Captive Brushtail Possums Research Articles

Sustained recall of bait acceptability in captive brushtail possums (Trichosurus vulpecula)

Non-toxic prefeeding is often used to increase the efficacy of aerial poisoning of possums (Trichosurus vulpecula) by increasing their willingness to accept toxic bait. An important question is whether that increased acceptance lasts more than a few days or weeks– if not, repeated prefeeding might be required if the poisoning was delayed by poor weather. We presented 48 captive possums with 100 g of non–toxic bait for 40 min on three or more occasions at a range of intervals (1–28 days), and measured bait consumption on each occasion. Bait consumption increased from a mean (±SE) of 23.3 ± 2.5 g on first encounter to 32.9 ± 2.4 g on the second encounter, but without further increase on the third encounter (32.7 ± 2.5 g). The amount of bait eaten increased (rather than decreased) as the time between exposures increased. When possums were subsequently offered toxic bait (sodium monofluoroacetate) at a range of intervals (10–50 days) after their last exposure to prefeed, there was no evidence that amount of toxic bait consumed declined as the time since last exposure to non-toxic prefeed increased (r = −0.23, d.f. = 45, P = 0.085). The results indicate that a single short period (40 min) of exposure to prefeed should be sufficient to induce a sustained willingness to accept bait that lasts at least 1 month.

MateID: Design and Testing of a Novel Device For Recording Contacts Between Free-Ranging Animals

The design and testing of an electronic device, MateID (Electronics Laboratory, Department of Conservation, Wellington, New Zealand), for recording close proximity between wild animals is described. Comprising a receiver and data logger in a collar mounting, the device detects period-coded radiotags at distances below 40 cm. It stores time-stamped identification data in memory for up to 2 years for later down-loading to computer for processing. Bench trials of the MateID device confirmed its reliability. It recorded presence of 98.9% of stationary single tags placed within 40 cm and identified all of these correctly. However, no tags were identified where ≥2 tags were present within a 40-cm range. Trials on MateID devices attached to neck collars on captive brushtail possums (Trichosurus vulpecular) were conducted in large pens. Simultaneous video recordings confirmed that the devices accurately recorded all observed approaches (closer than 40 cm) by possums carrying MateID collars to a bait feeder fitted with a radiotransmitter. Contacts between a radiotagged possum and three carrying a MateID device also were accurately recorded. The device “MateID” provides a new technique for studying social and mating systems of free-ranging vertebrates. It also can be useful for monitoring or research of captive animals. (WILDLIFE SOCIETY BULLETIN 34(1):203–207; 2006)

Conjunctival vaccination of the brushtail possum (Trichosurus vulpecula) with bacille Calmette-Guérin

AIM: To determine the efficacy of conjunctival vaccination of captive brushtail possums (Trichosurus vulpecula) with bacille Calmette-Guérin (BCG), as measured by immunological responses to vaccination and response to intratracheal challenge with Mycobacterium bovis. METHODS: Nine adult male brushtail possums were vaccinated by the instillation of a suspension of BCG strain Pasteur 1173P2 into the conjunctival sac of each eye. Each drop contained approximately 2.5 × 105 colony forming units (cfu). At 8 weeks post-vaccination (pv) the vaccinated possums and 10 unvaccinated possums were challenged by intratracheal instillation of approximately 100 cfu of M. bovis. Cellular immune responses to bovine purified protein derivative (PPD) antigen were measured using the lymphocyte proliferation assay (LPA). Possums surviving to 50–51 days after challenge were euth anised and subjected to detailed post-mortem examination, including histopathology, to assess protection against tuberculosis. Sections of lung and spleen were cultured for M. bovis. RESULTS: No conjunctival inflammation or other adverse reactions to the administration of the vaccine were evident macroscopically. The vaccinated group showed a systemic cellular immune response to bovine PPD antigen at 4 and 8 weeks pv, and the response at 8 weeks was significantly greater than at 4 weeks (p<0.05). Conjunctival vaccination induced significant levels of protective immunity, measured as less mass of tuberculous lesions in lung (p<0.05) and less dissemination of disease in vaccinated compared with unvaccinated possums (p<0.05). CONCLUSIONS: Conjunctival vaccination with BCG induced a significant level of protective immunity against M. bovis infection in possums. This route of vaccination, together with intranasal aerosol vaccination, could be utilised in the delivery of an aerosolised vaccine using a device that sprays the vaccine suspension into the eyes and nose of possums.

