Elastic pinch biomechanisms can yield consistent launch speeds regardless of projectile mass

Energetic trade-offs are particularly pertinent to bio-ballistic systems which impart energy to projectiles exclusively during launch. We investigated such tradeoffs in the spring-propelled seeds of Loropetalum chinense, Hamamelis virginiana, and Fortunearia sinensis. Using similar seed-shooting mechanisms, fruits of these confamilial plants (Hamamelidaceae) span an order of magnitude in spring and seed mass. We expected that as seed mass increased, ejection speed would decrease. Instead, ejection speed remained relatively constant. We tested if fruits shoot larger seeds by storing more elastic potential energy (PE). Spring mass and PE increased as seed mass increased (in order of increasing seed mass: L. chinense, H. virginiana, F. sinensis). As seed mass to spring mass ratio increased (ratios: H. virginiana = 0.503, F. sinensis = 0.653, L. chinense = 0.842), mass-specific PE storage increased. Conversion efficiency of PE to seed kinetic energy (KE) decreased with increasing fruit ma..., ,