Deslorelin implants control fertility in urban brushtail possums (Trichosurus vulpecula) without negatively influencing their body-condition index

Article (PDF Available)inWildlife Research 36(4):324–332 · January 2009with117 Reads
DOI: 10.1071/WR08050
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.
Deslorelin implants control fertility in urban brushtail
possums (Trichosurus vulpecula) without negatively
inuencing their body-condition index
Cheryl A. Lohr
A,C
, Harriet Mills
A
, Helen Robertson
B
and Roberta Bencini
A
A
School of Animal Biology, The University of Western Australia, 35 Stirling Highway, Crawley, WA 6008, Australia.
B
Perth Zoo, 20 Labouchere Road, South Perth, WA 6151, Australia.
C
Corresponding author. Email: cheryl.lohr@gmail.com
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 ghts with conspecics. A contraceptive implant, which
contains the gonadotrophin releasing hormone (GnRH) agonist deslorelin, could be a potential method of managing this
population. We tested the efcacy of the implant and its impact on the body-condition index of treated possums with
KaplanMeier 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.
Introduction
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 ghting
with conspecics. The injuries caused by these ghts 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;
DOcchio and Aspden 1996; Gong et al. 1996; Aspden et al.
1997; DOcchio 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
(DOcchio 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 4.7 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
untreated animals.
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
CSIRO PUBLISHING
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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 ODriscoll 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 efcacy 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.
Study site
The study was conducted across ~18 months (December 2005
April 2007) within the 18 ha of Perth Zoo, Western Australia
(31
58
0
S, 115
51
0
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 elds.
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 g 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 exuosa).
Trapping protocol
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 Shefeld live-capture traps
(220 220 550 mm,ShefeldWireProducts,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.
Animal handling
Each animal caught was marked with two numbered ngerling
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 signicantly
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 classied as mature adults as per
Wayne et al. (2005). The pouch of all females captured was
examined and classied 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).
Efcacy 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
deslorelin (((6-D-tryptophan-9-(N-ethyl-L-prolinamide)-10-
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 rst 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.
Analysis
A KaplanMeier 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 KaplanMeier 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 nal outcome, i.e. duration without young,
was observed. Censored data occurred because some possums
were not captured in the nal 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 rst 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
rst 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 rst young
(140 days) until their nal 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
possums 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
(Fig. 2):
Table 2. The fate of the 60 female brushtail possums treated with the
deslorelin implant
Fate n No. with expired
implant
Unknown not caught after given implant 15 n.a.
Dead 8 0
Last caught before November 2006 17 1
Caught in the nal 6 months of study
(Nov. 2006Apr. 2007)
20 4
Birth 1
Duration between births
Nominal weaning
140 days
Duration between
young
Suckling young
Conception Birth 2
17 days
Fig. 1. Relationship between birth dates, weaning date and the duration
between young.
Table 1. The number of treated and untreated female brushtail possums and their reproductive state when they were rst 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.
Expectedweight for:
adultfemale¼664:1þ26:98averageheadlengthðmmÞ;
adultmale¼169:49þ17:92averagehead lengthðmmÞ;
subadultfemale¼1931:59þ39:58averagehead lengthðmmÞ;
subadultmale¼1915:56þ40:49averagehead lengthðmmÞ:
The slope and the intercept for the regression equations for
adult male and female possums were signicantly ( P < 0.01)
different and also differed signicantly (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
data points.
Results
Efcacy of the deslorelin implant
The deslorelin implant signicantly increased the duration
without young. The Kaplan
Meier survival curves for duration
without young in treated and untreated possums were
signicantly (P < 0.001) different (Fig. 3).
Some of the implants failed to prevent reproduction in treated
female possums. The rst 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 nal 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
0
500
1000
1500
2000
Body weight (g)
Head len
g
th (mm)
0
500
1000
1500
2000
2500
3000
0
500
1000
1500
2000
50 60 70 80 90
50 60 70 80 90
70 80 90 100
70 80 90 100
0
500
1000
1500
2000
2500
3000
(a)
(b)
(c)
(d )
Fig. 2. Linear regression of head length versus weight for four groups of
possums: (a) adult female, n = 489, r
2
= 0.15; (b) adult male, n = 813, r
2
= 0.07;
(c) subadult female, n = 51, r
2
= 0.80; and (d) subadult male, n = 136, r
2
= 0.57.
