Australia: The Land Where Time Began

A biography of the Australian continent 

The Red Kangaroo

The red kangaroo is the largest living marsupial, at 2.5 m from nose to tip of tail and weighing about 85 kg. They live in the arid parts of Australia, in the inland woodlands and deserts. Like other marsupials they are well adapted to their arid environment, shutting down their reproductive system when conditions are too dry to breed successfully, and rebooting it only when conditions improve. They are one of the most highly adapted animals to desert conditions, especially their reproductive strategy. The females usually have a production line of young, one out of the pouch but still drinking milk when necessary, a baby still developing attached to the other nipple, and a fertilised ovum that remains at the blastocyst stage of development until the young attached to the teat reaches the stage were it leaves the pouch, the blastocyst them recommences development. Each nipple varies the composition of the milk it produces to suit the stage of development of the young drinking from them. They don't have a definite breeding season, the female being ready to mate the next time there is a male nearby as soon as the blastocyst resumes development. This reproductive strategy, in which there are young at 3 stages of development allows the female to reproduce at maximum speed in good times and in drought, if she reaches a stage when she can no longer support young, the milk supply stops and, in the most severe conditions, 1 or both of the young feeding from nipples can die, following which the blastocyst resumes development. Each young takes 600 days to reach the stage where it can live independently, but in good times a female can have a young becoming independent every 240 days. There is a high mortality at the stage when they leave the pouch, but in good times they can build up their numbers rapidly.

They drink from stock watering places when available, but they are capable of surviving with no surface water to drink, getting all they need from their food. They usually feed between dusk and dawn, spending the hot part of the day resting in whatever shade they can find. They usually move around in groups of about 20 with a single dominant male. In the dry season mobs of several hundred are not uncommon. They can live more than 20 years, but most don't, many not getting past a couple of years. Their numbers fluctuate widely with the seasonal conditions. With the return of good conditions their reproductive strategy means they can breed up very rapidly in the good times.

This species is the most common of the large kangaroos, and exemplifies the 3 main adaptations the macropods made, that have been referred to as 'the 3 steps to success', to survive, and even thrive, in Australia's drying climate, that has made Australia's central regions a land of drought, high temperatures and what little rain there is, is unpredictable. To the Aranda People, who have occupied parts of central Australia for thousands of years, the red kangaroo was Ara, one of their totemic animals, associated with the being of that name from the Dreamtime. This species is restricted to the arid zone, with an annual rainfall of less than 800 mm, and a mean annual temperature above 15^oC. It occupies mostly the plains country, that is better watered, and open woodland, its range being centred on western New South Wales, though it extends through most of the central regions, except for the true desert areas, and as far north as the northwest coast of the continent. It is absent from areas with predictable rainfall, such as the southwest corner of the continent and southern and eastern areas and Tasmania, and from the tropical latitudes north of 14^oS. It occurred as far south as the southern coast 12,000 years ago, at a time when the rainfall pattern or the mean temperature differed from the present (Flannery & Gott, 1984; Caughley et al., 1987b).

Between 1959 and 1962 the ecology of the red kangaroos in MacDonnell Ranges area was studied by Alan Newsome. The study began during the longest drought since European settlement, so the ecology of the species during drought could be studied, and the effects of the presence of cattle could be observed. Newsome also collected Dreamtime stories from the local people, and those recorded by Theodore Strehlow (1947,1971). According to Strehlow, in drought times the tribes retreated to sites of permanent water, evacuating up to 7/8 of their land.

As a result of his studies of the red kangaroos response to drought he realised that the Dreamtime stories and songs were actually based on a deep understanding of the red kangaroos adaptations to the harsh climate (Newsome, 1980). He noticed that of the 14 places he had found to be refuge areas of the red kangaroo during drought, places with grassy plains near the range margins, that receive runoff from the ranges and so receive more water than the open plains, and stream lines, 10 were also sacred sites connected with the Ara myths.

