Myrrh trees (Commiphora) are useful things...

Most Commiphora have distinctive peeling bark, Eyasi Aug 2011.

Having last week given you the bad news about the biological warfare that plants with thorns are engaging in, I thought it only fair to share some tips that may help you stave off those tropical nasties threatening to kill you... So the good news is that some of those very same thorny trees that are out to get you also hold the cure in their sap. Traditional healers and many folk still living in the country have long known about the beneficial effects of the sap of Commiphora trees. Indeed, the earliest recorded use of Commiphora to treat infections goes back to 1100BC where Sumerians were recorded using myrrh (for that is what Commiphora is) to treat tooth infections and intestinal worms. It's use is also clear in the Bible, as one of the three famous gifts presented to the infant Jesus. Today it's still commonly used in village communities to treat an endless list of infections and maladies, and it's also pretty good as a mosquito repellant if you need emergency cover!

Why is the African Savanna so full of thorns?

Giraffe lick leaves between thorns. Note how obvious the white thorns are.

Spinescence. Now there's a word! It simply means having spines and one of the first things many visitors to the African savannah notice is that everything is covered in thorns. Or, in other words, Africa is spinescent. It's not a wise idea to brush past a bush when you're walking, and you certainly want to keep arms and legs inside a car through narrow tracks. These are thorns that puncture heavy-duty car tyres, let alone delicate skin. But why is the savanna so much thornier than many of the places visitors come from? Or even than other biomes within Africa, such as the forests?

This post I've just written as a guest blog over at "Nothing in Biology Makes Sense". I'm incuding it here too, but do go and check that blog out if you're interested in evolution! You can read the rest here, so skip to the story there if you want...

Why is Africa so full of thorns?

Assorted browsers and browse lines, Selous GR, June 2010

I'm actually away this week, but left this to post itself whilst I'm off and keep people interested! One of the places I'm headed to is Mwiba Ranch, south of Serengeti and, as you might know, mwiba is swahili for 'thorn'. No doubt I'll have some specifics to talk about when I'm back, but one of my 10 things to talk about topics is thorns. Why? Well, many visitors from the north live in places where tere aren't lots of thorny trees, so going to a walking safari and discovering that just about every bush and tree is covered in massive needles is a bit of a shock, even if those of us lucky enough to live here barely notice them (until they get infected, of couuse...).

Giraffe (and shorter!) browse line, Arusha NP, June 2010

So, why should Africa, or at least African savannahs in particular, be so thorny? The answer, of course, is fairly simple - why do tourists visit the savannah? To see the animals, and what do the animals eat? Well, rather a lot of them like to eat bushes, which is no fun at all if you happen to be a bush. In fact, in many places the grazing pressure is so heavy, very distinct browse-lines form and the plants take on structures as if someone was pruning them into interesting sculptures. So, defending yourself against Africa's abundan browsing animal population is a very good idea, and I've always thought thorns must be pretty nasty things to eat. But, you say, if the thorns are a defense against browsing, how come I still see impala and giraffe and all the others happily choping on thorn trees? Obviously the defence doesn't work? That's a good point, and allows me to introduce one of the many ways in which nothing in biology makes sense except in the light of evolution. Imagine living in an Africa before there were any thorns - all the bushes are undefended, and equally appealing to a browser. Now imagine there's a little mutation in one of the offspring of those bushes, that means it grows small spines - all the other bushes are still nice and undefended. If you were an impala, which bush would you eat? I suspect you'd go for any of them, except the one with the small spines. Which, of course, means that the spiny bush is going to do very well and will produce lots of babies, also with spines. In time, all the bushes will have little spines and the poor impala, if he wants any lunch, just has to tuck into that thorny bush. But, of course, one bush might have a mutation making it's spines a little harder, a little longer, a little nastier - and you can see immediately what's going to happen now - bigger spines evolve.

Greater Kudu, carefully nibbling around thorns, Kruger NP, May 2011.

Of course there might also be heritable variation in the impala browsing technique or mouths - maybe for thicker skin, or a narrower nose that can squeeze between the thorns. Giraffe, of course, have evolved a huge long sticky tongue so they practically lick the leave out from between the thorns, rather than have to go to close. But both species certainly are aware of the thorns, even though they have no choice but to eat the prickly trees, of course - imagine what would happen to the poor thorn tree that, though some mutation, had no thorns. Ooops, poor thing! So clearly the thorns do have an impact - what's interesting to me to look for in places with very heavy browsing is evidence that the thorns do work, even though the plants get eaten.

Heavily browsed yellow-barked Acacia, Arusha NP, June 2010

To see this, you need to look at the shapes of trees and bushes. Here's a nice Vachellia xanthophloea (see, I'm trying to get you used to the new names!) that's been very hevily pruned - a favourite with the girafe. So how's it ever going to make the leap from heavily pruned bush to fully fledged tree? The secret is to grow wide, before growing tall. While the bush is still short and relatively narrow it doesn't stand a chance - any giraffe will bend down, and chew off the top bits. But if it can get wide enough the giraffe don't like the thorns on their skin, and they'll just nibble to top bits within easy reach - leaving a tuft in the centre to break away. And once that is done, those short, wide branches at the base of the tree are no longer important and soon die back, in favour of the taller tree. Not clear? She here's a little diagram showing how to escape giraffe broswing pressure...

