Superorganisms = organism

People like to talk about ants as ‘superorganisms’. Of course, we’re all kind of superorganisms, built out of a structure of captured cells and using home-grown bacteria to function. But when we talk about ants, wasps, and termites, we mean something else. Each insect on its own seems to be an independent organism – though they can’t all survive away from the colony – but in reality, the colony is the organism.

In PNAS, Hou et al. apply the metabolic scaling law to eusocial insect colonies. Individual organisms have a metabolic rate that scales as the 3/4 power of body mass. Along with this are laws scaling reproductive organ mass, total biomass, and organism (colony) growth with metabolism. Individual ants do not follow this law: worker ants have almost no reproductive organs while the queen has tons of ’em.

The figure above shows the body mass vs. metabolic rate for some (non-ant/wasp/termite) insects and for various colonies as a whole. And the curves are the same! If you look at the other predictions, they all turn up exactly on the curve, too. So it is only when you consider the colony as a whole that eusocial insect act as a proper organism. Iain Couzin once mentioned to me that ants in a colony act analogously to neurons in a brain, though I can’t for the life of me remember how that was. But see, the individual ant is just like a lonely neuron: meaningless.

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Bugs in our gut

Stumbling along the cybernet, I came across this intriguing article about the extinction of our intestinal flora. The tag line here is:

Having evolved along with the human species, most of the miniscule beasties that live in and on us are actually helping to keep us healthy, just as our well-being promotes theirs. In fact, some researchers think of our bodies as superorganisms, rather than one organism teeming with hordes of subordinate invertebrates.

The human body has some 10 trillion human cells—but 10 times that number of microbial cells. So what happens when such an important part of our bodies goes missing?

It’s hard to appreciate just how symbiotic our relationship is. In fact, the flora has a collective metabolic activity equal to a virtual organ within an organ. Our body is well attuned to this fact: immunosensory cells can actually distinguish pathogenic bacteria from the helpful ones. So what are the functions of this virtual organ?

Well, they actually help us to break down some undigested carbohydrates; rodents raised in a sterile environment without gut flora need to eat 30% more calories. They are also able to repress pathogenic bacteria from colonizing the gut through a “barrier” effect. They help prevent allergies. They’re even able to help prevent cancers.

It’s interesting that we’ve been able to selectively allow bacteria to colonize our bodies – and it’s not just us mammals; certain ants have gut flora that allow them to obtain nutrition from honeydew. In fact, the reason that ants are able to be herbivores at all is due to their intestinal flora. Completely unrelated herbivorous ants have bacteria from the same order – as Myrmecos puts it, “The bacteria are not mere evolutionary hitchhikers passively tracking the genealogy. They show up in a highly non-random fashion in ants that have need of nitrogen.” But it gets even more interesting! Leafcutter ants not only employ antibiotics in their fungus gardens, they use colonies of the bacteria Klebsiella to capture atmospheric nitrogen and make it available to the fungus and the ants. I wonder how similar the evolutionary symbiosis of these bacteria is?

And now, here are a few of our friendly flora.

Ants ants ants

Man I’m behind on my ant blogging. OK, here’s what I’ve learned recently, thanks to Science. Why does Science have so many more ant articles than Nature? It’s quite an unfortunate oversight.

To start things off, we have a fascinating bit of research about dancing lizards. See, fire ants like to ants1eat some nice hearty meals, like lizards. They surround the lizard, pry up the scales, and inject venom. To combat them, some lizards have discovered that they can give some hearty shakes and wiggles, and throw the ants off. Lizards who live longer around fire ants are more likely to do this (and also have sturdier hind legs!). So then – is this a genetic influence? Or a lizard cultural one?

Now let’s turn to the ant queens. In order to cement their social status, ant queens give off a squeal that elicits increased benevolence from their workers. This is in addition to the normal communication signals of “semiochemicals” (ie, pheremones). Some insects have been known to infiltrate ants society by mimicking their semiochemicals. After all, if your only identification method is chemicals, it’s easy to get tricked. But even worse, these other insects have learned to mimic the sounds made by queens so they are instantly treated as royalty – even their pupae are given the royal treatment. Apparently, vocal signals are only used to signify caste status, so if you’ve got a good voice, you can be anyone in society that you want to be.

Next up is an article questioning how ant eusociality (ie, being a superorganism) evolved. How exactly does an insect become caste-like? The consensus in the field has long been that it is because of altruism to kin – ants will be sterile in order for the queen ant to pass on some portion of their genes. Intergroup competition leads to this being a successful strategy. But recent findings contradict that theory. Among other evidence, some bees that are social come in pairs: one bullies the other to become a worker while the first becomes the queen. As time goes on, it is the kin least related to the queen that stay and the more related kin that leave. The most important bit here is an evolved behavior to stay near a nest. Of course, the vast majority of the field still disagrees but there ya go, that’s E.O. Wilson’s opinion.

ants2The final bit is less about ants and more about general evolution. A paper in science is attempting to show that evolution is less random than it might appear at first glance, and that its direction can be almost predictable. Their first bit of evidence is convergent evolution – how vastly different species can evolve behaviors or appearances that are nearly identical (homologous) – or even parallel evolution – when different species evolve the same gene in parallel. But these parallel evolutions often act on the same gene, even when there may be eighty or more genes that could mutate to do the same thing. Why? The authors suggest the kinetics and chemistry of the transcription factors and signaling molecules – the “genetic network” – constrain evolution in such a way that only a few genes are at all likely to be targeted for evolution. They give plenty of examples of how evolution is constrained and explore their hypothesis that evolution is predictable – it’s interesting and definitely worth a quick read.

Finally, a series of videos of trap jaw ants. Their mandibles are so powerful that they hurl themselves in the air when they close. Sweet.

Naked moles

Naked mole rats have to be one of the ugliest and strangest creatures on the planet. I mean, just look at the guys: evolution is a cruel mistress. The mole rats are particularly cool because they are the only eusocial mammal – they have a highly structured social life, with a queen and different castes. This behavior is common in insects with ants, bees, termites, but mammals?  Weeeird.

These mole rats are found in eastern Africa and live entirely underground. Their eyes are so devolved that they can only tell whether it is light or dark. Thanks to these little guys, I learned there is more to this whole “herbivore” vs. “carnivore” dichotomy than I thought: they are “radicivores” meaning they eat only tubers. But in fact they farm tubers! When they find one, they will eat the inside and then let it grow back. In this way, a single tuber can feed a colony for weeks to months. Also, strangely, the mole rats eat their own poo in order to get the maximal amount of nutrients out of everything they eat. Oh, and they don’t drink water and can’t feel pain. Didn’t I say they were strange?

nacktmull
The rats – even though they aren’t really rats, they are more closely related to hamsters – have a social structure with a queen at the top of the hierarchy. The rest of the colony is split into castes with some specialized as tunnelers, some as warriors, some as foragers, etc. The fattest and laziest caste sits around until one day they decide to wander off and start a new colony. After the queen of a colony dies, the remaining females fight to be the queen, upon which the winning female grows an inch or so and starts popping out huge numbers of babies. Because of this structure, individual colonies are incredibly inbred and their individual DNA is virtually identical. In fact, they are so socialized that if they are kept alone they will die. They need their hive!

The foragers use their giant teeth to dig through the dirt and create large underground tunnels, attempting to run into tubers at random. Great strategy. Their teeth are actually on the outside of the mouths, and can be moved individually, like chopsticks. In fact, up to a third of their cortex is devoted to their mouth area! That is ridiculous, oh silly naked mole rats. Now I suggest you watch this movie about them, and gaze in some awe.