Behavioral Ecology
doi:10.1093/beheco/arj041
Advance Access publication 1 February 2006
Exclusive core areas in overlapping ranges
of the sleepy lizard, Tiliqua rugosa
Gregory D. Kerr and C. Michael Bull
School of Biological Sciences, Flinders University, G.P.O. Box 2100, Adelaide,
South Australia 5001, Australia
Where animal home ranges overlap extensively, objectively identifying exclusive areas within individual ranges has been difficult,
particularly in species lacking overt territorial behaviors. By analyzing the overlap between successively smaller core areas among
individuals in a population of the long-lived Australian skink, the sleepy lizard (Tiliqua rugosa), we objectively determined exclusive
areas within animal ranges. Using 4-year radio tracking data, we found that ranges consisted of relatively large sally zones (mean
66–80% total range), around home ranges with multinucleate cores strongly associated with key refuge sites. Total range and
home range area varied significantly among years, being smaller in a drought year. Total ranges overlapped extensively between
and within sexes, but for both sexes, intrasexual overlap of inner range cores rapidly approached zero, suggesting intrasexual
territoriality. Intersexual inner core overlap reflected this species socially monogamous mating system. But, male overlap of female
ranges and inner cores was consistently higher than female-male overlap. Refuges and/or mates may be defended resources within
these core areas, although aggressive behavioral interactions were rarely observed. In the extensively overlapping sally zones, males
shared space with females other than their principal partner. In productive years, with larger home ranges and more extensive
overlap, some lizards associated with extra partners, suggesting that males opportunistically use sally zones for polygyny. Consequently, we suggest that benefits to females from male association may change with environmental quality, such that if food resources are low, monogamy may be favored if females increase foraging efficiency as a consequence of male vigilance during pairing.
Key words: home range-territory continuum, lizard, population spacing system, sex, social structure. [Behav Ecol 17:380–391 (2006)]
here animal home ranges overlap extensively, the presence of territoriality is not always easy to determine. This
may be because the definition of what behaviors define territoriality is debated, the behaviors are difficult to observe, or
the techniques used to determine the presence of territoriality
are not always definitive.
Territoriality and the absolute dominance of hierarchies in
coexisting conspecifics are at opposite ends in a continuum of
dominance behaviors. Territoriality is a form of social dominance where the relative dominance between individuals is
reversed across territory boundaries (Kaufmann, 1983), with
a territorial animal having priority of access to a spatially related resource gained through social interaction. Authors differ in the degree of interaction required to define a territory,
ranging from overt defense-evoking escape and avoidance in rivals (Brown and Orians, 1970) to mutual avoidance (Kaufmann,
1983). In some cases, the extent of social interaction among
conspecifics may be difficult to determine by direct observation.
This may be the case if the operating dominance hierarchy is
established through past interaction (Lopéz and Martı́n, 2001),
if social recognition is achieved through visual or olfactory cues
that require limited contact between individuals (Aragón et al.,
2001a) or where social recognition results in different responses
to neighbors and to unfamiliar individuals or nonterritorial
‘‘floaters’’ (Schradin, 2004; Temeles, 1994), or if the study animals are secretive or cryptic.
If there is extensive range overlap in a population of longlived and sedentary individuals, two general social structures
are possible. Firstly, individual ranges may be randomly positioned and undefended. The sharing of resources may be in-
W
Address correspondence to G.D. Kerr. E-mail: greg.kerr@flinders.
edu.au.
Received 14 June 2005; revised 14 December 2005; accepted 29
December 2005.
The Author 2006. Published by Oxford University Press on behalf of
the International Society for Behavioral Ecology. All rights reserved.
For permissions, please e-mail: [email protected]
dependent of spatial location, and instead, social behaviors
such as dominance hierarchies may control access to resources
(Kaufmann, 1983). Secondly, there may be internal structure
to the ranges where territorial defense is limited to specific
sites associated with core resources. The extent of territoriality
within an animal’s home range may vary from a small exclusive
area, such as a nest, to the exclusion of conspecifics from
a large portion of the area occupied (Burt, 1943; Kaufmann,
1983) and may change temporally (Maher and Lott, 1995).
