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Giraffes, being one of the tallest mammals in the world, have a failsafe called rete mirabile to prevent them from dying from excessive blood pressure while lowering their head.[1]

Due to their long necks, a formidable amount of blood pressure is needed (quoted from animals.about.com):

A giraffe's heart has special adaptations to enable it to pump blood up the animal's long neck to its head. A giraffe's heart has the formidable task of pumping blood at high enough pressure so that it can flow up the giraffe's neck to the brain. To accomplish this, a giraffe's heart is specially adapted. It can weigh up to 10 kg (22 lb) and generates twice the blood pressure of other large mammals. Having enough blood pressure to pump blood to the brain when the giraffe's neck is extended upward is one challenge, but when the animal lowers its head it risks injury due to excessive blood pressure. To counter this, giraffes have a pressure-regulating system known as the rete mirabile which restricts the amount of blood that rushes towards the brain when the giraffe lowers its head.

How did this evolve? I believe in evolution but I can't wrap my head around this. If an "overload" of blood would reach the giraffe's brain while it is, for instance, drinking water and either instantly killing the animal or severely wounding the animals mental and physical capabilities by rupturing the brain, how could a failsafe like this evolve?

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    $\begingroup$ How could it not evolve? Taking the most laymanic interpretation of evolution by mutation, a proto-giraffe without the mutation of the heart would surely die. Those with it wouldn't and thus able to pass on the mutation to its descendants! $\endgroup$
    – Jamiec
    Nov 6, 2012 at 11:31
  • $\begingroup$ A giraffe's rete mirabile is located somewhere "behind" the brain, as a complex system of veins and tissue to form some sort of sack to store the excess blood, hence it's not the heart taking care of this matter. A proto-giraffe with this mutation would surely be the one to pass the genes on. But since this function, the rete mirabile, isn't really needed until the moment right before the sudden death/wound of the giraffe, how did it possibly evolve in the first place? $\endgroup$
    – Simon Carlson
    Nov 6, 2012 at 11:48
  • $\begingroup$ Where the thing is located is irrelevant. Mutation is purely random, and if that mutation gives you a step up on your neighbour, then your genes go on and his (possibly) don't. Again this is a huge oversimplification of Evolution. $\endgroup$
    – Jamiec
    Nov 6, 2012 at 11:56
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    $\begingroup$ According to Wikipedia, en.wikipedia.org/wiki/Rete_mirabile, these structures are widespread in vertebrates, and usually play a part in equilibrating two parts of the bloodstream (heat, ions). So this would simply require the adaptation of a prexisting structure to take on an additional role, with obvious selective advantage - if there is a selective advantage to having a long neck then there will also be one to changes that accommodate it. $\endgroup$
    – Alan Boyd
    Nov 6, 2012 at 13:03
  • $\begingroup$ Thank you for moving it to Biology. Im still navigating myself around StackExchange. Reading these comments they all make sense, but I feel my intention with asking this question is far too widespread and traces too far back in evolution, where it's more or less impossible to answer. Thanks for your time and input :). Question can be closed. $\endgroup$
    – Simon Carlson
    Nov 6, 2012 at 13:32

1 Answer 1

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I think what leads to the confusion here is the assumption that the neck was evolved before the rete mirabile. Giraffes evolved from creatures with shorter necks. As evolution occurred nature was selecting for longer neck creatures (being able to eat more leaves) but at the same time creatures with longer necks had problems with blood pressure. At this point this would also not be lethal because we are talking about animals with much shorter necks, just in some way disadvantageous to the animal. So over time necks got bigger, blood pressure went up, and blood vessels developed that could better and better deal with the change of pressure. Rete mirabile also occur in other animals, so they are not a rare evolutionary phenomenon of giraffes.

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  • $\begingroup$ its also true that since some reptiles and birds have Rete mirabile, that these short necked giraffes already had a mechanism to adjust the blood pressure rapidly. Its not hard to imagine that theirs simply adapted to more extreme conditions as the necks got longer. giraffes whose necks got longer without a well adapted compensation of blood pressure just didn't make it. $\endgroup$
    – shigeta
    Nov 6, 2012 at 17:13
  • $\begingroup$ Thank you, this made a lot of sense! This'll make it easier to fall asleep tonight :) $\endgroup$
    – Simon Carlson
    Nov 7, 2012 at 13:01

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