This is a story about why lancewoods look more like umbrella carcasses than trees.
See what I mean?
This is also about why, after an awkward adolescence of 10-15 years they undergo an ugly-duckling transformation into much more elegant trees with bushy mop-tops.
But first, let’s take a closer look at the juvenile lancewood’s umbrella spines/leaves:
That is one unappetising leaf, given that lancewoods evolved in the complete absence of mammals that could eat it.
Part of the reason why New Zealand has such an odd assemblage of plants and animals is because until recently, mammals were never much of a success there. Until 800 years ago, when humans and their beloved pigs and rats arrived, the islands experienced many blissful millions of years of mammal-less natural selection. There were no deer-like browsing animals, or rat-like opportunists, or wolf-like predators. It was this lack of ground predators that led to New Zealand’s most distinctive quirk, the prevalence of flightless birds. Like the Moa.
There were nine moa species, ranging from a modest,
turkey- rhea-sized species, to the giants that inspired paleontologist Richard Owen to name them Dinornis (‘terrible bird’). Giant moa could reach over 3 m (10 ft) and could get about as heavy as a baby grand piano.
Despite their nasty-looking claws, moas were herbivores, eating the leaves, fruit, seeds and twigs of many different kinds of plants. They probably exerted a formidable selective pressure on the plants they ate and on the other herbivores they competed with. In other words, all kinds of plants and animals in New Zealand were adapted to life with moas, and one possible example is that scrappy lancewood.
The moa/lancewood hypothesis goes like this:
The weirdness of the leaves of lancewood saplings was an adaptation that discouraged moas from eating them. The juvenile leaves would make an awkward meal, even for a giant bird, because they are long, inflexible and have pointy edges. Birds don’t have teeth, don’t chew and have to angle elongated objects down their throats before swallowing, so the whole operation of eating the umbrella carcass phase would have been extremely annoying. To help moas recognise just how annoying, lancewoods have pale patches that highlight each spine, and a racing stripe down the length of each leaf.
Here’s the cool part. Since moas could fly about as well as a baby grand piano, the leaves of a fully-grown lancewood would be permanently out of reach for even the giant moa species. So the strange adaptations that were beneficial to the juvenile were useless to the adult plant, and adult leaves could revert to the ancestral shape and colouration. Adult lancewood leaves are indeed broader, softer, much less spiky and lack the spine ‘highlights’.
But how can we test this idea? Sadly, there are no moas left alive that could be subjected to feeding preference trials. No-one will ever get to see the result of a moa vs. lancewood wrestling match because they were hunted to extinction within a few hundred years of humans arriving. I doubt we will ever get hard evidence supporting/rejecting the hypothesis, but here’s some circumstantial evidence:
- Fadzly et al. (2009) found that the spine highlighting should have been quite visible to moas, based on the visual sensitivities of ostriches and other living birds.
- Chatham Island was never occupied by moas, but has a native species of lancewood that evolved from the mainland lancewood. However, the island lancewood has mostly lost the umbrella carcass phase. The juvenile leaves don’t have the distinctive stiff, elongated shape and they also don’t have the highlight colours.
- The only other hypothesis I consider plausible is that the juvenile leaf form is better adapted for life in the shaded understory, and the adult leaf form is better adapted for life in the canopy. But a comparison of juvenile lancewoods with a related species that lacks the umbrella carcass form found that the spiky lancewoods actually do worse in shady environments (Someone more qualified than me would be a better judge of whether the comparison species they picked was a fair choice – do tell me if you know).
So it seems that the lancewood looks like an umbrella carcass to avoid being eaten by a herbivore that has been dead for 500 years. One of the reasons that I like this story is that it is a visual reminder of one fact about natural selection that we should remember more often. No organism is adapted to its environment; it is adapted to its ancestors’ environment. Just like us.