Brains. They think with their feet.
Although the above answer is sometimes true. Horses, Pigeons and yes Rats, actually lack a Gall Bladder. They do not have one, interesting, eh?
horses
lungs
What organ systems do echinodermes lacking
Neither rats nor horses have the ability to regurgitate (throw up). As gross as it sounds, the ability to throw up something bad is a life-saver.
Horses, Bats, and Rats are all mammals, give birth to live young, and nurse their live young.
aplasia
If you are talking about other versions, rats or white mice were turned into the horses. The original story was slightly different.
Deer do not have a gallbladder. Scientists have still not been able to establish why they lack this organ that's responsible for producing bile that aids in digestion.
Yes - only rats and horses do not have a gall bladder.
Rats do not have a gallbladder because their liver produces bile continuously, so they do not need a storage organ like the gallbladder to store bile.
Time and lack of rats which also died.
Rats, lack of sanitation and lack of understanding of how sanitation keeps germs from spreading.