The emu, or Dromaius novaehollandiae, is the only extant species of the Dromaius genus and are part of the ratite family. Ratites are flightless running birds and also include the ostrich, kiwis and cassowaries. Emus are found throughout the continent of Australia with a relatively abundant population which the IUCN lists as least concern despite the fact that they are extinct on the Australian islands and are threatened by habitat fragmentation and loss. Although they live throughout Australia they shy away from living in extreme habitats like deserts and rainforests. Emus are the second largest flightless bird in the world and the largest in Australia weighing 80-120lbs and growing up to 6 feet tall with the females generally growing larger than the males.
Emus, unlike other animals that care for their offspring, have paternal care instead of maternal care. During mating season the male emu both builds and incubates the nest. After the male emu builds the nest the female will lay 5-15 eggs and the male will then sit on the nest and not move, aside from adjusting the eggs, for the next eight months. The female, however, may mate again and lay up to three clutches during a breeding season. During this nesting period the male can lose up to one third of his body weight as he won’t leave the nest for food and his only water source is the dew. Luckily, emus also have special adaptations to help them go long periods of time without water. The emu’s extra-long intestine helps it retain water and is 7-8 times its body length, significantly longer than their relative, the cassowary’s.
Emu feathers are adapted to help survive the harsh Australian climate and to retain water after swimming. The emu feathers have several key differences from the feathers of flying birds to provide, these adaptions make the emu feather less hydrophobic. while other birds typically want their wings to resist water emus want to retain water to prevent dehydration. First the emu feather is double shafted, meaning two feathers, instead of one, grow out of the cuticle. Next, the cuticle lack the outgrowths in the feathers of flying birds that make the cuticle appear “hairy”. Those outgrowths are believed to aid in aerodynamics so it’s thought that the outgrowths are an adaption and that the emu feather’s lack of outgrowths may be ancestral. Further, the cortex of the feather does have barbules like flying birds’ feathers but the barbules lack interlocking hooks making the emu feathers look hairy. Finally the cortex of the emu feather isn’t thickened like in flying birds and has a large sinus volume. The large sinus volume is what allows the emu feathers retain water. By retaining water in their feathers after swimming they are able to prevent their skin from losing water because the water in their feathers will evaporate first to prevent dehydration.
Not only are emus excellent swimmers, they are also can jump up to 7 feet straight up and run about 30 mph. When emus run they use their feet and toes to give the power to push off while they use their thighs in adduction, bringing their legs back to their body’s center. Here is a video showing the emu running, note the how the toes give the power to the stride. Emus can run so fast because their musculature is upside down compared to their flying relatives. Flying birds’ upper body and the emu’s lower body are both developed for efficient locomotion. In birds that can fly 10-35% of their muscle mass is in their upper body, similar to the emu’s 25% of their muscle mass from their legs. Further, the type of muscle in flying birds’ upper body and the emu’s lower body is similar in that it contains little, if any, slow fibers. Slow fibers are the ones that aid in posture while fast fibers are responsible for locomotion. There are also higher levels of myoglobin in the upper body of flying birds and the lower body of emus to help maintain high rates of aerobic metabolism. So while emus and flying birds utilize different forms of locomotion they have similar muscle make up to aid in efficiency of their specific locomotion. Unfortunately, even though emus have specially adapted legs to run quickly and efficiently they still cannot run or even walk backwards.
On the other hand, while emus have some amazing adaptions to aid in their ability to thrive in the Australian outback, they still haven’t managed to shake off the dead weight that is their wings. While emus can move their wings, their wings don’t hold any function and are thus considered vestigial structures. Many examples of vestigial structures taught in textbooks only remain as small bones not visible from the outside, while emus’ wings are still very apparent. Emus are thought to be currently in the process of evolving to no longer have these wings at all. It’s believed that the emu’s wings have been evolving from a common ancestor of ratites from about 25 million years ago because of a shared single digit. More so, there is not only a high degree of variance of the vestigial wings within the ratite family but also a high degree of variance in emus. Emu wings are unique in that not only do they vary from individual to individual they also vary from wing to wing. The asymmetrical wings in emus suggest the environment has no impact on selecting for the phenotype of the wing and that there isn’t a certain wing phenotype that aids in fitness levels, further supporting the idea that the emu wing is in fact a vestigial structure.
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