Bushes escaping browsing. Honest!

Nearly there! This bush behing the giraffe has started to escpe from the centre, Lake Manyara NP, April 2010

Not the most artistic ever, I admit - but I'm an ecologist, not an artist... (It's a giraffe bending down on the left, not a funny, long-necked kangaroo). So on the left you have a tiny seedling been chewed by a giraffe. This makes the bush grow flat, like in the photo above. Then, when the plant is wide enough, the throns around the edge stop the animals from being able to reach the centre and in the third part of the diagram a new shoot is escaping browsing, and finally in the last picture a mature tree has grown, with just a little whisp of the original short, fat bush that had to escape browsing remaining as evidence.

Made it! Just a few whisps of short left, West Kili, April 2010

So, that all sounds very nice, but why, you ask, do some plants get away without thorns, even in the savannah? Both Terminalia and Combretrum are fairly thorn-free, but a typical savannah plants. So how do they do it? Well, the difference here is the nutrients. Remember that Acacias (or whatever we're going to start caling the group now) are legumes and are absolutely full of nutrients, whereas these other two species live on the nutrient poor ridges and represent pretty low-quality browse. So, if you're a tasty plant growing in nutrient rich areas, you're going to be browsed much more than those specialising in nutrient poor environments, and here you want to invest more in defence.

And that, for now, is that. Hope you're having as much fun as I will be having in Mwiba!

Plant signaling

Perhaps the subject that most surprised guides when we were chatting about things to talk about when there are no lions came up in my session on thorns. Thorns and other plant defences are quite fascinating and I'll certainly talk more about them in the future. But most people were more impressed to hear about how plants signal, than they were about thorns themselves, so I thought I'd give a bit more information about plan signaling in this post instead.


Now, imagine you're a plant that's getting browsed. Not much fun, huh? You'd want to do something about it if you could, wouldn't you? So, let's say you can detect browsing (how would you do that? Easy, as it happens, just look out for plant chemicals that should be contained within cells, in places they should be - if there's cell contents in places it shouldn't be, the chances are you'vebeen damaged - and we all know how damaged plants can smell), it would be nice to have a quick response and produce more nasty tasting chemicals straight away, and when you regrow, it would make sense to be extra thorny in this area. Even better if you could somehow warn other branches that you're been eaten and communicate with the other side of the tree, don't you think? And as it happens, plants can do this - rather than always producing lots of costly thorns and nasty chemical defenses, plants tend to just produce a minimal output, and then up the defenses if they actually come under attach. Very sensible really. And one of the key ways they have of signalling that they're under attack is through the use of a plant hormone. Now there are several plant hormones, but the simplest is called ethene (or ethylene, it's the same and I'll use the two interchangeably here) and is a colourless gas, consisting of two carbon atoms, and four hydrogen atoms:


H              H
    \          /
      C = C
    /           \
H              H


if you're into your chemical formulae. It's a very simple organic molecule, and plants use it for almost everything you can imagine: signalling that it's time to ripen fruit, so they all ripen at the same time (fruit importers in the west use this trick - they get people to pick unripe green fruit in the tropics, stick it on a boat to Europe (it last well if it's green), then gas it with ethene so everything ripens nicely before getting into the shop. Which explains why fruit tastes much better here than in UK... You can also use ripe bananas to speed up the ripening of other fruit, if you stick them together in a paper bag.); seed germination; fertilization, etc., etc. But for us, right now, we're interested in how they use it to signal stress, and in particular herbivory.

This avocado is ripening thanks to a ethylene signal from the ripe bananas

To be fair, it's not actually the main signal process for herbivory - it's probably too general in function for that task – but it does play a role and the concept is the same for the other signalling processes. Once chewed, a plant rapidly (within seconds) with start to produce ethylene. Being a gas, it can drift all around the plant, and receptors in other parts of the plant pick it up, decide what it means (“help!, I'm being eaten!”) and tell the plant to get on with appropriate defences.

All well and good. But the observant among you will already have picked up on one thing – there's nothing to stop the signal moving out of the plant being eaten and into the neighbouring plant. And if that plant can pick up the signal, then all of a sudden, plants in a neighbourhood can communicate with one another. And, in fact, this is what they do. Pretty impressive for a plant, I'd say! Since most plants use the more or less the same set of signals it's quite possible for the signal to come from an Acacia, but be picked up by a Balanites – inter-specifc plant communication. And you thought plants were just sitting there and taking it! I'll save the 'what can a plant do about it' question for another time, but for now just remember that plants can signal, even to other plants, that they're being eaten and perhaps there's something here to talk about next time you watch a bush being hammered by an elephant...

This poor Acacia xanthophloea is showing quite how many thorns it will grow when there are lots of nasty giraffe around. Arusha NP June 2011.