If internal skeletons didn't adapt to the ever larger forces exerted on them as organisms grow, they would eventually break under the increasing stresses and strains. But fortunately bones change in response to an organism's growth and movement. Having studied how the bones of goats change as they grow, Russell Main and Andrew Biewener turned their attention to a model of two-legged locomotion, the emu. Emu are ideal creatures to study because they increase in mass by around 65 times as they grow from 25 cm tall hatchlings weighing 0.74 kg to 55 kg adults up to 2 m tall. `The goal was to examine growth and development in the emu hind limb skeleton and see how it is affected by changes in locomotor mechanics and body mass during development',says Main (p. 2676).
To monitor the movements of the limb joints and forces generated by different sized emu, the team placed external markers on the leg joints and monitored their movements with a video camera as the emu ran down a runway with force plates embedded in the floor. They found that both big and small emu walked or ran in very similar ways, and that the maximum force they generated as their feet hit the ground remained very similar in proportion to their body weight.
Having shown that how emu run doesn't change much as they grow bigger, the team attached strain gauges to the two largest leg bones: the femur and the tibiotarsus. During a delicate operation, the team attached either a single strain gauge, pointing down the length of an emu's bones, or a `rosette' of three strain gauges arranged on top of each other, each at a 45° angle to the one above it, which could measure strain in three different directions at once. Wires from the strain gauges travelled under the skin to an external cable at the hip.
Following surgery, the emu still generated the same forces with their operated legs as they ran on a treadmill. Concentrating on the strain patterns the gauges recorded, the team found that at each of the sites they measured in both bones, the patterns of strain were similar in different sized emu. However the amount of strain differed in some parts of the bones with age. The strain over the front and back surfaces of the femur and over the tibiotarsus increased as emu grew. This means that as emu get bigger, their bones experience relatively larger strains. The team found that the leg bones stayed a similar size and shape relative to increasing body mass, rather than becoming relatively more robust as they grow, which might otherwise be expected to support the extra strain.
Other measurements that the team made on the bones gave them some clues as to how the emu partly deal with these increasing strains as they grow. They found that as the emu grew both leg bones became straighter, which decreases the strains that a bone experiences. They also found that the bone mineral content increased from 50% in small emu to 70% in large emu, which makes the bones much stronger and better able to support an emu's increasing bulk.