Estimating the Age of Wild Rock-wallabies by Dental Radiography: a Basis for Quantifying the Age Structure of a Discrete Colony of Petrogale assimilis
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Wildl. Res., 1995,22,547-59
Estimating the Age of Wild Rock-wallabies by
Dental Radiography: a Basis for Quantifying the Age
Structure of a Discrete Colony of Petrogale assimilis
R. ~ e l a n e f l ' and H. ~ a r s h ~
*~e~artment of Zoology, James Cook University of North Queensland,
Townsville, Qld 48 11, Australia.
B~epartment of Tropical Environment Studies and Geography, James Cook
University of North Queensland, Townsville, Qld 481 1, Australia.
'present address: Department of Zoology, University of Melbourne, Parkville,
Vic. 3052, Australia.
Abstract
The teeth of rock-wallabies from a wild population of Petrogale assimilis at 'Black Rock' in north
Queensland were radiographed in the field at intervals over two years in order to estimate their age. The
gross morphology of the dentition conesponds with that of browsing macropodids. The pattern of tooth
eruption conforms to the general macropodid model. Although P. assimilis reach sexual maturity at 18-24
months, full molar eruption apparently does not occur until individuals are at least seven years old. The
plane of the occlusal surface is only slightly C U N Most
~ of the cheek teeth are in occlusion at once. In all,
70% of wallabies examined at 'Black Rock' had a full complement of cheek teeth. P4 is well developed and
rarely lost. These attributes collectively suggest that the small amount of mesial movement of the molars
constitutes advanced mesial drift rather than true molar progression. Molar eruption is a reliable index of
age until full eruption occurs. Molar movement is likely to be of only limited use in estimating the age of
animals with a full complement of cheek teeth. The age structure of the colony at 'Black Rock' suggests a
low rate of recruitment into the adult population of P. assirnilis for most of the 1980s.
Introduction
Most of the methods used to estimate the age of macropodids have been based on dental
characteristics. Research has concentrated on species of Macropus (Sharman et al. 1964;
Kirkpatrick 1964, 1965; Ealey 1967; Kirkpatrick and Johnson 1969; Maynes 1972; Newsome et
al. 1977; Poole et al. 1982). There have been very few age determination studies of other
macropodids. The only studies of which we are aware are those on Setonix brachyurus (Shield
1968), Petrogale xanthopus (Poole et al. 1985), Petrogale lateralis (Kinnear et al. 1988) and P.
assimilis (Bell et al. 1989).
In all the macropodids studied, the molar teeth erupt sufficiently slowly for the ages of
individuals to be estimated well into adulthood (Kirkpatrick 1985). Relative to reference points
in the skull, the cheek teeth of all species show some movement forward with age (mesial drift).
Grazing species exhibit molar progression, a mechanism whereby less than the full complement
of cheek teeth is maintained in the tooth row and worn teeth are replaced by unworn teeth
(Sanson 1978, 1989). In several macropodids, the extent of molar progression has been used as
the basis for estimating the age of individuals, especially those in which all the teeth have
erupted (e.g. Sharman et al. 1964; Kirkpatrick 1964; Kirkpatrick and Johnson 1969; Newsome
et al. 1977).
This paper describes a method for radiographing rock-wallabies in the field as a basis for
determining age from molar eruption status. The pattern of eruption of the cheek teeth of wild
P. assimilis corresponds to the general macropodid model. The small amount of mesial
movement of the molars constitutes advanced mesial drift rather than true molar progression.
The age structure of the colony, revealed by this study, indicates a low level of adult recruitment
throughout the 1980s.R. Delaney and H. Marsh
Materials and Methods
Study Animal
Petrogale assimilis is a medium sized rock-wallaby that is classified as a homomorph according to the
criteria of Jarman (1989). At 'Black Rock', adult females weigh an average of 3.54 kg, adult males average
4.07 kg (Delaney 1993).
Capture and Tagging
Petrogale assimilis were trapped during 34 monthly field trips between June 1986 and June 1989 at
'Black Rock' (19"05'S, 14t027'E, elevation 619 m), an isolated sandstone outcrop in savanna woodland
about 250 krn west of Townsville. The trapping techniques and field site are described by Delaney and
De'ath (1990) and Horsup and Marsh (1992). Between 12 and 40 (mean = 27) adults were caught per trip.
