Koala yawning and showing the tongue and teeth used to consume its diet of eucalypt leaves. (Photo: Steven Lesser)
social and solitary. Social behaviours include vocalisations, different forms of aggressive behaviour, scent-marking, reproductive and maternal behaviours, but it is in solitary behaviours that we see how the koala spends the majority of its day—feeding, travelling and resting. In this chapter, we explore a day in the life of the koala.
Non-social, or solitary, behaviour is clearly the dominant form of koala behaviour. During the breeding season, koalas have been observed to spend 86 per cent of their time alone. In the non-breeding season, a koala can spend up to 93 per cent of its time alone, although a female may have a joey with her.2 As we saw in the previous chapter, the koala gets so little energy from its food that it must deliberately minimise its energy expenditure. The best way to do this is to sleep some 20 hours per day. Though they can be seen in clusters, the koala invariably rests alone, most often sitting vertically in the fork of a tree, with its head dipped forward toward the tree trunk and its arms held in next to the body to conserve heat.3 Most other arboreal or tree-dwelling marsupials prefer to nest in tree hollows, but the koala's hunched sleeping position keeps it warm in cold weather. When it rains, the water simply runs off its thick fur. On windy days the koala prefers to rest on a lower, thicker and therefore more stable branch of its tree.
Given that it spends such a lot of its time immobile, the koala has other resting postures. In warm weather, it can often be found reclining against a branch, or lying on its stomach or
Resting postures of koalas. (a) basic posture. (b)—(d) variant tree postures. (e) Normal ground posture. (f)—(h) different ground postures. (Taken from Smith (1979))
even its back, with its arms extended either side of the branch to increase heat loss. In the height of summer koalas will come down to the ground to seek shade, but even in the hottest weather the koala can go for long periods of time without water, and will urinate only once or twice a day. The koala climbs backward down its tree and, once on the ground, has a bounding gait and can move surprisingly quickly from tree to tree, in order to evade predators such as dingoes and wild dogs. When climbing, the koala uses its large, strong claws to scale the trunk rapidly, and the hooked 'thumbs' on each hand can leave deep scratches in the tree's bark.
Different resting positions and its soft, thick fur helps the koala to regulate its body temperature. Robert Degabriele and Terry Dawson discovered that the fur on a koala's back offers better insulation than that of any other marsupial. Its short, compact structure means that it is impervious to wind as well as rain, further increasing its effectiveness as an insulator. In contrast, the fur on the koala's stomach is only half as dense as that on its back and therefore provides considerably less insulation. As it is white, when the koala lies on its back and exposes it's abdomen to the sun, it is actually cooling down.4
So how do koalas pass the few hours in a day when they are not asleep? Over a 24-hour period, koalas have been observed, in the wild and in captivity, feeding for between one-and-a-half and four-and-a-half hours.5 As we discussed earlier, it's the koalas metabolic rate is so slow that to save energy it must sleep for almost 20 hours in a day, so it becomes clear that the average koala really can do very little other than sleep and eat. Feeding bouts can last for anything from 20 minutes to more than two hours, and hunger pangs can strike at any time of the night or day. Typically, both captive and wild koalas prefer to feed just after dusk and just before dawn. Michael Robbins and Eleanor Russell observed that 66 per cent of koala feeding behaviour took place during the night. The koalas' preferred time for feeding during the day was in the late afternoon, but some feeding was observed during the middle of the day.6 A subsequent study by Lynda Sharp identified three favourite feeding times—two-and-a-half hours before sunset, two-and-a-half hours after sunset and three hours before sunrise. Sharp did not observe any feeding behaviour during the middle of the night.7
A feeding koala will try to access the furthest reaches of its branch, because that is where there is the most food. A small koala can venture almost to the very tip of a branch, while larger animals such as adult males are restricted to the thicker bases of branches. A feeding koala holds on to the branch with both feet and one hand, using its free hand to grab branchlets of foliage. A koala out on a limb certainly does look precarious and branches can break or give way, interrupting the koala's meal. Despite the heights at which it rests and feeds, the koala has a remarkable ability to bound away unscathed from such dramatic plunges. As we saw in Chapter 4, the koala's brain is surrounded by a protective layer of fluid, a bit like an inbuilt crash helmet.
Studies on the reproductive status of free-ranging female koalas has a direct effect on their feeding behaviour. Murray Logan and Gordon Sanson discovered that lactating females not only consume more leaf material than non-lactating females, but also spend more time chewing each leaf.8 A lactating female is in a no-win situation, however. She must eat more to meet the increased demands being made on her energy reserves, but in order to eat more she has to expend more energy. The fact that she has to spend more time each day feeding means that she has less time to rest, or replenish those reserves of energy. The female koala's long lactation period is one of the reasons that she usually breeds only every second year.
In another study, Logan and Sanson found that tooth wear also has a significant impact on the amount of time a koala spends feeding. Animals with low tooth wear spent more time on the move, both within and between trees, and had much larger home ranges than those with high tooth wear. Animals with high tooth wear were found to feed for longer periods, which meant they were less likely to engage in social behaviour and therefore less likely to mate. Koalas with high tooth wear not only had longer feeding bouts but spaced these bouts more evenly throughout each 24-hour period, so were less active at night.9
A healthy koala's coat always seems impeccably kept, even though it appears to spend very little time maintaining its appearance. Malcolm Smith studied various koala behaviours, including grooming, in a series of insightful papers on the captive koalas at Lone Pine Koala Sanctuary near Brisbane. During the observational sessions (which lasted for two hours) the koala typically made seven grooming motions, most of which were only a single scratch. While the koala would occasionally use its hand or mouth to groom, its preferred tool was its inbuilt comb—the syndactyl toes on its hind feet.10 This comb's effectiveness is demonstrated by the fact that there are remarkably few ectoparasites in a koala's fur, although there can be a build-up of ticks during the warmer months, especially within the ears. Old koalas that are not as diligent in their grooming can develop matts and even dreadlocks in their fur.
The koala cannot be considered a social animal, with some researchers finding that social behaviour, whether physical interaction or mere vocalisation, is limited to as little as 15 minutes per day.11 The extent of overlap of the home ranges, however, appears to vary throughout the species distribution, with home ranges in Victoria being small and clustered with extensive overlap. In contrast the home ranges in central Queensland are widespread, large and have minimal overlap.12 Instead of being related to social factors, it was proposed by Peter Mitchell13 that home range area was inversely related to population density and positively related to the density of trees.
Ellis, Hale and Carrick14 described the koala population dynamics as consisting of longer-term 'residents'—mostly females—and 'transients', usually males. The few 'resident' males achieve their dominance through sheer body size and appear to benefit from territoriality. Ellis, Hale and Carrick also proposed a dominance-based breeding system, based on their observations of males that travelled frequently and widely, and who initiated and dominated encounters with other males. Their observations would seem to suggest that the dominant resident males would sire more young, but DNA analysis revealed that successive young produced by individual females to have different sires, so being the dominant male does not appear to provide any reproductive advantage.
Male koalas have a larger home range and travel further than females, especially during the breeding season, but every area has an alpha male that is larger than all the other males nearby. Peter Mitchell observed a dominance hierarchy among male koalas, with size being the defining criterion. Those higher up in the hierarchy were larger than those near the bottom, but the alpha male was dominant over all other males he encountered. Mitchell also observed male koalas travelling outside their home ranges into areas used by other clusters of animals and that even outside their own home ranges, alpha males were still dominant over other males.15
Koalas are non-gregarious and both sexes leave their mothers after weaning, though a high proportion of males emigrate when two to three years of age (88 per cent compared with 51 per cent of females).16 The aggressive behaviour of female koalas towards their adolescent young may be a deliberate strategy, to encourage the young to find their own home range.17 There are different theories as to koalas' mating behaviours; ranging from male koalas maintaining harems, through males being territorial and waiting for females to come into
Adult male scent-marking. (Taken from Smith (1980b))
their territories, to travelling males with no fixed home range, mating with available females as they come across them. The male koala displays no obvious courtship behaviour, but at the beginning of the mating season he will be more vocal and will advertise his presence by scent-marking trees.18
Every male koala has a scent gland on its chest, known as the sternal gland. During scent-marking, the male koala grasps a vertical object, such as a tree trunk, and rubs his chest against it. This stimulates the sternal gland to release a strong-smelling secretion, the scent of which is unique to each koala. This behaviour first appears when the koala is three years old and reaches a peak by the age of four or five. It is often observed at the same time as bellowing, especially in response to an
Figure of koalas during an aggressive encounter. (Taken from Smith (1980d))
aggressive encounter or the bellowed challenge of a rival male.19 Both sexes will signal their presence by urinating at the base of trees, and when travelling across the ground a koala typically stops briefly to sniff at these calling cards.
Social interaction between adult koalas is often aggressive, especially between males. An aggressive encounter usually starts by the aggressor climbing a tree to attack its resident male. The koala under threat (which is usually smaller than its aggressor) either retreats to the extreme end of a branch or tries to rush past the bigger koala and climb down the tree. If the aggressor can trap his quarry at the end of a branch, he attacks, grasping the smaller animal around the shoulders and biting it repeatedly, inflicting deep wounds with his sharp incisors. The aggressor's aim is to dislodge the weaker koala from its tree, and when the beaten animal has fled, the dominant koala bellows its victory and marks the tree with its sternal gland.
Malcolm Smith discovered that all aggressive behaviours are based on a standard move—hooking a foreleg around an opponent and biting.20 Aggressive encounters such as the one described above are at the upper end of the scale. The most common aggressive behaviour between koalas is the squabble, a brief, push-and-shove interaction sparked by one koala trying to climb past another. If it is a minor fight, either combatant may bite his opponent, but usually only once. Both combatants will stay in the same tree, snarling at each other. Between males, these minor fights are essentially intensified squabbles. Major fights between two male koalas involve wrestling, chasing and biting, and usually occur between strangers. Females can become aggressive during pregnancy and at the end of lactation. They will stand their ground and vocalise at other koalas, especially males, but attack only if the other animal comes within reach.
Koalas have a surprising range of vocalisations. A koala joey will signal distress by squeaking. As the koala gets older, this squeak becomes a squawk and can be a sign of either mild distress or aggression. The only vocalisation produced with the mouth closed is a low grunt, which is an expression of irritation elicited by a weak stimulus such as being climbed over by
Male koala bellowing. (Taken from Smith (1980a))
another koala who wants to get past. Fighting males will produce harsh, open-mouth grunts, which are much louder than ordinary grunts. The female koala's repertoire of calls includes snarls, wails and screams. The snarls are brief and guttural, but the other calls are distinct and well-structured. The female koala can use the same call either as a defensive threat, or as a cry of distress. The bellow is characteristic of the adult male koala, and consists of a long series of deep, snoring inhalations and belching exhalations.21 The hoarse, grating sounds of a male koala's bellow uttered during the breeding season have been likened to the 'noise made by a hand-saw cutting through a thin board'.22 The female bellow is produced more softly and used less often, typically during defensive encounters.
Vocalisation also appears to be influenced by age, as Logan and Sanson have found that bellowing behaviour is most common in mature animals.23
Koalas can make various facial expressions, however the three most common ones can be seen above. In the first, the top lip is raised in a curve and the ears are pricked forward. This expression is most commonly associated with the snarl, but also seen with the wail and the squawk. In the second expression, both lips are brought forward to make an oval aperture and the ears are sometimes raised. This expression is generally associated with the guttural cries produced by highly agitated females. In the third expression, the lips are retracted, widen ing the mouth, and the ears are often laid backwards. This is the expression associated with screams, and can also be seen during wailing.24
As we have seen, most aggressive and vocalising behaviours occur during the breeding season, when adult male koalas move between trees in order to meet more females. Dominant males use vocalisations and scent-marking to warn subordinate males that they are in the vicinity, perhaps in the assumption that the subordinate males will be too scared to attempt to mate with any nearby females. However, dominant males can only intervene if they are close by during the actual act of mating, and as this only lasts for a matter of minutes, many subordinate males are able to mate successfully.
