I would not say that rats has a complex emotion but they certainly have a lot of feelings! They get lonely if they are neglected, they get depressed if their cage mate disappears suddenly (or dies), they get agitated if they are abandoned on their own. I would say they are as emotional as dogs, but rats just can't do that tail wagging things that dogs do.

So how do we know when the rat is a happy one? Well, they would have less of red staining around their nose or eyes, they are curious and active, eating well and they come running to you when they spot you walking near their cage.  If you ever hear a rat teeth chattering, congRATurations, your are in with the rats! It's their way of saying "I am very pleased!" or the equivalent of a cat purring. What does teeth chattering sounds like? Good question.  It's sounds very similar to a Queen Alien out of Alien movie approaching the victim slowly and making this fast "tak tak tak tak" sound. 

Another one sounds very similar to teeth chattering but it sounds as fast as tak tak chattering noise but through the nostrils and you will see the eye balls vibrating. It kind of looks freaky but don't be afraid. The rat is happy!

So lets' thinks about what make a happy rat. I think rats enjoy a variable and exciting lifestyle, rather than being locked in a cage. I live in a flat and my rats hardly gets any contact with nature, so even the smell of London air gets them excited. They love a good ride on my shoulder to look around my messy flat. They love crawling in a washing basket and exploring. They actually enjoy being spoken  sweet nothings. They love being stroked and sometimes roughed up for a bit of wrestle with my hand.
 

But here is my best tip that I love to share with everyone.
Top Tip: Rats and the digging pot.
All you need is a deep and moderately sized plant pot, a chemical free sterile soil, and rat's favorite treats. Place plenty of soil in a pot and bury some treats in the soil. Rats will go wild! It tickles their instinct to burrow deep enough and they get treats, some excercise and fun! It also keeps these little sharp nails trimmed.  I also think the smell of damp soil is something comforting for them.

Make sure the pot is big enough for the rats to maneuver around and deep enough to be able to burrow. Expect some mess.  The soil might get dry after a while, so spray the soil wet but not the rat!