The arrival of cattle in the area had a number of effects on the native animals. The smaller species of macropod disappeared, but the red kangaroo expanded its range onto the treeless open plains once the water from bores was available, and the grazing of the cattle caused the growth or green shoots on plants that had been cropped, greatly increasing the favourite food of the red kangaroo, and their numbers increased greatly. The new habitat, on exposed plains where temperatures got very high during the day and with no trees to provide shade, spermatogenesis was impaired in the male kangaroos, as well as reduction in the size of the testosterone-producing interstitial cells. In the hot weather the numbers of females that had ovulated without becoming pregnant increased. This effect of hot weather occurs in other mammals, as well as loss of secretions from the pituitary gland. The red kangaroos that had occupied the new country, as it became available after the availability of water and increased amounts of green food, had a much reduced ability to breed. Among the Aboriginal People, hunting was prohibited in these refuge areas, and for some distance around them, that were associated with Ara, conservation many thousands of years before the word existed, also a clue to how these "primitive" people survived, sustainably, on the driest vegetated continent on earth. These sacred sites would now be called a network of conservation reserves in prime habitat.

Coping with heat

In the hot climate of central Australia the animals need to deal with the heat generated by their metabolic processes, but also a high heat load from the environment, where air temperatures can reach 40^oC and solar radiation can reach as high as 126^oC. As with most marsupials, kangaroos have a comparatively low standard metabolic rate, SMR, 70 % of the mean for mammals, and a low body temperature, an advantages in a hot, dry climate, as there is less metabolic heat to dissipate. Like the camel, though to a lesser degree, the red kangaroo can allow its body temperature to rise during the day, discharging the excess heat at night, dropping it by 1-3^oC soon after dawn, rising 10^oC during the day, 35-37^oC (McCarron & Dawson, 1989).

The problems dealing with the external heat load is much greater. The red kangaroos rely on their mobility to find nutritious, succulent food and the sparse shade of bushes. Red kangaroos are not found around rocky outcrops or caves, relying on the shade from small saltbushes and mulga bushes, where the temperature is usually 32-38^oC, with humidity of about 20 %, when the solar radiation reaches 60^oC. Evaporative cooling is enabled by the airflow under the bushes, the large subcutaneous veins of the forearms allowing the blood to dissipate heat to the passing air. They also keep activity to a minimum, and they may stand under the shelter, with their tail drawn under them, to get it out of the sunlight, while allowing the air to circulate around their bodies. Their pale colour also aids by reflecting part of the sunlight. The fur of the red kangaroo is finer, as well as being more than 3 times as dense as that of the euro, about 62 fibres/mm² compared with the 20 fibres/mm² of the euro. This fur increases the insulation of the fur by a large amount, in windy conditions, whether the weather is hot or cools (Dawson & Brown, 1970).

These mechanisms reduce the heat load and the need for evaporative cooling, which also saves water. When further cooling is necessary there are 3 ways of achieving evaporative cooling, used when the air temperature is high or they have been active, panting, sweating and spreading saliva on the skin of their forearms. Panting is the most important of these methods of heat reduction. In experiments with red kangaroos and euros, the temperature was increased from 25^oC to 45^oC. In both species the respiration rate increased 15-fold. The respiration under these conditions began through the nose, the nasal epithelium providing the evaporative cooling surfaces, open mouthed panting began when the air temperature was increased still further, the evaporative cooling taking place on the surfaces of the throat. Panting has a larger effect on the deep body temperature, especially that of the brain, than sweating, even while the skin temperature remained high as a result of being in contact with the hot air. The nasal turbinals give an indication of the importance of panting in the kangaroos of the desert areas, those of the red kangaroo being much larger than those of the grey kangaroos that inhabit areas with less extreme temperatures (Tyndale-Biscoe, 2005).

In the red kangaroos and euros, sweating takes place only while they are active, stopping when they rest, even if the body temperature is high, but they continue panting. The panting uses water much more efficiently than sweating, conserving water, which only cools the skin, which will soon be reheated by the hot air.

