According to Melissa Starling, a Postdoctoral researcher at the University of Sydney, who holds a BSc (Hons) in zoology and a Ph.D. in dog behavior, personality, emotions and cognition, we can often get a good idea if animals are happy or unhappy by looking at how they are behaving in general.

"A cat that purrs because it's happy may also be winding her body around your legs, or relaxed in your lap, have her tail high in the air, or roll over on her back," Starling said. "All this shows she is trusting and interested."

"Likewise, a rabbit grinding its teeth while relaxing will also likely be stretching its body out as well. You can tell how relaxed a rabbit is by how stretched out it is while resting. If a rabbit is in pain, it tends to hunch up and squeeze its eyes half shut like it is wincing. Animals that are relaxed and not tense are usually happy and content."

Starling said we can also see what animals like to do by what they choose to come back to over and over again.

"If your animal chooses to lie in the sun or look for tasty treats or dig holes, then you know when they are doing that, they are probably happy," the researcher explained.

"So, to know when an animal is happy, we need to look at more than what one body part is doing, and we might need to watch them to get to know them."

The question of how do we know if our pets—especially the prehistoric ones—are content has been explored for centuries.

In 1872, Charles Darwin published a whole book on the subject, suggesting that humans and many animals express emotions in similar ways—if anything, emotions actually evolved before our species did.

But philosopher Thomas Nagel said that we’ll never know if this is true or not. In his paper 'What is it like to be a bat?' Nagel wrote that even if we spent our days “hanging upsidedown by one’s feet in an attic,” it’s impossible to know how a bat feels, and not even Batman has that kind of inner access.

But just because we aren't 100 percent sure how an animal feels, doesn’t mean they don’t feel something.

Dolphins, cows, chimpanzees, dogs, and even squirrels have similar emotional processing centers in the brain as humans, so at the very least, they would experience basic emotions like fear, anger, grief, and joy.

One group of scientists was so sure of this that they signed the Cambridge Declaration on Consciousness in 2012, acknowledging that all mammals and birds (and some invertebrates, like octopuses) are conscious creatures.