Social-network analysis of Mycobacterium bovis transmission among captive brushtail possums ( Trichosurus vulpecula)

Wild brushtail possums ( Trichosurus vulpecula) are the main source of Mycobacterium bovis infection for New Zealand livestock. The disease is spread principally by infectious aerosol; therefore, social interactions determine disease transmission. In captive possums, den-sharing behaviour provided the greatest risk of tuberculosis transmission between animals. Den sharing between individual possums was used as the basic measurement for quantifying close proximity between animals over extended periods. Social-network analysis (SNA) was used to model patterns of social behaviour and to predict tuberculosis transmission. There was great diversity between groups in their social behaviour—but there were consistent trends in the SNA measures (closeness and flow-betweenness). With time, the social distance between possums in the same group increased, the social network became more homogeneous and the possums less differentiated from each other. Alteration of the physical environment of the pens (such as changing the number of dens or relocating the group to a new pen) had an inconsistent effect on social structure when comparing different groups. During the infection-transmission study, the possums that became infected had greater closeness and flow-betweenness scores than those that remained free of infection. Although standard statistical descriptive measures (such as the number of partners and the frequency of den-sharing events) were greater for the infected than the infection-free possums, the SNA-specific measures were more precise and could be compared across time and between groups.

Natural transmission of Mycobacterium bovis infection in captive brushtail possums (Trichosurus vulpecula)

AIMS: To examine natural transmission of bovine tuberculosis (Mycobacterium bovis infection) in captive brushtail possums (Trichosurus vulpecula) and to determine if this mode of transmission could be employed to challenge possums in vaccination studies. METHODS: Three experiments were conducted. In Experiment 1, 11 pairs of possums were housed together in cages, one of the pair having been experimentally infected with M. bovis. Of the in-contact possums 5/11 had been vaccinated with bacille Calmette-Guérin (BCG). In Experiment 2, three susceptible possums were placed in a colony of 19 possums that had been experimentally infected with M. bovis. In Experiment 3, the four most socially active possums in each of two colonies (24 possums in one colony and 23 in the other) were experimentally infected with M. bovis, and 10 of the remaining possums in each colony were vaccinated with BCG. RESULTS: In Experiment 1, transmission of M. bovis infection occurred in only 1/11 pairs. In Experiment 2, none of the three in-contact possums became infected. In Experiment 3, infection was transmitted to 5/20 in-contact possums in one colony and 12/19 in-contact possums in the other. The possums that became infected by natural transmission were significantly more socially interactive than those that remained free of infection (p<0.05). CONCLUSIONS: When susceptible and infected possums were randomly mixed, the rate of transmission of M. bovis was low, but when highly sociable possums were infected the rate of transmission increased markedly. The risk of transmission was dependent on the close proximity of infected and susceptible possums and the frequency and duration of their social interactions. Natural transmission from experimentally infected to incontact possums in a colony would be a useful way of studying the pathogenesis of tuberculosis in this species, and the social behaviour of the possums studied should be taken into account. The high degree of variation in the rate of natural transmission of M. bovis infection between possums makes this mode of transmission unreliable for assessing vaccine efficacy.

Social interactions among captive brushtail possums ( Trichosurus vulpecula)

A biological control strategy relying on a self-disseminating agent may provide the only affordable long-term technique for reducing brushtail possum ( Trichosurus vulpecula) numbers throughout New Zealand. The objective of this study was to determine the frequency and patterns of social interactions in single and mixed-sex groups of possums, in order to identify interactions that may assist in the dissemination of biocontrol agents. Thirty-two sexually mature wild-caught possums (16 female, 16 male) were housed in captive groups (of four) in enclosures during the breeding and non-breeding seasons. Groups consisted of either all females, females and males or all males. Four types of social interactions were defined: threats, fights, affiliative and sexual interactions. Interactions only occurred between two animals at one time and were most frequent in the non-breeding season. Within all female groups, possums readily engaged in interactions that had either an agonistic or affiliative nature. Mixed-sex groups interacted less frequently than females, but also engaged in both agonistic and affiliative interactions. In contrast, possums in all male groups rarely interacted, with only a few fights and no affiliative interactions observed. Some mixed-sex dyads appeared to ‘associate’ during the breeding period. Young were produced by three females that regularly associated with a male and one female that showed little associative behaviour. Given the different interaction patterns observed in each group type, biological control agents that rely on specific interaction patterns for dissemination, are likely to spread at different rates among different possum groups and in different seasons.