To identify exclusive areas within overlapping ranges, in species lacking overt territorial behavior, analysis of the overlap
of either entire ranges or overlap of subjectively defined core
areas within each of the ranges is used. Territoriality is then
interpreted from the degree of exclusivity of these core areas
with any available behavioral evidence (Chamberlain and
Leopold, 2002; Eifler DA and Eifler MA, 1998; Hass, 2002;
Kwiatkowski and Sullivan, 2002; Poole, 1995; Stone and Baird,
2002; Warrick et al., 1998). The subjective criteria used to define exclusive core areas may result in ambivalent conclusions
as to the presence of territories (Hall et al., 1997) and in difficulties defining when a home range becomes a territory
(Maher and Lott, 1995; Wittenberger, 1981). This makes it
difficult to compare spatial organization within or among species (Maher and Lott, 1995) or to identify other nonterritorial
spacing patterns (Brown and Orians, 1970). There is a need to
develop a means of quantifying spacing systems that allows
comparisons among species (Maher and Lott, 2000). Here
we use an ecological basis to analyze overlap between successively smaller core areas to determine objectively the exclusive
areas within an animal’s range. We adopt a modification of the
conceptual definition of territory of Maher and Lott (1995),
incorporating ecological evidence of exclusive range use combined with behavioral observations. We define territory as
a fixed space from which an individual or a group of mutually
tolerant individuals excludes competitors from a specific resource or resources through social interaction.
Kerr and Bull
•
Exclusive core areas in overlapping ranges
The sleepy lizard (Tiliqua rugosa) is well suited to investigate
the internal range structure, population spacing systems, and
territoriality using modern range analysis techniques. Individuals maintain long-term, stable home ranges (Bull and
Freake, 1999) that overlap extensively (Bull, 1994), and they
show little overt evidence of territorial defense (Bull, 1987;
Satrawaha and Bull, 1981). As their behavior is affected by
the presence of an observer (Kerr et al., 2004b), using observation of agonistic interactions to deduce spatial organization
may be problematic. Finally, they inhabit a relatively homogeneous habitat at our study site. Any variation in home range
structure across the study area is therefore unlikely to result
from ecological heterogeneity (Dutilleul, 1993).
The aims of this study were to investigate the internal structure of sleepy lizard ranges, to describe the spatial pattern of
adjacent total ranges and home ranges in a population, to determine the extent of home range and core area overlap during
the main season of lizard activity, and to determine the influence of sex and season on range use. If the sleepy lizards have
a territorial social structure, we predicted that there would be
exclusive inner core areas with no overlap. Determination of
the internal structure of lizard ranges and extent of overlap
should provide indirect insights into the social organization
of the sleepy lizard community. In this monogamous species,
a close association between members of a pair was expected,
with little difference between male and female range structure.
METHODS
Study animals
The sleepy lizard (T. rugosa) is a large (mean snout vent length ¼
30 cm), long-lived (Bull, 1995), and mainly herbivorous (Dubas
and Bull, 1991; Henle, 1990) skink from temperate regions of
Australia. During spring, sleepy lizards form monogamous
pairs for 6–8 weeks from early September until mating in early
November (Bull, 2000; Bull et al., 1998). During this period
pairs spend about 50% of their active time together (Bull
et al., 1991). In the pairing season, males on average take more
strides each day and move faster over a longer period to cover
nearly twice the distance as females (Kerr and Bull, 2006).
Within their home range, sleepy lizards use multiple refuge
sites (Kerr et al., 2003). They shelter under perennial woody
bushes, moving to larger, more dome-shaped bushes as the
season becomes hotter (Kerr et al., 2003). During extended
dry and hot periods, more extensive use is made of cooler rabbit and wombat burrows as refuge sites (Kerr and Bull, 2004a).
