Back off, extinct moa

Leaf color and shape may defend a New Zealand tree species from a long-gone giant bird

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

OUCH ALERT Bright spots mark the thorns on the long, narrow leaves of these lancewood saplings, possibly a leftover warning to extinct moa that these are hard to swallow. David Collings

Lancewood trees’ skinny, mottled-brown early leaves could still be defending themselves against the long-extinct moa, flightless birds that lived in New Zealand hundreds of years ago, report Kevin Burns of Victoria University of Wellington, New Zealand, and his colleagues.

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

From a moa perspective, small seedlings’ narrow, dark leaves would have been hard to see against their background of leaf litter, the researchers report in an upcoming New Phytologist. As the plants grow, newer leaves develop bright spots that mark hard-to-swallow spines. These snagging spines would have made swallowing awkward for a toothless bird working long objects into position to go down its throat, the researchers suggest. The brighter dots could work like other plant markings proposed as easy-to-remember, defensive warnings to browsers, the plant version of eye-popping colors on poison dart frogs.

More conventional leaves without 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 big moa species, 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 seedling leaf color 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’s leaves morph as a defense against moa. Holdaway, a paleobiologist at the independent research organization Palaecol Research in Christchurch, New Zealand, is analyzing food bits preserved in the digestive tracts of moa remains and says lancewoods do show up now and then.

Moa would have been the predators to guard against in ancient New Zealand because they were the islands’ only big, leaf-chomping animals. The only mammals were a few species of bats, Holdaway says.

Moa’s beaks were far more robust than ostriches’ and could slice through a lot of shrubbery. “Moa were built like bridge beams,” Holdaway says.

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 that might have defended them against browsing birds that are now long extinct. Now lancewoods join the list of organisms haunted by ghosts.

Susan Milius

Susan Milius is the life sciences writer, covering organismal biology and evolution, and has a special passion for plants, fungi and invertebrates. She studied biology and English literature.

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