The role of non-toxic prefeed and postfeed in the development and maintenance of 1080 bait shyness in captive brushtail possums

Shyness to sodium monofluoroacetate (1080) in cereal bait can persist in sub-lethally poisoned possum (Trichosurus vulpecula) populations for at least 2 years. We investigated the use of non-toxic cereal ‘prefeed’ and ‘postfeed’ as ways of inhibiting and overcoming such shyness. The postfeed result was also compared with changing to a non-cereal, gel-based 1080 bait. Prefeeding had a significant effect on the number of possums that became ‘bait shy’ following an approximate LD20 1080 dose, with 97% of non-prefed possums developing an aversion to 1080 cereal bait compared with only 22% of prefed possums. In contrast, postfeeding with cereal was relatively ineffective in reducing the number of 1080 bait-shy possums, with mortality of these possums being 30% compared with 0% of non-postfed possums. In contrast, the gel 1080 bait killed 64% of 1080 cereal bait-shy possums. These results suggest that 1080 bait shyness can be markedly reduced by prefeeding non-toxic bait to possums prior to each control operation. However, this may not be the most cost-effective control option, given the observed efficacy of follow-up baiting with 1080 gel.

Den sharing behaviour of captive brushtail possums (Trichosurus vulpecula)

Den sharing among wild brushtail possums (Trichosurus vulpecula) has important implications for disease transmission. This study investigated den sharing in captive possums, and measured interactions between possums sharing dens. Thirty‐four sexually mature possums (16 female, 18 male) were housed in single‐sex or mixed‐sex pairs in large enclosures that contained two dens. Daily patterns of den sharing were recorded for each pair over a 69 day period in the breeding or non‐breeding season. Social behaviour within shared dens was sampled using miniature infrared cameras. Male pairs rarely shared dens in the breeding or non‐breeding seasons (4% and 1% of days respectively) and usually engaged in ‘threats’ and ‘fights’ associated with den defence. Pairs of female possums (in both seasons) and mixed‐sex pairs housed together in the breeding season shared dens most frequently (between 84% and 91% of days), and also spent the most time together in dens each night. While sharing dens, affiliative interactions were frequent, including long periods of ‘touching’, and also ‘food sharing’ and ‘allogrooming’. The preference for den sharing and close contact shown by captive possums highlights the importance of den sharing as a potential route for disease transmission.

Seasonal Changes in the White Blood Cell Population and the Ability of Lymphocytes to Proliferate in Captive Brushtail Possums (Trichosurus vulpecula) .

Brushtail possums in our captive colony die more frequently in winter than in summer. The present study examined whether the increased number of deaths in winter is due to a decreased immune response. The number and proportion of white blood cells, the proliferation of lymphocytes in response to the T cell mitogen, phytohaemagglutin (PHA) and the plasma cortisol concentration were determined in six male and six female possums during summer and winter. The total numbers of white blood cells, the numbers of lymphocytes, the proliferation of lymphocytes and the plasma cortisol concentration did not vary significantly between summer and winter. However, significant differences were observed in the numbers of neutrophils and the ratio of lymphocytes to neutrophils (L:N ratio) between the two seasons. The numbers of neutrophils in male and female possums (n=6, P=O.Ol) and 3.15 x 10 /ml (n=6, P=0.04) respectively during winter. The L:N ratio in male and female possums changed from 93:7 (n=6) and 87:12 (n=6) during summer to 78:20 (n=6; P=0.0004) and 65:33 (n=6; P=0.03) respectively during winter. The changes in the blood parameters in possums during winter are similar to those observed in unhealthy possums that subsequently die. These results suggest that possums face more difficult conditions in winter and this may play a role in the greater number of deaths observed at this time of the year.