Study area
The study was conducted over 4 years (2000–2003) in a 1.5 km2
slightly undulating region of homogeneous chenopod shrubland (139 21# E, 33 55# S) within the previously described
Mount Mary study area (Kerr et al., 2003) in the mid-north of
South Australia. There was no obvious change in vegetation
type across the study area.
Annual rainfall at Bundey Bore Station (3 km N of the study
area) averaged 287 mm during the period 1970–1997. Rainfall
close to the average in 2000 (227 mm), 2001 (224 mm), and
2003 (280 mm) resulted in growth of annual plants in the
spring period and consequently relatively plentiful food supplies for the herbivorous lizards. An extended drought in 2002
(81 mm) resulted in very low food levels, low lizard activity,
little pairing and mating, and a 20% mortality, probably resulting from dehydration and starvation (Kerr and Bull, 2004a).
Radio tracking
We tracked 70 (27 females, 43 males) individual adult lizards
in total; 30 lizards in 2000 (12 females, 18 males) and 2001 (11
381
females, 19 males), 50 in 2002 (20 females, 30 males), and 35
in 2003 (14 females, 21 males). Lizards were followed in successive years with replacement where animals died. Ten radiotracked lizards died of starvation or dehydration in the 2002/
2003 drought, and a further 10 died from other causes (one
in 2000, two in 2001, three in 2002, and four in 2003), mostly
being killed on roads by cars. The animals followed represented all the lizards with home ranges within the central
study area and most lizards from its edges. In the latter group,
we could not determine the range overlap with all other lizards further out. Lizards were hand captured after random
encounter during searches in activity periods. We attached a
3.6-g radio transmitter (Sirtrack, Havelock North, New Zealand)
to the lateral surface of each lizard’s tail with surgical adhesive
tape. Radio transmitters represented less than 1% of minimum
lizard body mass. After release, we relocated the lizards using
a TR-4 receiver (Telonics, Arizona, USA) with a three-element
Yagi antenna. Lizards were followed intensively (up to 4 days
per week for 13 weeks) during their main period of activity in
the spring (late August to early December) of each year. In
2002 and 2003, lizards were also followed less intensively (twice
a month) from January to May.
We obtained 6359 location fixes from these lizards over the
study period (mean 6 SE [range] fixes per year: 2000, 57.9 6
2.23 [20–72]; 2001, 39.1 6 1.49 [20–49]; 2002, 47.7 6 2.41
[17–74]; 2003, 30.4 6 1.85 [11–47]). For each location, we
walked up to the radio-tracked animal. Active lizards usually
ceased activity when approached by an observer, and inactive
lizards remained coiled (Kerr et al., 2003). At each location,
we recorded lizard behavior, pairing status, and microhabitat.
Lizards were defined as inactive if they were coiled up or active
if their bodies were straight. Stationary but straight lizards
were classed as active. Each lizard location was determined
using a Garmin (GPS12) global positioning system (GPS),
with a mean figure of merit on all locations of 4.0 m (SE ¼
0.08). To verify the accuracy of the GPS fixes, we took readings
at one fixed position on 98 days in 2002 and 2003. Readings
varied by up to 65 m from the mean and had a standard
deviation of 1.94 m in the easting and 2.00 m in the northing.
We needed to obtain sufficient fixes of an individual to define its range, while leaving adequate time between fixes to
minimize the effect of autocorrelation on home range determination (de Solla et al., 1999; Harris et al., 1990; Swihart and
Slade, 1985a,b). Sleepy lizards move infrequently and for a relatively short part of the day, but when active they can move
across their home area in a day (Kerr and Bull, 2006; Kerr et al.,
2004a). Consequently, on each day, lizards were tracked once
during a period of activity and once in a subsequent period of
inactivity. This gave equal weight to active foraging and inactive
shelter locations when analyzing home range structure and
allowed biologically independent (Lair, 1987) and relevant observations (de Solla et al., 1999) of position. Because the times
of day when sleepy lizards are active vary over the spring (Firth
and Belan, 1998; Kerr and Bull, 2006), sampling times were
adjusted as the climate changed (Kerr et al., 2003). Surveys
of lizard locations continued throughout the day from dawn
to dusk. The order of tracking lizards was changed within and
among days to avoid structuring temporal autocorrelation into
the data (Harris et al., 1990).
