Introduction
The Emu, Dromaius novaehollandiae, is the largest native bird to Australia and the second-largest extant bird in the world, surpassed only by the ostrich. Besides it's size, the Emu's most recognisable and well-known characteristic is it's flightlessness.
Characteristics
During it's life span, the Emu can grow up to 2 metres (6 feet 7 inches) tall and 1 to 1.3 metres (3.2 – 4.2 feet) wide at the shoulders. This size causes the Emu to usually weigh between 30 – 45 kilograms (66 – 100 pounds). Emus are also flightless running birds similar to the ostrich with the capability of travelling up to 50 kilometres an hour at a fast economical trot. This animal can be easily characterized by its long powerful legs and long largely naked bluish-black neck. Although sexes are very similar in appearance, females generally have darker necks. Also, immature birds are dark brown with cream stripes accompanied with a dark head and neck. The plumage of an Emu consists of shaggy grey-brown soft-feathers, which resemble primitive coarse hair. Furthermore, emus also have wide and soft beaks ideal for grazing.
There are three separate subspecies which exhibit varying characteristics. The first is D. novaehollandiae novaehollandiae, which is locaced in the southeast of Australia and is characterized by it's whitish ruff when breeding. D. novaehollandiae rothschildi, which is found in the southwest, is characterized by having no ruff during breeding. Lastly, D. novaehollandiae woodwardi is found in the north and is characterized by its slender and paler appearance.
Distribution & Habitat
Emus are can be found throughout mainland Australia ranging from coastal regions to high in the Snowy Mountains. Although an Emu’s main habitats are sclerophyll forests and savannah woodlands, they generally stay in areas with sustainable food and water sources. This causes the bird to be found anywhere from open forests, woodlands, coastal dunes, wetland areas, tea tree plantations, open farms, and occasionally in littoral rainforests and very arid areas. Emus were previously widespread on the NSW north coast, but are now largely restricted to certain areas.
Status
The Emu population in the bioregion of the NSW north coast is listed as an endangered population under the Threatened Special Conservation Act. Several subspecies on Australia's surrounding islands have become extinct due to European settlement, this includes the Tasmanian subspecies, as well as two dwarf species of Emu that lived on Kangaroo Island and King Island. There are three extant subspecies still residing on the Australian mainland.
Diet
Emus are nomadic type animals and move according to climatic conditions. More specifically, the birds will reside in one area if there is sufficient food and water present. However, when the resources become more variable or sparse, Emus can travel hundreds of kilometres (12 – 25 km per day) in search of sustenance. Similarly, Emus feed on a wide range of plants (native and introduced species) and insects that differ in abundance from time to time and place to place due to seasonal availability. Insects may include grasshoppers and crickets, lady birds, soldier and saltbush caterpillars, bogong and cotton-boll moth larvae and ants.
Although Emu’s feed on a great variety of fruits, seeds, growing shoots of plants, flowers, insects, other small animals and green herbage of annual and perennial plants, they exhibit preference when a choice is available. For example, emus feed on seeds from Acacia aneura until it rains; after rainfall they eat fresh grass shoots and caterpillars; and during the winter, they feed on leaves and pods of Cassia; while in spring, emus feed on grasshoppers and fruit of Santalum acuminatum, a sort of quadong. In addition, since emus feed on seeds, they are an important agent for the dispersal of large viable seeds, which contribute to floral diversity.
Reproduction & Development
The breeding season for the Emu typically begins in November and lasts until May. During this time a male will make a nest and allow 7-12 eggs to be laid in it. After this time, the male will begin to incubate the eggs and will forcefully defend the nest from predators or even female Emus that attempt to lay more eggs in his nest. Each egg is approximately 13 x 9 cm in size and weighs 700g. The male will incubate the eggs for approximately 56 days, during which time he will fast and rarely move from the nest. When the eggs hatch, the male will care for and protect the chicks for another 18 months. Chicks will reach maturity and can begin breeding when they reach 2 years of age. The usual length of life for an Emu is around 25 years, but they can live longer in captivity.
Behaviour
Classically, the Emu’s social reproductive behaviour was categorized as monogamous. Emu’s were observed to pair up and stay paired until the male began to incubate the eggs. More recently this view was challenged by molecular data. A study done by Taylor et al (2000) showed that on average 51% of chicks in a nest are not fathered by the male that is sitting on the nest and only 11% of nesting males fathered all the chicks in their brood. Also, the males do not seem to be able to detect which chicks are their genetic offspring, so they cannot give preferential care. Another note of interest is the fact that the males have a very high parental investment in the chicks, as they go without food for almost 2 months while incubating the eggs, and are the sole provider after the eggs have hatched. It has yet to be determined how this reproductive strategy benefits the male in terms of reproductive success. Also of note, is that it was suggested in the study, that the earlier the male begins to incubate, the more likely he will father the majority of eggs in the nest.