The ears of emergent pouch young and older animals were tattooed. Adults (n = 79) were also marked with
fingerling tags. Independent visual observations made during a concurrent behavioural study suggested that
we succeeded in marking a high proportion of animals in the colony; only one rock-wallaby older than a
pouch young (an adult) remained untagged by June 1989 (A. Horsup, personal communication).
Anaesthesia
To reduce stress, it was necessuy to anaesthetise the wallabies before radiographing them. After initial
trials (Delaney 1993) the barbiturate thiopentane sodium (Abbot: Pentothal, maximum dosage of 12.5 mg
kg-') was routinely administered to effect via a lateral tail vein. Maximum dosages were rarely used.
Induction was rapid and the depth of anaesthesia controlled and no mortalities occurred during anaesthesia.
We later replaced Pentothal by another barbiturate, methohexitone sodium (Lilly: Sodium Brietal,
maximum dose 8.3 mg kg-') because the use of the latter was associated with less tissue necrosis.
After each animal had been radiographed, it was placed in a cage lined with hessian bags and monitored
as it recovered from the anaesthetic. Each animal was left in a comfortable position and regularly checked
and moved as it recovered. On cold days, two animals were placed together in a cage and a domestic heater
used for additional warmth. Recovery times were always less than 1 h.
Radiography
During each field trip from June 1987, the portable radiography machine (Atomscope 903-A Mikasa,
Japan) was installed in an open walled, dirt-floored shed and powered by a portable 240-V generator
(Honda ES3600). The radiography unit was mounted on a steel frame over a table of adjustable height. The
two operators wore radiation safety badges and standard lead safety aprons and gloves. Adjustable vertical
lead sheeting was placed between the animal and the operators during radiation exposure. The radiation
exposures measured by the safety badges were checked regularly by the Queensland Department of Health.
We used cassettes containing Kodak Min-R X-ray film and rare earth intensifying screens. In the field,
film was transferred between a light-tight box and the cassettes as required via standard darkroom bags.
In contrast to most other stationary machines, the kilo-voltage (kV) and milli-Amp (mA) settings were
locked together on the Atomscope. Initially, the kV m ~ - 'and film focal distance were kept constant and
only the exposure time was varied. These settings were then modified by trial and error until the optimal
exposures were obtained (Table 1).
One operator held the anaesthetised animal while the other operated the radiography unit. Two sets of
radiographs were taken for each animal: a dorso-ventral radiograph for molar progression and a lateral
Table 1. Exposure times for radiographs of P. assimilis of
various age classes at 80 kV per 20 mA and a film focal
distance of 900 mm
- - - -- - -
Age class Head length (mm) Exposure times (s)
Lateral Dorso-ventral
Juvenile 60-75 0.20 0.30
Subadult 70-95 0.35 0.40
Adult 95-1 15 0.45 0.50Age Estimation of Rock-wallabies
radiograph for molar eruption. The lateral view was also useful in interpreting the individual teeth in the
dorso-ventral radiographs. The dorso-ventral radiographs were taken with the animal placed in ventral
recumbency with its lower jaw on the cassette. The operator pressed down firmly on the vertebrae at the
base of the skull, forcing the occlusal plane of the teeth to be parallel to the X-ray film. Excellent lateral
radiographs were obtained by the operator adjusting and supporting the animal's head so that the sagittal
plane of the head was parallel to the radiographic plate. The exposed film was developed on return to the
laboratory.
Adult wallabies were typically X-rayed every six months from June 1987. Younger wallabies were
radiographed more regularly. Known-age animals were first caught as pouch young; their ages at first
capture were estimated from head and pes measurements (Delaney and De'ath 1990).
Molar Eruption
Cheek tooth nomenclature follows Thomas (1888, cited in Sanson 1980). Molar eruption stage (MES)
was scored from lateral radiographs of 82 rock-wallabies using the method of Newsome et al. (1977). Each
fully erupted molar was scored as 1. A partially erupted molar was categorised as either 0.2,0.4,0.6 or 0.8
as an index of its eruption status. Thus, an animal with the first and second molars fully erupted and the
anterior loph of the third molar just through the gum had a MES of 2.4.
Measurement of Molar Movement
Before the molar index could be scored, the average length of each molariform tooth was determined
from skulls and molar teeth of P. assimilis from 'Black Rock' (n = 11) and the following museum
collections: Australian National Wildlife Collection, CSIRO, Canberra (n = 80); Queensland Museum,
Brisbane (n = 16); and the Australian Museum, Sydney (n = 4). Measurements were made using vernier
callipers correct to 0.1 mm.
The molar index was scored as the number of upper molar teeth that were forward of the reference line
drawn across the anterior rim of the orbits (Kirkpatrick 1964). For example, a molar index of 3.3 indicated
that the first three molars and 0.3 of the fourth molar were anterior to the reference line. If M4 was forward
of the reference line, a molar index of more than 4 was calculated by assuming the presence of a fifth molar
of equal length to the fourth (Kirkpatrick 1964). To aid calculation of the proportion of the last molar, a
perspex plate etched with lines spaced at intervals corresponding to 20% of the average length of each
molar (calculated on the basis of the measurements of teeth in the skulls) was placed over the required skull
(12 = 21) or dorso-ventral radiograph (n = 452). In practice, the mesial edges of individual teeth were
obscured by the superimposition of bone and teeth, and it was easier to score the position of the distal edge
of the teeth spanning the reference line and to subtract each of these scores from one. There was good
agreement between independent estimates of molar movement and molar eruption stages obtained directly
from skulls and from the corresponding radiographs.
Methodological Limitations
The data from known-age animals are limited both by the small sample sizes and the length of this
study. Despite the 2 4 % sexual dimorphism in most morphological measurements in adult P. assimilis at
'Black Rock' (Delaney 1993), we were unable to consider the sexes independently in our analysis because
of small sample sizes. Sexual dimorphism does occur in the pattern of molar progression and the rate of
molar movement in Macropus agilis (Newsome et al. 1977), a more dimorphic species than P. assimilis
(see Jarman 1989 and Delaney 1993). Only three years (a small proportion of the lifespan of a rock-
wallaby) could be reported because of the time constraints on this study.
Rainfall Measurements
Rainfall was measured at 'Black Rock' when field workers were present (12-15 days per month) from
January 1988. The monthly rainfall records of 'Lyndhurst' homestead (16.5 km SSW) were compared with
'Black Rock' data. There was a strong linear relationship between the measured monthly rainfall at
'Lyndhurst' and 'Black Rock' (3 = 0.91), indicating that 'Lyndhurst' records were a reliable index of
rainfall at 'Black Rock'. The 'Lyndhurst' rainfall records for the years 1970-71 to 1988-89 (July-June, so
that each wet season was contained within one year) were considered in relation to the average annual
rainfall based on the corresponding records from 1886 to date, in order to place the climate of the 1970s and
1980s at 'Black Rock' in a long-term perspective.R. Delaney and H. Marsh
Results
General Pattern of Dentition
The dentition of P. assimilis corresponds to the normal macropodid pattern as described by
Kirkpatrick (1978). Young animals have two deciduous molariform teeth, P3 and dp4, which
are replaced by the sectorial permanent pre-molar, P4, in the adult wallaby. The adult dental
formula is I ~ , P ' , M ~No
~ .canine teeth were observed in any skull or radiograph.
The average length of each cheek tooth in the measured skulls is summarised in Fig. la. P4
was always longer than M1 in individual animals. Neither sex ( F = 2.73, d.f. = 1, 703, P =
0.100) nor side (left or right) (F = 0.01, d.f. = 1,703, P = 0.909) had a significant effect on the
length of individual cheek teeth (3-way ANOVA). The interactions [sex by side ( F = 0.07, d.f. 7
1,703, P = 0.792), sex by tooth ( F = 0.14, d.f. = 4,703, P = 0.965) and tooth by side ( F = 0.10,
d.f. = 4,703, P = 0.983)] were not significant.
The length of M1 did not decrease significantly with age (based on MES) (1-way ANOVA;
F = 1.48, d.f. = 937, P = 0.1765). However, the size of M1 was much more variable in older
wallabies than in young animals (Fig. lb). This may be an artefact of the small sample of
wallabies with less than a full complement of teeth (Fig. lb). Particularly in older animals, some
M1 showed obvious wear and tended to be (1) worn to the roots, (2) displaced labially, or (3)
markedly reduced in length (Fig. 2b-d). Such extreme changes were not observed in any other
cheek teeth. However, R. Close (unpublished data) has observed some museum specimens with
very worn M2.
