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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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