The sample size is 16, which should be enough. A random factor shouldn't have that big an effect if the trials were uncorrelated. To make the trials uncorrelated, they would need to interleave their trials. For instance, if they do all the 30-minute tests, then all the 45-minute tests, then all the 60-minute tests, each group of tests is almost completely correlated in environmental conditions. Because rats have such different senses than humans, it's impossible for a human to tell by observation whether something unusual to a rat is going on.

The expectation of the experimenter is another factor that can correlate results within a trial group. We usually require double-blind tests on humans, but not on rats.