Most of us have some idea about what life is and what, or when, death is although the latter is a concept that has changed over time with scientific development.
In terms of life, a lot of high school textbooks go with movement, respiration, sensitivity, growth, reproduction, excretion and nutrition. There are always exceptions, possibly designed just to annoy those high school teachers… But it works as a basic idea. Apparently, NASA’s definition is that ‘Life is something that undergoes Darwinian evolution’ (Dr Louisa Preston quoted in Death on Earth).
Defining death intuitively feels like it should be straightforward… In the past, your heart stopping would be the end of your life but today we have CPR and technology that brings the possibility of being revived. If your body cannot function for itself, you may be considered dead, or you may be considered brain dead and be reliant on machines to keep your body alive. Where is the defining line in these cases?
Jules Howard adds further complicating examples…
“Consider those creatures that undergo cryptobiosis, able to survive for long periods as lifeless shells or hardy eggs. Consider, for instance, the tiny sexless metazoans that live in birdbaths (among other places), the bdelloid rotifers, which expel all water from their bodies and form a hard stone-like ball when their puddles dry up… They can last for seven years in this dehydrated state. They undergo no growth or metabolism, nothing life that, in all that time. They are surely not alive in this state… but they are surely not dead either. They might revive. And then there are the sea monkeys (brine shrimps), which can undergo cryptobiosis like bdelloid rotifers but for far longer, perhaps for centuries in some cases. Not all of these dehydrated life forms will find water. Many of them may blow away or be buried in places without water, and many will break down over years or decades, eroded by the elements.”
With these cases, when did the organism finally die? Wood frogs are another weird case, seeming to die and be revived each winter:
“Wood frogs stop breathing and their hearts stop beating entirely for days to weeks at a time. In fact, during its period of frozen winter hibernation, the frogs’ physical processes—from metabolic activity to waste production—grind to a near halt.”
– National Geographic
To throw another spanner in the works, let’s have a look at a case involving a zombie caterpillar…
Trees have developed tactics to win the war against pests, and some can influence parasites into attacking caterpillars. The tree influences the rate that caterpillars are infected by baculovirus. Once infected, the baculovirus enters the caterpillars gut and multiplies before overwhelming the entire body. The caterpillar swells because of this internal flood of the virus. The virus then manipulates the caterpillars behave, in a way that zombie writers would be proud of. The growth cycle of the caterpillar is halted and the caterpillars mind is essentially taken over. Instead of carrying out normal caterpillar behaviour, they seek out light, struggling up the treat and eventually burst. This is great for the baculovirus as it creates a virus shower that covers the tree, the leaves and all the other caterpillars who live there… Baculovirus 1 – Caterpillar 0.
Does the caterpillar die when it explodes? Or does it die when it’s mind is taken over by the virus? Or when it gets infected?
If we return to the school definition, the caterpillar is moving, respiring but is no longer growing or reproducing and I would have questions about whether it is sensitive any more…
Given that there are over 100,000 species of parasitic wasps, compare that to the less than 10,000 species of mammals, the case of the zombie caterpillar becomes important to our question about when is death. And spoiler alert, I probably won’t be answering that question…
Aside: If you want to explore a related question, why death is, then I recommend Death on Earth.
It seems like viruses may well be making zombies out of all of us. Apparently if you are infected with the flu, but aren’t showing any symptoms, you are more likely to engage in social activities and hence spread the virus. The rabies virus changes personality and causes aggression which again helps it move hosts.
Toxoplasma is something that cat owners may be familiar with, at least on an intimate level, possibly unawares. It loves cats because the only place they can have sex is inside the cats digestive system. This means they have to get from current host to cat, and they do this by manipulation. If they’re in a mouse or a rat, for example, they control the host’s behaviour and turn animals which are fearful of cats into animals that seek out cats. They travel further, explore more and have less anxiety about unknown or dangerous situations. Ultimately, toxoplasma is hoping the host will get eaten by a cat, turning the host suicidal.
“Animals, on the whole, don’t kill themselves unless their parasites want them to.”
Just for fun, let’s have a look at some of the other zombies that are currently making our planet their home…
There is a type of parasitic barnacle that sets up home in a crabs reproductive system, cutting off all chance of the crab reproducing. The barnacle has such power over the host that it can cause a male crab, who doesn’t normally take care of eggs, to care for the barnacle as if it was a brood of eggs.
Ants and caterpillars can get taken over by fungi, essentially becoming a fertilizing, transport vessel.
And for one final example, there are cicadas who end up pumped full of hallucinogenic drugs and have to face the horror of their abdomens falling off… Despite this, males then become hyperactive and hypersexual. Personally, sex is the last thing that would be on my mind if half my body had fallen off…
I’d love to know your thoughts about the when is death question. Until I started looking into zombie creatures, I’d not really thought much about it.