To be sure that only those teeth that were lost during life and not after collection for
museums were included in our estimates of tooth loss, all museum specimens were excluded
from these calculations. The upper and lower jaws of 78 adult wallabies (MES a 2.2) from
'Black Rock' were examined from radiographs. Eight (2.56%) P4 teeth from seven animals, 14
M1 (4.49%) from 14 animals and three M2 (0.75%) from two animals were missing. Most
animals (70.5%) had a full complement of teeth. No animal had lost more than two teeth.
The cheek teeth of most P, assimilis apparently remain in occlusion throughout life (Fig. 2a,
b, d). However, in some of the study animals, the fully erupted posterior molars were not
occluded while the worn anterior teeth were not occluded in others (Fig. 2c). Imaginary lines
through the crests of both upper and lower cheek teeth were only slightly curved, and the
ventral margin of the mandibular ramus was parallel to the dorsal margin (Fig. 2a-d). The roots
of teeth in some animals with full MES were deflected slightly backwards (Fig. 26).
Molar Eruption
Extensive sets of observations were available from five known-age animals (2 captive reared
females and 3 wild males). Shorter sets of observations were available from another 12 animals
(1 captive female and 1 captive male, and 3 wild females and 7 wild males) (for details, see
Delaney 1993).
Two of the three pairs of upper incisors were fully erupted and the third was erupting by the
time of permanent pouch emergence. The first molar was fully erupted at 226-268 days (n = 2).
P4 erupted between the time of full eruption of M2 and M3 (n = 8). Two captive females
reached reproductive maturity (eversion of the teats) at MES of 2.0 (20 months) and 2.6 (23
months). Full molar eruption had not occurred in any of the known-age animals at the end of
this study. The oldest known-age animal (31 months) was a captive animal with a MES of 2.6.
Two wild animals did not have a full complement of cheek teeth when at least 4.5 years old.
A logarithmic equation was used to describe the MESS of wallabies aged between 175 and
948 days. Individual curves were constructed from the five animals measured extensively. The
fits assessed by residual plots were satisfactory. A single curve of the same functional form was
fitted to the pooled data for these animals. The individual curves were significantly different
from the pooled curve ( F = 9.740, d.f. = 6,17, P = 0.0001), indicating individual differences in
the timing of eruption. However, in view of the small sample of animals that were extensivelyAge Estimation of Rock-wallabies
P4 M1 M2 M3 M4
Cheek tooth
3.0
Molar eruption stage
Fig. 1. Length of cheek teeth measured from skulls of P. assirnilis (a) for each cheek tooth (mean
2 95% C.I.) and (b) for the length of M1 measured in skulls of different molar eruption stages.
measured, a pooled curve was fitted to all known-age animals (where M is MES and A is age in
days) and its form was
M = 1.2784 * In A - 6.0512
(?= 0.90, range = 175-948 days).Fig. 2. Comparison of lateral radiographs of four P. ussirnilis from 'Black Rock'. (a)9 Molar eruption stage (MES) 3-2, (b) 9 MES 4.0,(c) d MES 4-0,(6) 9 MES 4.0; showing variability in the size and wear of M1 (mowed); the occlusion of cheek teeth; and backward deflection of the roots of some molars (see 4. The dotted lines represent imaginary lines through the crests of both upper and lower cheek teeth.
Age Estimation of Rock-wallabies
The inverse of Equation 1 should not be used to predict age from MES as the error structures
of the inverse equation and that of the curve independently derived for predicting age from
MES are different. Since for most wild populations, age is predicted from MES, the data from
the 13 known-age wild animals were used to generate a second pooled curve to describe the
relationship between age (A) in days and MES (M):
In A = 0.8587M + 4.9629
(2= 0.90;range = 175-581 days).
Molar eruption stages potentially estimate age very well until full molar eruption (Fig. 3a).
The asymptotic age for full molar eruption predicted by Equation 2 is 9 years 2 months (95%
Known age (years)
Known age (years)
Fig. 3. Raw data and regression curves for P. assirnilis of known-age showing ( a ) molar
eruption stage and (b) molar index. 0,'Black Rock'; A, data from Bell et al. (1989).R. Delaney and H. Marsh
C.I.: 8 years 2 months-10 years 1 month). Our estimates of rock-wallaby age were converted to
months so that we could derive a curve of the same functional form as that used by Bell et al.
