Scientists have explained why emus don't fly

Scientists from Kyushu University have compared the development of emu and chicken embryos and found out why emus do not develop a keel - a protrusion on the breastbone to which powerful pectoral muscles are attached in flying birds. In chickens this protrusion is formed, but in emus the growth of the necessary structure stops too early. The work is published in Nature Communications.

Important: the study does not claim that this is the only reason why emus don't fly. Flight depends on wings, body mass, muscles, and the entire evolutionary history of the bird. But scientists have shown an important mechanism: emus fail to form one of the key skeletal "parts" needed for active flight.

Details

Most flying birds have a keel - a longitudinal bony protrusion - on their breastbone. It acts as a strong support for the large muscles that move the wings. Without this protrusion, it is much harder for a bird to develop the strength needed to take off and fly.

The researchers compared the embryonic development of a chicken and an emu. It's a convenient pairing for comparison: the chicken flies poorly but retains the skeletal structure of a flying bird, while the emu is a large, flightless bird from Australia.

In the early stages, the development of the sternum in these birds appeared to be very similar. The cells that later form the breastbone appear on the sides of the embryo's body and then converge in the centre. The difference begins later, when in the chicken the cells continue to divide and form the rudiment of the keel, while in the emu this process quickly dies out.

The TGF-β signalling pathway plays a key role. In simple words, it is a molecular system that helps cells receive a command: to continue dividing and building tissue or to stop and mature. In both birds, this signalling is active up to a certain stage. But in the emu it switches off earlier, while in the chicken it works longer - about two more days of development. During this time, the cells have time to "pull" the future keel downwards.

It is this small difference in time that gives a big result. The chicken forms a breastbone protrusion, while the emu does not. Therefore, scientists do not talk about different "sets of genes", but about different schedules of development: a similar programme works for different times.

This mechanism is called heterochrony - when a change in the timing of development leads to a noticeable difference in the adult organism.

Why it matters

The study helps to better understand how birds gained or lost the ability to fly during evolution. Sometimes big differences in the appearance of animals start with small shifts in embryonic development.

This is important not only for the history of emus. The keel of the breastbone is one of the main features of the skeleton of flying birds. If we understand how it forms, we can better understand why some birds became excellent flyers while others - such as emus, ostriches or casuars - stayed on the ground.

The authors also note that the work could be useful for a broader understanding of thoracic bone development in vertebrates. But it's too early to draw medical conclusions here: the study was conducted on birds, not humans, and it doesn't offer a way to treat thoracic deformities.

Background

Birds have lost the ability to fly many times during evolution. This happened for a variety of reasons: on islands without large predators, a transition to running, increased body mass, or a change in lifestyle.

The emu is one of the large flightless birds. They don't fly, but they run well and are adapted to life on the ground. Therefore, the question "why emus don't fly" cannot be reduced to a single detail. However, the keeled breastbone is a really important part of the flight apparatus.

A new study shows how such a detail can disappear not because of a complete "broken" gene, but because of a shift in timing: the right signal is switched off early, and the keel simply doesn't have time to form.

Source

Seung June Kwon et al, "Heterochronic activation of TGF-β signalling drives the diversity of the avian sterna", Nature Communications, 2026. In the study, the scientists compared sterna development in chicken and emu embryos. They used analyses of gene activity over time and space, as well as experimental models of cell growth. The authors showed that the TGF-β signalling pathway in both birds is active at an early stage, but in the emu it switches off earlier, because of which the cells of the future sternum stop actively dividing and the keel does not form.