The Kangaroo Family: Superfamily Macropodoidea
Kangaroos and their kin are characterised by powerful hindlimbs and long hind feet which are usually employed in a fast hopping gait. Until recently, most authors included all of them in a single family, Macropodidae, but reassessment of the diversity of the group has led others to upgrade the
taxonomic status of the group to a superfamily, Macropodoidea. The semi-technical term, 'macropod' (more strictly 'macropodoid'), remains a useful general term for all members of the group. The Macropodoidea includes two families, the Potoroidae, often loosely referred to as 'rat-kangaroos' and comprising rat-kangaroos, potoroos, and bettongs; and Macropodidae, comprising the kangaroos, wallabies, hare-wallabies, nailtail wallabies, rock wallabies, pademelons, swamp wallaby, quokka, tree kangaroos, and the forest wallabies of New Guinea.
With the exception of the Musky Rat-kangaroo, macropods bound on their hindlegs when travelling at speed. Such a gait is marginally more efficient in energy expediture than quadrupedal running or galloping but it is ineffective at low speed, so much so that kangaroos must use their tails in a
clumsy 'pentapedal' gait when moving slowly. The weight of the body is taken on the forelegs and the down-pressed tail while the hindlegs are ponderously swung into a forward position. A kangaroo cannot walk, nor can it easily move backwards.
Why, then, do kangaroos hop? There is no certain answer to the question but, as this method of locomotion is found elsewhere among mammals only in small animals such as hopping-mice, it may be that the habit arose in small ancestors of the macropods and was retained because there was no great evolutionary pressure to revert to a more generally effective quadrupedal gait. It is possible to imagine a kangaroo-like animal that could run on its hindlegs like an
emu or a frill-necked lizard but the neuromuscular coordination of most kangaroos is such that the hindlegs must move together when used in terrestrial locomotion. An exception is found in the tree kangaroos which can move the hindlegs alternately when walking along a branch, but use them
together when hopping on the ground. A further interesting exception, demonstrating that limitation of movement is imposed by the nervous system, is that a swimming kangaroo kicks its hindlegs alternately.
Around 100 million years ago, marsupial and placental mammals diverged in their evolutionary paths. Placentals evolved a system of nurturing young inside the female's body, while marsupials gave birth to embryonic offspring and then nursed them outside the female's body, usually in a pouch. The earliest marsupials in Australia date back 23 to 30 million years, arriving via a land bridge connecting Antarctica and America. Eventually Australia and New Guinea split away, enabling the marsupials to evolve, without competition, into the broad range of unique animals we see today.
Marsupials appear to have originated as an arboreal group and typically they have prehensile tails. Prehensility has been lost in the largely terrestrial dasyurids and bandicoots but is present in sufficient degree in potoroids to permit them to carry nesting material with the tail. No macropodid has a prehensile tail, this structure serving as a balancing counter-weight when an animal is hopping, as a prop when standing upright and as a fifth limb in slow locomotion. Tree kangaroos would be much more efficient arboreal animals if their tails were prehensile but it seems that, having evolved from terrestrial kangaroos that had lost this capacity, they were unable to redevelop it.
Most macropods are nocturnal but the larger species may be active in the early morning and late afternoon. Potoroids construct nests but no macropodids do so, although many retire to dense vegetation during the daytime. The Red Kangaroo requires no more than the shade of a tree or bush but other kangaroos of the arid regions retire to caves or rock crevices in the heat of the day. Such behaviour reduces heat stress and water loss due to evaporation and is an essential component of the strategies by which they cope with desert life.
The pouch of macropods (like that of all diprotodonts except wombats and the Koala) opens forwards. It contains four teats but it is usual for only one young to be born at a time. In most macropods the female becomes sexually receptive immediately after giving birth (post-partum oestrus) and mating usually leads to the fertilisation of another ovum. Since gestation occupies no more than 3-5 weeks, this would seem to lead to the production of a new young every month or so. However, the presence of one suckling young in the pouch exercises an effect on the maternal endocrine system such that the second fertilised egg ceases development after it has become a spherical sac about 0.3 mm in diameter, known as a blastocyst. The blastocyst normally remains in this quiescent stage until a few weeks before a young animal permanently vacates the pouch (at an age ranging from about 90 days in the Brushtailed Bettong to about 200 days in the Red Kangaroo). The phenomenon is known as delayed implantation or embryonic diapause.
If a pouch young should die or be lost, the quiescent blastocyst resumes development and proceeds to birth. If development of the pouch young proceeds normally, the quiescent blastocyst resumes development a few weeks prior to permanent vacation of the pouch by its occupant. Timing of events is such that the individual developing from the blastocyst is born almost immediately after the pouch is vacated by its predecessor. This birth is normally followed by mating which leads to the production of yet another quiescent blastocyst.
The situation is complicated by the fact that a young macropod does not cease to suckle immediately after leaving the pouch, weaning extends over several weeks, during which period it suckles by inserting its head into the pouch. From the time of birth to the completion of weaning, it feeds from the same teat, so its younger sibling must therefore attach itself to one of the other three teats. It is extremely unusual for a mammal to cope with two suckling young of very different ages and the situation is even more remarkable in that the mammary glands supplying each of the two functioning teats produce milk of quite different composition.
Under normal circumstances a typical female macropod is almost continuously pregnant from the time it reaches sexual maturity. After the birth of its first young it is also almost continuously lactating and carrying a quiescent blastocyst. Malnutrition arising from drought may, however, cause a female to terminate its oestrous cycle temporarily. In this condition it does not mate after giving birth but will nevertheless produce one more young from its quiescent blastocyst.
