Bullying in a rat cage rarely looks like a battle. A younger, faster cage mate simply arrives first, takes prized pieces of food from the senior rat's paws, and monopolizes the warm sleeping spot, and the older rat, who moves more slowly and tires sooner, quietly eats less and less. Watching works when it rests on defined signs, checked on a schedule, rather than on a general feeling that the younger rat seems pushy.
Learn the Watch Set by Heart
Check for the same watch set every time: weight loss, eating only when the younger rat sleeps, waiting by the door, and wounds or barbered fur. Weight loss is the sign to measure rather than guess, so weigh the senior rat weekly on the same scale, at the same point in the day, and write the figure down, since a downward drift across two or three entries confirms what a hand on the ribs only suspects. Eating only when the younger rat sleeps shows the senior has stopped contesting food altogether and is fitting meals around the other rat's schedule, and waiting by the door, greeting you eagerly at the front of the cage while ignoring food placed deeper inside, often means the door is where food arrives without competition. Wounds or barbered fur, small bites, scabs, or patches chewed short on the back and shoulders the senior cannot reach alone, show that the pressure has become physical, even if you never witness a scuffle. Note which signs you see and when, in a few plain words, because a single sign can have other explanations while a cluster of them, growing over the same weeks, describes a cage dynamic that needs changing.
Feed So Food Cannot Be Guarded
Redesign feeding so one rat cannot own the food supply. Scatter the daily ration widely through the bedding instead of serving it in one bowl, so finding food becomes foraging that two rats can do in different corners at once, and place food at several points on different levels the senior can reach without climbing past the younger rat. Add a second water bottle at the senior's preferred level for the same reason. Then add separate supervised senior meals: lift the senior out, or feed them in a secure play area, for a calm meal of their normal ration plus any food the veterinarian has suggested for an older rat, while the younger rat feeds in the cage. Stay and watch the senior eat at their own pace, noting appetite and chewing as you do. If the senior eats eagerly and fully at these meals while barely eating in the cage, the food itself is fine and access is the problem, which points your fix squarely at the shared setup.
Separate for Safety Without Cutting Companionship
If the watch set persists after feeding has been opened up, or the senior keeps losing weight, separate the rats for the senior's safety, but separate with companionship rather than into isolation. House them side by side in adjoining cages, or divide a large cage with a barrier that allows sight and smell contact, so the senior keeps the social presence of another rat without competing at every meal, and consider whether a gentler companion arrangement suits the senior better than life beside this particular younger rat. Keep the supervised meals and weekly weighing running after separation, and expect the senior's weight and willingness to eat in the open to tell you within a few weeks whether removal of pressure was the answer. Take the record to the veterinarian if weight loss continues despite free access to food, if wounds appear, or if the senior becomes withdrawn or stops eating, since possibilities the vet may need to explore include pain, dental trouble, and other illness in an aging animal. A dated run of weights and watch notes is far more useful in that appointment than a summary from memory. That run of notes is exactly what KinsLe Pet Passport keeps together in KinsLe, alongside visit summaries and weight entries, for the veterinarian to read.