How can we tell zoo animals are happy?

In most cases, days are the gone where animals are kept in tiny enclosures with nothing to do, but now scientists have come up with a method of measuring how happy they actually are. We now have a vast array of chemicals that we can actually measure to assess this, one of the best examples is cortisol. Cortisol is a hormone secreted by the adrenal cortex in response to stressful events. We can measure this through taking blood, but for zoo animals, if they're potentially distressed anyway it's better to look at the levels in urine, faeces or saliva. However scientists need to be careful that they don't misinterpret the levels because they can be influenced by things like the age or sex of the animal. We could also use preference tests to see what's best for the animal, so from different foods to sleeping materials. Although we need to make sure we don't misinterpret those results because like us, what we prefer may not be what's best!