Deslorelin implants control fertility in urban brushtail possums (Trichosurus vulpecula) without negatively influencing their body-condition index
Wild brushtail possums (Trichosurus vulpecula) occur in large numbers in the grounds of Perth Zoo, Western Australia. These possums are a problem because they consume feed the zoo buys for its captive animals, damage seedlings and trees and many need to be treated for injuries sustained during fights with conspecifics. A contraceptive implant, which contains the gonadotrophin releasing hormone (GnRH) agonist deslorelin, could be a potential method of managing this population. We tested the efficacy of the implant and its impact on the body-condition index of treated possums with Kaplan–Meier analysis and a mixed model with residual maximum likelihood. We implanted 60 female possums with deslorelin and monitored reproductive success of treated and untreated possums for the following 18 months. At the conclusion of the study, 80% of 20 treated females recaptured had shown no evidence of breeding activity, giving an average minimum duration of effective contraception of 381 days. The implant did not have a negative impact on the body-condition index of treated possums during the course of the study. Our results suggest that deslorelin implants could be an effective management tool for brushtail possums at Perth Zoo and in other urban environments.
Deslorelin implants control fertility in urban brushtail
possums (Trichosurus vulpecula) without negatively
inﬂuencing their body-condition index
Cheryl A. Lohr
, Harriet Mills
, Helen Robertson
and Roberta Bencini
School of Animal Biology, The University of Western Australia, 35 Stirling Highway, Crawley, WA 6008, Australia.
Perth Zoo, 20 Labouchere Road, South Perth, WA 6151, Australia.
Corresponding author. Email: firstname.lastname@example.org
Abstract. Wild brushtail possums (Trichosurus vulpecula) occur in large numbers in the grounds of Perth Zoo, Western
Australia. These possums are a problem because they consume feed the zoo buys for its captive animals, damage seedlings
and trees and many need to be treated for injuries sustained during ﬁghts with conspeciﬁcs. A contraceptive implant, which
contains the gonadotrophin releasing hormone (GnRH) agonist deslorelin, could be a potential method of managing this
population. We tested the efﬁcacy of the implant and its impact on the body-condition index of treated possums with
Kaplan–Meier analysis and a mixed model with residual maximum likelihood. We implanted 60 female possums with
deslorelin and monitored reproductive success of treated and untreated possums for the following 18 months. At the
conclusion of the study, 80% of 20 treated females recaptured had shown no evidence of breeding activity, giving an average
minimum duration of effective contraception of 381 days. The implant did not have a negative impact on the body-condition
index of treated possums during the course of the study. Our results suggest that deslorelin implants could be an effective
management tool for brushtail possums at Perth Zoo and in other urban environments.
Brushtail possums (Trichosurus vulpecula) can be a pest in an
urban environment where they interact with people. Possums are
causing problems at Perth Zoo, Western Australia, because they
consume feed bought for captive animals and damage seedlings
and trees, some of which are threatened species (H. Robertson,
Perth Zoo, pers. comm.). Attempts to reduce the damage caused
by brushtail possums by collaring palm trees, encircling seedlings
with wire mesh, making buildings ‘possum proof’ and trapping
and relocating possums to bushland have been unsuccessful
(Patt 1995). Veterinarians at Perth Zoo euthanase many
possums each year due to injuries sustained while ﬁghting
with conspeciﬁcs. The injuries caused by these ﬁghts have
raised concern for the welfare of these animals. For these
reasons, staff at Perth Zoo would like to reduce the size of this
population of brushtail possums (H. Robertson, pers. comm.).
There are lethal methods of control for brushtail possums.
However, because Perth Zoo is a popular tourist destination
receiving ~600 000 visitors every year and is surrounded by
residential and commercial areas, shooting and poisoning
campaigns cannot be used as a method of control for brushtail
possums because they pose a risk to public safety. Fertility
control is often regarded as a more socially acceptable
method of managing an over-abundant vertebrate species,
particularly when the species to be managed is endemic to the
area. Deslorelin, when administered via a slow-release implant,
prevents reproduction by interrupting the action of reproductive
hormones at the level of the pituitary gland (Karten and
Rivier 1986; Melson et al. 1986; Aspden et al. 1996;
D’Occhio and Aspden 1996; Gong et al. 1996; Aspden et al.
1997; D’Occhio et al. 1997; Trigg et al. 2001). Deslorelin could
be a more appropriate tool than lethal methods, for managing
urban populations of brushtail possums.
Deslorelin has been shown to be an effective contraceptive
in females of a wide variety of mammals including cattle
(D’Occhio et al. 1996, 2000), exotic carnivores such as lions
and cheetahs (Bertschinger et al. 2001, 2002), dogs (Trigg et al.
2001), cats (Munson et al. 2001) and marsupials (Herbert et al.
2004a, 2005, 2006b; Eymann et al. 2006). Deslorelin does not
prevent reproduction in male marsupials (Herbert et al. 2004b;
et al. 2007). However, modelling has shown that
sterilising female brushtail possums is likely to be many times
more effective for population control than is sterilising male
possums (Barlow 1994).
