Articles in the news that relay scientists' new discoveries of the complexities of other animals' mental lives occur fairly frequently. One today discusses the results of tests done with the famous African Grey Parrots. Other stories abound, and continue coming out.
The moral of all these stories? In general, we are usually underestimating the richness of other animals' mental capacities and subjective experiences. Perhaps this all goes back to the fact that animals can't talk, and humans use speech as a way of evaluating how smart or even valuable other beings are. If you can't talk, you must be fairly dumb, and thus your subjective experience must be something easy to dismiss, from a moral standpoint. Humans don't do this just to other animals, of course- they judge other people in this way all the time (for one example, look up the origin of the word "barbarian").
Given what we know about biology, about where emotions and other features of mental lives arise from, and the frequent studies that fairly consistently reveal how we underestimate other animals, one can take it as a rule of thumb that our (under)appreciation of what animals are thinking and experiencing is a substantially inaccurate representation of reality. It's fairly reasonable to conclude that to the degree that an animal shares our underlying biology, the experience that that biological function is responsible for will manifest similarly across species. Thus if we imagine the pain and distress we would feel being a lab animal undergoing painful procedures, or being caged, beaten, and sick in a factory farm, then it's safe to assume that other animals are going to feel this way too. It's called biology.
This heightens all the more our moral duties to fellow inhabitants of our planet. They don't just deserve our respect; they deserve to be free from the actions of ours that so frequently cause them pain and distress.