The mottled brown colour of seedlings matches the background leaf litter, says the study.This may have been a defence against New Zealand's largest herbivore, the now extinct moa.
Credit: Kevin Burns
SYDNEY: One species of New Zealand tree evolved camouflage to escape being eaten by the now extinct giant moa, says a new study.
Although camouflage is common in animals such as insects, snakes, and fish, this is the first time it has been reported for a tree. The finding is published this month in the journal New Phytologist.
"There isn't much quantitative evidence for camouflage in plants, so our result for seedlings is new. The mottled colour of the margins of leaves make their true size and shape difficult to distinguish," said lead researcher Kevin Burns, an ecologist at Victoria University of Wellington in New Zealand.
Metamorphosis
Plants have long been known to use defences such as poisonous chemicals – including cyanide and strychnine – or nasty thorns to deter predators. But matching colours to the background environment has rarely been recorded.
The New Zealand native araliaceae tree Pseudopanax crassifolius undergoes several colour transitions throughout its life cycle, from blotchy brown leaves as a seedling; to brown leaves, with green highlighted spikes as a sapling; to plain dark green leaves as an adult.
While this variation could be due to changes in the environment, Burns and his team suspected it evolved to help the young trees evade the now extinct moa, a giant bird that inhabited New Zealand until around 750 years ago.
Prior to the arrival of people, the islands had few native land mammals and were instead home to a wide variety of flightless birds, including the moa a giant relative of the ostrich and emu.
Moa adaptations
To test the theory, they collected one leaf from 10 seedlings, 10 saplings, and two adult araliaceae trees on both the mainland and the Chatham Islands, which were never home to the giant bird. The leaves were compared to both each other and the leaf litter in their natural environment.
On the mainland, the mottled brown colour of seedling leaves matches the background leaf litter, said Burns, while in saplings, defensive leaf spines are bright green and clearly distinguishable from the brown leaf litter background.
Leaves from adult plants – which are taller than moa's maximum height of three metres – were spineless and plain dark green, he said.
Chatham Island tree leaves did not show the same pattern of changing colour and clearly stood out against the leaf litter, which suggest that aliaceae leaf patterns could be adaptations to the presence of moa, said Burns.
He explained that for small seedlings camouflage may be the best strategy to avoid being eaten. For larger saplings, which are between 10 cm and three metres tall, having vicious spines signposted with a bright colour may have been more effective as moas did not have teeth and swallowed leaves whole.
"The plant appears to switch strategies, from hiding from predators while they are really young, to advertising defences when they are juveniles, to lacking colour-based defences altogether when they grow above the height of the tallest known moa," Burns told Cosmos Online.
"Although this type of metamorphosis is common in insects and other types of animals, it has never been documented in plants."
Angela Moles, an ecologist from the University of New South Wales in Sydney, Australia, finds the study compelling.
"This was a clever test of an original and interesting idea. The researchers made use of a natural experiment set up by plants on different islands to test an exciting evolutionary theory. This paper provides some of the first evidence for plants using camouflage to hide their most vulnerable leaves from herbivores," she said.
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