Eymann et al. (2007) measured the maximum duration of
contraception for an implant containing 4.7 mg of deslorelin on
three captive female brushtail possums from New Zealand. They
found that the implant prevented reproduction for 259 to
>734 days, without any obvious negative side effects. The
implant did not affect the weight of treated animals relative to
Brushtail possums in south-western Australia are a smaller
and different subspecies (Trichosurus vulpecula hypoleucus)
from those used by Eymann et al. (2007) and those present in
Wildlife Research, 2009, 36, 324–332 www.publish.csiro.au/journals/wr
CSIRO 2009 10.1071/WR08050 1035-3712/09/040324
New Zealand (Pracy 1974). The population of possums at Perth
Zoo is also unusual in that it is one of a few populations where
breeding occurs all year round, with seasonal peaks in breeding
rate (Sanderson and O’Driscoll 1985; Patt 1995; Hetherington
2007). In 2006, the breeding rate for brushtail possums in
Perth Zoo peaked in February and October (Hetherington
2007). In other populations, breeding is typically in the
autumn months, with a possible second spring breeding
(Green 1984; Kerle 1984; Isaac 2005).
We tested the efﬁcacy of a 4.7-mg deslorelin implant at
preventing reproduction in the free-ranging female brushtail
possums at the Perth Zoo. Since the Western Australian
subspecies have a smaller bodyweight than its eastern
Australian counterpart, we expected that this dose would
inhibit reproduction for at least the 259 days reported by
Eymann et al. (2007). We also expected that the implant
would not affect the body condition of the treated females.
Materials and methods
To test our hypotheses, urban possums living in the Perth
Zoological Gardens were captured and females were implanted
with a 4.7-mg deslorelin implant. A monthly trapping regime was
then conducted to monitor reproduction and body condition of
treated and untreated females.
The study was conducted across ~18 months (December 2005–
April 2007) within the 18 ha of Perth Zoo, Western Australia
E). Perth Zoo is located ~5 km south of the Perth
CBD and is surrounded by busy city roads, residential and
commercial buildings, and large sporting ﬁelds.
There is a large variety of exotic and native plants within the
grounds of Perth Zoo. The species’ distribution within the
grounds is based on the theme of a particular area. For
example, the ‘bushwalk’, an open native mammal exhibit, is
dominated by Eucalyptus sp., Agonis sp. and Acacia sp. Palms
(e.g. Archontopoenix alexandreae) and bamboo (e.g. Bambusa
balcooa) dominate the rainforest exhibits. The ‘safari’ exhibit
hosts a variety of exotic plants, including Erythrina lysistemon
and Dracaena marginata. Three species of ﬁg tree (Ficus sp.)
are located throughout the grounds. There are also three areas of
lawn in the northern half of the zoo dotted with the Western
Australian peppermint tree (Agonis ﬂexuosa).
In December 2005, a trap layout consisting of 240 trap sites
(~20 m apart) was established within the accessible parts of the
Perth Zoo. The areas that could not be accessed included the
enclosures and the staff working areas of dangerous animals.
The zoo grounds were divided into quarters, with 60 trap sites
per quarter. Each night, 60 Shefﬁeld live-capture traps
(220 220 550 mm,ShefﬁeldWireProducts,Welshpool, WA)
were set in one quarter of the zoo, so that during four
consecutive nights per month, all 240 trap sites were opened.
The traps were baited with rolled oats, peanut butter and apple
segments and were covered with hessian sacking.
Each animal caught was marked with two numbered ﬁngerling
ear tags (National Band Tag Co, Newport, Kentucky). Head
length was measured to the nearest 0.1 mm with vernier
callipers. The animals were weighed to the nearest 25 g with a
spring balance. The weight of any pouch young (n = 250) was
subtracted from that of their mothers to calculate the weight of
adult females. The weight of each pouch young was estimated
by using the head length of the pouch young measured to the
nearest 0.1 mm and the nomogram published by Lyne and
Verhagen (1957). In cases where the length of the head could
not be measured easily the short pes and/or crown-to-rump length
were also recorded and used to estimate weight with the same
nomogram. Although the nomogram was developed using
Tasmanian possums, it was the best available estimate for our
purposes (and only 68 of the 250 pouch young were 100 days
old, at which point growth curves begin to differ signiﬁcantly
among different populations).
The length and width of one testis (excluding epididymis) of
male possums were measured through the skin to the nearest
millimetre. Males with a testis measurements greater than or
equal to 10 mm 15 mm were classiﬁed as mature adults as per
Wayne et al. (2005). The pouch of all females captured was
examined and classiﬁed as undeveloped (subadult, very small
pouch and non-everted teats), parous (adult, not active, fully
everted teats and developed pouch), active (clean and moist
with no pouch young), suckling young (young present) or
lactating (enlarged teat, no pouch young) according to Wayne
et al. (2005).
Efﬁcacy of the deslorelin implant
Female brushtail possums (n = 49) captured between the months
of December 2005 and March 2006 without a pouch young
were given a Suprelorin implant (Peptech Animal Health Pty
Ltd, Sydney, Australia) which contained 4.7 mg of
deglycinamide))GnRH) (Trigg et al. 2001). The implants were
placed subcutaneously between the shoulder blades or at the
base of the tail with a sterile single-use commercial implanting
device. The injection site was sealed with VetBond (3M, St Paul,
MN, USA). An additional 11 possums with pouch young
between 90 and 135 days of age were implanted, also
between the months of December 2005 and March 2006.
In total, 60 female brushtail possums were implanted with
deslorelin (Table 1).
