That is a very philosophical question and one I have pondered for years.
I think the first question that would need to be asked is whether or not animals are sentient beings. In my own introspection I am still trying to make sense of that one. In my view I am open to the idea but I'm also highly skeptical. I do know that for a being to be sentient it must have a point of view outside of its physical nature to be aware of itself. Being a Bible believing Christian I understand the nature of humanity to be spiritual which would satisfy that condition, and that our spiritual nature is an integral part of our design and purpose. In my view animals have been assigned different roles in the history of the earth (to make the way for advanced civilization before the creation of humanity, to aid the growth and advancement of our society in agriculture, and to demonstrate God's wisdom and creative power.) Therefore we can expect to see in some animals, especially those playing an active role in human society, traits that allow us to relate to them on some level of mutual understanding. It is possible that some kinds of animals are sentient in order to satisfy that relationship but I would still question the necessity of sentience there.
It would not surprise me that one day we will be able to simulate the brain functions of mammals such as rats or mice, (or chinchillas although they are typically not the laboratory animal of choice), and reconstruct their intelligence artificially to a very high fidelity because their minds are strictly mechanistic. However, with that being said there are some aspects of interacting with pets in my own experience that leads me to think there is something more there than a highly advanced computer program crunching through bits of data.
If we can somehow establish that animals are sentient then perhaps we can explore what animal emotions might be, but even then I am not sure how that could be done. It is a question that I am not sure is answerable with any degree of certainty and one that will be discussed until the end of days.
