A recent study suggests that rats can be pessimistic. Apparently, they can assume novel events are going to be positive or negative. You see, scientists trained rats to respond to two different tones.
If they pressed a lever in response to one tone, they got a food pellet. This was the positive event. When they heard the other tone and pressed the lever, they heard an unpleasant noise--so here they learned not to press the lever to avoid a negative event.
Then scientists moved half the rats into housing conditions that changed unpredictably. For example, the cages were unfamiliar, or lights went on at unusual times. Then they tested the rats to see how they responded to a novel tone that fell between the positive and the negative tones. And guess what?
The rats from unstable environments were less likely to press the lever. When the rats heard the novel tone they were less likely than the rats from the stable housing to associate the novel tone with food and press the lever. The next step, of course, is to study the brain processes involved more thoroughly.