Australasian Science: Australia's authority on science since 1938

Moa Affected Plant Evolution

The feeding habits of the moa may have influenced the evolution of warning and defence mechanisms by a plant species in New Zealand.

As part of his PhD studies at Victoria University of Wellington, Patrick Kavanagh looked at the colour and shape of the lancewood. “The lancewood is pretty amazing and unique,” he said. “It starts out with rigid, saw-like leaves when it’s juvenile, but at about 3 metres in height the leaves become wider and more rounded in shape. It’s no coincidence that 3 metres is the same as the maximum height that the largest moa species was able to reach.”

While this theory has been around for some time, Kavanagh has added weight to the argument by examining colour changes in the lancewood leaves as the plant matures, arguing that they are used as a warning signal to deter moa from eating the developing saplings.

Kavanagh explains that the lancewood leaves “possess spines down their margins that are largest when plants are saplings, potentially to deter New Zealand’s largest known herbivore – the moa”.

Small green spots on the tops of the leaves are associated with these spines. “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.”

Kavanagh “also noticed that the underside of the lancewood leaf changes colour as the plant develops. Small seedlings are light green underneath the leaf, but that turns dark red when it reaches sapling stage. It changes back to green when the plant is fully grown.”

All warning colours and leaf spines are absent in adults, after they grow above the height of the largest moa species.

Kavanagh used spectral analysis techniques to test whether the dark red colouration makes the sapling’s leaves more conspicuous to herbivores looking up from below. “The higher contrast of dark red against the other green foliage happens when the leaves are most spikey and therefore best defended. 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.”

In contrast, “spots on the upper side of leaves were strongest when plants were small and poorly defended, providing a dishonest warning signal”.

Kavanagh says that the production of honest and dishonest warning signals as the lancewood plant grows vertically reflects the changing visual perspective of herbivores, and is therefore an example of how the moa has shaped the evolution of plants in New Zealand.

The research was published in the Biological Journal of the Linnean Society (www.tinyurl.com/jqvn3v8).