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Re: [dinosaur] Avian flight: Aerobic performance in tinamous limited by small heart (free pdf)
It's been known for a long time that tinamous have a small heart and lungs (in relation to their body size).Â But the prevailing view is that their undersized cardiovascular system was a tinamou 'quirk', and associated with their secondarily weak (and rather weird) flight abilities.Â However, this study instead proposes that these weak flight abilities (short bursts with a powerful jump, described as "non-sustained flapping flight") are primitive for crown birds (Neornithes) and solely retained by tinamous.Â The authors further propose the idea that "the ground-up, short and not sustained flapping flight displayed by tinamous represents an intermediate step in the evolution of the aerobic sustained flapping flight of modern birds".Â But this hypothesis is at odds with the excellent flight abilities inferred for many non-neornithine ornithuromorphs and enantiornithines.Â Advanced/sustained flight abilities appear to have arisen near the origin of Ornithothoraces, long before Neornithes came onto the scene.Â So I suspect the small heart/lungs and non-sustained flight abilities are tinamou quirks after all, not primitive for Neornithes.
The proposed 'small heart' hypothesis also requires that the various ratite lineages all acquired their 'big' hearts independently, associated with the need for cursoriality and running endurance.Â Although multiple loss-of-flight events can account for the evolution of flightlessness across paleognaths, it's not known how and why the various flightless lineages gave up flight in the first place.Â (Note that according to the authors, data is apparently lacking regarding the size of the heart in kiwis... which I find surprising).
The authors might be on to something insofar as tinamou-style flight could be primitive for birds (Avialae): ground-up jumping (take-off) leading into short, non-sustained burst of flight.Â This sort of flight might have been exhibited by _Archaeopteryx_, jeholornithids, confuciusornithids, etc, though for different anatomical reasons than tinamous (as far as the flight apparatus is concerned).Â Tinamous have well developed keeled sternum and pectoral muscles, but rather short wings and insufficient cardiovascular capacity for sustained flight.Â Basal avialans lack the keeled sternum, triosseal canal and expanded pectoral muscles for sustained flight, but had relatively large wings and long, powerful hindlimbs - so they were possibly capable of take-offs (initiated by the hindlimbs) and non-sustained flight. Like tinamous, these short bursts of flight in basal birds would have been sufficient to escape a pursuing predator on the ground.Â