Tuesday, January 21, 2014

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.


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.


Holly Dunsworth said...

My comment is called "What does Anne use torture?" because that's what that video was ... I was so squirmy and lumpy in my throat until I laughed and it erased the torture like shaking an etch-a-sketch. I'm so glad you were attacked and tortured though because what a great post!

Anne Buchanan said...

Oh, I forgot to include a warning about the video! I did mean to! In fact, I haven't been able to watch it through myself! I'm told there are worse ones, too, but haven't searched for those and probably won't.

Manoj Samanta said...

It cannot be defense, because even on bare skin, the guy said it did not hurt much, but had tingling feeling. Most wild animals have a large layer of hairs on top of skin. Being eaten has never been the top concern of plants elsewhere.

Being able to spread far away could be one explanation. If water is sparse, the plants cannot just spread their seeds around in immediate vicinity and let all offsprings die due to competition. It could also be water conservation, or both.

Ultimately the best answer is "I do not know". That bothers me, because I want to live in the know-it-all predictable Franciscan world (world of Francis Collins) and cure diabetes by finding SNPs :)

Ken Weiss said...

Anne will tell you that it hurt like hell! I was there! I agree with your last paragraph. We as scientists are supposed to 'know'. But false Franciscan promises are sins against Nature. Our bird-brain post refered to the phrase 'truth for now' which sounds like a good watchword for honorable scientists not living by pandering. But I think even that's misleading, since we don't know what 'truth' is. I'd prefer 'the explanation for now', which I think is clearer.

Jim Wood said...

Being eaten has long been a top concern of plants, Manoj, which is almost certainly why so many have not only spines but bitter, toxic chemicals in their tissues. (You don't have to be a rabid pan-selectionist to believe that.) Imagine how those spines would feel in an herbivore's sensitive mouth. On the other hand, I really like your final paragraph.

Hollis said...

Belated thanks for this discussion of the difficulty of confirming adaptation, selection, etc. It's part of my contribution to this month's botany carnival: