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The 2019/2020 Australian bushfires resulted in 10 million hectares of wildlife habitat burning, affecting 1.25 billion animals. It is estimated that 30% of the Koala population worldwide were affected, putting the species under new levels of threat (Gonzalez-Astudillo et al., 2019; World Wide Fund for Nature, 2020). While bushfires have become a massive threat to the species recently, the Koala population has been decreasing since the 18th Century – since European settlement in Australia. This decline in the population can be partially attributed to several introduced threats which the population is anatomically ill-equipped to deal with.
Due to their diet, the Koalas are arboreal animals meaning that much of their locomotion consists of climbing and moving between tree branches. The functional anatomy associated with climbing in the Koala is best described when compared to its closest terrestrial relative – the Wombat. Koalas and Wombats are both marsupials (Louys et al., 2009) and have many similarities including molecular data, dentition, posteriorly directed marsupium and a vestigial tail which evidence their close relationship (Grand and Barboza, 2001). However, due to their relative environments and diets – trees and Eucalyptus foliage for Koalas, burrows and grasses for Wombats – they have many derived traits.
The Koalas sedentary position when climbing and feeding has led to medially facing soles and palms allowing them to grip the trunk or branches of trees, whereas the Wombat has plantigrade palms and soles allowing stability and pressure distribution when weight bearing (Fig.1)(Young, 1881). Supination of the Koala forelimb is facilitated by the deeply bowed radius whereas the compact radius of the Wombat restricts supination (Fig.2)(Grand and Barboza, 2001). While this skeletal orientation aids climbing, it hinders the Koalas ability to travel between habitats in search of quality food sources and mates – yet terrestrial locomotion is becoming more important as human interference results in an increasingly fragmented Eucalyptus habitat (Grand and Barboza, 2001; Narayan and Williams, 2016).
Compared to Wombats, Koalas also have an elongated humerus’ and a diminished olecranon (Fig.2), this allows them to fully extend their forelimbs, increasing their stride length (Grand and Barboza, 2001). Increased stride length is common in arboreal species and is hypothesised to reduce destabilising peak forces when traversing narrow branches. This morphology further prevents excessive terrestrial locomotion as the slender humerus is not designed for weight bearing on solid surfaces with greater resistance. The lack of a notable olecranon also reduces stability against hyperextension in the elbow joint during weight bearing (Gaschk, Frère and Clemente, 2019).
In addition to the medial rotation of their limbs, Koalas also have an opposable hallux on each foot and two opposable digits on each hand to give them extra grip strength. All their digits have large, curved claws which further enable them to secure their position in a tree and grip onto bark (Young, 1882; Grand and Barboza, 2001; Gaschk, Frère and Clemente, 2019). However, these also hinder terrestrial movement as they force the hands and feet to be splayed and digits to be hyperextended when in contact with the ground, resulting in significant lateral displacement as the limbs are swung forward; this is very inefficient when considering speed relative to energy consumption (Grand and Barboza, 2001; Shipley, Forbey and Moore, 2009). These claws reduce the speed of terrestrial locomotion as they increase the necessary stance phase due to an increased reliance on precise digit placement in order to correctly apply pressure for propulsion (Gaschk, Frère and Clemente, 2019).
Similar to the forelimb, adaptations in the musculature of the hindlimb mean it is also inverted. The lack of an iliac attachment for gluteus externus, the absence of soleus, and a single origin of biceps femoris along with the occasional attachment of gracilis to the marsupial bone all encourage the medial rotation of the hindlimb (Young, 1882). These muscular arrangements also result in reduced mobility of the hip joint which contributes significantly to the biomechanical inefficiency of terrestrial movement (Grand and Barboza, 2001).
Due to their slow terrestrial speed and anti-social behaviour – causing most koalas to travel as individuals rather than in a pack – Koalas are particularly vulnerable on the ground as they are exposed to road deaths and predatory attacks, particularly from dogs (Lassau et al., 2008; Gentle et al., 2019). However, recent climate changes and developments in human settlement mean the Eucalyptus habitat is becoming increasingly fragmented (Narayan and Williams, 2016). Therefore, the Koala is being forced to spend extended periods of time moving over land between suitable habitats in order to search for food, water and mates (Gaschk, Frère and Clemente, 2019). As Koalas have very few natural predators, they do not have the necessary adaptations to escape or defend themselves when under threat. Around peri-urban areas, wildlife habitats have been replaced with an area of intense human activity full of threat which Koalas are unable to thrive in.
