In love with animagus forms effect on humans, so I did some research into Red Deer, and here's something hilarious:
Humans have a very balanced, pretty constant level of testosterone. Stags, do not.
Stags shed their antlers every year, in March/April. This is important for antler health - if they don't, they get too heavy and grow wrong and they don't grow nice new ones. From art and our assumptions, James has very healthy antlers, which means to some extent (perhaps not as extreme due to his human side), he has a testosterone cycle.
This means that in October, he goes into rut (quiet, omegaverse). He's more aggressive, eager to fight other males, and has a higher libido. (I've also just realised Harry being born in August means he was conceived around October/November sooo.... canon) It's likely the worst time for anyone to annoy James, but especially the dog and wolf that he lives with, and his temper is hair-trigger (poor Snape).
Then in March his testosterone drops dramatically. This causes shedding, which I reckon would be an itch or headache in human form that he has to turn into a stag to get any relief from, when his antlers finally fall. But also, the drop in testosterone would make him irritable, fatigued and lethargic. Which is fun right before your exams!
And then in the summer he'd hate head injuries even more cause the skin where the antlers regrow is very sensitive. Also, cause he's regrowing his bones, he'd crave calcium rich foods like cheese lol.
Ooh, and fun metaphor - while Remus' cycle is monthly and thus related to the moon, James' is yearly, and thus related to the sun 🌞
And for some extra animagus fun, let's look at what they can't eat! (They'd probably be more intolerant than deathly allergic due to their human sides, but still):
Dogs: chocolate obviously. Grapes/raisins, onions and garlic, macadamia nuts and avocado weirdly (it's the 70s he's fine on that one), caffeine and funnily, alcohol. Let's just say it makes Sirius a massive lightweight.
Stags: he's kind of alright, since while they're mostly herbivores they're opportunistic and can eat most things. However they are not equipped to eat with human levels of starch or fat, so James would probably end up eating a very low carb and low fat diet, no full English breakfasts for him!
Rats: wow I thought they were indestructible but at least for pet rats, the list is long. Chocolate, citrus, mango, uncooked beans, blue cheese, caffeine, licorice, poppy seeds, raw cabbage, Brussels sprouts, peanuts, rhubarb, dried corn. However this list is partially due to the dose, so given the size translation between a rat and a human, he'd probably be more fine with his intolerances, since for example they say "dont give rats caffeine" cause any human amount would kill them cause theyre just so small. They also apparently can't vomit, which is fun.
It depends on the headcanon cause it could just mean that they need to be careful not to eat these things too soon before transforming, and that their human stomachs are fine with it still. But it'd also be hilarious if they had to change their diet overnight cause a bunch of things suddenly make them quite ill. Also James being the only one able to digest caffeine is funny, at least in safe amounts. He wouldnt be chugging wizard red bulls because apparently, deer are already prone enough to just dying from over-exertion or stress that caffeine would be a bit of a nightmare for his heart!
More things I discovered in my research: dogs are chemically predisposed to feel unsafe without their pack 🥺 he'd have genuine physical separation anxiety due to his loyalty to his friends 🥲 (dont think about canon dont think about canon). Also dogs don't sleep in 8 hour blocks, so he'd nap a lot during the day instead.
And I've seen people talk about Sirius as a seeker before, and it's great for both the Gryffindor-Slytherin brotherly rivalry, and also because dogs are born predators even more than humans are, and have both faster vision and mental target lock for this. What I'm saying is I bet Harry wasn't the first to catch the snitch in his mouth! Meanwhile James as a chaser works really well cause stags both have herding instincts (good team captain) and being prey have high awareness and hyper-vigilance and good peripheral vision (which is more ironic cause glasses and bad eyesight give you absolutely none)
(Feel free to fact check me on all of this, deer, dog and rat experts of tumblr. I just googled things and id love to hear more from the experts!)
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