Basic biology / ecology

A summary of the biology of the western ringtail possum based on observations made between 1969 and 1974 of captive animals and two natural populations claimed to be the first published account. (Ellis and Jones, 1992)
More than 40 years later, many aspects of the ecology and biology according to Kaori Yokochi still remain unknown, mainly due to the difficulty of capturing these strongly arboreal animals. (Yokochi et al, 2015a)

Even though the animals’ distinctiveness from the common ringtail possum is now well known and widely accepted (Woinarski et al, 2014) publications as recent as 1991 still listed P. occidentalis as a synonym of P. peregrinus (Strahan, 1991) and on various websites it is still called a subspecies of P. peregrinus.

Western ringtail possums are small arboreal animals with short, dark brown fur above and a grey or creamy white middle strip of varying width on their tummy. They lack sexual dimorphism - both genders look the same (apart from their reproductive organs). Albinos exist (Wayne et al, 2005) but are extremely rare.

The literature usually describes the adult weight as ‘up to 1.3 kg’ (Wayne et al, 2005) however in 10 years of rehab work I have hardly ever encountered an animal that heavy (unless of course there was a disease background leading to the high weight such as water retention). Every animal over 1 kg would be in very good body condition which is a rare sight nowadays. 
In the early 1970s field weights were given as up to 1250 grams (Ellis and Jones, 1992) or even as 1330 grams (Wayne et al, 2005) in the South West Jarrah Forest.
It can be hypothesised that food quality was better in those wetter decades and therefore the average body condition and weight higher.

The poorer WA food supply (peppermint) could be one reason for the larger size of the western ringtails in comparison to the common ringtail – a larger animal can eat more.

Declining average size can be an evolutionary response to worsening environmental conditions. Under climate change, declining body sizes can be expected as a higher surface area-to-volume ratio facilitates heat loss. (Cabrelli et al, 2015)

Their small rounded ears with short, fine fur on the back distinguish them easily from common brushtail possums even on monochrome photos.

Their fore feet gave them their scientific name ‘Pseudocheirus’ meaning ‘false hand’ as the first 2 toes are opposable to the other 3 – like a grasping hand. The hind feet have an opposable thumb, followed by 2 short and fused digits and then 2 long toes.

The long, slender and highly prehensile tail aids when climbing, balancing or nest building and is often seen curled up into a ring – which gave the animals their common name.
The tail usually has some white part at the tip but the length of it is highly variable – from 3/4 white to just a little white ring or a centimetre of white.  In a decade I have seen only one animal with no white at all, the tail was actually almost black at the tip.

All eyes have two types of receptor cells – cones and rods. Cones allow colour vision and are concentrated in the central portion of the retina, while predominance of rods provide greater sensitivity in low light conditions such as at night and give an advantage in detecting movement. In total darkness no animal can see.  (Robinson and Thomson, 2016)
Nocturnal animals have a highly reflective layer of pigment behind the retina of the eye, the tapetum lucidum, which increases the light picked up by rod-shaped photoreceptors in the eyes and which causes them to glow in the dark when a light is shone into them. (Kerle, 2001)
Very bright light (such as our LED lights) temporarily blinds possums and it can take up to half an hour to restore full vision. (Kerle, 2001)

A recent discovery of three spectrally distinct cone photoreceptor types has hinted at possible colour vision in Australian marsupials (Arrese et al 2005) but it is questionable how useful colour vision would be for nocturnal animals.

Western ringtail possums are highly arboreal folivores – or tree-dwelling leaf-eaters.
Even though their natural diet consists primarily of the leaves of peppermint (Agonis flexuosa) and in inland areas of myrtaceous species such as jarrah and marri, they adapt to their environment and in urban areas can live on a variety of garden plants.

Their digestive system and low metabolic rate is well adapted to a diet with low nutritional values such as leaves.
Water requirements seem low in higher rainfall habitats and/or with food sources containing low secondary metabolite levels. However, while the even more specialised eucalyptus feeders such as the koala and greater gliders can derive almost all their water from their leaf diet (Hume, 1999), ringtails in our drying climate increasingly seem to need water.

Marsupials and eutherians/placentals are equally successful groups of mammals and mainly distinguished on the basis of their reproduction. (Archer and Kirsch, 2006)
Surprisingly not all marsupials have pouches but all have placentas.

Gestation is very short in marsupials – an estimated 2 weeks for ringtail possums. When marsupials are born their organs have formed but much of the growth and differentiation happens in the pouch, where the main part of development takes place.  (Nelson and Armati, 2006)

Maternal antibodies are transmitted via milk until the youngster’s own immune system has developed sufficiently. (Krockenberger, 2006) Symbiotic bacteria without which the gut cannot develop adequately and enable the offspring later-on to survive on an adult leaf-diet are provided throughout pouch life, possibly via the mother’s caecotrophes.

The peak seasons for births of western ringtail possums is April to July and September to November in coastal areas. (Jones et al, 1994b)  Females can double-breed if they lose a recruit or if environmental conditions are very favourable.
Seasonality of breeding is more pronounced in inland populations of the Jarrah Forest around Manjimup with most births occurring in May/June and to a lesser degree in October/ November. Females there breed only once a year and the commonly observed sex ratio is 1:1. (Wayne et al, 2005)

Seasonality of breeding is often linked to food availability and only peppermint can potentially provide a year-round food source which might make year-round breeding possible. 

10% to 17% (in best habitat) of births in the coastal area are twin births. In inland areas 1 offspring per adult female per year is the norm. (Jones et al, 1994b, Wayne et al, 2005)

At their time of birth, tiny offsprings do not place high nutritional demands on their mothers. Late lactation and weaning is the most critical and demanding time which coincides with spring and early summer when foliage is plentiful.  (Jones et al, 1994a, Shaw, 2006)
In October nitrogen levels in peppermint are at their highest level, which would also make it a very nourishing food source while lactating. (Jones et al, 1994a)

There is a risk in suboptimal habitat areas that the foliage does not provide sufficient nourishment to support the young through the final stages of growth and development. Either a low number of transient animals or sink populations consisting only of non-breeding animals can be the consequence. (Tyndale-Biscoe, 2005)

The young stay in the pouch for around 100 days and are fully weaned at a weight of 550-650 grams.  They are able to breed at approximately 1 year of age.  (Jones et al, 1994b)

Lactation starts when the young starts suckling and any female could theoretically therefore foster a pouch young. (Kerle, 2001)

Folivorous animals such as ringtails differ from other marsupials as their milk does not become increasingly concentrated in late lactation. Milk lipid levels do not rise as the animals’ food source is very low in energy. Lactation is however prolonged, supplementing the offspring while already on a leaf diet. (Krockenberger, 2006)

The mean daily water intake can almost double during late lactation. Caecotrophy is critical for water conservation and to survive on a leaf diet, but particularly during late lactation when a high percentage of protein is exported in the milk to support the young. (Tyndale-Biscoe, 2005) Extended dry periods and no access to water might lead to diminished breeding success and even pose a risk for the species’ long-term survival.

To maintain a steady population size every female western ringtail possum would need in minimum 2 offspring to survive to reproductive age. (Wayne et al, 2005) This is hardly likely as the mortality rate for dependent young common ringtail possums is given as 45-95% and probably similarly high for western ringtails. The average survival rate to maturity for P peregrinus is 30% but this is highly variable between the years and changing environmental conditions and particularly low during drought years. (Wayne et al, 2005)
There are no age-specific survival rates or reproductive rates available for western ringtail possums so far. (Clarke, 2011)

Western ringtail possums are generally classified as short lived throughout the literature. 3-6 years seem realistic in the wild.
Tooth wear is the best non-destructive means of estimating age (Wayne et al, 2005) and is widely used in science.

Ringtails’ molars have four major cusps shaped into crescentic blades that cut the food into tiny pieces. When premolars and molars are worn down in old age, the leaves cannot be grinded finely enough and the animal will eventually die from starvation. (Hume, 2006)

The oldest confirmed wild ringtail was 9 years old when she died showing clear signs of old age (partially blind due to cataracts). (De Tores et al, 2008)
The oldest ringtail in captivity I know of was 12 years old but apart from badly worn down teeth and discoloration she was still going strong until cataracts made euthanasia advisable.

Mortality rates are at their highest in late summer and autumn. Body conditions are then usually bad which increases the risk of being predated. (Grimm and
De Tores, 2009

Social System
Western ringtail possums live in a matriarchic society. (Ellis and Jones, 1992) The females choose their partner which of course depends on the availability of males and their desirable heritable qualities.

Western ringtail possums are relatively solitary, except during mating (Jones et al, 1994b) which is a typical feature of arboreal browsing herbivores. (Croft and Eisenberg, 2006)

Western ringtail possums – common brushtail possums
brushtail possumringtail possumWestern ringtail possums are one of two larger possums in the South West – however both are only medium size. The other local possum, the common brushtail is actually the smallest subspecies of the Trichosurus vulpecula species – hypoleucus. Adults usually weigh only approx. 1.5 -1.8 kilograms.

The species are easily distinguishable on the basis of bulk, pelage and colouring,  ear shape and size  and their distinctive tails.

The tail of a common brushtail possum is much thicker, shorter and less prehensile than the tail of a western ringtail possum and ends in the characteristic brush-like tip. The tip can be black or white but in brushtails the white part is usually only short while it is highly variable in ringtails.

Common ringtail possums – western ringtail possums
Adult western ringtail possums are larger than common ringtails - approximately 40 cm head-body length to 30-35 cm head-body length in the common species. The weight is accordingly different (900-1300 to 700-1100 grams).
The western species is usually darker and less reddish in colouring.  (Van Dyck and Strahan, 2008)

The most important difference between the species is most likely their breeding potential – the highest of the large possum species in common ringtails and one of the lowest in western ringtails. For common ringtails the highest number of offspring was recorded as 5 and females can rear 2 litters in a year. The fathers share the care work and can often be seen with a youngster on their back. Common ringtails are necessarily more gregarious and family groups share dreys (nests). (Kerle, 2001

The other main difference is the far broader diet of the common ringtail.