• Question from:Nicolas

The question may sound strange, but I was wondering whether there are animals, plants, insects or other living creatures that, by some standards (heart, breathing, pulse or others) would not fit the definition of life of their respective category, i.e. that could be considered dead?

The more general underlying question (if you've got time to spend answering this) is, of course, how would life be defined in the first place: are there strict standards that are agreed upon in biology? Then how can we account for cases that do not fit that definition (if there are any, of course)?

References (for non-biologists - I am in communication studies) would be welcome, as I am really growing fond of the issue by way of all those zombie movies I've been watching. Thanks a lot.

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  • University of Bristol
  • Posts: 2,462

The most obvious example of of what you are talking about and often sparks questions on what we mean by dead/alive are viruses - see below

http://www.askabiologist.org.uk/answers … php?id=101

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  • QMUL, London
  • Posts: 1,022

There are frogs in North America that freeze in winter. Turn to ice. No heartbeat, no nothing. When they thaw out in spring they somehow come back to life and carry on. I don't think it unreasonable to call these 'dead' in their winter phase. so there is a vertebrate that fits your definiton.

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  • At home in Ruardean, Gloucestershire, England
  • Posts: 625

Then there are "zombie ants", which have been taken over by a fungus that modifies their brain to make them behave in ways advantageous to the fungus.  See for example this report in Scientific American: http://www.scientificamerican.com/article.cfm?id=fungus-makes-zombie-ants

  • University of Bristol
  • Posts: 349

And there are quite a few examples of ‘mind-controlling’ parasites! e.g., see: http://www.livescience.com/technology/0 … velgy.html
http://www.damninteresting.com/mind-con … ie-spiders (and see reference to scientific article therein)
Also, there may be evidence that ‘mind-controlling’ parasites date back millions of years!! (http://www.msnbc.msn.com/id/38747237/)

Last edited by Steve Lolait (2nd Nov 2010 12:37:20)

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  • South London
  • Posts: 527

To expand slightly on Dave's example of frozen froggies (and going off at a slight tangent!), the whole process of, for example, deep 'hibernation' is known as cryptobiosis (hidden life) and is exemplified by things like fairy shrimp ('space monkeys') and especially, tardigrades. They've been frozen to near absolute zero (~-270 Celcius or 3 Kelvin), superheated, blasted into space vacuum, huge atmospheric pressures; you name it, these little critters have taken everything scientists have been able to throw at them and still 'come back to life' when conditions are right. Brilliant!

In other tangential news, Ampulex compressa does rather a good job in creating 'zombie' cockroaches by inhibiting the flight reflex and leads them to their doom by dragging on their antennae (like leading a cow to the slaughterhouse). Once in the burrow, the wasp lays an egg on the 'roach, which is now food for the endoparasitoid larva. I'm sure an internet search engine of your choice of this jewel wasp will lead to more interesting info...

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  • QMUL, London
  • Posts: 1,022

Oh yeah, tartigrades, they are good, nice shout. The rotifers are pretty good like that too, they take a quite a bit of killing when in the dormant state.

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  • At home in Ruardean, Gloucestershire, England
  • Posts: 625

But tardigrades aren't zombies, they just have superpowers.

(Has anyone made a superhero, Tardigrade Man, yet?  If not, it might be time for me to get into the webcomic business.)

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  • Institut de Biologie du Développement de Marseille-Luminy (IBDML), Marseille
  • Posts: 79

To continue off on a bit of a tangent, DARPA (the US defense department research agency) is working on methods of suspended animation to use treating battlefield injuries.  I think the idea originally comes from the observation that people who "die" when they are cold and their blood is high in CO2- such as someone smothered by an avalanche - have comparatively high chances of reviving when they are warmed up.  Here is a link to the Danger Room blog post that discusses the research:

http://www.wired.com/dangerroom/2009/12 … ating-gis/

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  • University of Arkansas for Medical Sciences
  • Posts: 159

My personal favorite is Deinococcus radiodurans, a nifty little
bacterium that can take enough radiation to melt glass by letting its
DNA get shredded, which it then just puts back together and trucks on.
There are lots of bacteria that form endospores, which are almost
impossible to kill short of torching them to ash.

But these
aren't up and moving around, even if they are tough to kill while in
their dormant states. The fungus and parasite controlled animals are
mobile, but they aren't dead, even if they are effectively brain-dead.
They are more like the 28 Days Later type of pseudo-zombie.

So we don't really have walking dead sorts of creatures that really fit the whole movie zombie package.

The
definition of life is a tricky thing, though. So far, every definition I
have seen can be contradicted in some form by something. So, to your
question, no, there is not a strict definition that is accepted by all
biologists. Most definitions involved reproduction and metabolism of
resources for growth and maintenance, but there is a lot of debate.

You'd
think that something so basic to biology (it is the study of life after
all) would be worked out. Sadly, it just goes to show how complex the
subject is. but that's the whole fun of it.