compare between exhibits When sampling from animals in different exhibits of the same zoo, first, think about what your unit of replication is. This is not a trivial as it sounds (an exhibit with 3 animals in it is not necessarily n = 3!). A common mistake is to have all members of Group 1 in one encolsure, and all members of Group 2 in another. The unit of replication in this case would be enclosure, not individual (i.e., one enclosure would constitute n = 1!). The reason this is important is because the variability within each enclosure will usually be lower than the variability between each encolsure. So, any effect that you observe might be an effect of enclosure, rather than an effect of your treatment. Think about it: there may be one exhibit where the public are in plain sight, and another where the animals can hide!

One way to deal with this is to employ a phase design:

AB
ABA
ABAB

where 'A' is baseline and 'B' is your treatment (x).

These types of design are very useful as the allow you to return to baseline to ensure that any effects of treatment are not due to uncontrolled factors. Also, you can randomize the order of 'A' and 'B' between the different enclosures:

Week 1 Week 2
 Week 3
 Week 4
Enclosure 1
 A
 B
 A
 B
Enclosure 2
 B
 A
 B
 A

Finally, don't forget that as you have not used random sampling (i.e., all of the animals have come from one zoo) you cannot generalise your findings beyond this study! To do this, you would need to collect a random sample from the population.