An additional 28 female possums that were already carrying
pouch young during the time the implants were administered
were not given an implant because lactation typically prevents
possums from conceiving a second young (Tyndale-Biscoe and
Renfree 1987). These possums, along with females ﬁrst caught
after March 2006 (n = 79) were assigned to the ‘untreated control’
group (Table 1). Untreated animals did not receive a placebo
implant as previous studies have already shown that the implant
matrix does not affect a marsupial ’s weight or its ability to
reproduce (Herbert et al. 2005, 2006a; Eymann et al. 2007).
The pouches of all females caught in the subsequent months
were checked for pouch young. The head length of the pouch
young was used to estimate the birth date and hence the date that
Deslorelin in urban brushtail possums Wildlife Research 325
the implant ceased to prevent reproduction, assuming a
pregnancy of 17 days. We estimated the date of birth for
pouch young from the growth curve for south-western
brushtail possums (Wayne et al. 2005), as follows:
Head length ðmmÞ¼0:3211 age ðdaysÞþ7:0:
If the head-length measurement for a pouch young was
<7 mm then it was assumed that the young had been born on
the day it was captured.
A Kaplan–Meier estimator with a log-rank test was used to assess
the difference in duration without young, between the treated
and untreated group (Kaplan and Meier 1958; Bland and Altman
2004). The Kaplan–Meier estimator produced a survival
function for the deslorelin implant. This method is frequently
used in medical research because it allows the use of censored
data, i.e. those collected from possums that disappeared from
the sample before the ﬁnal outcome, i.e. duration without young,
was observed. Censored data occurred because some possums
were not captured in the ﬁnal 6 months of the study, or died or
failed to breed before the end of the study (Table 2).
The ‘duration without young’ for possums without young
when treated with the implant was calculated as the number
of days between receiving the implant and the date they
conceived their next young. If a possum was carrying a pouch
young when it was treated with the implant, then the ‘duration
without young’ was calculated in the same way as for the
possums without young, minus the time required for the
current pouch young to reach 140 days of age. The duration
without young for untreated females was calculated as the
number of days between the birth date of their ﬁrst observed
young and the birth date of their second young, minus 140 days
for lactation and 17 days for gestation (Pilton and Sharman 1962;
Tyndale-Biscoe and Renfree 1987; Fig. 1).
Five months or 150 days is reportedly the minimum length
of time for lactation in brushtail possums (Tyndale-Biscoe 1973;
How and Hillcox 2000). Because lactation usually inhibits
oestrus in brushtail possums (Pilton and Sharman 1962), it
should also be the minimum time that a possum spends raising
one young before it can breed again. However, our data
suggested that young may stop suckling before 150 days of
age. On two occasions, a possum was caught with a dependent
back-riding young (175 days of age) and a pouch young
(40 days of age). Therefore, 140 days of lactation was
used in these calculations. A pouch young was assumed to
have died if a female gave birth to a second young before the
ﬁrst had reached at least 140 days of age. These young
were removed from the dataset because the time and cause of
death was not known.
An average duration was used if a possum raised more than
two young. Seven untreated possums were observed with only
one young. The duration without young for these individuals was
calculated as the interval of time from weaning their ﬁrst young
(140 days) until their ﬁnal capture.
Impact of deslorelin implant
Body-condition indices (BCI) for treated and untreated possums
were calculated as a method of assessing general health. We used
1489 possum captures to develop a regression that allowed us to
predict the standard bodyweight for each possum by using its
average head length. It has been suggested that age and sex may
have an effect on condition (Riney 1955; Caughley 1967;
Bamford 1970; Viggers et al. 1998). Therefore, the possums
were divided into the following four groups: adult males, adult
females, subadult males and subadult females. Data for the BCI
analysis were collected only once per trapping session. An adult
possum’s head length was averaged if the possum was caught in
multiple trapping sessions. Head length for subadult possums was
not averaged because it was likely that the individual was still
growing. The data were analysed with a general linear model in
SAS Enterprise Guide (SAS Institute Inc. 2008).
The linear regression equations for each group were as follows
Table 2. The fate of the 60 female brushtail possums treated with the
Fate n No. with expired
Unknown – not caught after given implant 15 n.a.
Dead 8 0
Last caught before November 2006 17 1
Caught in the ﬁnal 6 months of study
(Nov. 2006–Apr. 2007)
Duration between births
Conception Birth 2
Fig. 1. Relationship between birth dates, weaning date and the duration
Table 1. The number of treated and untreated female brushtail possums and their reproductive state when they were ﬁrst captured
In the parentheses is the number of individuals whose implant failed to prevent reproduction
Treatment Suckling Lactating Parous Undeveloped Total
Treated 11 (1) 5 25 (2) 19 (2) 60
Untreated, caught prior April 2006 26 0 0 2 28
Untreated, caught April 2006 onwards 53 2 16 8 79
326 Wildlife Research C. A. Lohr et al.
The slope and the intercept for the regression equations for
adult male and female possums were signiﬁcantly ( P < 0.01)
different and also differed signiﬁcantly (P < 0.01) from the
equations for subadult possums; however, there was no
difference (P 0.8) between regression equations for subadult
males and females. The two equations were kept separate because
some females were subadults when they were treated with
deslorelin, whereas no male possums were treated with
deslorelin. These equations were used to predict the
expected weight for each possum. The BCI for each capture
was equal to the ratio of observed body mass to expected body
mass (Krebs and Singleton 1993).