Note that axes have different scales for adults and juveniles.
Duration without youn
g
(days)
Cumulative survival
500
400
3002001000
1.0
0.8
0.6
0.4
0.2
0.0
Treated-censored
Untreated-censored
Treated
Untreated
Group
Fig. 3. KaplanMeier 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 signicantly
(P < 0.001) shorter for untreated than treated possums. The
average duration between young increased from an average of
33.2 days (range 4212) to 148 days (range 2483; Fig. 4). In
total, 16 (of 20) treated possums captured in the nal 6 months
of the study (192483 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
381 days.
Impact of the deslorelin implant
We found a signicant (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 signicant
(P < 0.01) effect of time on the BCI of possums and there was
a signicant (P < 0.01) interaction between treatment and time
(Fig. 5), although treatment alone did not signicantly 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 signicant (P = 0.2).
Female possums not treated with deslorelin started with a
BCI of 1.08. Their bodyweight then uctuated rapidly, with
a minimum BCI of 0.88 occurring in September 2006. There
was a signicant (P = 0.01) difference between the minimum and
maximum BCI for untreated possums (Fig. 5).
Discussion
Efcacy 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 nal
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
g
(days)
Treated
Untreated
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.
0.85
0.90
0.95
1.00
1.05
1.10
Dec 05
Jan 06
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Feb 07
Mar
Apr
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 nal 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 denitively 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 denitively conrm 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, ve of the deslorelin implants failed to prevent
reproduction in treated possums. These ve 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 nal 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 calcied 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 nal 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 signicantly. However,
the BCI for treated possums did not uctuate throughout
the year as it did for untreated possums.
The BCI for untreated females may have uctuated
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 signicantly 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 rst 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 difcult, 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
bins.
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 methodology.
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 inuence
condition indices. For example, muscle mass may have an impact
on the condition of a possum, particularly in males who may ght
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 conrmation,
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
veterinary expertise.
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
reserves.
Acknowledgements
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 eld 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
(PZRE 200525).
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http://www.publish.csiro.au/journals/wr
    • "The presence/absence of pouch young and the response to GnRH challenges, confirmed the19, 7, 15, 14, 6, 11, 12 "
    [Show abstract] [Hide abstract] ABSTRACT: Populations of Australian marsupials can become overabundant, resulting in detrimental impacts on the environment. For example, the threatened black-flanked rock-wallaby (Petrogale lateralis lateralis) has previously been perceived as overabundant and thus ‘unwanted’ when they graze crops and cause habitat degradation. Hormonally-induced fertility control has been increasingly used to manage population size in other marsupials where alternative management options are not viable. We tested whether deslorelin, a superagonist of gonadotropin-releasing hormone (GnRH), would suppress reproduction in free-living adult female rock-wallabies without adversely impacting body condition. We trapped, synchronised reproduction and allocated female rock-wallabies to a placebo implant (control, n = 22), one (n = 22) or two (n = 20) subcutaneous implants of deslorelin. Females were then recaptured over the following 36 months to monitor reproduction, including Luteinising Hormone levels, and body condition. Following treatment, diapaused blastocysts reactivated in five females and the resulting young were carried through to weaning. No wallabies treated with deslorelin, conceived a new young for at least 27 months. We did not observe adverse effects on body condition on treated females. We conclude that deslorelin implants are effective for the medium-term suppression of reproduction in female black-flanked rock-wallabies and for managing overabundant populations of some marsupials.
