Replying to: F. Massol & P. Crochet Nature 451, 10.1038/nature06743 (2008).

The more an individual stands to lose, the more cautious that individual should be. We have shown¹ that this basic principle gives rise to consistent individual differences in risk-related behaviour whenever individuals have different future fitness expectations. To illustrate this, we considered a model where differences in fitness expectations result from a trade-off between current and future reproduction. Massol and Crochet argue² that the emergence of such differences depends on the shape of this trade-off. Their claim is based on the technical argument that our model has a 'branching point' only for a limited range of the trade-off parameter . In contrast, we show here that the emergence of individual differences is a robust phenomenon that does not depend on such details. Our analysis illustrates the important insight that a branching point is not needed for the emergence of polymorphism.

1. Theoretical Biology Group, Centre for Ecological and Evolutionary Studies, University of Groningen, 30 Kerklaan, 9751 NN Haren, The Netherlands
2. Santa Fe Institute, 1399 Hyde Park Road, Santa Fe, New Mexico 87501, USA
3. Department of Zoology, Stockholm University, SE-106 91, Stockholm, Sweden.
   Email: f.j.weissing@rug.nl