ABSTRACT. Five groups of Callithrixjacchusjacehus were studied in the field in north east Brazil.Group size and composition were similar to that described for other callitrichids, but changes in group membership occurred much more frequently than previously reported. There was a 50 ~ turnover in group membership in a six month period, and adult males changed groups particularly often. Most groups contained only one reproductively active female, but there was evidence that one group contained two females that were reproductively active simultaneously. Some groups had several adult females, and the role of reproductive female switched from one individual to another with time.Individual marmosets are faced with an unusually wide range of alternative life history strategies involving different combinations of location and reproductive condition. We argue that marmoset social organization is best studied at the level of the whole population within an area, rather than at the level of the individual group.

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A series of traction force microscopy experiments involving pairs of keratocytes migrating on compliant substrates were analyzed. We observed several instances where keratocytes that are about to collide turn before they touch. We term this phenomenon collision avoidance behavior and we propose that the turning is caused by the substrate mediated elastic interactions between the cells. A multipole analysis of the cell traction reveals that the left-right symmetry of the keratocyte traction pattern is broken during collision avoidance events. The analysis further shows that the cell migration direction reorients before the principal traction dipoles as the cells turn. Linear elasticity theory is used to derive the cell-cell interaction energy between pairs of keratocytes. The traction force applied by each cell is modeled as a two points (dipole) or three points (tripod) force model. We show that both models predict that cells that are about to collide in a head-on manner will turn before touching. The tripod model is further able to account for the quadrupole components of the traction force profile that we observed experimentally. Also, the tripod model proposes a mechanism that may explain why cells tend to scatter with a finite angle after a collision avoidance event. A relationship between the scattering angle and the traction force quadrupole moment is also established. Dynamical simulations of migrating model cells are further used to explain the emergence of other cell pair trajectories that we observed experimentally.

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