A native New Zealand plant evolved to outsmart the moa, giving it a better chance at survival, a new study suggests.
Horoeka, or lancewood, changed its colours as it grew, tricking the ancient flightless bird and stopping the plant from becoming a snack as a sapling.
The theory was put forward by Patrick Kavanagh as part of his PhD in ecology and biodiversity at the University of Victoria, Wellington.
"The lancewood is pretty amazing and unique. It starts out with rigid, saw-like leaves when it's juvenile but at about three metres in height, the leaves become wider and more rounded in shape.
"It's no coincidence that three metres is the same as the maximum height that the largest Moa species was able to reach," Dr Kavanagh, who is now based at Colorado State University, says.
While the theory has been around for some time, Dr Kavanagh's study looked at how colour changes could have affected their chances of survival.
When the plants are young, they have small green spots on the top side of their leaves associated with the spikes down the side.
Juvenile lancewood (Velela / Wikipedia)
"These spots are most conspicuous when the plant is poorly developed and therefore most vulnerable to predators -- the spots act as a kind of untruthful signal to deter moa and other herbivores from eating it," he says.
The underside of the plant's leaf changes from light green as seedlings to dark red and then back to green when it is fully grown.
Spectral analysis shows the higher contrast of dark red against the green happens when the leaves are at their spikiest and the plant's defence are at their highest, sending out a warning to animals and birds.
"In this phase of the plant's life, it's a more truthful warning to any bird planning to eat it there'd be painful consequences."
All warning colours and leaf spines disappear once the plant matures and is out of reach of the largest moa species.
Dr Kavanagh says a lot is known about the bright colours on animals such as frogs and caterpillars to ward off predators, but how plants use the tactic is more of a mystery.