    Full-text · Article · Dec 2015
    • "However, using fertility control must meet four criteria to be an acceptable means of achieving population regulation (Nettles 1997; Fagerstone et al. 2002): (1) it must not adversely affect the health of the treated animal, (2) it must affect the target species only, (3) it can be limited to welldefined populations, and (4) it does not alter the gene pool of the species (Anon 1994). Research on contraceptive GnRH implants indicates that they meet the first three criteria, and rate better on the fourth than lethal alternatives (Herbert et al. 2006; Lohr et al. 2009). However, there are some important aspects to consider on the genetic impacts of fertility control. "
    [Show abstract] [Hide abstract] ABSTRACT: Hormonally-induced fertility control is increasingly used to manage overabundant wildlife where political or ethical considerations exclude culling or translocations. By altering reproductive rates, fertility control risks accelerating the loss of genetic variation from isolated wildlife populations if control is biased against particular genotypes. This is most likely in spatially structured populations, but information on patterns of genetic exchange can be difficult to ascertain by observation or trapping. Fertility control was proposed to manage an isolated but overabundant population of the black-flanked rock-wallaby, Petrogale lateralis lateralis, at Mt Caroline Nature Reserve, south-western Australia. This species typically inhabits large granite outcrops, where refuges within outcrops are often patchily distributed, and there is strong potential for population structure. We used fine-scale analysis of microsatellite DNA variation to establish the pre-fertility control extent of population structuring and the relative extent of gene flow in males and females in an isolated population of black-flanked rock-wallabies. Bayesian model-based clustering and fixation statistics revealed that overall gene flow is extensive, but two areas of refuge were genetically distinct from distant neighbours. Spatial autocorrelation analyses indicated that female-mediated gene flow was less extensive than male-mediated gene flow. Our results indicate that it would be desirable to apply fertility control to female rock wallabies within defined genetic isolates, but fertility control could be applied in a spatially random manner to males.
    Full-text · Article · Oct 2014
    • "Subcutaneous implants that release fertility control agents into an animal over a sustained period of time have been successfully employed to induce infertility for 1–5 years in a variety of wildlife species (e.g. Plotka and Seal 1989; Nave et al. 2002a Nave et al. , 2002b Coulson et al. 2008; Lohr et al. 2009). Bio-bullets are biodegradable projectiles used to administer remotely various veterinary substances (DeNicola et al. 2000). "
    [Show abstract] [Hide abstract] ABSTRACT: As human populations grow, conflicts with wildlife increase. Concurrently, concerns about the welfare, safety and environmental impacts of conventional lethal methods of wildlife management restrict the options available for conflict mitigation. In parallel, there is increasing interest in using fertility control to manage wildlife. The present review aimed at analysing trends in research on fertility control for wildlife, illustrating developments in fertility-control technologies and delivery methods of fertility-control agents, summarising the conclusions of empirical and theoretical studies of fertility control applied at the population level and offering criteria to guide decisions regarding the suitability of fertility control to mitigate human–wildlife conflicts. The review highlighted a growing interest in fertility control for wildlife, underpinned by increasing numbers of scientific studies. Most current practical applications of fertility control for wild mammals use injectable single-dose immunocontraceptive vaccines mainly aimed at sterilising females, although many of these vaccines are not yet commercially available. One oral avian contraceptive, nicarbazin, is commercially available in some countries. Potential new methods of remote contraceptive delivery include bacterial ghosts, virus-like particles and genetically modified transmissible and non-transmissible organisms, although none of these have yet progressed to field testing. In parallel, new species-specific delivery systems have been developed. The results of population-level studies of fertility control indicated that this approach may increase survival and affect social and spatial behaviour of treated animals, although the effects are species-and context-specific. The present studies suggested that a substantial initial effort is generally required to reduce population growth if fertility control is the sole wildlife management method. However, several empirical and field studies have demonstrated that fertility control, particularly of isolated populations, can be successfully used to limit population growth and reduce human–wildlife conflicts. In parallel, there is growing recognition of the possible synergy between fertility control and disease vaccination to optimise the maintenance of herd immunity in the management of wildlife diseases. The review provides a decision tree that can be used to determine whether fertility control should be employed to resolve specific human–wildlife conflicts. These criteria encompass public consultation, considerations about animal welfare and feasibility, evaluation of population responses, costs and sustainability.
    Full-text · Article · Jan 2014
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