Social dominance and breeding success in captive brushtail possums, Trichosurus vulpecula

The brushtail possum (Trichosurus vulpecula) is New Zealand's most important mammal pest. Options for the biological control of possums are being evaluated, particularly fertility control. The functions of social dominance hierarchies in possums have not been determined, but in many species dominant males breed more successfully than subordinates. This paper describes the effect on breeding success in captive groups of possums of vasectomising the dominant male. Dominance was measured by recording the outcome of social interactions during monthly observations. In groups of two male and two female brushtail possums, the dominant male was vasectomised (n = 7) or left untreated (n = 5). Five other groups with one male and two females were used as additional controls. In the groups where the dominant male was vasectomised, its dominance status remained unchanged after treatment but there were no further births, whereas young continued to be born in pens where males were untreated. At least in captivity, dominant males are therefore capable of behaviourally or physiologically inhibiting reproduction by subordinate males. This is the first evidence for such an effect in possums.

Dominant and subordinate behaviour of captive brushtail possums (Trichosurus vulpecula)

Brushtail possums are generally solitary animals. They use a range of olfactory, auditory, visual, and tactile behaviour to achieve spacing between individuals and to establish dominance hierarchies. Existing descriptions of possum behaviour are either not widely available or incomplete, and the function of dominance hierarchies is unclear. Data presented here were collected incidentally during observations of captive possums to determine the effects on dominance status of suppression of reproduction for biological control. Dominant and subordinate behaviour are described and reviewed. The top‐ranking possums in dominance hierarchies were always females. Dominant possums displaced subordinates from food, water, and dens. In our captive setting, only dominant males attempted to mate with females, but dominant and subordinate females produced a similar number of pouch young. In the wild, where resources are limited, we predict that dominance behaviour should limit the frequency and success of breeding by both subordinate males and females.

Determination of oestrus and mating in captive female brushtail possums, Trichosurus vulpecula (Marsupialia : Phalangeridae), from urine samples

Reproductive data were collected from captive brushtail possums, Trichosurus vulpecula, by examining daily urine samples for the presence of non-keratinised and keratinised epithelial cells, polymorphonuclear leucocytes, and sperm. Around the time of oestrus there was a massive increase in the number of epithelial cells and leucocytes in urine. The pattern of cell shedding was similar in mated and non-mated females undergoing either spontaneous ovulations or those induced by removal of pouch young. There was no difference between fertile or non-fertile oestrous cycles. During the 2-month sampling period, 97% of females showed changes in cell numbers indicative of oestrus and 72% of females were confirmed to have been mated by the presence of sperm. Conception rate was 74% and offspring were born 16–19 days after mating. When pouch young were removed, 85% of females were mated 7–15 days later and 77% of mated females produced pouch young. Compared with vaginal smears, collection and examination of urine was easier and faster, only one animal handler was required, the method was less stressful for the possum, and cell types in the sample were scored directly without the need for fixation and staining.

Effects of experimental infection with Leptospira interrogans serovar balcanica on the health of brushtail possums (Trichosurus vulpecula)

Data on changes in the condition and feeding behaviour of captive brushtail possums (Trichosurus vulpecula) following an experimental infection with Leptospira interrogans serovar balcanica are presented. The daily food intake of nine experimentally infected possums was recorded for 47 days before and 24 days after balcanica inoculation (0.5 ml intraperitoneally containing about 108 organisms). Body weight was recorded seven times between 42 days before and 96 days after inoculation. The mean daily food intake of infected possums decreased significantly throughout the 24-day recording period after infection with balcanica. The mean body weight of infected possums had decreased significantly by day 26 after inoculation but had fully recovered by day 96 after inoculation. Uninfected and control possums showed no significant changes in mean daily food intake or mean body weight over the experimental period. These data suggest that the health of possums is temporarily affected by infection with balcanica, but that overt disease caused by balcanica is unlikely. These findings support previous research suggesting that balcanica has only subclinical effects on possums.

Induced 1080 Bait-Shyness in Captive Brushtail Possums and Implications for Management.

