Ratites; the ancients of the bird world. #2 The case of the flying moas

In introducing the wonderful and venerable ratites last time I made mention of another group of relatively little-known birds, the tinamous, long described as a 'sister group' of the ratites. There are 47 species of them in South, Central and North America, as far north as about central Mexico. Like the ratites they are Palaeognaths, forming with them an entirely different group of  birds, the 'ancient palates'. They fly poorly but can still do so and, unlike the ratites, thus retain the keel on the breast for anchoring the mighty flight muscles. They also differ from the ratites in having powder down feathers - amazing feathers which are never moulted, but which crumble at the tip to provide a permanent supply of conditioning powder (sort of like home-made talc) for cleaning and generally improving the feathers. For these reasons it has always been assumed that they separated from the ratites early in the piece, but travelled with the rheas when South America broke free of Gondwana. This means too that they couldn't actually be ratites, because that would imply that they had a common flightless ancestor with the rheas and regained flight in South America - such a reversal of all the acquired adaptations to a flightless lifestyle is universally regarded as an evolutionary impossibility.

Tinamous are notoriously shy, and to date I've only seen three species - all rainforest birds - two of them at one sitting, courtesy of the wonderful Aguas Verdes initiative in northern Peru, where forest birds are lured to a hide by offerings of corn. As a result the forest has been spared from conversion to coffee, and other landowners are taking interest; more on this story here. 

Cinereous Tinamou Crypturellus cinereus, Aguas Verdes, lower eastern Andes slopes, northern Peru.
This shows typical tinamou characteristics - strong running legs, and virtual lack of a tail,
which is associated with their reluctance to fly, and general lack of competence at it.
This is a medium-sized tinamou, weighing about half a kilogram.

The Little Tinamou Crypturellus soui is indeed one of the smaller tinamous,
weighing only 200-250 grams.

Tinamous, as I mentioned, are very hard to see normally, and this cryptic behaviour has saved them to some extent from dramatic losses suffered by other largish edible birds in their range. Nonetheless hunting pressures can be severe, along with steady habitat loss.

Great Tinamou Tinamus major at roost, Napo Lodge, Ecuadorian Amazonia.
This is a species that is facing problems, having been recently listed as Near Threatened.
It is a big tinamou, weighing well over a kilogram.

OK, you've been very patient - if indeed you're still reading! - while I go on about a group of birds that aren't really ratites. However, that's only what we used to think... I've been at some pains to introduce them because new understandings about them have changed pretty much everything we now think about ratites too. The nice neat story about ratites representing the archetypal Gondwanan story - an ancient group which inhabited all the southern lands before they parted ways, and drifted with the new continents to their new positions, was very appealing and was one I've told, in all good faith, many times over the years.

There have been some mutterings over the years along the lines of "OK, they're old, but not quite that old!". The mutterers had some good points - for instance Africa broke free of Gondwana by at least 110 million years ago. Madagascar was isolated by 88 million years ago – but from India rather than Africa! But still, how else could we explain what we see? Then in 2010 a paper was published by a group of Australian and New Zealand scientists which turned this particular comfortable understanding of the world inside out. They had available to them techniques denied to earlier researchers, including the ability to do a complete mitochondrial DNA analysis (mitochondria contain far less DNA than do cell nuclei, and this DNA seems to evolve more quickly than does that of the nucleus so tells good stories) and to look at such material from fossils.

The technique has rapidly become widely used to study a range of animal groups, comparing species to determine when their Most Recent Common Ancestor walked the earth ('MRCA' appears all over the place now). Phillips et al shocked us all by announcing that far from being associates of the ratites, tinamous are slap bang in the middle of them - they are ratites for any practical purposes. One existing hint (isn't hindsight wonderful?!) might have been that tinamous, like most other ratites, breed most unconventionally - a male waits to be visited by groups of roaming females who mate with him and lay their eggs into his nest, then move on and find another obliging dad for their eggs. (There are a few tinamou variations, but this is the norm.) It is generally regarded as a very efficient way of rapidly increasing populations.

Darwin's Rhea father and chicks - who between them have many mums - Torres del Paine NP, Chilean Patagonia.

From the conventional viewpoint however it gets worse - the closest relative of tinamous are not the obvious rheas, but the New Zealand moas! Moreover the same study showed that Ostriches and the recently extinct Madagascan elephant birds are not each other's closest relations, as logic would insist, but the elephant birds and kiwis are closer to each other than they are to anyone else... Ostriches are way out on the ratite fringes - and may have even arisen in Eurasia, arriving in Africa only some 23 million years ago.

The only way to make any kind of sense out of all this is to accept the seemingly preposterous idea that the ancestral ratites flew to their current continents, then all (except for the tinamous) subsequently lost the power of flight. Kiwis and emus/cassowaries parted ways only 60 million years ago - but by then New Zealand had been isolated for 20 million years. Moas and tinamous separated at about the same time.

It's a difficult concept to absorb. I've always been a firm believer in the parsimony principle – ie that the simplest evidence-based explanation is always likely to be the correct one. The more evolutionary steps that are required to explain something, the less likely it is that it happened that way. But once we accept, however reluctantly, all this explosive material, how on earth are we - or at least the researchers - supposed to explain such a bizarrely unlikely set of circumstances? What could possibly have happened right across the Southern Hemisphere to have triggered a series of wildly unlikely parallel events at about the same time?

Well, of course something did. Around 65 million years ago, already a time of colossal volcanic activity, a massive meteorite, an asteroid some 10km across, smashed into what is now the Yucatán Peninsula of Mexico, hurling vast quantities of dust and smoke into the atmosphere and dropping temperatures cataclysmically. Sulphates hurled into the air formed nucleation sites for brutal acid rain storms. Once the dust particles settled the huge quantities of carbon dioxide released raised worldwide temperatures for centuries. By the time things had started to settle down again, three quarters of all plant and animal species on earth had vanished, including all the dinosaurs (other than some of the birds of course). 

Southern Cassowaries, Etty Bay, North Queensland.
Their small flying ancestors survived the great meteorite strike.

It was an empty landscape, full of menace and opportunities. Among the now-empty niches was that formerly occupied by the birds' immediate ancestors, the erect running dromaeosours. We know that birds, especially on islands, are prone to give up the enormously energy-demanding flight habit when there is no longer a pressing need for it. So, it is not at all hard to conceive that members of a particular group of bird survivors, with the genes of the dromaeosaurs still within them, should have responded to landscapes suddenly largely devoid of predators and full of options, by giving up flight that had carried their ancestors across the oceans. Later they also grew larger in response to such predators that did eventually arise.

Darwin's Rhea chicks, Patagonia.
Their ancestors flew to South America, arriving quite separately from the (apparently later) tinamous.

An unlikely story, but the only one that fits the facts - and the job of science, and indeed of common sense, is to find theories to fit the facts, rather than deny the facts to fit a theory.

And doesn't it make a good story?!

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