(1989) for captive P. assirnilis. The values of the parameters of our curve were within the 95%
confidence limits of those of Bell et al. (1989) (J. Bell, personal communication) and hence are
not significantly different. As the curve generated by Bell et al. (1989) was based on more
complete data for older animals, we used their curve for age prediction (Equation 3) with In
rather than to log,,, as in Bell et al. (1989) as the former is more appropriate for mathematical
modelling:
In A = 0.670M + 1.727
(A is age in months, 2 = 0-97; range = 6-80 months).
Molar Movement
Insufficient data were available from known-age animals to compare individual curves
describing the change in the molar index (I)with age. Accordingly the data from all 17 known-
age animals from 'Black Rock' (6 females and 11 males) were pooled and a logarithmic curve
fitted (Fig. 3b):
I = - 4.2992 + 0.91463 * In A
(A is age in months, ? = 0.92).
No significant gender difference occurred in the estimated annual rate of molar movement
for known-age animals from 'Black Rock' (F = 0.148, d.f. = 2,39, P = 0.8629).
There were 23 animals from 'Black Rock' with fully erupted molars for which there were
good sequences of radiographs over at least one year, including 14 animals whose molar index
increased by 0 . 2 4 3 ; seven animals with an increase of 0.1 and two with no change in molar
position over the study period. Thus, the forward movement of teeth in this species is variable
and slight after full molar eruption has occurred. There was a strong correlation between molar
index and MES (2= 0.97, n = 66).
Age Structure
The year of birth of 82 radiographed animals at 'Black Rock' was estimated using Equation
3. A minimum age of seven years at first capture was assigned to animals whose teeth were
fully erupted when they were first caught on the assumption that tooth movement and eruption
are constant between individuals and over time at this site. This assumption is an untested
simplification. We have no data on the relationship between diet and dental status in P. assirnilis
apart from our observation that there is no significant difference between the relationship
between MES and age in wild and captive animals. Most animals at 'Black Rock' had full
MESs although this occurs some five years after reproductive maturity. Their MESs indicated
that most of the animals examined for the first time as adults (44 of 62) were born in or prior to
1981 (Fig. 4a, b).
Rainfall
The 99 years of 'Lyndhurst' records indicate that the annual rainfall in this region is highly
variable, ranging from 189 mm to 1612 mm (mean = 738 mm; C.V. = 40%). Our trapping
period was part of a time of below-average rainfall that had persisted since the 1981-82 wet
season except for 1983-84 (Delaney 1993). In contrast, in only two years during the 1970s was
there below-average rainfall (1977-78 and 1979-80).Age Estimation of Rock-wallabies
0 1 2 3
MES Score
1980 1981 1982 1983 1984 1985 1986 1987 1988
Year of birth 7
Fig. 4. (a) Frequency of rock-wallabies of different MES (Molar eruption stage) sampled within the
'Black Rock' colony between June 1987 and June 1989. Note the lack of young adult animals. The arrow
marks the MES at sexual maturity. (b) The estimated year of birth of animals whose ages are illustrated in
a. Animals with fully erupted molars have been assumed to be at least 7 years old. Hatching represents
individuals born in, or before, the given year. The arrow indicates the year sampling began at 'Black Rock'.
Discussion
Molar Movement in P . assimilis ~elativeto other Macropods
Sanson (1989) regarded the variations in dental morphology and the degree of molar
movement within the Macropodoidea as adaptations to diet and feeding. He described two basicR. Delaney and H. Marsh
patterns of premolar and molar morphology in the Macropodidae: (1) browsers, species that eat
non-abrasive, low-fibre plants such as herbs and forbs (but rarely grasses), do not show molar
progression and retain P4; and (2) grazers, species that feed mostly on abrasive, high-fibre
plants, such as grasses, exhibit molar progression and lose P4 (Sanson 1978, 1980). There is a
range of dietary preferences between these extremes. On the basis of dental characteristics,
Sanson (1989) placed most members of the genus Petrogale at the grazer end of the browser
grade. P. xanthopus, however, was placed at the browser end of the intermediate browser/grazer
grade.
The gross morphology of the dentition of P. assirnilis at 'Black Rock' (Fig. 2) is similar to
that of the browsing macropodids such as Wallabia bicolor and Thylogale spp. (Sanson 1980).