The probable cause of possums (Trichosurus vulpecula) becoming 'shy' towards 1080 [sodium fluoroacetate] bait, a growing problem in the control of this pest, was determined. Possums captured from North Canterbury, New Zealand, (n = 131) were offered sublethal baits (1 or 2.5 g) followed by lethal (6 g) baits 2 days later. Most possums became bait shy and the proportion becoming shy appeared to be related to the size of the initial sublethal dose. Most of a group of survivors retested after 3 months with toxic pellets were still shy. Shyness was not overcome by changing to a different mask (orange flavour, as opposed to cinnamon flavour) or toxin (brodifacoum), but changing to both a different bait base (carrot) and mask (orange) resulted in most shy possums eating a lethal quantity of bait. Possums therefore appeared to learn to recognise the bait base as the cue for avoiding poisoning. More shy possums than naive possums rejected non-dyed, non-masked, non-toxic pellets, confirming that shy possums recognised the bait base. Green dye appears to act as a secondary cue for avoiding pellets as a higher percentage of 'shy' possums than naive possums rejected dyed baits.

Fermentation in the Hindgut of the Greater Glider (Petauroides volans) and the Brushtail Possum (Trichosurus vulpecula): Two Arboreal Folivores

The pattern and rate of microbial fermentation were studied in captive and wild-shot greater gliders (Petauroides volans) and captive brushtail possums (Trichosurus vulpecula) fed Eucalyptus foliage. The cecum and the cecum/proximal colon were the principal sites of microbial activity in the greater glider and brushtail possum, respectively. Total short-chain fatty acid (SCFA) concentrations were 36 mM and 70 mM in captive and wild-shot greater gliders, respectively, and 75 mM in the brushtail possums. SCFA production rates in the hindgut were similar in vitro in the two species (19-20 mmol/L/h) but slower than in the hindgut of most herbi-vores, although this was of set by the large volume of digesta contained in the hindgut. The pattern and rate of fermentation were similar in captive and wild-shot greater gliders. The difference between the initial molar proportions of acetate, propionate, and butyrate and their proportional contributions to total SCFA production in both species suggested that SCFAs were absorbed in proportion to their chain length. SCFA production contributed similar amounts of energy to both species, but as a proportion of digestible energy (DE) intake, SCFA was 15% of the DE intake of the brushtail possum and 7% of the DE intake of greater gliders. The relatively low level of SCFA production in both greater gliders and brushtail possums is largely due to the lignified nature of the fiber of Eucalyptus leaves and, in the brushtail possum at least, the inhibitory effects of leaf phenolics.

Digestion and metabolism of high-tanninEucalyptus foliage by the brushtail possum (Trichosurus vulpecula) (Marsupialia: Phalangeridae)

The digestion and metabolism of Eucalyptus melliodora foliage was studied in captive brushtail possums (Trichosurus vulpecula). The foliage was low in nitrogen and silica but high in lignified fibre and phenolics compared with diets consumed by most other herbivores. The high lignin content was suggested as the main cause of the low digestibility of E. melliodora cell walls (24%); microscopic observations of plant fragments in the caecum and faeces revealed few bacteria attached to lignified tissues. The conversion of digestible energy (0.34 MJ X kg-0.75 X d-1) to metabolizable energy (0.26 MJ X kg-0.75 X d-1) was low compared to most other herbivores, probably because of excretion of metabolites of leaf essential oils and phenolics in the urine. When the inhibitory effect of leaf tannins on fibre digestion was blocked by supplementing the animals with polyethylene glycol (PEG), intake of dry matter, metabolizable energy and digestible fibre increased. These effects were attributed to the reversal by PEG of tannin-microbial enzyme complexes. It was concluded that the gut-filling effect of a bulk of indigestible fibre is a major reason why the brushtail possum does not feed exclusively on Eucalyptus foliage in the wild.

Aggressive and Nonaggressive Interactions among Captive Populations of the Brush-Tail Possum, Trichosurus vulpecula (Marsupialia: Phalangeridae)

A study of aggressive and nonaggressive encounters among a group of captive brush-tail possums revealed that the majority of the aggressive encounters involved the dominant male and the subordinate males. Selective aggression of the dominant male towards other males of the population may function to differentially exclude young adult males from the home territory. Removal of the dominant male for a short period did not produce any significant changes in the dominance hierarchy of the remainder of the group, and the male readily reasserted his dominance when reintroduced into the group. New animals introduced into the group generally occupied a low position in the hierarchy. The results of this study are consistent with observations of other mammalian species which suggest that conspecific aggression is predominantly an inter-male phenomenon.