A creature has evolved to protect the herd. Its blood contains chemicals that act like pepper spray, burning the eyes and throat of anything around it. However it wants to be able to get a decent coverage on a pack of predators.

So how large an area could the blood be sprayed into?

To clarify I'm picturing a weaker member of the herd being caught - perhaps a younger one - and when the predator's bite severs an artery the blood is released.

I suppose, really, the question can be interpreted as "How high a blood pressure is possible?"

Another few points:

  • Gravity: Similar to ours
  • Animals, same basic structure - hearts pumping blood around and such.
  • 2
    $\begingroup$ I wonder how such an effect might evolve... After all, being torn asunder by predators is not exactly something that gives you an advantage in mating... $\endgroup$
    – Burki
    Jun 6 '17 at 15:19
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    $\begingroup$ Neither does menopause. Evolution is about passing genes. Genes don't care if it;s you or your sister that has off spring. Mostly I don't understand how predators would eat such animals at all. They either evolve immunite or avoid them entirely. You can't eat 'around' the blood. $\endgroup$
    – Mormacil
    Jun 6 '17 at 15:22
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    $\begingroup$ Have you heard of the horned lizard? $\endgroup$ Jun 6 '17 at 15:27
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    $\begingroup$ So if the animal gets hurt by a non-predator, it still explodes, spraying the other members of its herd? Maybe you should make this voluntary. I'm btw not saying this is a bad question, I'm just curious because I like weird things $\endgroup$
    – Raditz_35
    Jun 6 '17 at 15:28
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    $\begingroup$ @Burki Imagine two herds, one has this mutation and the other doesn't. The herd with this loses one of their weakest and the rest of the predators are unable to keep up the chase. The other herd gets chased down and loses more members...and so is weaker. $\endgroup$
    – FreeElk
    Jun 6 '17 at 15:28

You're right that blood pressure can be used as a proxy for how far it could potentially be sprayed. Knowing that healthy human blood pressure should peak at around 120mmHg, and bottom out at 80mmHg, it can spurt around 15 cm vertically and 46cm laterally.

The giraffe has one of the highest recorded blood pressures in the animal kingdom at 240/180mmHg due to a host of cool physiological features, so you could loosely double the spurt range from an artery to 30cm vertically and 100cm laterally.

However, animals make use of other liquid projectiles:

Projectile use isn't common in the animal kingdom because of the energetic constraints of making the projectiles, but certainly has evolved before.

Every mammal can, of course, urinate (larger animals have higher flow speed and, I would assume, greater effective range) to excrete soluble wastes and mark territory. You could invent some interesting defence given that urinating in the face of danger is common to all animals, including humans, but the toxicity of urine is probably dependent on what is consumed in the first place.

  • 3
    $\begingroup$ I like the fiery pee idea... $\endgroup$ Jun 6 '17 at 18:18
  • $\begingroup$ human blood pressure during exercise (such as running from a big predator) can spike to 200 mm Hg or higher. $\endgroup$ Jun 6 '17 at 19:20
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    $\begingroup$ I should have clarified the blood pressures to be 'at rest'. I would assume the blood pressure of a giraffe also increases substantially when running away. (New event at the morbid olympics: Long Spurt?) $\endgroup$
    – Force4
    Jun 6 '17 at 19:32
  • $\begingroup$ Even at rest I would guess it gives me an order of magnitude idea, right? Like 100cm for giraffe at rest isn't going to turn into 1000cm for him running is it?. I'm still sticking with the blood idea for story reasons but thanks for the others. $\endgroup$
    – FreeElk
    Jun 7 '17 at 10:44
  • $\begingroup$ It does. I doubt you could find 10 metres naturally. Bonus reading: en.wikipedia.org/wiki/Autohaemorrhaging You could have some sort of specialized 'appendage' on the rear of the animal (or wherever is most likely to get snagged by a predator's claws) that swells massively with a certain finite volume of blood during escape, and if snagged begins to dislodge its contents. This may be difficult because predators would eventually learn of, and plan around, the blood-hose and only attack from a different angle. $\endgroup$
    – Force4
    Jun 7 '17 at 17:58

The Horned Lizard already does this after a fashion.

...are also able to squirt an aimed stream of blood from the corners of the eyes for a distance of up to 5 feet (1.5 m).

This is a single target squirt, not an area effect but nature has already found a solution that you can use as a model.

The main thing is that they pressurize the blood in the head to increase the range of the squirt.

If you are using simple blood pressure to provide the spray, you are inviting a bleed out situation any time they are injured. It might be better to have blood accumulate in "pressure vessels" or "blood sacks" under the skin that can be closed off when they lose pressure to prevent too much blood loss.

It could also have specially designed pores leading to those sacks. If the sacks collect blood waste products, the blood could be mildly toxic or irritating then. You could use the Cane Toad of Australia as an example. They don't really squirt (it is more like ooze) but there's no reason your critter can't use a similar mechanism under pressure.


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