The possums caught each month were divided according to
their treatment groups, female possums treated with deslorelin,
untreated females and males. We compared the average BCI
for each of these groups per month (see Fig. 5). A mixed model
with residual maximum likelihood was used to test for a
relationship between BCI, time and treatment group. If a
relationship was present then t -tests were used to compare two
Efﬁcacy of the deslorelin implant
The deslorelin implant signiﬁcantly increased the duration
without young. The Kaplan
–Meier survival curves for duration
without young in treated and untreated possums were
signiﬁcantly (P < 0.001) different (Fig. 3).
Some of the implants failed to prevent reproduction in treated
female possums. The ﬁrst implant failed to prevent conception
after 43 days. Four more possums conceived after 51, 57, 88
and 215 days. The possums whose implants failed after 51 and
57 days gave birth to three young each (including young
conceived on the 51st and 57th day) before the end of the
study. The other three possums gave birth to, and successfully
raised, one young.
Of the original 60 possums that were implanted at the
beginning of the study (captured between December 2005 and
March 2006), 20 were trapped in the ﬁnal few months of the
study (Table 2), between November 2006 and April 2007
(11 of the 28 possums added to untreated group between
December 2005 and March 2006 were recaptured in 2007).
Four of these possums were carrying pouch young indicating
their implants had failed to prevent reproduction. The other 16 had
Body weight (g)
50 60 70 80 90
50 60 70 80 90
70 80 90 100
70 80 90 100
Fig. 2. Linear regression of head length versus weight for four groups of
possums: (a) adult female, n = 489, r
= 0.15; (b) adult male, n = 813, r
(c) subadult female, n = 51, r
= 0.80; and (d) subadult male, n = 136, r
Note that axes have different scales for adults and juveniles.
Duration without youn
Fig. 3. Kaplan–Meier survival (of the contraceptive effect) plots for the
duration without young in female possums.
Deslorelin in urban brushtail possums Wildlife Research 327
not reproduced during the study and many of their pouches were
small and dry, with everted but small teats.
The average duration without young was signiﬁcantly
(P < 0.001) shorter for untreated than treated possums. The
average duration between young increased from an average of
33.2 days (range 4–212) to 148 days (range 2–483; Fig. 4). In
total, 16 (of 20) treated possums captured in the ﬁnal 6 months
of the study (192–483 days after treatment) did not breed
between receiving the implant and the end of the study. The
average minimum duration of infertility for these 16 recaptured
possums that had shown no evidence of breeding activity was
Impact of the deslorelin implant
We found a signiﬁcant (P < 0.001) relationship between the
head length and weight of possums at Perth Zoo (Fig. 2). By
including subadult possums in the general linear model we
explained 56% of the total variation. Omitting the subadult
possums would have explained only 15% of the total variation.
The mixed model revealed that there was a signiﬁcant
(P < 0.01) effect of time on the BCI of possums and there was
a signiﬁcant (P < 0.01) interaction between treatment and time
(Fig. 5), although treatment alone did not signiﬁcantly affect
the BCI. Male possums had their highest average BCI during
the summer months at the beginning and end of the study, which
is similar to the results found by Wayne et al. (2005) and Ji
et al. (2000). The BCI was 1.07 in December 2005 and declined
to ~0.95 late in 2006. However, the recovery was rapid, from
0.94 in September to 1.05 in December 2006, which represents
an average weight gain of ~200 g.
Female possums treated with deslorelin started with an
average BCI of 0.94, and this remained fairly constant
throughout the year until September 2006 (Fig. 5). They had a
minimum BCI of 0.91 in July 2006, after which it gradually
increased to a maximum of 1.1 in April 2007. The difference
between the minimum and maximum BCI for female possums
treated with deslorelin was not signiﬁcant (P = 0.2).
Female possums not treated with deslorelin started with a
BCI of 1.08. Their bodyweight then ﬂuctuated rapidly, with
a minimum BCI of 0.88 occurring in September 2006. There
was a signiﬁcant (P = 0.01) difference between the minimum and
maximum BCI for untreated possums (Fig. 5).
Efﬁcacy of the deslorelin implant
The deslorelin implant prevented reproduction in brushtail
possums at Perth Zoo. However, only 20 of the original 60
possums treated with the implant were caught in the ﬁnal
6 months of the study. Of these, 16 animals (80%) had not
bred since receiving the implant. The average duration
0 100 200 300 400 500
Duration without youn
Fig. 4. Box plots showing the variation in the duration without young
(days) in control possums (n = 34) and those treated with deslorelin implants
(n = 44). Data from 16 treated possums are not included in the graph because
15 were not trapped again after they were implanted and 1 died the day it was
implanted (Table 2). Box plots: the vertical line in the box indicates the sample
average, the left and right extents of the box indicate the 25th and 75th
percentiles, and the T-bars indicate the maximum and minimum values,
excluding outliers. The data point outside of the box plot is an outlier.
Average body condition index
Treated (n = 205)
Control (n = 291)
Male (n = 949)
Fig. 5. The variation in the average body-condition index for male and female brushtail
possums at Perth Zoo. Implanted females were treated with deslorelin between the months of
December 2005 and March 2006.
328 Wildlife Research C. A. Lohr et al.
without young for these 16 possums up to their ﬁnal observation
was 381 days. Because the deslorelin implant seemed to prevent
reproduction in 80% of possums for an average of 381 days, our
hypothesis was supported. In comparison, the average duration
without young in untreated possums was 52 days.
The average duration of 381 days is a conservative estimate
of the potential duration of contraception owing to the deslorelin
implant. The maximum duration observed in the present study
was 483 days. This particular individual did not breed before
the end of the study and the pouch and teats were small.
