Why don’t we see any four-winged feathered dinosaurs, these days? Time was, not so many hundreds of millions of years ago, that various theropods sported quite well-developed vaned feathers not just on their forelimbs, but on their hind limbs as well. The most famous, and perhaps most flamboyant of these animals (that we know of) was Microraptor, a small four-winged dromaeosaur known from fossils that, even given their tiny size by dinosaur standards, are among the most spectacular ever found. It looked really rather fabulous. So why isn’t there an equivalent among modern birds?
Well – and some of you will have known where this was going – there is. But it’s domesticated and fancy.
Specifically, pigeons. While some chicken breeds can possess some quite flooftastic foot feathering, none matches the amazing, truly wing-like arrangement seen in pigeons. This trait, which has been achieved through careful selecting breeding over generations, results in the pigeon’s second toe acting like an alula, with ‘primaries’ along the toes, ‘secondaries’ along the foot, and even ‘tertiaries’ on the shin. They are truly Microraptor-like birds, and the implications of the alula-like second toe are very interesting.
All of this is explained, and stunningly illustrated, by Katrina van Grow (Twitter) in her latest book, Unnatural Selection. It’s a follow-up to The Unfeathered Bird, a book I described as “undoubtedly the best ever to be inspired by a dead duck”. This time, it’s not all about anatomical illustrations, although there are still plenty of those if that’s what you’re in for. Rather, the book is an exploration of how humans have shaped animals through centuries of domestication; a look at what people have sought after and why, how domestic breeds have been created and shaped as a result, and what the underlying genetic processes are.
This includes fancy pigeons, her husband Hein’s particular fascination – from pouters, to brachycephalic weirdos, to the four-winged freaks we’ve been looking at here. It also includes other birds, like chickens, geese, ducks and finches, along with cats, dogs, pigs, cattle, sheep and much more besides. The apparent plasticity of domestic animals is endlessly fascinating. By selectively breeding animals over millennia, often for traits that would have served no advantage in the wild, humans have unwittingly helped expose the mechanisms of evolution. This much was apparent to Darwin, and it remains as relevant today. As Katrina points out, when people dismiss tiny toy dogs as abominations that run contrary to the principles of nature, they are entirely missing the point. Domestic dogs of every type are a huge evolutionary success story. There are millions of them, all perfectly adapted to their man-made environment. In the wild, there is no intelligence in control, but even ‘unnatural selection’ should be seen as a part of nature.
That said, I’m with Hein* – there’s nothing quite like those fancy pigeons. Although humans have attempted to, for example, ‘reverse-engineer’ cattle to recreate the extinct aurochs, such efforts have ultimately been in vain. All you’ll end up with is a big, angry, unmanageable cow. But those four-winged pigeons are something else entirely. They may well demonstrate that palaeontologists and others who wish to study the origin of birds should be careful about being led down the garden path. It’s well established, by now, that the line between bird and non-bird feathered dinosaur is so blurred as to essentially be meaningless, and recent discoveries pertaining to enantiornithines have only further complicated the story of bird evolution. As Albert Chen explained so well at TetZooCon this year, just when scientists think they have a handle on bird evolution, the cheeky little feathered buggers conspire to move the goalposts. As Katrina herself notes,
“…It sounds almost foolish to suggest that a ‘man made’, ornamental show bird can provide answers to such profound questions as the evolution of birds from non-avian dinosaurs. Yet the step that distinguishes scaled legs from feathered ones, forelimbs from hind limbs, is likely just a small one – not a difference in the genes themselves but in the regulation of genes – the way they’re expressed, or in the timing of respective developmental processes…There’s a dinosaur in the genome of every bird…The revelation in modern biology is not the differences between animals, but the similarities they share.”
So why the lack of well-developed hind limb feathers on modern birds? Katrina argues that it’s likely a matter of keeping cool. Feathers are highly efficient for insulation, but at the same time, flying generates a lot of heat. Nonavian dinosaurs like Microraptor were very poorly adapted for flight when compared with modern birds, and may have mostly been gliders. Katrina points out that, in studies where modern pigeons have had their scaly feet covered up, the animals quickly overheat when flying. Show pigeons with the whole ‘four winged’ thing going on hardly fly at all. Katrina suggests that the reason we don’t see Microraptor-like birds today is that hind-wings simply don’t work that well for powered flight. All the same, the gene combinations to produce them are still there, at least in some birds, and in pigeons, at least, the desire on the part of breeders to produce the fanciest birds for show has brought them out. As Katrina rightly points out, it’s pretty wild to think how the breeding of modern birds, purely so that people can show them off, can allow such insights into deep time.
Of course, all this pigeon business comprises only a small section of Unnatural Selection. This is a book that feels just as enormous as The Unfeathered Bird – if not moreso, given the way that it encompasses human history, domestication, anatomy, genetics, and more – but it was completed in a fraction of the time. It’s an astonishing accomplishment, and an absolutely sumptuous book just to flick through. You’d be doing yourself a disservice by merely admiring the pretty pictures, though. Katrina’s writing has a newfound confidence, personability and liveliness this time around. She manages to perfectly explain often quite heady topics related to genetics without alienating the dummy lay reader (like me). The insight into the processes behind various beloved breeds, and the mechanisms that give us black cats, sausage dogs, five-toed hens, four-horned sheep, shaggy cattle and elongated meat-havin’ pigs is fascinating in itself. This would be a remarkable book even without Katrina’s art. That it includes both makes it a must-buy.
*Now there’s something I thought I might never say.