reasons are or were true in the evolutionary past.  Botanists must have many clearer ideas about this than we do, of course.  But we think this illustrates why, even when the assumption that the trait is 'adaptive'--that is, is here ultimately because of natural selection--that assumption is hard to prove and in particular the reason is hard to be sure about.