Home range determination
We defined several components within a range (Figure 1). The
entire area occupied by a lizard is its ‘‘total range’’ (Linn and
Key, 1996). This total range is equivalent to the ‘‘map of locations’’ of White and Garrott (1990) and is defined operationally by the 100% minimum convex polygon (MCP). The total
range is composed of two main regions, the ‘‘home range’’ and
Behavioral Ecology
382
Figure 1
Range determination through
OEC and ICP analyses allowed
objective determination of internal range structure. Diagram
of a typical lizard home range
illustrating terminology used in
this paper. The total range is
composed of the home range
and the sally zone. Selected core
ICP isopleths (20, 40, 60, and
80%) illustrate the multinucleate internal range structure.
the ‘‘sally zone’’ (Linn and Key, 1996). In our study, home
range was determined from outlier-exclusive cores (OEC) that
were derived by cluster analysis using nearest neighbor distances (Kenward et al., 2001). This analysis assumes that movements inside and outside range cores involve different activity
patterns. This results in mean distances between locations
within range cores being smaller than distances between locations during excursive movements.
In this study, two methods were used to determine home
ranges. In the first, the truncated method, locations in the
largest 5% of the nearest neighbor distance distribution were
excluded. In the second, an iterative process excluded the location with the most extreme linkage distance if it was beyond
0.1% of the distribution estimated by the remainder. This process was repeated until all distances were within this alpha level
on a normal distribution (Kenward et al., 2003). Polygons were
plotted round clusters with no nearest neighbor locations beyond this distance. The range area resulting from the iterative
method is equivalent to the concept of home range as defined
by Burt (1943) representing the normal area of activity exclusive of occasional sallies outside the area (Kenward et al., 2001).
The sally zone is the remaining outer area of the total range.
Within the range, those areas receiving the most concentrated
use are defined as core areas (Samuel et al., 1985).
We determined core areas from hierarchical incremental
cluster analysis (Kenward, 2001), using Ranges6 v1.212 software (Kenward et al., 2003). To minimize the risk of Type 1
errors, we selected home range estimators a priori (Kenward
et al., 2001). We did not use fixed kernel density estimators
(Seaman and Powell, 1996; Worton, 1989) because the use of
a median smoothing parameter, to allow valid comparison
among home ranges, resulted in some home ranges having
individual rings around isolated locations and other home
ranges having contours approaching ellipses with single nucleus. Sleepy lizards use multiple shelter sites within their
home range (Kerr et al., 2003), and when moving between
shelter sites during the day, they forage as they move. Therefore, neither of these extremes would realistically represent
the areas actually used by the lizards. Instead, we used incremental cluster polygons (ICPs) that allowed us to define
multinucleate core areas within each range. To form ICPs,
locations that minimize the mean joining distance are linked
in clusters (Kenward, 2001). The analysis generates a series of
isopleths of diminishing percentage (i.e., 99, 95, 90, 85, . . ., 20)
that define increasingly central components of the range. In
these analyses, fixes are always located on the vertices of each
range core with no tendency to expand artificially into unused
areas (Kenward, 2001). This avoids any false increase in overlap. Both ICP and OEC analyses in Ranges6 provide parameters to describe range core structure (Kenward et al., 2001).
For comparison with previous studies, MCP areas are also presented in the results.
Minimum sample size
We used graphs of range area against number of observations
to determine the minimum sample size needed to estimate
range sizes of lizards (Harris et al., 1990; Kenward, 2001). We
Kerr and Bull
•
Exclusive core areas in overlapping ranges
383
used the number of points needed to describe 80% of the 95%
ICP range area asymptote (Rose, 1982; Stone and Baird, 2002).
