The evolution of flight in birds required dramatic modifications in anatomy and physiology. Many of the key adaptations, such as the swap of forelimbs for wings, are obvious. For this reason, it is commonly assumed that the earliest birds, which were equipped with wings, were probably capable of flying. But true sustained flight also required other changes deep below the feathers. Notably, powerful flight muscles need to be provisioned with large amounts of oxygen, which is pumped around the body in the blood by the heart. However, reconstructing the cardiovascular systems of extinct, early birds is no mean feat.
Fortunately, there is an ancient division within modern birds that helps shed light on the earliest avian origins. ‘Neognaths’ are represented by the majority of modern flying birds, whilst ‘paleognaths’ include ratites (flightless birds such as ostriches, emus and kiwis) and tinamous. Tinamous are peculiar chicken-sized residents of Latin America that are capable of short bursts of flight but not sustainable, long-distance flapping flight. They are also characterised by having much smaller hearts than other birds. This prompted Jordi Altimiras from Linköping University, Sweden, and his colleagues to detail the cardiovascular physiology of tinamous in Bolivia and Chile in order to predict how the hearts of early birds may have worked.
By weighing the hearts of tinamous of different ages, Altimiras and colleagues first confirmed that their hearts are tiny: chicken hearts are about twice as big. Indeed, tinamou hearts are more comparable in mass to those of similarly sized alligators, which represent the non-flying reptilian ancestors that birds evolved from. The team also measured cardiovascular function in anaesthetised birds and revealed that the smaller hearts of tinamous generate cardiac output that is 2.5–3 times lower than that of chickens, although they have similar blood pressure.
The researchers also made metabolic measurements in tinamous and chickens after they chased the birds into flying for 3 min. The tinamous were physically exhausted by this and took far longer than chickens to return to resting levels of oxygen consumption. Lactate, a marker for anaerobic metabolism, also reached about twice the concentration in the tinamou blood after exercise, strengthening the view that their small hearts could not provide enough oxygen to adequately sustain flight muscles.
Based on this evidence, Altimiras and his team suggest that the evolution of flight must have coincided with the evolution of an enlarged heart, which they term the ‘heart to fly hypothesis’. They further suggest that this most likely evolved after the split between paleognaths and neognaths. This is, however, controversial. Whilst tinamous have small hearts, most ratites, despite being flightless, have much larger hearts that are comparable to those of flying birds. If Altimiras and colleagues are correct, ostriches and their kin must have evolved their large hearts independently from other birds. This is possible – it may be an adaptation for their famous running abilities – but is difficult to prove. Nonetheless, the data provide a unique insight into the cardiovascular changes that were required for birds to conquer the skies and the hypothesis deserves further study.