A male koala is capable of breeding at 18 months of age but until he reaches his peak weight, at around four years of age, older and larger males generally prevent them from gaining access to females. Female koalas are able to breed from the age of 18 months and show they are in oestrus by head jerking, bellowing and being mildly aggressive towards males. They can also demonstrate pseudo-male behaviour, such as mounting other female koalas.25 During the breeding season, a male koala who visits a female's tree is greeted by a low snarl or weak bellow that becomes louder and longer as he gets closer. Sometimes the female will retreat. Once the male has caught his female, he tries to pin her between him and the trunk of the tree grasping the fur of her head or neck with his teeth. The usual response of a female koala is to struggle away from the male resulting in many failed mating attempts. She will strike him and bite his head, all the while squawking and screaming loudly, but if she is in the right position, that is, vertical, the male will try to mate. The actual act of mating lasts only a minute or two, from start to finish, and afterwards the male usually beats a retreat, bellowing as he goes. He will often remain in the tree to re-mate with the female a second or third time.
Mating typically occurs between September and March, and the study of koala mating patterns and success rates carried out by Fred Bercovitch and his colleagues at San Diego Zoo gives some insights into the criteria for mating success in koalas. They discovered that although male koalas are typically between 50 and 75 per cent larger than females, body mass or relative size dimorphism had no effect on reproductive success. They also found that reproductive output was highest when males were slightly older than females, which led them to propose that females in the wild possibly assess a male koala's age through his bellowing behaviour and scent-marking.26
There are many factors that affect koala fertility rates, and it can vary markedly between different populations and even within a particular population. Melzer and Houston, for example, found fertility rates ranging from 0 per cent at the Grampians in central Victoria (because of the introduction of koalas infected with Chlamydia), through 22 per cent on
French Island, to 92 per cent in central Queensland.27 Similarly the decreased fertility rates observed by Roger Martin showed the low fertility rate at Walkerville (13 per cent) and Phillip Island (22 per cent) in Victoria to be the result of reproductive failure among females older than three years of age, probably as a result of widespread Chlamydia infection. The particularly low rate at Walkerville was proposed to be the result of a combination of factors including Chlamydia, poor nutrition and a heavy tick burden.28
A successful mating between fertile adults will result in the birth of a joey, which develops inside its mother's pouch until it is about six months old. When it leaves the pouch, it is at a similar stage to a placental newborn, clinging to its mother's stomach or back, and returning to the pouch to feed. The young koala explores its mother's body, including her shoulders and head, all of which is well tolerated by the mother. When the mother is moving the joey clings to her back, not riding jockey-fashion, but clinging on with both legs and arms. The whole body of the young koala is flattened against its mother's back.
A young koala who gets separated from its mother will display considerable distress—squeaking, urinating and stretching out towards her until she returns.29 The koala joey soon outgrows this dependence, however, and will happily ride on the backs of other, unrelated koalas, which show a remarkable tolerance towards the youngster. Even adolescent koalas have been seen staggering under the weight of joeys clinging to their backs. Captive female koalas with joeys can often be seen with one, two or no young at all, as the youngsters swap between parents.
When the young koala is nine to ten months old, it begins to leave its mother for short periods, typically less than ten minutes at a time. During these early forays the young koala will explore its immediate surroundings, but never stray more than a metre away from its mother. Malcolm Smith recorded that a major stage in a joey's development is when its mother turns away from it when it is either feeding nearby or sleeping with its back to her. The observed responses depended on the joey's age. Very young koalas would squeak loudly and reach out for the nearest adult, while older joeys squeaked only if they got into difficulty.30
Many researchers do not consider marsupials capable of 'playful' behaviour, but young koalas have been observed to display behaviour that would appear to contradict this. Young koalas will play-fight with the hand of the person hand-rearing them and Malcolm Smith observed one juvenile making peculiar, jerky movements away from its mother, which Smith could only describe as 'skipping'. Returning to its mother, the young koala nibbled her fur or made sudden bites at her body, especially if she made any abrupt movement such as raising her arm. Smith proposed that the rapid nibbling movements were like those of a playful puppy.31
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