The organ of the rat that serves the same function is the cecum
Rats are strictly herbivores and therefore do not get much animal fat and the gall bladder is not necessary. The cecum is a large, thin-walled, blind pouch shaped looks like a comma, highly constricted at the middle, the rat cecum is different from others rodents as it is devoid of internal septa. It is used as a fermentation chamber. In the cecum there are bacteria that are able to digests the cellulose in the seeds. The food sits in the cecum and essentially rots.