Fig. 1. A time chart of animal preparation and experiments. Pairs of animals share aquaria (rectangular boxes), where they can be isolated from each other by an opaque barrier (gray vertical bar), kept apart by an open divider that allowed pairs to smell, see and touch each other but not to fight (broken gray bar), or interact freely (gray double-headed arrow). The sequence of treatments is shown vertically, in rows labeled from A to K. Tests are indicated by light coloring of the rectangles. Animals were divided into two groups, Isolates and Social Group. (A) All animals were separated from an aquarium partner by a closed divider (i.e. isolated) for four weeks. (B) (two rows) Isolates were separated for two more weeks and thereafter through the tests. Social pairs were allowed to interact freely and form dominance relationships for 30 min day^–1; at other times they were kept apart by an open divider. Dominance status was determined during each 30 min of interaction by scoring the numbers of aggressive and submissive behaviors performed by each member of a pair (see equation in Materials and methods and Fig. 3). (C) At the end of the two weeks, the social pairs were re-isolated overnight (15 h) to prevent further agonistic interactions. (D) On experimental Day 1, Isolates and the socially paired animals were tested while separated. (E) Isolates and socially paired animals remained separated overnight. (F) On Day 2, Isolates remained separated while social pairs were allowed to interact freely with their partners for 30 min. (G) All animals were tested, Isolates while separated and social pairs while in the presence of their partner. (H) Social pairs were kept apart by an open divider for 30 min and then (I) tested over a 2–3 h period. (J) Social pairs were then re-isolated by a closed divider for 30 min and then (K) tested again over 2–3 h while isolated.