I'm undergraduate physics student, but I've always been interested in biology. So I have a couple of questions about an application of the evolutionary principles to practice.

  1. Agony as the last stage of dying.
  2. Hydrophobia as symptom during Rabies.

Now I'm going to clarify what exactly I do not understand. In the first case it's not clear for me how this mechanism helps to survive, how it can help at all? In the second case it's not obvious why this virus causes such a symptom. As I think virus always create favorable environment for himselve (for reproduction). But how could water prevent it?

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    $\begingroup$ These might be better posed as separate questions. $\endgroup$
    – kmm
    Mar 17, 2013 at 14:51
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    $\begingroup$ I'm with Kevin. These seem like separate questions. To give you an idea of the first - Evolution via Natural Selection doesn't really 'care' how you die, as long as you've passed on your genes (reproduced) beforehand. $\endgroup$
    – MCM
    Mar 17, 2013 at 20:28
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    $\begingroup$ Agony is not always associated with the last stage of death, many pass peacefully - and the agony when it does occur is likely to be caused by the cause of death (ie a symptom) and possible exacerbated by the fact the person knows they are dying... And viruses/disease can have weird side effects on neural pathways which could be why we suffer from unusual neural episodes/behaviors $\endgroup$
    – rg255
    Mar 17, 2013 at 21:05
  • $\begingroup$ Maybe you mean fear of death? I don't see that agony always accompanies death. Could you cite some reference or example? $\endgroup$
    – shigeta
    Mar 17, 2013 at 22:30
  • $\begingroup$ Viruses and bacteria have a tendency to care mainly about eating and/or dividing. If the host dies, it's often a biproduct of toxin or foreign body accumulation. Agony in death, however, can be either physical or psychological and I'd count that as either too broad, or inaccurate (not everyone agonizes, as in painful symptoms). I wouldn't count the first question as having to do with biology/evolution. $\endgroup$
    – CKM
    Feb 3, 2015 at 0:52

2 Answers 2


The hydrophobia symptom is most likely a bi-product of the effects the virus has on the brain and not an evolutionarily derived strategy to create an appropriate environment to reproduce in. Just the same as other symptoms the infection causes, like fever for example, is a reaction the body has to the presence of the virus and not something the virus is doing on purpose to set up a better environment.

A bit old, but check out [1]: "Hydrophobia may represent an exaggerated respiratory tract irritant reflex with associated arousal potentiated by the selective destruction of brain stem inhibitory systmes."

So basically the virus is indirectly causing a fear of water. The destruction of brain cells and inflammation caused by the presence of the virus in the central nervous system cause a reflex in the respiratory tract which manifests as muscle spasms and renders the patient, who is already pretty out of it at this stage, unable and very unwilling to drink any fluids [2].

[1] http://europepmc.org/abstract/MED/982512 (Warrell DA, Transactions of the Royal Society of Tropical Medicine and Hygiene [1976, 70(3):188-195])

[2] http://publichealth.lacounty.gov/vet/rabiesmanual/clinical.htm

  • $\begingroup$ It's not actually a fear of water. It's that they have spasms on swallowing. Rabid animals will not swallow their saliva either. Hence the frothy mouth. $\endgroup$ Feb 3, 2015 at 7:24
  • $\begingroup$ @anongoodnurse Because the brain prepares the body for swallowing before it occurs, thinking about swallowing may trigger the same reaction, which may appear as a conscious fear of water. $\endgroup$ Apr 3, 2017 at 18:39
  • $\begingroup$ The fact that hydrophobia is a biproduct of the virus's effects on the brain doesn't necessarily mean that it is not evolutionarily selected for. Rabies is transmitted via saliva in bites, so difficulty swallowing could very well have been selected for by evolutionary pressure (along with other behavioral effects such as increased aggression and hypersalivation). In effect, the host's altered behavior can be considered part of the "extended phenotype" of the virus! $\endgroup$
    – augurar
    Apr 20, 2017 at 8:32

If your first question means: why is later-life ageing usually a long and painful process of degradation (rather than why do we have existential agony) with diseases of ageing like osteoporosis, heart disease, dementia - then I can offer one answer.

There is an absence (almost) of any selection pressure after reproductive age. Once individuals are past reproductive age, any such diseases, unless they place a great survival burden on the immediate (related) community/family, do not influence fitness of the population.

In other words, perhaps evolution has nothing to say about diseases of ageing, and no 'imperative' to purge our species of these afflictions.


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