There is a general consensus that remembering everything forever is not ideal: there are physiological costs as well as costs with potential interference with the recall of memories. We are looking at memory length as an evolutionarily adaptive behavior, specifically asking: when is it optimal to forget? We have published models and previous work with blue jays and pinyon jays. Current work in the lab is using bees and flies, comparing forgetting curves and other aspects of memory for different sources of information, and how this interacts with certainty about the environment.