Multivariate Coevolution Shapes Life-History Strategies Across Amniotes

Life-history traits jointly determine survival, growth, and reproduction, yet their evolutionary interdependence remains poorly understood. Most comparative studies focus on pairwise trade-offs or composite axes, overlooking the complexity of multivariate coevolution. Here, we integrate phylogenetic path analysis and multivariate Ornstein-Uhlenbeck models to investigate the coevolution of five key life-history traits across amniotes. We show that life-history evolution proceeds through modular coevolutionary pathways with distinct tempos among traits and taxa. Despite loading similarly on the fast-slow axis of mammals and birds, clutch size and clutch frequency evolve independently at both instantaneous and long-term timescales, with clutch frequency adapting rapidly, whereas clutch size evolves slowly over macroevolutionary time. Lifespan, development time, and offspring size exhibit positive coevolution at instantaneous scales across clades, but follow distinct adaptive modes with varying directions over longer timescales. These patterns become evident only through the integrated multivariate comparative approaches.