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It's a lizard that has the ability to burst blood vessels in it's eyes and shoot blood as self defense.

How can this behavior be explained evolutionary? Wouldn't it mean that there had to be some intermediate evolutionary step where the lizard could bleed from it's eyes only slightly? How did that not cause it's extinction?

To generalize more, how does a specific trait evolve when it seems the intermediate traits are detrimental to survival?

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I like this question. Hope someone has a good, scientific answer. – J. Musser Jul 20 '14 at 6:06
each specific case is a separate question. – user813801 Jul 20 '14 at 6:33
@user813801 I'm asking this about this specific case then. – Darwin Jul 20 '14 at 6:34

The behavior can be explained evolutionarily just like any other trait. It came about either through adaptation, neutral evolution, or as a by-product of another adaptation.

An intermediate step, like the one you suggested, is not required. But if it did occur, there's no reason to believe that such a trait would cause extinction. Bloody eyes, whether they squirt or not, could be beneficial in some species. We know that blood-squirting eyes appears to be a defensive trait. Take away the ability to squirt and the blood could provide defense in some other unexpected way, or it may have had nothing to do with defense at all. Maybe the blood was just a lubricant before, or maybe the blood somehow enhanced vision during a stressful event. Or maybe it was just there and didn't do anything; nothing good, but nothing bad.

There's nothing special about this lizard. It does have a unique trait, but there's something novel in every species.

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I'm speculating here, but it's likely that the taste of the blood has something to do with this. The blood of the horned lizard tastes foul to feline and canine predators. Therefore, a small amount of blood leaking out of the eye might put predators off immediately after the first attack, if that attack was aimed at the head.

Obviously, this is an advantage for the lizard, but since a large percentage of attacks to the head would result in debilitating injuries, there is a selective pressure to take this further so that the taste of the blood is "communicated" to the predator before the initial attack.

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There isn't always an intermediate step like "oozing blood eyes", I suspect that this particular trait is pretty easily explained by having an area of weakness in a blood vessel that made the lizard bleed under high stress situations.

maybe in early version the lizard had to get some trauma or could intentionally hit its head on a rock or something... I don't know, but it doesn't really require any novel structure.

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