The adaptive significance of spines on pine cones

Besides woody cone scales, certain species of wind-dispersed pines (Pinus) have spines on their scales as a putative form of defense against seed predators. We tested whether spines differentially deterred seed predators foraging on closed and open pine cones. Red Crossbills (Loxia curvirostra) were selected as the seed predator because they commonly forage on these cones. We timed crossbills foraging on closed and open pine cones with and without spines. Crossbills did not require more time to remove seeds from closed ponderosa pine (P. ponderosa) cones with spines. However, crossbills required significantly more time (18-34%) to remove seeds from open ponderosa pine and Table Mountain pine (P. pungens) cones with spines than from cones whose spines had been removed. Moreover, experiments designed to isolate the effect of spines on the perching and probing behavior of crossbills revealed that spines hindered both activities additively. These experimental results were consistent with our phylogenetic analyses of 21 species of hard pines (subgenus Pinus). Whereas the evolution of changes in the length of time seeds are retained in closed cones and that of changes in the presence of spines appear independent, changes in the length of time seeds are retained in open cones were associated with changes in the presence or development of spines. Therefore, pines that retain seeds in open but not closed cones for extended periods tend to have well-developed spines. This illustrates the complementarity of experimental approaches and explicit phylogenetic models in elucidating ecological processes.