P4 is a well-developed sectorial tooth that erupts late during the eruption of M3. It is longer than
M1 and usually retained throughout life. The mandibular rami are relatively straight, with only
slight curvature along the occlusal surface. Most of the tooth row is in occlusion. However, a
few P. assirnilis from 'Black Rock' with very worn teeth (Fig. 2b-d) showed some of the
features that are characteristic of grazing macropodids exhibiting molar progression (e.g.
Macropus; Sanson 1982): (1) the roots of the molar teeth projected posteriorly at an acute angle
into the jaw; (2) P4 was lost; (3) P4 and M1 were worn and no longer occluded. Further, M1
was often displaced labially behind P4, indicating a latent capacity for progression. Measurable
amounts of molar movement occurred in some animals from 'Black Rock' but not in others. P.
assirnilis thus shows features of advanced mesial drift.
These observations support Sanson's (1989) placement of Petrogale at the grazer end of the
browser grade. They also accord with the diet of P. assirnilis at 'Black Rock', where it eats
mainly forbs and browse (Horsup and Marsh 1992). On average, grass comprised only 9% of
the diet. It was eaten in significantly higher proportions in the early wet season when new
growth was available.
Phylogenetically, Petrogale is closer to Thylogale and to Setonix than to Macropus
(Baverstock et al. 1989). Sanson (1989) classified Thylogale and Setonix as browsers. Our
results support the conclusions of other workers regarding the evolution of Petrogale.
Maximum Lifespan
Data on the maximum lifespan of P. assirnilis are sparse. W. Davies tagged an unknown
number of rock-wallabies at 'Black Rock' in 1973-76. When we commenced studies at this site
ten years later, only one of these tagged individuals was confirmed alive. As it was tagged as an
adult, it lived for at least 12 years (W. Davies, personal communication). An individual reared
in captivity lived to almost 8 years (J. Bell and R. Close, personal communication) and a second
semi-captive P. assirnilis is reported to have lived for 11 years 2 months, but this age remains
unconfirmed (P. Johnson, personal communication). One female of the closely related species,
P. rnareeba, lived in captivity for almost 10 years (R. Close, unpublished data). On the basis of
a trapping study, Kinnear et al. (1988) reported that some Petrogale lateralis lived for at least
12 years. Robinson et al. (1994) reported wild P. xanthopus living until at least 10 years old.
Estimating the Age of P. assimilis
Molar eruption has proved to be a valuable basis for estimating the age of P. assirnilis well
into adulthood, despite significant individual differences in the timing of eruption of specific
teeth. Full molar eruption is not attained until at least 6.5 years (Bell et al. 1989). R. Close
(unpublished data) reports that the molars of another captive P. assirnilis were fully erupted at 7
years and 11 months. The female offspring of a cross between P. rnareeba and P. assirnilis had
a MES of 3.6 when aged 6 years while the molars of a female P. rnareeba were still not fully
erupted (MES 3.8) at age 9 years and 10.5 months (R. Close, unpublished data). Kinnear et al.
(1988) reported that wild P. lateralis with four erupted molars were at least 6 years old.
Although there were no significant differences between the data from captive animals (Bell
et al. 1989) and wild animals from 'Black Rock' in the ages of molar eruption, this conclusionAge Estimation of Rock-wallabies
should be treated with caution as the curve generated for the 'Black Rock' animals had to be
extrapolated.
After all teeth have erupted, the forward movement of teeth is slight in P. assirnilis, a result
more consistent with mesial drift than molar progression. This result suggests that
measurements of molar movement have limited application for age determination in this
species, despite their use for age estimation in P. xanthopus (Poole et al. 1985). However, in his
review, Sanson (1989) considered that P. xanthopus tends slightly more towards the grazer
grade than other Petrogale spp. This conclusion is consistent with faecal analyses that suggest
that P. xanthopus in the Flinders Range of South Australia consumes more grass than P.
assirnilis at 'Black Rock' (Copley and Robinson 1983; Horsup and Marsh 1992).
