Terminally ill ants choose to die alone

i-32ddc6e97f21cef06bc537f50b2f18d6-Captain_Oates.jpgIn 1912, Antarctic explorer Captain Lawrence Oates willingly walked to his death so that his failing health would not jeopardise his friends' odds of survival. Stepping from his tent into a raging blizzard, he left his men with the immortal words, "I am just going outside and may be some time." It was a legendary act of heroism but one that is mirrored by far tinier altruists on a regular basis - ants.

Like Captain Oates, workers of the ant species Temnothorax unifasciatus will also walk off to die in solitude, if they're carrying a fungal infection. In fact, Jurgen Heinze and Bartosz Walter found that workers, regardless of the reason for their demise, take their last breaths in a self-imposed quarantine. A Temnothorax worker may spend its life in the company of millions, but it dies alone.

In nature, old age is a luxury that few individuals can afford. Most often, death comes at the hands of predators or parasites. In the latter case, dying individuals pose a massive threat to their peers. In the closely-packed, humid environment of a nest, infections can spread like wildfire. Metarhizium only becomes infectious a few days after its host succumbs - it takes that long to produce new spores. And by that point, the ants are long gone.

Heinze and Walter treated 70 workers with spores of the parasitic fungus Metarhizium anisopliae. Three-quarters of them were dead within ten days. Of these, at least 70% voluntarily left the colony either hours or even days before that point and died well away from their nestmates. Another 21% were found dead outside the nests. It wasn't clear if they had left themselves or been evicted but certainly, the other ants don't treat infected workers any differently.

The tiny hermits never try to return home. They don't forage for food or water. They never try to get in touch with nestmates. If they're returned home, they'll actively try to flee again. As Heinze and Walter say, this appears to be "an active and, in most cases, adaptive response of the dying ant to its own condition."

i-1a6aad1aa5d3dc690863c819aac94f3f-Temnothorax.jpgIt's not just fungus that prompts these quarantines.  Heinze and Walter found that they could trigger the same behaviour by exposing young and apparently healthy workers to extremely high levels of carbon dioxide, a treatment that causes ants to age prematurely.

Even ants that are dying of old age will leave the nest. The duo kept an eye on over 1,600 workers from 28 different colonies and found that 92% of those that died spontaneously left the nest beforehand. The only one that died back in the nest was mistakenly carried there by another worker! Unlike the infected workers, they started their reclusive spell around 1-15 days before they actually died, probably because their health wasn't failing as quickly.

The fact that workers showed the same behaviour, regardless of the cause of death, suggests that the self-imposed exiles aren't just part of the parasite's manipulations. Parasites can famously twist the wills of their hosts to increase their chances of finding another. For example, some fungi and liver flukes can make ants climb to the tips of grass stems before dying, so that the spores can be dispersed on the wind or the flukes could be eaten by another host - a bird. Either way, it looks very much like the ant has willingly marched off to die alone but its strings have actually been pulled by the parasite.

That's clearly not the case for the Temnothorax workers. Although the colony doesn't really benefit if a poisoned or old ant dies elsewhere, Heinze and Walter think that these are just side-effects of a general rule that says, "If you're dying, leave, in case it's something serious that could kill the others."

Similar strategies may be common in other social insects, like Temnothorax, that have very small colonies. With so few individuals, they can ill afford the risk of quarantining infected individuals in the nest or of assigning "undertakers" to evict sick ants to specific sites. Perhaps this behaviour explains why bees disappear from hives that are associated with colony collapse disorder - it's a case of morbid seclusion en masse.

Leaving the nest isn't just a passive end either. It actually accelerates the hermit's demise. If Heinze and Walter blocked the entrance to the colony after poisoning workers with carbon dioxide, they found that the affected individuals actually survived for longer than if they had managed to escape. Like Captain Oates, they accelerate their own deaths for the good of their kin.

Reference: Heinze and Walter. 2010. Moribund Ants Leave Their Nests to Die in Social Isolation. Current Biology http://dx.doi.org/10.1016/j.cub.2009.12.031

Image: from AntWeb

More on ants:

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not to be a bitch, because this is an incredible story, but the sheer amount of grammatical errors makes it very difficult to take this blog seriously sometimes. :(

By monkeysharpie (not verified) on 28 Jan 2010 #permalink

Very interesting, are there other hive/social creatures besides insects that show similar behavior?

I like this piece very much, as it makes me question altruism. The instinct of the ant, would in humans, be accredited to bravery and selflessness. We would normally not make such connections to ants, leaving me to believe that even with us, this act is not a very sophisticated advanced feature of the brain. Fascinating.

Cats do it too. And since they hide ailments until they are terminal (or close to) one has little if any warning. Since this happened with one of our older cats a few years ago we take special pains to be sure we know where they are every evening and morning.

By Gray Gaffer (not verified) on 28 Jan 2010 #permalink

I like good grammar as much as the next person. But consider that I basically spend an extra 1.5-2 working days every week on this blog, often in the middle of the night. Proofreading every piece is something I'd love to be able to do, but it's a luxury that the need for sleep sometimes curtails. If you find grammatical errors, feel free to email me about them.