Kangaroos also cool themselves by spreading saliva on their forearms (Dawson, 1973). The maxillary glands secrete this saliva, its composition differing from that of the saliva secreted by the parotid glands during feeding, containing lower amounts of phosphates and protein than are found in parotid saliva. The saliva helps cool the blood returning to the heart in the large subcutaneous veins of the forearm. This is an extravagant use of water, though it aids with evaporative cooling in the dry desert air, and it lowers the deep body temperature more directly than sweating. When this method of cooling is used, the water must be replaced by eating succulent vegetation or drinking. It is used by kangaroos that are active at times of high air temperatures, though the production of saliva stops if water is restricted. In the tropics, this method of evaporative cooling is used by antilopine wallaroos, though they spread the saliva on their inner thighs, not their forearms.

The frequency of drinking by red kangaroos and euros is an indication of their wellbeing, as while they can find enough green food they can subsist with less drinking.

Red kangaroos in the arid environment

The red kangaroo needs to eat better quality food than the euro because it cannot escape the heat by resting in caves in rocky outcrops, as euros, do so needs to use evaporative cooling, panting and forearm licking, with the result that its needs to replenish the water, from succulent shoots when water isn't available, and also has a greater need to conserve body water where it can. The water conservation is assisted in red kangaroos by having kidneys that concentrate the urine to a greater degree than the kidneys of euros, and the difference is even greater in summer (Dawson & Denny, 1969a, b). Water conservation is also assisted by the ability of kangaroos to recycle urea by returning it to the forestomach where it can be utilised by the bacteria, with the result that less water is required to excrete it than would be required to flush it out in the urine.

Red kangaroos have a preference for green forage, especially in new shoots of grasses and forbs, being able to select these parts, even in vegetation that appears dry and brown. Green grass makes up about 75-90 % of the their diet in Central Australia, consisting mostly of kangaroo grass, Eragrostis sentifolia, 54 %, a plant that remains green even in dry conditions (Newsome, 1975). This grass and bottle brush, Enneapogon avanaceous, accounted for 21-69 % of red kangaroo food in western New South Wales. The presence of a higher proportion of these plants in the stomachs of red kangaroos than grows in the area indicated that, in this area at least, they were being selected by the animals (Baily et al., 1971; Cartwright et al, 1987a). In dry conditions, red kangaroos gather on open plains and along water courses because that is the only place they can find green grass on seepage lines to meet their needs for both water and protein. They switch to chenopods, Basia diacantha, and black bluebush Maireana pyramidata, and occasionally browsing shrubs in some areas, when their preferred food of grass and forbs is scarce. Some chenopods are always avoided, such as round-leaf chenopod, Kochia, even at times when this plant is abundant, though the plant is eaten by euros, (Ellis et al., 4977).

Red kangaroos occupy a much larger home ranges than euros, as they don't have the requirement of having nearby caves, allowing them to move to different types of country in different seasons, depending on where their preferred food is available. It has been found (Croft, 1991a) that red kangaroos in western New South Wales had weekly home ranges of about 259-560 ha, the largest ranges being occupied by large animals, that were mostly males. They travel about 25-30 km to find more nutritious food in times when rainfall is patchy and the forage is poor (Frith, 1964; Bailey, 1971; Coughley et al, 1987a). It has been found that in Central Australia 66 % of red kangaroos remained within 1 km of persistent forage in dry periods, moving up to 28 km to feed on the new growth of ephemeral grasses and forbs after the fall of effective amounts of rain (Newsome, 1965).

It has been found the large numbers of kangaroos move to areas that have received rain away from their normal home range. In February 1982 there were 5211 red kangaroos in a study site, then after rain 30 km away in April, many of the animals, 188 of which had been radio-collared, moved to the new area, 1336 remaining in the study area. When rain fell in the study area in late June they returned to the study area, including 185 of the collared animals, 3 having been shot before they returned.  The females tended to return to their home range when possible (Croft, 1991a).

In drought, when there are long periods without rain, the starving kangaroos become too weak to move to new areas if rain does fall away from their home range, eventually dying on their home range if rain doesn't come in time. Red kangaroos have a very high capacity to survive starvation and dehydration, losing more than  20 % of their body weight, much more than most large mammals, which is comparable to the camel, that can lose more than 30 %, but much higher than cattle.

