My cat was on protection mode earlier this morning from something entering the room. I am wondering when animals start having sensitivity to the otherside.

You know I'm in on this one lol.
Animal have greater senses then people. It's a known fact. In saying that, they are super sensitive of the paranormal. Often times they sense things way before we do.
For example: While I was a keeper aid at my local zoo, I was cleaning stalls of the sloth bears. The male was familiar with my presence, my smell ect. I had some strange things happen that were paranormal while in that particular part of the zoo. For some strange reason, Franci would charge in my direction for no reason. Before or after the charging, I would feel someone tap on my head or arm when no one else was with me. I believe it was a spirit and the bear sensed it.

Even if it is a non threatening spirit, animals react defensively when they sense something off around them...unknown energy.

They do! My cat Shadow has never done that before, even when I go to bed he is always either at the foot of my bed or next to me. He has been aware of the paranormal ever since he was little. The cats that I have seem pretty sweet and not threating to me.

Maybe something came to you with a stronger energy that Shadow felt more then anything else before. Think of it like this...if a cat is outside and another animal crossed it's path or was just curious, the cat would go into typical defensive/protective mode...back arched, hissing and growling, fur standing up...against something it has never encountered before.

Since your cats know you and have been with you for a long time, they will protect you when they feel a potential threat. You know, cats are sacred and highly worshiped according to Egyptian history...cats rock! lol.

Oh yes they do! Also I have felt paws from one of the ghost cats at the foot of the bed coming up and sitting at my feet, while my cat was downstairs sleeping.

I have totally had this same experience. I swear I have cats around me all the time in spirit and in life. I know there is a black cat that protects me, I have seen him before when i was going through a tough time and needed protection from neg energy. I know he still comes to visit me and is around me. that's pretty cool u felt it too, pretty cool!

I have seen a black cat and a multi-colored one a lot of times. The black cat looks exactly like mine and he is a fluffy maine coon, siamese, bermeise mix. I see the black cat go into the kitchen like he is getting a bite to eat and then disappears. The multi-colored cat just sits upstairs on my bed waiting for me to come home It's so cute!

What makes animals so sensitive to the paranormal? That is what I have been wondering ever since I was little.