Competitive social feedback amplifies the role of early life contingency in male mice

Contingency (or “luck”) in early life plays an important role in shaping individuals’ development. By comparing the developmental trajectories of functionally genetically identical free-living mice who either experienced high levels of resource competition (males) or did not (females), we show that competition magnifies early contingency. Male resource competition results in a feedback loop that magnifies the importance of early contingency and pushes individuals onto divergent, self-reinforcing life trajectories, while the same process appears absent in females. Our results indicate that the strength of sexual selection may be self-limiting, and they highlight the potential for contingency to lead to differences in life outcomes, even in the absence of any underlying differences in ability (“merit”).