One of my favourites is the recurrent laryngeal nerve: In most mammals, this already is a pretty poor situation.
The nerve, instead of travelling directly from the brain to the larynx, it typically travels down the neck, around the aortic arch in the heart, then up to the larynx. In humans, this makes it ~5x longer than the ideal route. In giraffes, it extends the nerve to nearly 4.5 meters (~15 ft).
In addition to that, the distance between their feet and their brains, they have built in lag (about 100ms), meaning they need their spinal muscles to manage to process of finding footfall, rather than thinking about it.
Giraffes and nerves are kinda weird, circulation is a whole other level of messwittery. Firstly, they need to get blood to their brains, as a result, their hearts are huge. Consequently, their blood pressure is extremely high, the highest of any animal and their heart rate rests at about 170bpm. This is fine for getting blood up to the brain, but is problematic for the lower body.
In order to prevent turning into a whirling dervish of blood spewing madness every time they get a leg injury, and to prevent blood simply pooling in their legs, they've adapted extremely tight skin on their legs, and a series of one way valves to keep blood running in the right direction.
So that's not so bad, right? I mean, it's weird, but it's to be expected.
Problems start when it has to drink; when a giraffe lowers itself down to drink, it has to contend with the blood now rushing towards its head. Without compensation, it would die as soon as its head got below a certain threshold.
Firstly, when a giraffe bends its head down, valves in its neck shut down, preventing excessive blood flow to the brain. For the extra load from blood re-entering the brain, they need to distribute it in a spongy network of blood vessels. As they stand again, they use this system to maintain a steady blood pressure in their brain as they stand again. I suppose the upshot of this, is that throughout history, genetically weak giraffes have died in absolutely hilarious ways.
That's all great, but it's nothing on my favourite evolutionary arms race (barring ducks).
The Acacia tree vs giraffes:
The first line of defense for an acacia tree is the fact that it's covered in huge spines.
Giraffes get around this with their massive prehensile tongues, which they can use to avoid spines and still strip leaves.
The next line of defense for the acacia is tannin. Tannin tastes terrible (it's also toxic, and can kill other herbivores), when a tree senses it is under attack, it ramps up production of tannin in order to make it less attractive to eat.
The simple solution to this for the giraffes is just to rotate trees.
Now multiple trees are at risk, the acacia plays its next trick: it communicates with other trees in its vicinity by releasing chemicals into the air (fun fact, the lovely smell of fresh cut grass is also a distress marker). As the trees pick up these chemical markers they all ramp up production of tannin until the threat is gone.
It's at a point where giraffes now need to stalk acacia trees, approaching them only from downwind to avoid the trees that have been alerted.
The acacia has one more trick up its sleeve: Some species have developed heavily modified spines which house aggressive ants. The acacia have developed a symbiotic relationship, feeding the ants on nectar and housing them, in return the ants aggressively defend the tree.
As for the other stuff I can't fit in here: some of you may recall a terrifying image of a leatherback turtle's throat (actually just fleshy appendages - they eat jellyfish). Anyway, long story short: giraffes have them too. They've also got extremely strong esophagus muscles to facilitate regurgitation of food and you can make hallucinogenic drugs out of them.
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