This suggests that the implant has the potential to prevent
reproduction in possums for longer than 381 days. Eymann
et al. (2007) reported that one possum treated with deslorelin
(4.7 mg) had not bred after 734 days. Unfortunately, we were
unable to deﬁnitively conclude when the implants ceased being
effective due to time constraints.
Although some of the deslorelin-treated possums bred
again during the timeframe of this study, in the present and
three other studies on deslorelin-treated marsupials, at least
one individual failed to breed again when the other study
animals did (Herbert et al. 2004a, 2006b; Eymann 2006).
These observations may indicate that deslorelin can cause
permanent infertility in some individuals, although this was
not explicitly reported nor tested for in any of these studies.
As we do not know why these few individuals did not breed again
following treatment we recommend that another long-term
captive study be used to ascertain the total duration of
deslorelin-induced sterility, and to deﬁnitively conﬁrm that
these implants do not cause permanent sterility in brushtail
possums and other marsupials. It is important that wildlife
managers and the managers of captive-breeding programs
fully understand the tools they are using and use this
information to develop robust plans. This is particularly so in
the management of endangered species, where temporary sterility
might be desirable whereas permanent sterility disastrous.
In total, ﬁve of the deslorelin implants failed to prevent
reproduction in treated possums. These ﬁve implants may
have expired prematurely as there was a large gap in duration
between the last implant to expire (215 days) and the average
duration for treated possums captured in the ﬁnal 6 months of
the study with implants that did not expire (381 days). These
implants may have expired for several reasons. The animals
may have developed an abscess or calciﬁed tissue around
the implant, which may have prevented an effective dose of
deslorelin from entering their bloodstream. The implant may
have broken during or after insertion, creating a larger surface
area, therefore releasing the deslorelin faster than normal.
In two of the eight cases where an implanted female had died
the implant was found to have broken into two or three pieces.
There is also the more remote possibility that the implant was
lost. Many of the implants could be detected under the skin
while handling the possums. Unfortunately, we were not
able to perform autopsies or GnRH challenges (Scheele et al.
1996) on the possums whose implants failed to prevent
reproduction, as none died during the study and they were not
held in captivity.
Our estimate of the average duration of the implants may
have been affected by the natural fertility of possums assigned
the treated and untreated groups. Most of the treated possums
(82%) were given a deslorelin implant because they were not
carrying a pouch young. Therefore, some of the possums treated
may not have been carrying a pouch young because they were
already infertile. However, 71% of untreated possums at Perth
Zoo did not have a pouch young between December 2005 and
March 2006 when the deslorelin implants were being
administered (Hetherington 2007). Because 90% of untreated
possums bred at least once a year during the study (Hetherington
2007), the chance of getting a high proportion of infertile
females in our treated group was very low. In total, 4 of the
16 possums caught in the ﬁnal 6 months that did not breed after
being treated were carrying young at least 102 days of age at the
beginning of the study. Therefore, our estimate of the average
duration of the implants may be biased by naturally infertile
possums, although it seems unlikely that there would be many
naturally infertile individuals.
Impact of the deslorelin implant on body condition
The hypothesis that the deslorelin implant would not affect the
body condition of treated possums was not strongly supported,
as possums treated with deslorelin increased their body condition
throughout the study, although not signiﬁcantly. However,
the BCI for treated possums did not ﬂuctuate throughout
the year as it did for untreated possums.
The BCI for untreated females may have ﬂuctuated
because of the stress of reproduction. Female possums may
have had their highest body condition in the summer months,
owing to an abundance of food and a lack of dependent
young. Females may also have had their lowest body
condition in late winter because the winter months coincided
with high energetic demands owing to lactation and because
food was less abundant.
Wayne et al
. (2005) found that females in Chairup, Western
Australia, did not vary signiﬁcantly in condition over time. They
hypothesised that the females were able to offset increased
nutritional demands associated with breeding with the
seasonal peak in food availability. Females at Perth Zoo breed
more rapidly than those at Chairup, with 40% producing
two young per year (Hetherington 2007). In September and
October, a second peak of births occurred at Perth Zoo
(Hetherington 2007). Therefore, many untreated females were
weaning their ﬁrst young of the year, after declining in
body condition because of the earlier energetic demands of
lactation, and giving birth to their second during a period
when there may have been less food available, which may
have prevented them from offsetting the nutritional demands.
It is difﬁcult, however, to estimate the possible food abundance at
the Perth Zoo because there were many different food sources
from Eucalyptus spp. to animal feed and food scraps from rubbish
The method used to allocate female possums to treated and
untreated groups may have affected the initial body-condition
estimates. Most female possums were allocated to the untreated
group if they were carrying a pouch young. In December 2005,
untreated possums were on average in better condition than
treated possums, most of which were not carrying pouch
Deslorelin in urban brushtail possums Wildlife Research 329
young. Because the probability of breeding in brushtail
possums declines rapidly as body condition falls below
average (Bell 1981; Ramsey et al. 2002), the initial difference
in body condition between the two groups may be an artefact of
The use of a BCI to assess the impact of a method of fertility
control on treated animals is somewhat controversial. The basic
assumption underlying body-condition indices is that individuals
of higher condition will have better reproductive rates, lower
mortality rates and/or will be able to better cope with
environmental stresses, starvation or cold temperature (Brochu
et al. 1988). Therefore, condition indices are frequently
considered a predictor of fat reserves (McCance and
Widdowson 1951; Frost and Kipling 1967; Bailey 1968).
