The appetite changes that matter most are about hay and droppings
Hay intake is the appetite change that matters most in an aging rabbit or guinea pig, because both species are built to graze almost constantly. A hay pile that barely shrinks overnight, pellets left in the bowl, and droppings that become smaller, drier, or fewer are the early warnings worth acting on, long before either animal looks obviously thin.
Weight can mislead with these two. A rabbit or guinea pig can hold a similar weight while eating far less fibre, so the droppings and the hay rack tell the truth earlier than the scales do.
Selective refusal often tells its own story
Selective refusal is a useful clue in an aging rabbit or guinea pig: an animal who still wants soft greens but avoids hay and hard pieces is often finding chewing uncomfortable. Back teeth in both species grow continuously, and when chewing becomes uneven the mouth can become sore in ways you cannot see from outside.
A guinea pig adds one more signal. Turning away from the vitamin C-rich greens he normally races for is a notable change for a guinea pig specifically, because those greens are both a favourite and a nutritional need, and losing interest in them rarely means simple fussiness in an older animal.
A general shutdown is different from pickiness
A complete loss of appetite in an aging rabbit or guinea pig is an urgent change, not a picky phase. When grazing stops, the gut slows with it, and a rabbit or guinea pig who has eaten nothing for many hours, sits hunched, or produces almost no droppings should be seen by a vet the same day.
Keep offering familiar hay and greens in the meantime, keep the animal warm and quiet, and do not wait for a second day to confirm the pattern. With hindgut grazers, the timeline for help is measured in hours, and early treatment is far simpler than late treatment.
Making eating easier for older mouths
Easier access keeps an aging rabbit or guinea pig eating when stiffness or sore teeth raise the effort. Put hay at ground level in several spots rather than in one high rack, chop long strands shorter, offer greens in smaller pieces, and check that water bottles flow freely or swap in a heavy, low bowl the animal can use without stretching.
Weigh weekly as well, in the same way each time, and pair the number with a line about hay and droppings. That combination catches the slow drift of an older grazer far better than weight alone.
What to bring the vet about appetite change
Bring a short, dated record when an aging rabbit or guinea pig sees the vet for appetite change: which foods are refused, which are still eaten, how droppings have changed, and the last few weights. That detail helps the vet distinguish a mouth problem from a gut that has slowed or another age-related cause without guessing.
Note changes within a day of noticing them rather than reconstructing the week from memory at the surgery. Storing the log alongside vaccination and visit history in KinsLe's Pet Passport means the whole picture travels with your rabbit or guinea pig to every appointment.