The current scientific “consensus” is that the vertebrate group we now refer to as birds had origins in the late Jurassic Period more than 145-million years ago. Having evolved from bipedal dinosaurs belonging to the theropod clade into an Archaeopteryx-type winged flyer, birds had conquered the air long before the infamous mass extinction event at the end of the Cretaceous Period. Often referred to as the ‘KT Extinction Event,’ a massive environmental upheaval approximately 65-million years ago wiped-out the majority of animal life on the planet, including what at that time was the dominant vertebrate group, the non-avian dinosaurs. The exact cause of the KT Extinction is still a topic for debate with asteroid impacts and volcanic activity viewed as the likely leading contributors, but what is absolutely certain is that the resulting mayhem collapsed the Earth’s ecosystems into complete chaos. Birds, the only members of the dino-clan to survive the cataclysm, emerged from the ruin to occupy a world in disarray but non-the-less rife with newly revealed opportunity.
In addition to leaving behind a ravaged planet, the KT Extinction also instigated the abandonment of numerous ecosystems that had previously been ruled over by the dinosaurs during their Mesozoic habitation. Though left in shambles, these ecosystems had also been vacated by many of the predators and competitors that initially pushed the theropods to the sky during the Jurassic Period. In the absence of rivalry, birds had been granted access to ‘open ecosystems’ and presented with the option to undertake a second conquest of the land. Several species of birds seized on the prospect of life on terra firma and through evolutionary time traded their wings for the physiology of sure-footedness. One modern grouping of these ground dwelling, flightless dinosaurs is called the ‘ratites’ and boasts the South American rheas among its extant members.
There are two species of rhea; the ‘Greater rhea and the ‘Darwin’s rhea,’ which is also known as the ‘Lesser rhea.’ Despite their flightless status, both the Greater (Rhea americana) and the Darwin’s (Rhea pennata) display wings as identifying anatomical features; however, due to the lack of a ‘keeled’ sternum to anchor to their already reduced breast muscles, as well as the presence of a diminished wishbone (furcula) to provide skeletal support, these South American natives are bound to a grounded existence. As an alternative to employment in the aviation industry, the process of natural selection has resulted in enhanced legs that have been optimized for a life on the run. While rhea’s legs have undergone adaptation for speed and agility, they have not yet evolved the ability to avoid entanglement by humans, or for that matter, to steer clear of the occasional dinner table.
On a more historical time scale, one of the most widely used implements of rhea leg entanglement were hunting weapons called ‘bolas.’ Bolas, derived from the Spanish word for ‘ball,’ refers to a weapon constructed with lengths of rope or braided cord with a weighted ‘ball’ attached on each end. The bolas are swung to build-up momentum and then thrown at the legs of fleeing game, tended cattle, or - more to the topic - a running rhea. The momentum of the weighted ends entangles the targeted animal by wrapping the adjoining ropes around its legs; this causes the animal to fall to the ground immobilized, or at least to be sufficiently slowed as to permit dispatch by other means (i.e. gun, knife or club).
Rendering of Darwin's rhea by John Gould.
Gould also identified 'Darwin's finches.'
While traversing South America during the voyage of the HMS Beagle, Darwin had observed Greater Rheas in the wild on many occasions. In fact, with the aid of a few gaucho guided bolas, the abundant Greater Rheas had become a mainstay of his diet.
In conversing with the gauchos in regards to habits of these ratites, Darwin had learned that there was a second variety of rhea roaming the continent. This second type, which the gauchos called ‘Avestruz Petise’ was reportedly a smaller version of the Greater and exhibited a darker plumage and skittishness behavior that made it readily distinguishable from the ones he had seen in the wild. Unfortunately, following Darwin’s discussion neither he nor the gauchos were able to collect a specimen of this “lesser” flightless bird for formal description, and in time it faded from Darwin’s attention. The lesser rhea went unspoken of until finally one evening Darwin was enjoying a quiet dinner with Mr. Marten, the Beagle’s contracted artist, who had shot and killed what they believed was a medium sized Greater rhea. As it turned out, the fully cooked ‘medium rhea’ that graced their plates as entrée on closer examination turned-out to be something else entirely; as Darwin himself detailed on pages 108 and 109 of Voyages of the Adventure and Beagle, Volume III:
“When at Port Desire, in Patagonia, Mr. Martens shot [a rhea]; and I looked at it, forgetting at the moment, in the most unaccountable manner, the whole subject of the Petises, and thought it was a two-third grown one of the common sort. The bird was cooked and eaten before my memory returned.”
Fortunately not all was lost. Recognizing the blunder, Darwin quickly gathered the table scraps and remaining carcass from the garbage. He continued,
“…the head, neck, legs, wings, many of the larger feathers, and a large part of the skin, had been preserved. From these a very nearly perfect specimen has been put together, and is now exhibited in the museum of the Zoological Society.”
Phillips, M., Gibb, G., Crimp, E., & Penny, D. (2009). Tinamous and Moa Flock Together: Mitochondrial Genome Sequence Analysis Reveals Independent Losses of Flight among Ratites Systematic Biology, 59 (1), 90-107 DOI: 10.1093/sysbio/syp079
Image from Wikipedia - Public Domain