Aside from their anatomical adaptations to an arboreal lifestyle, the other main components of a Koalas anatomy which put them at a significant disadvantage in today’s environment are the adaptations which have manifested as a result of their diet. As a hindgut fermenter the Koala is one of the few species which can digest the highly fibrous Eucalyptus leaf and extract the limited nutrition and energy from the source. In order to enable the Koala to extract the maximum nutrition, the species has an enormous caecum relative to its body size (Grand and Barboza, 2001; Shipley, Forbey and Moore, 2009).
Koalas are regarded as one of the most highly specialised mammalian folivores (Shipley, Forbey and Moore, 2009). With over 93% of their diet consisting of 120 species of tree within the Eucalyptus genus, they are the most obligate of the Eucalyptus consumers (Grand and Barboza, 2001). It is hypothesised that Koalas adapted to eat the Eucalyptus leaf due to the lack of competition for the food source as it is highly toxic to many other species (Moore and Foley, 2000).
The Eucalyptus diet of the Koala is notoriously low in energy and difficult to digest. With a diet high in indigestible waxes, toxic terpenoids and fibre mass, but low in calories, the Koala requires 19-22hrs/day of eating and sleeping just to fulfil basal energy requirements and fund an average of 4 minutes a day of active movement (Grand and Barboza, 2001; Johnson et al., 2018). They are unable to spare the energy for excessive movement or socialising causing them to lead a sedentary and antisocial lifestyle. The low energy available to the Koala, means they need to have a basal rate of metabolism which is significantly lower than the average for other mammals of a similar size (Grand and Barboza, 2001). This has caused them to be metabolic conservationists in as many ways as possible.
In order to minimise the energy spent regulating body temperature, Koalas have a highly insulative pelt which keeps them warm in the windy environment of the treetops (Degabriele and Dawson, 1979). When trying to keep cool, Koalas rely on the shade provided by the Eucalyptus canopy and an area of thin skin over their sternum which they press against cool areas of ground or bark to reduce body temperature (Smith, 1979). However, while these reduce basal energy cost, their highly insulative pelt caused the species to be exploited by the pelt trade in the 19th and 20th centuries leading to millions of fatalities (Johnson et al., 2018).
In addition to this, as climate change escalates, causing more extreme weather conditions, and their habitat continues to fragment, it is unlikely that the species will be able to maintain a healthy body temperature using these methods alone. The increasingly elusive nature of shade and cool surfaces pose a particular threat; the high temperatures and low availability of water may lead to many Koala fatalities as a result of dehydration or overheating.
91% of the energy that can be utilised from the Eucalyptus by Koalas comes from the digestion of the cell contents rather than the cell wall (Lanyon and Sanson, 1986), so it is important that the cellulose walls are broken down as much as possible to expose maximum quantities of content. This is done during mastication in Koalas, the grinding motion of the jaw breaks down the cell walls and maximises the surface area to encourage fermentation later on. As there is such an importance placed on the exposure of the cell contents, the dentition of the species has adapted to maximise this (Lanyon and Sanson, 1986). Koalas use lateral grinding mandibular movements to crush the leaves to a pulp before swallowing, they also have a disproportionately large oral cavity for their skull size allowing more food to be masticated at any one time. As a result of this, a decreasing brain size can be seen in fossil records as more emphasis was placed on the process of mastication causing masticatory muscles and their attachment sites to increase in size (Grand and Barboza, 2001; Louys et al., 2009). Grand and Barboza (2001) described the skull of a modern Koala as “a chewing machine housing a small brain”.
Smith (1979) hypothesised that the brain size of the koalas, which is one of the smallest brains relative to body size seen in any mammal, may be responsible for the Koalas lack of adaptability and social skills. Smith suggested that, as they are not opportunistic animals, like primates or carnivores, they are able to simply analyse whether or not what’s put in front of them is a palatable food source or not, based on appearance, smell and moisture content with no need to evaluate their surroundings in detail. These findings are supported by Moore and Foley (2000) and Ellis et al. (2010). This lack of ability to process alternative environments, caused by their small brain size, may further limit the Koalas ability to adapt to new habitats, as different species of the Eucalyptus genus may present differently, leading to Koalas rejecting perfectly good food sources based on their differences in appearance (Moore and Foley, 2000).
