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kiwi evolution - a new take on an icon's ancient past

'The' kiwi (Apteryx spp.) has always been a bit of an enigma, not least for the fact that it lays an absolutely enormous egg in comparison to its body size. In one of the essays in his book Bully for Brontosaurus (1991), Stephen Jay Gould argued that this differential in egg/body size was due to the impact of scaling: kiwi, he believed, had 'downsized' from a moa-like ancestor but had retained the large moa-type egg. This idea was quite widely accepted, even though later genetic evidence indicated that kiwi were in fact more closely related to the Australian emu than to NZ's now-extinct moa. But new research suggests quite a different evolutionary trajectory - and I rather suspect that Gould, great scientist that he was, would be delighted to see his hypothesis robustly challenged :)

The research reported in this news article from scoop.co.nz will be published in the Proceedings of the 8th International meeting of the Society of Avian Palaeontology and Evolution - you'll find the abstracts of the conference papers here. A newly-described fossil, from what's known as the 'St Bathans fauna' of Central Otago turns out to be a new genus and species of kiwi, but a tiny one by today's standards. Paul Scofield, one of the paper's authors, is quoted in the scoop report as saying that 

[this] fossil from the early Miocene, about 20 million years ago, shows us that it's a tiny bird about one third the size of a small kiwi today. It suggests the opposite [to Gould's hypothesis] is, in fact, the case - that the kiwi has developed towards a larger size, a trend that is seen in many birds from the early Miocene.

So, how would an ancestral kiwi have arrived in New Zealand? The suggestion is that they flew. This is based on the evidence that a) kiwi and emu are more closely related than kiwi and moa and b) the emu-ish early kiwi arrived here after NZ and Australia were separated by the developing Tasman Sea.

Finding the wing bones of this new fossil species would help to confirm/deny this proposal. Although - having read the abstracts for the conference, I can't help wondering if a proxy might be the size of a part of the brain known as the 'cerebellar flocculus', as suggested in another presentation by Walsh et al. It's an intriguing possibility, anyway! And I'm wondering - how may we then explain that anomalous kiwi egg? 

I'll look forward to getting hold of a copy of the paper by Worthy and his colleagues, once it's published. 

 

Gould, S.J., (1991) Of kiwi eggs and the Liberty Bell, pp 109-123 in Bully for Brontosaurus. Penguin Books, London. 

Walsh, S., Iwaniuk, A., Knoll, M., Bourdon, E., Barrett, P., Milner, A., Abel, R., & Dello Sterpaio, P. (2012) Can the size of the avian cerebellar flocculus be used as a proxy of flying ability in extinct birds? 8th Internat. SAPE Meeting, 11.-16. June 2012 Naturhistorisches Museum Wien

Worthy, T.H., Tennyson, A.J.D., Salisbury, S., Hand, S.J., & Scofield, R.P. (2012) A fossil kiwi (Apterygiformes) from the early Miocene St Bathans fauna, New Zealand. 8th Internat. SAPE Meeting, 11.-16. June 2012 Naturhistorisches Museum Wien

 
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3 Comments

Really interesting stuff, but I suspect this story still has a way to run. As you say, it would be good if the wing bones of this new fossil species could be found. At this stage we don't have any idea of kiwi diversity 20 million years ago; the fossil record is very sparse. It's possible that there were several species around, and that this tiny species was flightless and descended from larger ancestors, and is an extinct cousin of modern kiwi rather than a direct ancestor. I still struggle with the idea that ancestral ratites were strong fliers, capable of crossing hundreds of kilometres of open ocean, and wonder if we're missing or misinterpreting something.

As an aside, the supposedly exceptional size of kiwi eggs may not be something that requires too much of an explanation, because the size is not that exceptional. The largest eggs relative to body size are laid by storm petrels, not kiwi - see http://www.teara.govt.nz/en/petrels/page-2 But then storm petrels are the smallest members of their order (they're closely related to albatrosses) so maybe their proportionately huge eggs are also the result of evolution to smaller body size - perhaps Gould was right after all?

Yes, I think the previous oldest-kiwi-fossil was only 1-a few million years old? The flying idea may have some legs (wings? haha), given that in the early stages of its development the Tasman would have been more of a gutter than a sea.

Let's get this straight: kiwi eggs are exceptional, because they're 25% of body mass. An egg 25% of body mass isn't exceptional if you're a rifleman or a hummingbird (or even a storm petrel, at 29%), because small birds ALWAYS have proportionately large eggs. It's REALLY unusual for a 1–2 kg bird like a kiwi. You always, always have to factor in body size in evolutionary biology.

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  • Mike Dickison: Let's get this straight: kiwi eggs are exceptional, because they're read more
  • Alison Campbell: Yes, I think the previous oldest-kiwi-fossil was only 1-a few read more
  • David: Really interesting stuff, but I suspect this story still has read more