A eucalyptus-like tree that grows in New Zealand is still defending itself from a giant bird that died out about 500 years ago. The lancewood tree changes its appearance twice in its lifetime--an adaptation, a new study suggests, that prevented it from being eaten by flightless moas.
As a seedling, the lancewood tree (Pseudopanax crassifolius) sprouts small, brown, blotchy leaves. Then, as a sapling, its leaves grow into footlong spears with tiny barbs along the edge. Finally, the adult lancewood, which can reach a height of 20 meters, sports rounded, nondescript green leaves. Many scientists think that the tree evolved these metamorphoses to avoid moas, the main herbivores on the islands and a relative of emus and ostriches that humans hunted to extinction.
To test the moa hypothesis, Kevin C. Burns, an evolutionary ecologist at Victoria University of Wellington in New Zealand, and colleagues compared lancewood leaves with those from the similar tree Pseudopanax chathamicus. This species, which scientists think evolved from lancewood, grows on the Chatham Islands 800 kilometers east of New Zealand, where moas didn't live.
To determine how the trees would appear to a moa, the team measured the wavelength and brightness of light reflected by the leaves. Light from the lancewood seedling foliage differed from that of the Chatham trees but matched the light from dead leaves on the forest floor, the researchers report online this month in New Phytologist. Because the seedlings stand only 10 centimeters tall, the scientists concluded that the leaves' color would help a young, tender plant blend in with leaf litter and hide it from a browsing moa.
When Burns and colleagues examined saplings from both trees, they found that lancewood leaves had intense bright spots near their barbs. The researchers suspect that these splotches highlighted the spikes and warned moas that the leaves would be difficult to eat. If a moa tried to gulp down one of these leaves, Burns says, it would be like "swallowing an arrow backwards."
Leaves from adult trees of the two species showed similar reflectivity to each other. Burns thinks that the adult lancewood stops morphing because its leaves would have been out of reach of even the largest moas. While the lancewood changes appearance early in its life, the Chatham doesn't. The team suggests that the selection pressure to change leaf color and shape was absent on the moa-free islands. "So the plants didn't have to pay the physiological cost of making these leaves, and they resorted back to normal-looking leaves," Burns says.
David Lee, a tropical botanist at Florida International University in Miami, says that although the evidence is speculative, the study suggests that "to understand the evolution of plant traits, you also need to look at extinct herbivores and their interactions with the plants." Thomas Givnish, a plant ecologist at the University of Wisconsin, Madison, adds that the hypothesis could be strengthened by "exposing these plants to ... emus or ostriches to demonstrate that these traits deter browsing by birds."