In terms of productivity, embryonic diapause leads to somewhat greater efficiency than a 'normal' cycle of continuous reproduction. In the latter case, even if mating occurs immediately after a birth, production of a new young is delayed for the period required for gestation while in the former, one is produced almost immediately. Whether this advantage is significant is by no means clear. If it were, one might ask why it is such a rare phenomenon, even among marsupials. Certainly, embryonic diapause is a device that enables a female to replace an infant that has been lost from (or died in) the pouch, but the time thus saved is not great. A number of other possible benefits have been postulated but none appears to be of sufficient magnitude to justify the complexity of the phenomenon.
Macropods occupy almost all Australian terrestrial environments but do not occur above the snow-line. The group is well represented in New Guinea and nearby islands.
The Red Kangaroo, Macropus rufus, is the largest of the macropods and with a head and body length of up to 1.4 metres in males and 1.1 metres in females, with a further .9 to 1 metre tail. Males can weigh up to 80 kilograms while females achieve only half that weight.
The Red kangaroo is readily identified by a red to blue-grey coat on its upper body and distinctly white coat underneath. Adults can be distinguished from other kangaroos by their white underparts, black and white patch at the side of the muzzle and a broad white stripe from the comer of the mouth to the base of the ear. Their rhinarium is naked, sharply delineated and dusky in colour.
An abundant species, it was previously known by the scientific name Megaleia rufa and has other common names of the Plains Kangaroo, Marloo and Blue flier (female).
It is a grazing animal with a preferred diet of green herbage, including grasses and dicotyledonous plants, and its local distribution is frequently influenced by the availability of this food. Feeding takes place mostly at night but may extend into the late evening and early mornings. The fore-stomach, like that of other kangaroos, is adapted for digestion of food rich in cellulose.
|The Red Kangaroo, one of the largest living marsupials, occurs over most of the central part of Australia in areas of less than 500 mm mean annual rainfall. Within its range, which includes Mulga and mallee scrub, shrubland, grassland and desert, it has a preference for open plains habitats but is seldom found in regions totally devoid of shade from scattered trees. Mobs are mobile and semi-nomadic and movement is not restricted by artificial barriers such as stock fences.|
Although frequently observed to drink at stock watering places, the Red Kangaroo appears to be able to exist in the absence of fresh surface water, provided that it has access to at least some green herbage. On mild days it may bask in the sun but under hot conditions it seeks any available shade.
Not all Red Kangaroos are red. Most males are pale red to brick red in colour but occasional blue-grey males occur. Females are usually blue-grey and in consequence are often called 'blue fliers' but, in some areas, both sexes are reddish.
Under favourable environmental conditions females become sexually mature when 15-20 months old, males when about two years old. Adult coat colour is established at this stage but, for several years thereafter, both sexes continue to increase in size and to cut more molar teeth. A mature female which is appropriately nourished and which is not suckling a young in its pouch, comes into oestrus at approximately 35-day intervals and is thus, like the male, potentially fertile throughout the year. However, periods of extreme drought may delay the onset of sexual maturity and lead to suppression of the oestrous cycle. After two years of drought a population may include females aged three years or more which have never produced young but, following rainfall and growth of new herbage, these animals come into breeding condition almost immediately.
Courtship of a female in oestrus is simple. A male held in the same enclosure as a female routinely sniffs her vent and pouch but does not actively follow her until about about two hours before copulation. Mounting is from the rear, the male grasping the female around the middle of her body and inserting his penis alongside her tail into the urogenital opening. Copulation usually lasts 15-20 minutes and is accompanied by multiple ejaculations.
Pregnancy does not interrupt recurrence of oestrus. The young is born 33 days after mating and mating occurs again a day or two after this. The embryo resulting from this post-partum mating remains a quiescent blastocyst until the previous young is about to vacate the pouch. During this period the suckling stimulus prevents the recurrence of oestrous cycles. If the young is lost prematurely from the pouch the quiescent blastocyst resumes development (as is normal at the end of pouch life) and cyclic reproductive
activity is resumed.
Prior to giving birth, the female licks her pouch clean and assumes a birth position with the tail passed between her hindlegs, hindquarters resting on the ground and back supported against a tree or other object. The hairless new-born young, which weighs less than one gram, makes its way to one of the 4 teats in the forward-opening pouch, attaches to it and suckles from this teat throughout, and beyond, its 235-day pouch life. It leaves the pouch in the form of a miniature adult, weighing 4-5 kg.
Following its mother at heel, the young animal continues to suckle for a further 4 months. Its younger sibling in the pouch meanwhile suckles from another teat. A remarkable feature of lactation is the concurrent production from different mammary glands of milk of different chemical composition. The teat on which the pouch young feeds delivers milk with much less fat than the one which serves the young outside the pouch.
Population densities vary remarkably over the range. The usual group consists of a dominant male, a number of adult femiles, and juveniles of both sexes, but it may include as few as two animals or as many as several hundred. Small groups of young males are not uncommon and old, scarred males may live as solitary individuals. Almost half of all young that are born fail to reach two years of age and 90 per cent die before reaching the age of ten years. A few may reach the age of 20 years.
It has been estimated that in 1975 there were about 2,000,000 Red Kangaroos in the New South Wales plains country (an area of 496,000 km2). In the same area and in the same year, licences were issued for the slaughter and harvesting of 48,000 animals (2.3 per cent of the population). In Queensland, which has a much greater area of suitable habitat, the 1975 harvest was 95,000 animals. There are legal harvests also in South Australia and Western Australia.
'Complete Book of Australian Mammals' - Australian Museum, 1983.
Australia's Wilderness Heritage - Flora & Fauna, 1988.
The Red Kangaroo is featured on Australia's Coat of Arms and on the following Australian coins:
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Last modified: 05 December, 2007