GoKawiilAnimals
Male and female C57BL/6J mice (8 to 12 weeks old; Janvier Labs, France) or DAT-iCre mice on a C57BL/6J background were group-housed under standard conditions (12 h:12 h light:dark, ~22 °C, ~50% humidity) and maintained in triads prior to testing (see Supplementary Methods for details). All experiments and procedures were performed in accordance with European Commission directives 219/1990, 220/1990 and 2010/63, and approved by the ESPCI and the ethical committee no. 059 under APAFIS #34335-2021121318085835.
Lone condition and microsociety experiment
Mice were placed either individually or in triads in a 50 × 50 cm large environment and continuously tracked using the Live Mouse Tracker system36, allowing monitoring of identity and behaviour of individual mice over extended periods. Before the experiment, all mice were implanted with a radio frequency identification (RFID) microchip (Biomark APT-12 PIT Tag, Biomark) under the shoulder skin.
Triads (3 males, 3 females, or 2 females and 1 male) stayed for 8 days and 7 nights in the environment, whereas lone mice were housed for 5 days and 4 nights.
The cage was composed of different zones that were freely accessible by all the mice at any moment. The arena contained a lever on one side and a food dispenser/magazine on the opposite side. Each lever press delivered a 20 mg pellet (TestDiet 5TUL/1811142 purified, Bio-Concept) and the lever becomes inactive for 5 s. A nose poke in the magazine leads to a beam break that is a proxy of the consumption of the pellet by the mouse. Outside the 5 s delay after each lever press, all the mice can press at any moment and consume the pellet at any time following its release. Mice were weighed and health-checked daily. No mouse was removed from the task either for weight loss or for aggression.
Social replacement experiment
To test role stability, we performed a reconfiguration experiment in which a male previously identified as a Scrounger during the first week of group housing was rehoused with two task-naive males (Scrounger–naive–naive condition). Mice were kept in the same semi-natural environment as described above, and behaviour was recorded continuously for an additional 7 days. We retained only triads in which a Scrounger was reliably identified at the end of the first week (13 out of 15 triads). To track individual trajectories across time, the retained Scrounger from week 1 was linked to its corresponding behavioural position in week 2, allowing comparison of lever press counts, distances to archetypes, and profile classification before and after reconfiguration.
Live Mouse Tracker system
Behaviour was monitored continuously (24 h daily, 7 days a week) using a dual acquisition pipeline combining time-stamped, animal-identified operant events (lever, magazine/dispenser TTLs (Transistor-Transistor Logic); MedAssociates) with continuous video tracking to quantify locomotion and social/spatial organization (Live Mouse Tracker)36. Individual lever presses, nose pokes and complete sequences were extracted from the database by matching TTLs to mouse identity within predefined lever/magazine zones, and a complete sequence was defined as a lever press followed by the same mouse reaching the magazine within 6 s, otherwise the sequence was not counted (typically if another mouse was already at the magazine waiting for the food) (see Supplementary Methods for details). Gain was defined as retrieving a pellet within 6 s after a conspecific pressed the lever, whereas loss was defined as a pellet retrieved by a conspecific within 6 s after the subject’s own press.
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