Red Squirrels Cause Balancing Selection on the Length of White Spruce Cones

Seed predators act as agents of selection on many conifer species through non-random foraging, but their effects on conifers that use mast seeding as a defense strategy are unknown. Optimal foraging models predict that when resources are scarce predators should act to maximize total energy, while when resources are abundant they should maximize efficiency. Here, I examined North American red squirrel (Tamiasciurus hudsonicus) seed predation on white spruce (Picea glauca). The length of white spruce cones exhibited a trade-off between total energy content and the rate of energy extraction. During mast years there was a low opportunity for selection and no evidence of selection on cone length. In non-mast years, squirrels disproportionately harvested longer cones farther from caches, but shorter cones closer to caches. Red squirrels harvested cones in a manner consistent with central-place foraging theory. However, these opposing relationships on cone length led to weak net selection.