Fixes for 80% asymptotes varied among years (2000, 41; 2001,
29; 2002, 30; 2003, 23). In 2000, two lizards had less than 41
(but .30) locations. These lizards were retained in the overlap
analysis to avoid artificially reducing the extent of range overlap with other lizards. Consequently, for each year, a minimum
sample size of 30 locations per lizard was used in determining the range area and overlap. This sample size corresponds
well with the recommended minimum number of locations
needed for home range size to stabilize (Kenward, 2001). In
general, there were sufficient locations per lizard in each year
to obtain range area values approaching the asymptote.
90, and 99), and in each year (2000–2003). Data for each year
were analyzed separately. Lizards were treated as subjects in
the analyses, with three fixed factors of sex, sex overlapping
(Sexlizo), and range core. Sex was a between-subjects variable, and both Sexlizo and core were repeated measures. An
unstructured covariance matrix (UN) was used. Conservative
Sidak post hoc tests were used where significant effects were
detected.
Pearson product moment correlation coefficient and linear
regression were used to determine the levels of association for
each sex, between the mean OEC home range area and the
proportion of lizards observed with more than one partner for
each year.
Change in overlap for each sex in each 10% range core
RESULTS
To determine if the range of each lizard contained an exclusive core area, we measured the extent of overlap among all
lizard range cores. This was done for a sequence of different
size cores. For example, for each lizard, we first counted the
number of other lizards with 99% ICP ranges overlapping
their 99% ICP range and calculated the extent of this overlap
(as a percentage of the 99% ICP area). For the cores of one
individual, the total percentage overlap by the corresponding
cores of all other adjacent lizards can sum to more than 100%.
We then repeated these calculations of number and extent of
overlap among their 90% ICP range cores and in sequence for
their 80, 70, 60, 50, 40, 30, and 20% ICP cores.
Range structure
Statistical procedures
SPSS V12.0.2 was used for all statistical analyses. One-way t tests
were carried out to determine if ICP core areas contained a
higher proportion of inactive locations than would occur at
random within each core area. A multivariate approach to repeated measures ANOVA with repeated contrasts was used to
determine if 90 and 99% ICP range size increased with the
addition of successive years of locational data. If lizards maintained stable ranges across years, then range core size should
have reached a maximum area as successive years of data were
added to the range determination. The 90 and 99% ICP areas
were calculated for each lizard using locational data combined
for 1 (2000), 2 (2000/2001), or 3 (2000/2001/2002) years.
Linear mixed effects were used to analyze the change in
range size among years, change in total home range overlap
between sexes and among years, and change in total overlap
within and between sexes for each range core. This form of
analysis handled many levels of repeated factors, thus addressing pseudoreplication, and accommodated missing repeated
measures data due to loss and replacement of lizards (von Ende,
1993). Prior to analyses, data were natural log transformed,
where appropriate, to address assumptions of normality and
homogeneity of variance. Autoregressive covariance matrices
(AR1 and ARH1) addressed temporal autocorrelation among
years. To determine the effect of year, sex, or OEC method
(iterative [a ¼ 0.001] and truncated [a ¼ 0.05]) on each OEC
parameter (Cloc%, ln OEC area, Cnuc, Cpart, CS loc, and CS
area; Supplementary Appendix Table 2), lizards were treated as
a random factor, with sex and year as fixed factors. To examine
the effect of sex and year on home range overlap, we determined the total overlap for each lizard by each sex for each year.
Lizards were treated as a random factor, with sex, sex overlapping, and year as fixed factors. Bonferroni corrections were
used for pair wise comparisons of year as sample sizes differed.
To examine how total range overlap changed within and between sexes for each range core, the dependent variable (total
percentage overlap) was calculated for each lizard, for each sex
overlapping, at each ICP range core (20, 30, 40, 50, 60, 70, 80,
Total range
Parameters of the 100% MCP home ranges of lizards in the
study are given for comparison with other studies (Supplementary Appendix Table 1). Figure 2 shows 99% ICP total
range areas (ha) for each year of the study. These differed
significantly among years (F3,40.5 ¼ 3.22, p ¼ .032). Mean total
ranges in 2002 were smaller than 2000 (Bonferroni post hoc:
p ¼ .033). Total range size within a year did not differ between
sexes (F1,46.0 ¼ 0.045, p ¼ .833), and there was no significant
interaction effect between sex and year (F3,40.5 ¼ 0.253, p ¼
.859). Figure 2 also indicates that 99% ICP areas measured
over all years were larger than those in individual years, perhaps because of range shifts from one year to the next.
