Abstract: 
The idea that animal groups can have collective memories has been raised a number of times in different contexts. The information centre hypothesis predicts that communal roosts allow for information exchange between individuals so that successful foragers, for example, can be followed by others to promising sites. A different approach to collective memory has been taken in the field of collective behaviour where it has been shown that information regarding predation risk, for example, can be structurally encoded in the network topology of the interacting individuals. While the information centre hypothesis is empirically well supported, the full information can be carried by a single individual which questions the description as a “collective” memory. Structurally encoded information, on the other hand, meets the criterion of a collective process but empirical studies have demonstrated encoded information only for as long as the risk was present which may not satisfy the criterion of a memory. This raises the question of whether collective processes exist that can encode information regarding environmental states or events even after they are no longer directly observable. In this talk I will present data from project 41 which suggest collective wave formation in sulphur mollies as a potential example of a collective memory.