Hi Winston. I am so sorry.
I do believe animals grieve but maybe not the way we perceive grieving. I think when they are used to having a lifelong partner, and that partner is missing, they search for them.
I have always had many animals at the same time so I do not think that they have missed the others that have passed. What I can tell you however is that my two siamese are very bonded and when I was fostering Dobie and when he left, I did notice a change in Blu. When Dobie came back (the people did not want him afterall), Blu greeted him with such enthusiasm. They both ran off together, got on the bed and started grooming one another. Since then (as you know) I ended up keeping Dobie because he was very important for Blu's well being (mind you, he is good for mine as well
). Witnessing this, I can only assume that there is a void in their lives when another is missing and does not return.
I think that Blu would have been fine in time, but I also know that the reunion was something to be seen and rather sad at the same time as it was obvious that they longed for one another.
I don't really know what you should do, but giving the remaining animal alot of attention and reassurance is the right thing to do in my opinion.
Again Winston, I am so sorry for your loss. I wish there was a better way of saying it.