An alternative is to choose one focal individual, and follow that animal (identified by markings, perhaps) and use the single-case design described here.
Typical designs include (but are not limited to) phase designs such as:
where 'A' is baseline and 'B' is 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.
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.