Bird spikes

A tree in New Zealand grows downwards-facing spikes for the first 15 or 20 years of its life; this is thought to be a remnant defence against gigantic now-extinct birds.

The horoeka, also known as the lancewood tree, has a very weird growth pattern. For the first couple of decades of its life, the leaves of the tree are long, stiff, and are an oddly mottled colour. They look like spikes pointing downwards, and their unusual colouration causes the whole tree to blend into the undergrowth. As the tree develops and matures, it sprouts leaves that are more like normal: smaller, softer, and greener.

Why the change? There’s a theory that the horoeka’s juvenile form evolved as a defence against the moa, a massive extinct bird. (By massive, by the way, I mean really big: up to 3.6 metres high.) Moas were herbivorous and because of their weight – up to 230kg – presumably voracious. Their beaks, like secateurs, could strip low-lying leaves clear off young trees… but long stiff camouflaged spiky leaves? Not so easy.

There are two good pieces of evidence for this hypothesis: the height at which the horoeka becomes more like a normal tree is right above moa-browsing height; a related tree that evolved on a different island without moa does not display the same spiky defensive behaviour. The moa have been extinct for more than five hundred years. The horoeka’s defence, much like that of the pronghorn, lives on.

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