Tree keeps vigil for extinct moa; leaves may have defended against big, chomping birds.

Odd shape shifts and color changes during a New Zealand tree's lifetime may be a botanical form of paranoia.

Skinny, mottled-brown early leaves could still be defending lancewood trees against the long-extinct moa, flightless birds that lived in New Zealand hundreds of years ago, says Kevin Burns of Victoria University of Wellington in New Zealand.

Using information about the visual system of the ostrich, moa's closest living relative, Burns and his colleagues tested what the leaves of lancewoods (Pseudopanax crassifolius) might have looked like to a moa.

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From a moa's perspective, small lancewood seedlings' narrow, dark leaves would have been hard to see against the background leaf litter, the researchers report in an upcoming New Philologist. As the plants grow, newer leaves develop bright spots that mark hard-to-swallow, snagging spines. "Moa would have to be sword swallowers to get them down," Burns says. The brighter dots could work like other plant markings proposed as easy-to-remember, defensive warnings to browsers, the plant version of eyepopping colors on poison dart frogs.

More conventional leaves without such defenses don't show up until lancewood trees are taller than 300 centimeters, the researchers report. That's the probable top of the browsing reach of the biggest moa, according to paleontologists' calculations.

Researchers also looked at the leaves of a species descended from the New Zealand lancewood. Growing on the Chatham Islands, this species, Pseudopanax chathamicus, never had moa to menace its foliage and doesn't show the same defenses, the researchers report. Its seedlings' greener leaves wouldn't have blended in with the background as well as the moa-zone species does, and its sapling leaves don't grow as narrow. Without moa, the offshoot may have lowered its guard.

"Plausible, but how are you going to test it?" Richard N. Holdaway says of the idea that lancewood leaves served as a defense against moa. Holdaway, a paleobiologist at the independent research organization Palaecol Research Ltd. in Christchurch, New Zealand, is analyzing food bits preserved in moa remains and says lancewoods do show up.

Moa would have been the predators to guard against in ancient New Zealand because they were the islands' only big, leaf-chomping animals. Moa's beaks were more robust than ostriches' and could slice through a lot of shrubbery, Holdaway says. "Moa were built like bridge beams."

Various lines of research suggest prey species can keep their defenses for thousands of years after the last of a terrorizing predator has vanished. Pronghorn still run far faster than their modern pursuers. And some of the Cyanea plants in Hawaii still sprout prickles that might have defended them against now-extinct browsing birds. Lancewoods now join the list of organisms haunted by ghosts.