Uncovering the Process of Sexual Segregation: Male Early‐Life Individual Tactics Shape Its Onset and Affect Phenotypic Quality in a Large Mammal

Sexual segregation is widespread in vertebrates, yet little is known about the mechanisms (i.e., when, why, and how) driving subadult males to leave female groups to join adult males, including impacts on phenotypic quality. Using 6-year longitudinal data from 144 fallow deer males (from birth to after segregation), we examined the effects of neonatal personality, body condition, sociality, and space use during the first year of life on sexual segregation timing and excursions back to female groups, if any. Path analysis revealed via a number of direct and indirect pathways that males born heavier at birth, with more risk-prone neonatal personality, and with weaker social bonds with peers within natal groups, displayed a larger first set of antlers, left natal groups earlier (a.k.a. "early leavers"), and integrated more successfully into adult male groups, with fewer excursions back to the natal range. Interestingly, the initial antler length disadvantage of "late leavers" disappeared within 3 years after detachment, likely due to recovery in body condition. This research provides novel insights into the complex interplay between early-life traits, sexual segregation patterns, and consequences in a large mammal, enhancing understanding of this key ecological and evolutionary phenomenon in vertebrates.