[velma]

As you have already noted, standards change as more behaviour is noted. This is the case with hamsters as well as the gerbils you mentioned. When I first had a hamster, it was reckoned that gerbils needed a bigger tank for burrowing but as hamsters ran on a wheel they didn't need a large cage size. This is obviously not recommended now so observation does make a difference but obviously this does not tell us enough when based on individual hamsters.
The problems with a study of this kind are the scale needed to draw a conclusion - both of size and time. Ethics aside, hypothetically you would need large numbers of hamsters, presumably from the same lines to minimise differences in temperament, to be able to observe if behaviours across the board alter. Too small a sample size and you are unable to tell much as changes in behaviour could come down to individual temperament. Other variables are toys included. Some hams will favour enrichment over space so you would most likely have to study each hamster in a variety of situations at several months each. Different cages, different style of cage, different levels of enrichment. Do you consider handling, taming & out of cage time with a story such as this. How little interaction do you have with the hams to ensure as little variance as possible in that regard across the board? Of course, with such short life spans, and altering behaviours over those life spans, then this is another variable that would need to be factored in. Gender would be another consideration. Some hamsters are a lot more sedentary & anecdotally this laziness seems to be more (but not universally) a male trait so you need to have either gendered studies or an evenly split mixture but still recording those differences. A lot of stories of this kind are observational but are of a large enough scale to see repeat behaviour patterns. As to recording things such as cortisol levels, you would need to account for other variables that could affect this. The stress of being tested could skew those results. A good example of this is sometimes knows as white coat syndrome in humans. Some people when getting their blood pressure tested will have an unusually high result because they are stressed about seeing a doctor & having their blood pressure tested. When given a chance to calm down & have it tested again the blood pressure is less high. It's not a direct comparison but it is something to consider.
Studies are so important to get right as their results can be quoted & used for a long time - Sometimes incorrectly if the original study was flawed. My favourite example of this in humans is the "fact" we all know about losing 80% of our body heat through our head. While the chances are that a good amount is lost through or head as it is an extremity and has a large surface area, the original study was massively flawed - the subjects were clothed but hatless. They lost 80% of their heat through their head as it was the only real place for it to disperse!