Deslorelin implants control fertility in urban brushtail
possums (Trichosurus vulpecula) without negatively
influencing their body-condition index
Cheryl A. LohrA,C, Harriet MillsA, Helen RobertsonBand Roberta BenciniA
BPerth Zoo, 20 Labouchere Road, South Perth, WA 6151, Australia.
CCorresponding author. Email: email@example.com
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
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.
Wild brushtail possums (Trichosurus vulpecula) occur in large numbers in the grounds of Perth Zoo, Western
Brushtail possums (Trichosurus vulpecula) can be a pest in an
urban environmentwhere theyinteract 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
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 ~600000 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;
Eymann 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 4.7mg 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, 324–332www.publish.csiro.au/journals/wr
? CSIRO 200910.1071/WR080501035-3712/09/040324
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
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 18ha of Perth Zoo, Western Australia
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
(~20m 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.
per quarter. Each night, 60 Sheffield live-capture traps
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 0.1mm with vernier
callipers. The animals were weighed to the nearest 25g 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 0.1mm and the nomogram published by Lyne and
Verhagen (1957). In cases where the length of the head could
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 10mm?15mm 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
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
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
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
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 lengthðmmÞ ¼ 0:3211 ? ageðdaysÞ þ 7:0:
If the head-length measurement for a pouch young was
<7mm then it was assumed that the young had been born on
the day it was captured.
the difference in duration without young, between the treated
and untreated group(Kaplanand 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
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
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
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
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).
Table 2. The fate of the 60 female brushtail possums treated with the
No. with expired
Unknown – not caught after given implant
Last caught before November 2006
Caught in the final 6 months of study
(Nov. 2006–Apr. 2007)
Duration between births
Conception Birth 2
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 LactatingParous UndevelopedTotal
Untreated, caught prior April 2006
Untreated, caught April 2006 onwards
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
some females were subadults when they were treated with
deslorelin, whereas no male possums were treated with
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
Theseequations wereusedto predictthe
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
Body weight (g)
Head length (mm)
5060 7080 90
50 6070 8090
70 80 90100
Note that axes have different scales for adults and juveniles.
Linear regression of head length versus weight for four groups of
Duration without young (days)
duration without young in female possums.
Kaplan–Meier survival (of the contraceptive effect) plots for the
Deslorelin in urban brushtail possums
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 ~200g.
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
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
Duration without young (days)
(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 werenot trappedagainafter theywereimplanted and1diedthe dayitwas
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.
Box plots showing the variation in the duration without young
Average body condition index
Treated (n = 205)
Control (n = 291)
Male (n = 949)
possums at Perth Zoo. Implanted females were treated with deslorelin between the months of
December 2005 and March 2006.
The variation in the average body-condition index for male and female brushtail
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
(4.7mg) 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.
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
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,
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
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.
the Perth Zoo because there were many different food sources
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
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
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
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
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, theyare 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
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 populationsome chanceof contributingto
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 (4.7mg) can
prevent at least 80% of female brushtail possums from giving
birth for at least 381 days, without any negative impacts on the
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 whovolunteered tohelpwiththe fieldwork.Our thanksalsogo tothe
this paper were approved by The University of Western Australia Animal
Ethics Committee (05/100/514) and the Perth Zoo Animal Ethics Committee
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