In terms of other social behaviours the Emu females will compete over males as the breeding season progresses and less non-sitting males are available. However, violent outbursts only seem to occur when the male feels his brood is threatened. In general the Emu is a docile and curious animal, but it tends to also be largely solitary. The Emu will usually only been seen in groups during the breeding season and when food is scarce.
Anatomy
The anatomy of the Emu consists of an interesting variety of structures, which help this large flightless creature to survive. Similar to other flightless birds, Emus have no keel on their sternum. This flat breastbone feature does not allow the bird's wing muscles to attach and anchor, thus disallowing these birds to fly even if they had suitable wings. Although the Emus lack flight, they have the ability to run at fast speeds due to their specialized pelvic limb musculature. The pelvic limb has several features which reflect the Emu's ability for sustained running at high speeds.
- Emus have on each foot three forward facing toes and no hind toe.
- A feature exclusive to the Emu is the powerful muscle in the shank call the Gastrocnemius muscle which contains four muscle bellies, instead of three.
- The contribution to total body mass of the pelvic limb muscles of the Emu is similar to that of the flight muscles in flying birds, whereas the pelvic limb muscles of flying birds constitute a much smaller proportion of total body mass.
Emus also have small vestigial wings, approximately the size of a hand. Although unable to allow flight in emus, these wings are used to regulate body temperature as they can be used to fan during hot weather. Feathers also allow emus to regulate temperature and cope with extreme changes in weather. A typical bird feather contains only a single rachis (main shaft), whereas the Emu feather contains a double rachis emerging from a single main shaft. The shafts and tips of the feather are also black, allowing solar radiation absorption. The loose-packed inner plumage also insulates the skin, which prevents heat from flowing to the skin. Thus, it allows the bird to remain active during the heat of the day. Another feature that Emus can use to maintain their body temperatures is the use of the lungs as evaporative coolers. Emus also utilize an inflatable neck sack to vocalize sounds of booming, drumming and grunting. These calls can be audible over two kilometres away from the source.
A Glimpse into Emu Evolution
This is an exploration of the clues that were found on the evolution of the Emu. The below comparison of various Emu anatomy (such as the skin or wings) to other birds that can fly helps to uncover differences which will help determine the Emu's evolutionary path. Birds that can’t fly are known as ratites and birds that can fly are referred to as volant birds.
Skin
When the skin of the emu was investigated with a light microscope, it was found that the epidermal and dermal layers have a similar arrangement to other avian species. Although the skin structure was similar, there was one difference that is unique to the Emu which is the number of lipid spheres that are located at the lower level of their epidermis or skin. This layer of fat may be an evolutionary adaptation to support the large feathers and also enhance insulation against ambient subzero temperatures. Also, since volant birds do not have this fat layer, it is hypothesized that it was developed after the loss of flight, or that all ancestral birds had this layer of fat but lost it when they took to the air.
Lungs
A close look at the size and physiology of an Emu's lungs has shown that they are poorly adapted for gas exchange when compared with lungs of other birds. The capillaries that are required for gas exchange are covered by granular epithelial cells, so we find the cells dividing the blood and the gas to be unusually thick. Combined with the Emu's poor ability for oxygen intake and it's large body size, this suggests that the Emu evolved in a warm environment with few predators. If there were few predators, the Emu would not have had a selective pressure for highly efficient lungs for flight for fast escapes.
Wings
The Emu has lost almost all need for the function of its wings. Through evolutionary time, their wing skeleton has degraded to only a single functional digit. In accordance with a reduced skeleton, there have been significant reductions in the muscles of the wing, even when comparing the Emu to other non-flying birds. Still, many muscles show diversity in their shape, where they are attached to the bone ,and whether they are even present or not. Evolutionary theory predicts that relaxed selection on vestigial organs should allow more variation to persist in the population, and this corresponds to what is observed in Emu populations.
Genome
When an important shift occurs in life-history strategies of an evolutionary lineage, they are accompanied by genetic and/or phenotypic changes. This means whenever there is a dramatic and evident change in a species, it should be at least visible in the genome, if not in the visual assessment of the organisms. We can then find these changes and compare them between bird species. More specifically, we can compare the differences of genes between birds that can fly and birds that can't. This is a way to map out how the Emu evolved into the bird it is today. In a study done by Roots & Baker (2002) it was found that ratite bird species actually posses a larger genome size compared to volant birds. They hypothesized that one reason for the smaller genome size in volant birds may be high metabolism requirements for flight. This metabolism requirement may have selectively trimmed away genes that were not necessary, thus making the birds more efficient and more reproductively successful. Also, ratites are believed to have diverged early from the bird family tree, so they are thought to still carry a fairly intact ancestral genome which was much larger than that of the descendants of later branching lineages. As more bird genomes are sequenced, more concrete information about the ratite and volant bird genomes will become available.
