July 9, 1999 A weekend hiker might reach for a pine cone from the forest floor, only to be rewarded by a prick from its sharp spines. It is interesting to learn that birds and other forest creatures face the same dilemma when feeding on the seeds that these cones harbor. A new study found that pine cones, which bear the progeny of their parent tree, have evolved highly specialized ways to ward off predators, ensuring the dispersal of their seeds.
The study, published in the June issue of Ecology, focuses on the evolutionary importance and modern function of spine development in pine cones. The study is novel because it combines both experimental data and analysis of evolutionary development to answer the question of why pine cones develop spines.
Kimberly Coffey, Craig Benkman and Brook Milligan from New Mexico State University, investigated the relationship between pine cones and the foraging efficiency of a finch, the Red Crossbill (Loxia curvirostra).
The researchers removed spines from some open and closed ponderosa pine (Pinus ponderosa) and open Table Mountain pine (Pinus pungens) cones. Other cones were left with their spines intact.
Crossbills were then allowed to eat seeds from the different types of cones, and the time taken for birds to successfully acquire a seed was recorded. In the open pine cones without spines, crossbills could remove a seed much more quickly than when spines were present. It took 18-34% more time for birds to get a seed from the spiny pine cones. They found that spines on pine cones made it difficult for birds to perch on the cone. Spines also impeded crossbills when they tried to reach for seeds between the cone scales.
By studying the evolutionary development of spines on pine cones, the researchers also found that the amount of spine growth has co-evolved with the length of time seeds remain in open pine cones. Therefore, in open pine cones where seeds stay longer, a greater degree of spine growth is observed. This finding answered the question of whether spines developed as a predatory defense, and were not just a welcome side effect.
"We believe," says Benkman, "that Red Crossbills' bodies have adapted over time to become more successful pine cone predators." Modifications such as stronger legs and reshaped mouthparts allow the finches greater perching and seed retrieval skills. Many other predators would have less success obtaining seeds, making the pine cones' defenses even more effective.
"The most significant element of this study," says the researcher, "is the integration of experimental results and analysis of evolutionary development. Combining these elements allows us to more effectively answer ecological questions which were previously difficult to explain."
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