However, although some authors have found a correlation
between condition indices and fat reserves (Bamford 1970;
Viggers et al. 1998), others have not (Krebs and Singleton
1993). Therefore, factors other than fat reserves may inﬂuence
condition indices. For example, muscle mass may have an impact
on the condition of a possum, particularly in males who may ﬁght
for females and territory. A larger muscle mass may make
possums more competitive, allowing them to secure better-
quality home ranges and mate with more or better-quality
females; however, this hypothesis remains untested. Although
body-condition indices may be controversial, they are the easiest
method of assessing the general health of wild animals and the
data can be collected within a few minutes without sedation.
Implants containing deslorelin may be a viable option for
managing urban populations of possums. Possums cannot be
culled in an urban environment without serious public concern.
Translocating animals can lead to the spread of disease (Viggers
et al. 1993) and Pietsch (1994) showed that many brushtail
possums die when they are translocated. Surgical sterilisation
is an expensive and invasive technique, with associated risks of
anaesthesia and post-surgical infection (Tuyttens and
MacDonald 1998). Immunocontraception is not yet available
for managing marsupial species.
Deslorelin is an effective contraceptive. It is relatively cheap
and Australian-made and therefore readily available for
managing wildlife populations in Australia. The implant does
not negatively affect the general health of the treated possums.
Although there is a need for formal experimental conﬁrmation,
the data suggest that the implant is non-permanent, giving all
individuals within the population some chance of contributing to
the next generation and maintaining genetic diversity. The
deslorelin implant is easy to administer and does not require
Our results indicate that a deslorelin implant (4.7 mg) can
prevent at least 80% of female brushtail possums from giving
birth for at least 381 days, without any negative impacts on the
treated animals. This suggests that deslorelin implants could be an
effective tool for managing populations of brushtail possums,
especially those in urban environments or in small, enclosed
Our most sincere thanks go to the staff at Perth Zoo, particularly Dr Karen
Payne, Dr Simone Vitali, Dr Paul Eden, Dr Rebecca Vaughan and Caroline
Lawrence. Also to Kevin Murray, for his help with the statistics and to those
people who volunteered to help with the ﬁeld work. Our thanks also go to the
referees of this paper for their input. All experimental procedures described in
this paper were approved by The University of Western Australia Animal
Ethics Committee (05/100/514) and the Perth Zoo Animal Ethics Committee
Aspden, W. J., Rao, A., Scott, P. T., Clarke, I. J., Trigg, T. E., Walsh, J., and
D’Occhio, M. J. (1996). Direct actions of the luteinizing hormone-
releasing hormone agonist, deslorelin, on anterior pituitary contents of
luteinizing hormone (LH) and follicle-stimulating hormone (FSH), LH
and FSH subunit messenger ribonucleic acid, and plasma concentrations
of LH and FSH in castrated male cattle. Biology of Reproduction 55,
386–392. doi: 10.1095/biolreprod55.2.386
Aspden, W. J., Van Reenen, N., Whyte, T. R., Maclellan, L. J., Scott, P. T.,
Trigg, T. E., Walsh, J., and D’Occhio, M. J. (1997). Increased testosterone
secretion in bulls treated with a luteinizing hormone releasing hormone
(LHRH) agonist requires endogenous LH but not LHRH. Domestic
Animal Endocrinology 14, 421–428. doi: 10.1016/S0739-7240(97)
Bailey, J. A. (1968). A weight-length relationship for evaluating physical
condition of cottontails. Journal of Wildlife Management 32, 835–841.
Bamford, J. (1970). Estimating fat reserves in the brushtailed possum,
Trichosurus vulpecula Kerr (Marsupalia : Phalangeridae). Australian
Journal of Zoology 18, 415–425. doi: 10.1071/ZO9700415
Barlow, N. D. (1994). Predicting the effect of a novel vertebrate
biocontrol agent: a model for viral-vectored immunocontraception of
New Zealand possums. Journal of Applied Ecology 31, 454–462.
Bell, B. D. (1981). Breeding and condition of possums Trichosurus vulpecula
in the Orongorongo Valley, near Wellington, New Zealand, 1966–1975.
In ‘Proceedings on the First Symposium on Marsupials in New Zealand’.
(Ed. B. D. Bell.) pp. 87–139. (Zoology Publications from Victoria
Bertschinger, H. J., Asa, C. S., Calle, P. P., Long, J. A., Bauman, K.,
DeMatteo, K., Jochle, W., Trigg, T. E., and Human, A. (2001).
Control of reproduction and sex-related behaviour in exotic wild
carnivores with the GnRH analogue deslorelin: preliminary
observations. Journal of Reproduction and Fertility. Supplement 57,
Bertschinger, H. J., Trigg, T. E., Jochle, W., and Human, A. (2002). Induction
of contraception in some African wild carnivores by down regulation
of LH and FSH secretion using the GnRH analogue deslorelin.
Reproduction 60(Suppl.), 41–52.
Bland, J. M., and Altman, D. G. (2004). The logrank test. British Medical
Journal 328, 1073. doi: 10.1136/bmj.328.7447.1073
Brochu, L., Caron, L., and Bergeron, J. (1988). Diet quality and body
condition of dispersing and resident voles (Microtus pennsylvanicus).
Journal of Mammalogy 66, 338–347.