Most red kangaroos that have been tracked stayed within about 30 km of their release site, but there have been some exceptions (Tyndale-Biscoe, 2005). The fastest known migration was of a young female from Western Australia, which travelled 338 km in 15 weeks (Oliver, 1986). The longest known distance travelled was of a young male that moved to a new site 300 km away in 25 years (Bailey & Best, 1992). This pattern of travel, involving a minority of individuals, has shown no correlation with age or sex (Tyndale-Biscoe, 2005).

Reproduction

Red kangaroos are opportunistic breeders (as are euros), their reproduction pattern varying according to whether their territory is experiencing good times, drought, or after a drought has broken. In favourable times female red kangaroos (and euros) have high fecundity, breeding continuously throughout the year. At all times these females have a youngster at foot, but still suckling, though from outside the pouch, a less well developed young in the pouch attached to the other teat, and a third one in embryonic diapause, at the blastocyst stage, waiting for a teat to become available. The young at heel is fully weaned, though remaining close to its mother, by the time the pouch young is about 120 days old. The blastocyst resumes development at about 200 days, developing to the stage at which it is ready for birth in about 33 days; the birth taking place a day or 2 after the older young has been excluded from the pouch. At this point, the mother tightens the opening to the pouch whenever the older young tries to enter it. Following this, post-partum oestrous and ovulation occurs, another egg being fertilised. From fertilisation to independence, the red kangaroo takes almost 600 days, but because of their production line method of reproduction, a young red becomes independent every 240 days from the same mother, while the good conditions continue.

Reproduction of red kangaroos is progressively affected as conditions deteriorate, and the quantity and quality of their preferred food declines. The young at foot is the first to feel the effects of harder times. At this stage it is depending less on milk and more on herbage. It is growing fast, and needs to maintain its body temperature, which in red kangaroos depends to a large extent on evaporative cooling. The SMR of this young is twice that of an adult, and its body temperature is higher, with significantly higher evaporative heat loss, especially at the highest summer temperatures (Munn & Dawson, 2001), resulting in a higher water requirement than adults in the same environment. In good times, there is enough water in the pasture, so milk is not essential. The high metabolic rate associated with rapid growth at this stage of development, as well as the high water costs of maintaining body temperature; make it difficult for the young kangaroo to get enough water, energy and protein when it is weaned in deteriorating conditions. While the mother is able to supply milk it can manage (Munn & Dawson, 2003a), but if the conditions are bad enough, the milk dries up, leaving a dry, elongated teat that cannot support the young kangaroo. Such young, who lose the support of milk, soon die. The mother still has a pouch young feeding on the other teat, and a blastocyst in embryonic diapause.

If conditions don't improve, the milk supply to the teat feeding the pouch young declines and fails, leading to the inevitable death of the remaining young. This usually occurs when the pouch young is about 2 months old, at the point where the milk supply is about to increase. As the pouch young declines the blastocyst is triggered to resume growth, the birth occurring as the older one dies. This pouch young then grows for 2 months, after which the milk declines and stops if conditions haven't improved, and this young also dies. If the drought doesn't break, this process can occur several times. Once rain falls the pouch young continues development. The presence of young in the pouch of red kangaroos soon after the breaking of a drought has led to a widespread belief that kangaroos know when a drought is about to break (Tyndale-Biscoe, 2005). This adaptation of the macropod reproductive  pattern allows red kangaroos to continue breeding well into a drought, and to take advantage of better conditions as soon as possible after the breaking of the drought (Tyndale Biscoe, 2005).

In 2 places in western New South Wales, the same reproductive pattern has been found in the eastern wallaroo, in response to poor nutrition. One site on improved pasture with fertiliser, and the other with Poa and no fertiliser (Taylor, 1982). On the untreated native pasture, 12 % of females had young at foot, whereas on the treated pasture it was 30 %. As no other factors were known to distinguish the 2 sites, it has been suggested that it was probably the less nutritious nature of the untreated pasture that was the determining factor leading to lower breeding success.

If a drought continues for longer than 6 months, both red kangaroos and euros stop breeding, ovulation not occurring after the end of the final pregnancy, after which they become anoestrus, a condition which persists either until the drought breaks or they die. Studies have shown that the same condition occurs in tammar wallabies if gonadotrophic hormones are not produced, and it has been suggested this would occur under extreme drought conditions (Tyndale-Biscoe, 2005).

