Pets often form strong attachments to one another, and the survivor of such a pair may seem to grieve for its companion. Cats grieve for dogs and vice versa.
You may need to give your surviving pets some extra attention and love to help them through this period. The survivor may appear to be ‘sad’, lethargic or they may become inappetent. Occasionally they will transfer their strong attachment to an owner and become anxious when separated from them. This may need some behavioural modification.
Remember that, if you are going to introduce a new pet to your home, the current pets may not accept the newcomer right away, but new bonds will grow in time.
Pets observe every change in a household and are bound to notice the absence of a companion.