Age Structure of the Colony
Assuming that the age distribution of the trapped animals was representative of the adult
population (which seems probable in view of the very high proportion of tagged animals), the
age structure of independent rock-wallabies in the 'Black Rock' colony during this study was
skewed in favour of very old animals (27 years). We trapped surprisingly few subadults or
young adults (Fig. 4a), a result that is consistent with concurrent behavioural observations
(Horsup 1994). This age distribution was quite different from the usual pattern for a mammal
with a lifespan of about 12 years. For example, 59% of animals in a shot sample of 324 eastern
grey kangaroos were younger than three years (Quin 1989).
Most of the wallabies in the colony during our study would have been born in or before 1981
(Fig. 4b). There is little evidence of other than occasional movement of wallabies between the
'Black Rock' colony and a small colony 2 km away although this has not been studied
explicitly. Low recruitment is thus unlikely to be the result of emigration, especially in view of
the few subadults or juveniles observed by Horsup (1994).
The low adult recruitment to the colony since the early 1980s (Fig. 4b) pre-dates the start of
our study in 1986 and thus is not solely attributable to handling stress as suggested for a captive
colony of P. xanthopus (Poole et al. 1985). It is, of course, possible that handling stress
exacerbated other stresses on these animals.
Several factors have probably combined to cause the low rate of adult recruitment at 'Black
Rock' from the early 1980s; however, these factors are confounded and their effects cannot be
separated. In contrast to the 1970s, which was a decade of generally above-average rainfall,
local landowners consider the 1980s to be a decade of almost continuous drought. Below-
average rain fell for seven of the ten years between 1979-80 and 1988-89 (Delaney 1993).
Manly-Parr mark-recapture estimates (+ variance) of the rock-wallaby population at 'Black
*
Rock' fell from an estimated 54.62 6.82 at the beginning of 1987 to 41.77 & 1.04 in mid-1989
(S. Delean and H. Marsh, unpublished data). Only 39.5% of pouch young monitored between
June 1986 and June 1989 survived to permanent pouch emergence (Delaney 1993). Drought has
been recorded as seriously affecting the recruitment patterns of other macropodids in seasonal
environments, most notably Macropus rufus (Newsome 1977) and Macropus giganteus
(Kirkpatrick and McEvoy 1966). These studies also reported a severe or total reduction in
survival of pouch young leading to low recruitment into the breeding population coincident with
long drought periods.
Macropus rufus enters anoestrus in response to drought (Frith and Sharman 1964). However,
at 'Black Rock', P. assirnilis bred continuously and the proportion of females with pouch young
was high throughout this study (87%, Delaney 1993). There was no evidence of the population
entering anoestrus although four adult females were without suckling young for periods of
4-9.6 months (Delaney 1993). The cause of this was unknown. All of these animals had been
caught regularly for more than one year before entering 'anoestrus' so it was unlikely to be due
to handling stress.
The effects of drought on rock-wallabies are probably exacerbated by predation. A feral cat
was sighted at 'Black Rock' in July 1986. At least eight rock-wallabies including five juvenilesR. Delaney and H. Marsh
were eaten by a feral cat at 'Black Rock' between January 1989 and June 1990 (Spencer 1991).
Kinnear et al. (1988) present convincing evidence that fox predation caused declines in colonies
of P. lateralis in Western Australia. They consider that predation poses a greater threat during
severe drought than at other times.
We agree with this assessment but cannot evaluate the relative importance of predation and
climate on the age distribution of P. assirnilis at 'Black Rock'. This will require an experimental
approach to predator control at several colonies along the lines of that adopted by Kinnear et al.
(1988).
Acknowledgments
This research is Paper No. 7 from the 'Black Rock' Project, a longitudinal study of
P. assirnilis funded by the grants from Australia Research Council and James Cook University
to Helene Marsh and her students. This work forms part of the doctoral research of Robyn
Delaney who was supported by a James Cook University Scholarship. W e thank the
management of 'Lyndhurst' for allowing access to 'Black Rock' and to their rainfall records;
Wally Davies who conducted research on rock-wallabies at 'Black Rock' as part of his post-
graduate research at the University of Queensland for access to unpublished data; Robin
Waterhouse for use of his Radiography Clinic; Keith Barry for helpful advice on radiographic
techniques and interpretation; Glenn De'ath for statistical advice; Peter Spencer and numerous
volunteers for field assistance; and Rob Close and an anonymous referee for helpful comments
on the manuscript.
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Manuscript received 22 July 1994; revised and accepted 14 March 1995You can also read