The results of 2 studies have indicated that female kangaroos responded to drought-breaking rain after a year-long drought, before the resulting plant growth. The studies were carried out 2 weeks after the drought broke. At this time 65 % of the female kangaroos were in oestrus (Frith & Sharman, 1964; Sharman & Clark, 1967). Ovarian follicles take 10 days to mature, which indicated that the animals had responded directly to the rain, and not to the appearance of new growth that resulted from the rain. It has been suggested the petrichor (the strong smell given off by dry soil after rain) (Bear & Thomas, 1966) may be the immediate stimulus, from the olfactory nerves to the pituitary gland, in a similar manner to the mechanism of hormones, though this suggestion had still to be investigated at the time of writing (Tyndale-Biscoe, 2005).

It is believed the main controller of the red kangaroo population is mortality of the young at foot and the late stage pouch young, which is a result of the quality of forage available to the females. The mothers need to be able to supply enough milk to support the young at foot at a time when they are most vulnerable because of their high water requirements. It has been found in studies at 3 sites in New South Wales that the mortality rate for young leaving the pouch was 83 % at the driest site, Gilruth Plains, 53 % at a Mt Murchison site that was not as dry, and at Toganmain, where there was plenty of forage, the mortality rate was 15 % (Frith & Sharman, 1964). The result is a large variation, from year to year, and place to place, in the recruitment to the red kangaroo population.

Social behaviour

Among red kangaroos, the basic group tends to be comprised of 2-4 individuals, displaying less agonistic behaviour than is seen among euros, and a female with young at foot is the most common grouping. When they congregate in large groups of higher density, it is usually females and a single male (Frith, 1964; Croft, 1981b; Johnson, 1983). The males are usually nearly twice the size of females and have proportionately much larger shoulders and arms (Jarman, 1983), used in establishing dominance hierarchies. The most common agonistic behaviour is seen between young adult males, in the form of ritualised 'boxing' (Tyndale-Biscoe, 2005). In these bouts to establish dominance ranking, they hold the opponents shoulders while balancing on their tails and kick and rake the abdomen of the opponent. The male reaching the top spot of alpha male has first access to oestrus females.

Once a male reaches the status of alpha male he engages in less agonistic behaviour, concentrating his energy on sexual behaviour, unless he is challenged by a subordinate. An alpha male that is defeated by a challenger avoids further close contact with other kangaroos, living in isolation. A study has found that a high proportion of the largest males produced no sperm in their ejaculates. It has been suggested that these may be former alpha males that were displaced (Sadlier, 1965). Subordinate males do mate with females after the dominant male has finished. Any possible competition between the sperm of the dominant male and that of the subordinate has only been studied in the tammar wallaby.

The red kangaroo and the euro are both macropods that have very successfully adapted to an arid environment, though they have based their adaptation to aridity on different aspects of macropod physiology. The euro conserves water by remaining close to caves, which requires a degree of sedentary behaviour, but reduces the need for evaporative cooling in hot weather. They must also rely on what forage is available within easy reach of the caves, often of low nutrient value. Euros have adapted to their low nutrient food by an increased level of urea recycling and a very low nitrogen requirement and basal metabolism. The red kangaroo has adapted by remaining on the open plains, making the most of what green food is available by moving to other areas to access plants that have sprouted green shoots after rain, returning to their home range when possible, though they have a higher water requirement than the euro because of the open nature of their habitat. It has been suggested these 2 species have diverged comparatively recently from a common ancestor, the key factor in the divergence being diet, one becoming nomadic to follow the green shoots they require, the other becoming sedentary, adapting to a low nutrient diet of spinifex (Newsome, 1975). The 2 species can interbreed, but the offspring are sterile.

Both red kangaroos and euros use embryonic diapause to maintain populations in some of the most difficult conditions on a mostly arid continent.

Sources & Further reading

1. Tyndale-Biscoe, Hugh, 2005, Life of Marsupials, CSIRO Publishing.
2. Mary E. White, Listen...Our Land is Crying, Kangaroo Press, 1997