The toxic nature of the Eucalyptus plant is partially caused by plant secondary metabolites (PSMs) and has made the food source abundant for Koalas, who have increased detoxification abilities which enable them to digest it (Moore and Foley, 2000). However, this does not mean that the PSMs have no effect on the Koalas, they may have uncontested access to food, but it comes at a cost. The PSMs are still absorbed into the bloodstream in high concentrations and it has been recently proven that high concentrations of PSMs act as potent immunomodulatory components in several animals including Koalas. The PSMs significantly reduce expression of key signalling glycoproteins which communicate between immune cells. This decreases or completely negates the effectiveness of certain immune response pathways, making the species particularly vulnerable to new diseases.
However, the suppression of certain immune pathways may have facilitated adaptation to such a low energy food source by eliminating some energetically demanding immune responses. While this may have been an advantage in the past, relative to the benefits of an uncontested food source, it is possible that this trade off no longer acts in their favour. Habitat degradation reduces the availability of dietary variation which is important in regulating the concentration of PSMs in the system, and new diseases associated with the introduction of new species and habitats pose massive threats to the population (Marschner, Krockenberger and Higgins, 2019). This threat can be seen in the effects of Chlamydia pecorum and Chlamydia pneumoniae on the species population; these diseases are thought to have been introduced to Koala populations via sheep and cattle during early human settlement and are now a leading cause of mortality in the population as the species does not have the immune responses needed to cope with this disease (Johnson et al., 2018).
Of the two chlamydial strains seen in Koalas, Chlamydia pecorum (C.pecorum) is the most prevalent and pathogenic, and infection can result in severe symptoms such as blindness (caused by keratoconjunctivitis), urinary incontinence, cystitis, infertility, reproductive tract lesions, bursitis, pneumonia and, in severe cases, death (Gonzalez-Astudillo et al., 2019). It has been proven that up to 88% of Koala populations are infected with one or both of these chlamydial strains, this has serious implications for the fertility of the population as C. pecorum is well documented to result in infertility in the female reproductive tract (Robbins et al., 2018).
With 66% of male carriers being asymptomatic, it is very difficult to provide treatment which may help manage the infection among populations as it can be difficult to diagnose in the wild (Hulse et al., 2020). In addition to this, due to the increased detoxification abilities of the Koala as a result of their diet, treatments for diseases such as chlamydial strains are processed and removed from the system very quickly, therefore Koalas require an extended 35-40 day treatment cycle which is harder to administer in the wild. Kollipara et al. (2012). state that the only way to fully control or eradicate the disease in the Koala population is to create a vaccine with as few booster immunisations as possible. However, capture, restraint and treatment of the animals is both expensive and traumatic, so this solution is still a long way off.
Narayan and Williams (2016) theorised that immunosuppression is exacerbated by humanisation of nearby areas causing acute stress in Koalas which disrupts their hormone regulation (Fig.3).This is supported by Hulse et al. (2020) who found the highest prevalence of C.pecorum infection in peri-urban areas, which are known to be stressful to wildlife. This stress can lead to decreases in reproduction, feeding and development eventually resulting in increased mortality.
Finally, as a result of reduced fertility, fragmented habitat and repeated relocation, a higher rate of inbreeding is also being seen amongst Koala populations, particularly in South Australia where island populations have been founded from as few as two or three individuals. Low levels of genetic variation can lead to morphological abnormalities, decreased reproductive success, and inbreeding depression, meaning populations with a high inbreeding coefficient are less likely to be able to thrive in harsh environmental conditions, leaving isolated wild populations particularly vulnerable to extinction in the changing climate (Seymour et al., 2001).
Overall, the Koala species has adapted very well to one specific environment but due to human interference this environment is changing, and their previously advantageous adaptations now pose significant disadvantages in the modern environment. The threats faced by the species are unprecedented – Koala death could be the result of disease, bushfires, introduced predators, an inability to cope in the harsh climate, a lack of genetic diversity or a search for food, water or a mate. Koalas lack the phenotypic plasticity to adapt at the same rate at which the environment is changing so, unless elements of human interference are removed, it is likely that the population of this species will continue to decline into the foreseeable future.
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