Let's see, Ed has an exciting post on colours in dino feathers, another amazing post (on so many levels) about rotifers escaping parasites and still being able to shuffle their DNA without sexual reproduction, and this wonderful post on the 'altruistic' behaviour of terminally ill ants; and someone is complaining about grammar??! Send him an email with actual mistakes rather than a random drive-by attack. Sheesh!

I'm a wildlife biologist/ecologist and your blog, Ed, produces stories that make me, despite all my experience, still say "Wow!". I read these three posts tonight and now I will be copying the links and sending them off to friends and students. These types of stories are the ones that got me hooked on biology a few decades ago when I was a child. Man, there's so much to still learn--what a journey of discovery! Thanks, Ed.

By Daniel J. Andrews (not verified) on 28 Jan 2010 #permalink

The Captain Oates story is a good one, but very likely invented after the fact. A great deal of the folklore around the Scott expedition was fabricated. Because it's so good, we may treat it like any great line from fiction; but who's the author?

There's a good case that Oates's death amounted negligent homicide, as a result of Scott's aggressive incompetence: what Oates and his colleagues, including Scott, were dying of was scurvy, the prevention and cure for which had been known for a century. Scott didn't bring along anything that had any vitamin C in it. Amundsen's group ate fresh seal meat, while Scott's crew subsisted on pemmican.

Anyway, three cheers for Ed! I'm half inclined to tell everybody who spells differently from Ed to get with the program.

By Nathan Myers (not verified) on 28 Jan 2010 #permalink

You're both too kind. The "Wow" factor that Daniel describes is still the reason why I write this stuff.

I'm puzzled. Seeing as how all members of the expedition died, how could the legend of Oates' heroism appear after the fact? I thought the evidence of it came from the journals of the other members, who did not survive much longer. Hard to believe that they would fabricate the story. Are you suggesting that they murdered him, and agreed on a cover story? Sorry for the OT, BTW. Great story. Loved the footage of infected ants in Planet Earth-anyone else catch it?


re your grammatical errors: I have never noticed, probably because schools stopped teaching English grammar 50 or 60 years ago where I live. To me, your posts are well-written, thought-provoking, and use vivid language to translate complex ideas from dry academic papers with great clarity. Your posts capture the wonder of science and nature and express the excitement and enthusiasm that (I assume) the researchers feel.

I'm going to ask a couple of questions about 2 recent posts, if you have time to answer them, then hibernate back into my usual lurking form.

By Reviewer 3 (not verified) on 30 Jan 2010 #permalink

Ed, don't bother trying to explain to people like monkeysharpie that we blog on our spare time and don't have editors. They've apparently got nothing better to do than nitpick. I infer that the 4th circle must be a pretty boring place. ;)

@Gray Gaffer #6 Cats do it too. And since they hide ailments until they are terminal (or close to) one has little if any warning

Actually, in that case I think it's simply because cats are modified from solitary animals. They don't actively avoid contact when ill, they simply don't show signs. And the reason for that is simply that there wouldn't've been any need to show such signs in their wild ancestors. Social animals like humans have evolved instincts to let our companions know when we are ill or injured, in order to receive assistance. A wild cat would have no need to show signs of injury or illness. In fact, it might even be advantageous to hide such signs - a rival who noted that he or she was becoming weak would surely take advantage of that weakness. Feigning health might allow an injured cat to bluff his way out of confrontations until he's healthy again.

By Christina (not verified) on 30 Jan 2010 #permalink

The thread has kind of split three ways it seems like, so here's my 3-in-1 comment:

1. Ants - they are an endless source of amazement. How they seem to know exactly what to do in complex situations when you know they have the brain of..well, an ant - is a fascinating area to study (I would imagine), and certainly fascinating to read about. There's probably some real transformative lessons about systems and information dispersal via networks that could greatly benefit society waiting to be discovered.

2. Your choice of stuff to write about - always fascinating. You get really high marks in this area as others have noticed. You seem to have a talent for identifying that magical area of intersection between your work and what the public will actually be interested in reading.

3. Your grammar - spellcheckers don't catch everything, even if they have grammar function built in. It seems to me that you have to decide how formalized you want your blogging to be. If you have time (5 minutes) to make one final scan through your posts just looking for grammar errors, that would be an effective patch measure - but after all, it's blogging, not publishing in a journal.

Um - I'm sorry, but what grammatical errors?

I like your article. well presented and interesting. keep up with the good work. Thanks.

Ed's grammatical errors are so rare as makes no never-mind. What the quibblers are quibbling over are spelling errors. Of course spelling has only the most glancing acquaintance with grammar, and to complain about the one after noticing the other is the height of ... well, something unpleasant I wouldn't care to mention here.

About legends of the Scott expedition... Most of the fabrications were performed by Scott's widow and expedition sponsor. His actual log was not released until many decades later. I don't know if this particular story was fabricated by them. If it appears in Scott's actual log, it still might have been made up by Scott himself to make Oates look better. Scott should have felt he owed that to Oates, considering he was personally responsible for Oates's farcically unneccessary death.

By Nathan Myers (not verified) on 31 Jan 2010 #permalink

@monkeysharpie - In your complaint about the grammar in this blog, you incorrectly used the word "amount". It should read: "the sheer number of grammatical errors". Just FYI if you ever feel the need to complain about the number of grammatical errors again.

@NERS - I enjoyed this article and it leaves me itching to learn someday what mechanisms cause this response. Thanks for another good read!