Early Life Luck Gives Male Mice Competitive Edge

Early life "luck" plays a pivotal role in shaping individuality and success, particularly for males, according to a new study in mice. In male animals, competitive social dynamics amplified small initial differences into lifelong disparities in fitness. The findings highlight parallels between biological competition and societal inequalities and they demonstrate how chance events can drive divergent outcomes even among genetically identical individuals. Contingency (colloquially, "luck") refers to the role of chance in shaping outcomes. It is a critical factor in both biological and social sciences, particularly during early life, when pivotal events can set individuals on diverging trajectories. This is especially pronounced in social animals, where interactions within groups amplify the impact of early contingent events. This often creates self-reinforcing disparities in access to resources and success, akin to the "Matthew effect" discussed in social sciences in which those who achieve early success tend to achieve ever greater success in the future. To date, studying the role of contingency in life outcomes has been challenging due to the difficulty of experimentally isolating and testing the effects of early life circumstances under controlled conditions. Matthew Zipple and colleagues compared the developmental trajectories of genetically identical, free-living mice to better understand how microenvironmental differences during development interact with competitive social processes to shape individuality and behavior over time. Zipple et al. found that competition among male mice amplifies minor early life differences, laying the foundation for divergent life paths that affect adult success in areas such as resource access, body condition, and reproduction. In contrast, females – who do not compete for resources – did not show this pattern of contingency amplification. These findings suggest that competitive social dynamics play a critical role in the earlier and more pronounced development of individuality in males compared to females. "Our results add to sociological and biological literature that underscores the potential importance of unpredictable, uncontrollable experiences in generating differences in outcomes even when differences in underlying quality (or "talent") are small or nonexistent," write Zipple et al.