Caughley, G. (1967). Growth, stabilisation and decline of New Zealand
populations of Himalayan thar (Hemitragus jemlahicus). Ph.D. Thesis,
University of Canterbury, Christchurch, New Zealand.
’Occhio, M. J., and Aspden, W. J. (1996). Characteristics of luteinizing
hormone (LH) and testosterone secretion, pituitary responses to LH-
releasing hormone (LHRH), and reproductive function in young bulls
receiving the LHRH agonist deslorelin: Effect of castration on LH
responses to LHRH. Biology of Reproduction 54,45–52. doi: 10.1095/
D’Occhio, M. J., Aspden, W. J., and Whyte, T. R. (1996). Controlled,
reversible suppression of estrous cycles in beef heifers and cows using
agonists of gonadotropin-releasing hormone. Journal of Animal Science
330 Wildlife Research C. A. Lohr et al.
D’Occhio, M. J., Sudha, G., Jillella, D., Whyte, T., Maclellan, L. J., Walsh, J.,
Trigg, T. E., and Miller, D. (1997). Use of a GnRH agonist to prevent
the endogenous LH surge and injection of exogenous LH to induce
ovulation in heifers superstimulated with FSH: a new model for
superovulation. Theriogenology 47, 601–613. doi: 10.1016/S0093-
D’Occhio, M. J., Fordyce, G., Whyte, T. R., Aspden, W. J., and Trigg, T. E.
(2000). Reproductive responses of cattle to GnRH agonists. Animal
Reproduction Science 60–61, 433–442. doi: 10.1016/S0378-4320(00)
Eymann, J. (2006). Management of urban common brushtail possums
(Trichosurus vulpecula). Ph.D. Thesis, Macquarie University, Sydney.
Eymann, J., Herbert, C. A., and Cooper, D. W. (2006). Management issues
of urban common brushtail possums Trichosurus vulpecula: a loved
or hated neighbour. Australian Mammalogy 28, 153–171.
Eymann, J., Herbert, C. A., Thomson, B. P., Trigg, T., Cooper, D. W., and
Eckery, D. C. (2007). Effects of deslorelin implants on reproduction in
the common brushtail possum (Trichosurus vulpecula). Reproduction,
Fertility and Development 19, 899–909. doi: 10.1071/RD07046
Frost, W. E., and Kipling, C. (1967). A study of reproduction, early life,
weight–length relationships and growth of pike, Esox lucius L., in
Windermere. Journal of Animal Ecology 36, 651–693. doi: 10.2307/2820
Gong, J. G., Campbell, B. K., Bramlet, T. A., Gutierrez, C. G., Peters, A. R.,
and Webb, R. (1996). Suppression in the secretion of follicle-stimulating
hormone and lutenizing hormone, and ovarian follicle development in
heifers continuously infused with a gonadotrophin-releasing hormone
agonist. Biology of Reproduction 55,68–74. doi: 10.1095/
Green, W. Q. (1984). A review of ecological studies relevant to
management the common brushtail possum. In ‘Possums and Gliders’.
(Eds A. P. Smith and I. D. Hume.) pp. 483–499. (Australian Mammal
Herbert, C. A., Trigg, T. E., and Cooper, D. W. (2004a). Effect of deslorelin
implants on follicular development, parturition and post-partum oestrus
in the tammar wallaby (Macropus eugenii). Reproduction 127, 265–273.
Herbert, C. A., Trigg, T. E., Renfree, M. B., Shaw, G., Eckery, D. C., and
Cooper, D. W. (2004b). Effects of a gonadotropin-releasing hormone
agonist implant on reproduction in a male marsupial, Macropus eugenii.
Biology of Reproduction 70
, 1836–1842. doi: 10.1095/
Herbert, C. A., Trigg, T. E., Renfree, M. B., Shaw, G., Eckery, D. C., and
Cooper, D. W. (2005). Long-term effects of deslorelin implants on
reproduction in the female tammar wallaby (Macropus eugenii).
Reproduction 129, 361–369. doi: 10.1530/rep.1.00432
Herbert, C. A., Trigg, T. E., and Cooper, D. W. (2006a). Fertility control in
female eastern grey kangaroos using the GnRH agonist deslorelin. 1.
Effects on reproduction. Wildlife Research 33,41–46. doi: 10.1071/
Herbert, C. A., Trigg, T. E., and Cooper, D. W. (2006b). Fertility in female
eastern grey kangaroos using the GnRH agonist deslorelin. 1. Effects on
reproduction. Wildlife Research 33,41–46. doi: 10.1071/WR04113
Hetherington, C. A. (2007). Use of deslorelin to manage an urban population
of brushtail possums (Trichosurus vulpecula). M.Sc. Thesis, The
University of Western Australia, Perth.
How, R. A., and Hillcox, S. J. (2000). Brushtail possum, Trichosurus
vulpecula, populations in south-western Australia: demography, diet
and conservation status. Wildlife Research 27,81–89. doi: 10.1071/
Isaac, J. L. (2005). Life history and demographics of an island possum.
