Health
The only western ringtail study that included a health investigation component mainly looked at evidence whether infectious disease potentially limits translocation success. (Clarke, 2011).
The animals underwent comprehensive health and disease screening and were tested for toxoplasmosis, leptospirosis, salmonellosis and chlamydiosis. All possums were negative.
There was however an observed relationship between the ‘white blood cell counts’ prior to release and early mortality. Also ‘white blood cell counts’ in some ringtails increased over time. It is unknown whether ringtails with higher WBC were suffering from sub-clinical infections or inflammatory processes or carrying some unknown pathogens which could have lowered their survival chances. (Clarke, 2011)
Haematological and serum biochemical values were considered normal for the released possums; however this study was the first to actually establish reference ranges for those parameters.
Western ringtail possums struggle with high ambient temperatures and tend to suffer from overheating at around 35°C particularly during an extended stretch of high temperatures. In a study with captive animals, evaporative water loss became biologically significant at temperatures above 32.5°C. (Yin, 2006)
This weakness coupled with a decrease in leaf quality under climate change conditions and the sensitivity of many pathogens to increased temperatures which might increase their severity (Cabrelli et al, 2015) could render western ringtails one of the earliest victims of climate change.
All animals can potentially suffer from a myriad of health issues and identifying those with potential conservation implications for the species would be essential.
According to my own experience there is a high occurrence of health problems in western ringtail possums in connection with a need for water (polydipsia, urinary tract infections, large amounts of non-staining crystalline material in urine, calculi, renal necrosis with intra-lesional oxalate crystals).
Heavy drinking over an extended time frame can render an animal unreleasable.
The potential relevance of the formation of bladder stones in ringtails was acknowledged by including the issue into the nomination for an upgrade of the conservation status to ‘critically endangered’ under the EPBC Act 1999. (http://www.environment.gov.au/biodiversity/threatened/nominations/comment/western-ringtail-possum)
Crippling parasitism - mites, fleas, ticks, cestodes – and in some habitats even all of the above simultaneously is clearly limiting survival, however we do not know whether parasitism is mainly a secondary problem, particularly in stressed animals. Emaciation, dehydration and parasitism can sometimes be simultaneously observed without a definitive disease process and the primary problem is unidentifiable.
Parasitism also played a negative role in mortalities in translocation research. (Clarke, 2011)
General body condition which is usually best in spring/ early summer and worst in late autumn/winter clearly influences the prevalence of parasitism. However, the condition of the coat follows other patterns and is usually at its best in cold weather when good insulation is needed. Most animals with bad fur loss particularly on the rump were spotted in spring and summer. This could however be unrelated to parasitism and instead be the result of fights over habitat in the breeding season and during dispersal.
Bite injuries even from another ringtail possum often lead to abscesses and septicaemia. These injuries are hard to treat particularly as Enrofloxacin seems to be the only safe antibiotic.
Also, investigations in to brush-tailed rock wallabies found that a high number of individuals in care carried resistance genes typical of those found in human pathogens while no animals from isolated wild populations carried any resistance genes.
There is obviously a risk that after release antibiotic resistance could potential be transmitted into wild populations. (Gillings, 2015)
Any mammal that is raised in a human environment might potentially carry this risk factor.
Any exposure to biological and/or environmental changes can act as a stressor, particularly in a creature of habit such as our western ringtail possum.
Susceptibility to infection or the risk of shedding of infectious agents can increase; even parasites that have co-evolved with the host can become an overwhelming burden, the immune system might suffer and the risk of mortality from any health issue is heightened under stress. (Hing et al, 2016)
Stress linked to extreme weather events – particularly prolonged heat - has also been suggested to increase mortality and an investigation into the relationship between stress and disease in wildlife is particularly important in our changing climate.
Some studies also see a link between infection patterns and the physiological stress responses of wildlife to habitat loss, disturbance and fragmentation. (Davies et al, 2013)
Similar to stress being strongly linked to disease, mortality and low adaptation capabilities, low genetic diversity can also reduce a population’s ability to adapt to environmental changes such as climate change. (Yokochi, 2015)
Reduced genetic diversity in many threatened species of Australian mammals is a consequence of small and often isolated or fragmented populations over multiple generations. (Eldridge and Herbert, 2015) Genetically uniform populations are more vulnerable to extinction as their capacity to evolve in response to changing environmental conditions is reduced.
Western ringtail possums already seem to have fairly little genetic variation. Genetic analyses so far have identified 3 genetically discrete subpopulations of western ringtail possums– Upper Warren region, Busselton and Gelorup/Dalyellup. (Wilson, 2009)
However, populations around Albany have not been sampled and some discrete locations in the greater Bunbury region – e.g . Binningup – might possibly also represent another genetically discrete population. (De Tores, 2009)
The finding that the Busselton and the Gelorup populations are genetically discrete is interesting as they are only 30 km apart and there are no geographic barriers to gene flow. (Wilson, 2009)
On the other hand, significant genetic divergences were already noticed in a population in the southern Busselton area that is divided by a 30 m artificial waterway (45 m gap in canopy). (Yokochi et al, 2015a)
Some threatened species obviously persist while lacking genetic variation (e.g. Koalas, Tasmanian Devils) but these species tend not to thrive and are under dire threat when conditions change.
Further declines in population size then lead to more losses of genetic variation and increases in inbreeding (mating between relatives). A steady decline in fitness is the probable consequence of increased homozygosity and an elevated risk of the expression of deleterious recessive alleles. (Eldridge and Herbert, 2015, Weeks et al, 2015) Reduced fitness of the offspring (inbreeding depression) can potentially hasten population extinction. (Frankham, 1995)
In theory 1000 individuals or more are needed for adequate adaptive potential in the face of environmental change. (Weeks et al, 2011)
Reintroduction programs for threatened species notoriously suffer from low availability of individuals and this also applied to western ringtail programs. With only 3 genetically distinct subpopulations available and initial resistance to gene pool mixing of animals between coastal and inland ringtails and between Busselton and Albany animals, reintroduced populations will have a very limited number of founders and lack genetic diversity from the onset.
However, the risk of outbreeding depression is now considered as having been overstated in the past (Eldridge and Herbert, 2015) and Swan Coastal Plain ringtails have been translocated to inland locations and can mix with Upper Warren animals that have a greater genetic diversity. (De Tores, 2009)
As translocations usually become necessary due to habitat destruction, it cannot even be safeguarded that the breeding individuals coming from one habitat are unrelated.
A significant percentage of male Koalas in a population with a high inbreeding coefficient has for instance been observed to have abnormal testes. (Seymour et al, 2001)
There has unfortunately not been a scientific investigation into inbreeding in western ringtail possums yet; however some observations in a small fragmented inner Busselton colony indicate similar issues.
