You will know which changes are expected in the surviving animal, which ones call for a vet, and what nobody can honestly claim about what it feels.
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One animal dies, another stays. The second starts doing new things, and you do not know what to make of it. You would like a clear answer about what it understands.
A shared home has a geography: occupied spots, routes, an order of passage. The loss of one animal releases all of that at once.
Some survivors take the other one's place immediately; others avoid it for weeks. Both reactions are described, and neither reads as proof of feeling.
The rhythm changes too: nobody sets off the getting up, the trips to the bowl, the moves towards the door. An animal that used to follow finds itself without the thing it followed.
A drop in appetite after a loss is often reported. It has several possible explanations, and grief is only one of them.
Eating alongside another animal shapes how an animal eats: the competition, the pace, the company beside the bowl. Removing the other animal changes the meal itself, not just its atmosphere.
The practical rule is the one that applies to any refusal of food: a few days of not eating properly, in a cat especially, calls for a vet rather than an interpretation.
Many survivors check the usual places, wait by a door, get up at a particular hour. It is one of the most consistent observations there is.
That behaviour shows an expected element is missing. It does not show that the animal holds any idea of death, and nothing allows us to say that it does.
The distinction has a practical consequence: reassuring helps, explaining does nothing. What helps is steady timings and your presence, not words.
The question comes up every time, and the honest answer is that it is unsettled. Some vets suggest it; others see no demonstrated benefit.
What is reasonably established is that the scene should stay calm and short, and that no animal should be made to take part. An animal that steps away has said what it had to say.
Not doing it causes no known harm. It is your decision, and it does not deserve the guilt usually attached to it.
In a pair, one often starts things and the other follows; one keeps watch and the other rests. Those roles are not character traits, they are positions in a system of two.
When one position disappears, the other reorganises. A quiet animal can become demanding; a settled one can start watching things it never watched.
These changes usually take several weeks to bed in. Judging them too early leads you to decide the animal's personality has changed when it is simply finishing the move.
Your state is part of its immediate environment: your timings slip, your voice changes, you touch it less or a great deal more, you cry.
Part of what you are seeing in it answers that, rather than the other animal's absence. The two are hard to separate, and it is more honest to say so than to put everything down to mourning.
This does not ask you to hide. It mostly asks you to hold the concrete landmarks: meal times, walks, places. Those are what carry the animal while you are elsewhere.
The idea of a new companion to console the survivor arrives fast, often within days. It mainly relieves the person proposing it.
An animal that has just lost its housemate, living in a household turned upside down, is not in its best period for taking a stranger into its space. Waiting costs nothing.
And some animals do very well on their own. A one-animal household is not an incomplete one, and deciding not to take another is a defensible choice, not a surrender.