Australian Journal of Zoology 53, 195–203. doi: 10.1071/ZO05018
Ji, W., Clout, M. N., and Sarre, S. D. (2000). Responses of male
brushtail possums to sterile females: Implications for biological
control. Journal of Applied Ecology 37, 926–934. doi: 10.1046/j.1365-
Kaplan, E. L., and Meier, P. (1958). Nonparametric estimation from
incomplete observations. Journal of the American Statistical
Association 53, 457–481. doi: 10.2307/2281868
Karten, M. J., and Rivier, J. E. (1986). Gonadotrophin-releasing hormone
analogue design. Structure–function studies toward the development of
agonists and antagonists: rationale and perspective. Endocrine Reviews 7,
44–66. doi: 10.1210/edrv-7-1-44
Kerle, J. A. (1984). Variation in the ecology of Trichosurus: its adaptive
signiﬁcance. In ‘Possums and Gliders’. (Eds A. P. Smith and I. D. Hume.)
pp. 115–128. (Australian Mammal Society: Sydney.)
Krebs, C. J., and Singleton, G. R. (1993). Indicies of condition for small
mammals. Australian Journal of Zoology 41, 317
–323. doi: 10.1071/
Lyne, A. G., and Verhagen, A. M. W. (1957). Growth of the marsupial
Trichosurus vulpecula and a comparison with some higher mammals.
Growth 21, 167–195.
McCance, R. A., and Widdowson, E. M. (1951). A method of breaking down
the body weights of living persons into terms of extracellular ﬂuid,
cell mass and fat, and some applications of it to physiology and
medicine. Proceedings of the Royal Society of London. Series B.
Biological Sciences 138, 115–130. doi: 10.1098/rspb.1951.0010
Melson, B. E., Brown, J. L., Schoenemann, H. M., Tarnavsky, G. K., and
Reeves, J. J. (1986). Elevation of serum testosterone during chronic
LHRH agonist treatment in the bull. Journal of Animal Science 62,
Munson, L., Bauman, C. S., Asa, C. S., Jochle, W., and Trigg, T. E. (2001).
Efﬁcacy of the GnRH analogue deslorelin for suppression of oestrous
cycles in cats. Journal of Reproduction and Fertility. Supplement 57,
Patt, M. (1995). The free-ranging population of the brush-tail possum,
Trichosurus vulpecula Kerr, in the urban environment of The Perth
Zoo; population estimates and management implications. B.Sc.
(Honours) Thesis, Edith Cowan University, Perth.
Pietsch, R. S. (1994). The fate of urban common brushtail possums
translocated to sclerophyll forest. In ‘Reintroduction Biology of
Australian and New Zealand Fauna’. (Ed. M. Serena.) pp. 239 –246.
(Surrey Beatty: Sydney.)
Pilton, P. E., and Sharman, G. B. (1962). Reproduction in the marsupial
Trichosurus vulpecula. Journal of Endocrinology 25, 119–136.
Pracy, L. T. (1974). ‘Introduction and Liberation of the Opossum into
New Zealand.’ 2nd edn. Information Series 45. (New Zealand Forest
Ramsey, D., Efford, M., Cowen, P., and Coleman, J. (2002). Factors
inﬂuencing annual variation in breeding by common brushtail possums
(Trichosurus vulpecula) in New Zealand. Wildlife Research 29,39–50.
Riney, T. (1955). Evaluating condition of free-ranging red deer (Cervus
elaphus), with reference to New Zealand. New Zealand Journal of
Science and Technology 36, 429–463.
Sanderson, K. J., and O’Driscoll, M. (1985). Breeding season of brushtail
possums, Trichosurus vulpecula (Marsupalia : Phalangeridae), in
Adelaide. Australian Mammalogy 8, 139–140.
SAS Institute Inc. (2008). ‘SAS Business Intelligence Software and Predictive
Analytics.’ Available at http://www.sas.com/index.html [Accessed 2
Scheele, F., Hompes, P. G. A., Lambalk, C. B., Schoute, E., Broekmans, F. J.,
and Schoemaker, J. (1996). The GnRH challenge test: a quantitative
measure of pituitary desensitization during GnRH agonist administration.
Horumon To Rinsho 44, 581–586.
Trigg, T. E., Wright, P. J., Armour, A. F., Williamson, P. E., Junaidi, A.,
Martin, G. B., Doyle, A. G., and Walsh, J. (2001). Use of a GnRH analogue
implant to produce reversible long-term suppression of reproductive
function in male and female domestic dogs. Journal of Reproduction
and Fertility. Supplement 57, 255–261.
Deslorelin in urban brushtail possums Wildlife Research 331
Tuyttens, F. A. M., and MacDonald, D. W. (1998). Fertility control: an
option for non-lethal control of wild carnivores. Animal Welfare 7,
Tyndale-Biscoe, C. H. (1973). ‘Life of Marsupials.’ (Edward Arnold:
Tyndale-Biscoe, C. H., and Renfree, M. (1987). ‘Reproductive Physiology of
Marsupials.’ (Cambridge University Press: New York.)
Viggers, K. L., Lindenmayer, D. B., and Spratt, D. (1993). The importance
of disease in reintroduction programmes. Wildlife Research 20, 687–698.
Viggers, K. L., Lindemayer, D. B., Cunningham, R. B., and Donnelly, C. F.
(1998). Estimating body condition in the mountain brushtail possum,
Trichosurus caninus. Wildlife Research 25, 499–509. doi: 10.1071/
Wayne, A. F., Ward, C. G., Rooney, J. F., Vellios, C. V., and
Lindenmayer, D. B. (2005). The life history of Trichosurus vulpecula
hypoleucus (Phalangeridae) in the jarrah forest of south-western
Australia. Australian Journal of Zoology 53, 265–278. doi: 10.1071/
Manuscript received 3 April 2008, accepted 18 February 2009
332 Wildlife Research C. A. Lohr et al.