The 90% ICP areas did not change with years combined
(F2,20 ¼ 2.54, p ¼ .104, Figure 3) or sex (F1,21 ¼ 0.98, p ¼ .334),
with no interaction effect (years combined 3 sex: F2,20 ¼
0.94, p ¼ .406). However, the 99% ICP areas continued to
increase with years combined (F2,20 ¼ 12.2, p , .001), with
each additional year of data resulting in a larger total area
(repeated contrasts: 2000 versus 2000/2001 F1,21 ¼ 11.22,
p ¼ .003; 2000/2001 versus 2000/2001/2002 F1,21 ¼ 12.6,
p ¼ .002, Figure 3). Again, there was no effect of sex (F1,21 ¼
1.52, p ¼ .231) or interaction effect (years combined 3 sex:
F2,20 ¼ 3.25, p ¼ .060) in this analysis. These analyses supported
Figure 2
Mean (6SE) 99% ICP total range areas for each sex in each year and
for all years combined. Sex: filled square ¼ male, open circle ¼
female. The n varies with year (Supplementary Appendix Table 1).
Behavioral Ecology
384
Figure 3
Mean (6SE) 90 (open triangle) and 99% (filled circle) ICP areas for
1 (2000), 2 (2000/2001), and 3 (2000/2001/2002) years combined
fixes. The n decreases from 29 in the first 2 years to 23 in the third
year due to the death of lizards prior to obtaining sufficient fixes to
determine home area in 2002.
the distinction made between home range and sally zone and
suggested that behaviors differed between these zones. A plateau in 90% areas within the first year suggested that the sleepy
lizard home ranges were fully defined in that year. However,
different parts of the overall sally zone were used (or detected)
among years so that total ranges shifted around a core area
between successive seasons, resulting in total ranges still expanding after 3 years of locational data. There was a strong
trend for males to be recorded from a greater area over the
whole study than females (Figure 2), probably representing
more sally zone excursions.
Home range
No significant interaction effects were found for any OEC
home range parameter (Supplementary Appendix Table 2)
among year, sex, or OEC method. However, for all parameters,
there were significant (p .002) main effects of OEC method.
Outlier exclusion through truncation resulted in the inclusion
of a greater percentage of fixes, giving larger estimates of
home range area, and fewer nuclei with lower levels of patchiness, lower diversity of use, and lower diversity in nuclei area
than the iterative method. There was a significant main effect
of year (p , .05) for all parameters except Cloc%. Home
ranges in all years were multinucleate. In 2002, home ranges
were significantly smaller (Bonferroni post hoc, p , .01), and
their fragmentation indices indicated that they had more activity nuclei than in 2001 (p ¼ .022) that were dispersed further
apart than in 2000 (p ¼ .002) and 2001 (p ¼ .002), with significantly greater equitability of use than in 2000 (p , .001).
Range internal structure
In each year, core areas were only a small component of the
total range, with an asymptotic decline in area from the 99%
ICP range to the inner core (e.g., 2000, Figure 4). Moving in
from the 99% ICP outer edge, the average number of nuclei
increased from just above one to approximately six by the 75%
range core (Figure 4). The number of nuclei remained at this
level until about the 40% range core, from where they declined. For males and females, the pattern of nuclei enclosed
within each core was the same. Internal structure of home
areas followed similar patterns in the years 2001–2003.
Figure 4
Filled square ¼ mean (6SE) ICP core areas (ha) for each 10% range
core in 2000. Open circle ¼ number of nuclei (mean 6 SE)
enclosed by each 10% range core in 2000. n ¼ 29.
Shelter sites and core areas
Refuge use formed a major component of home area determination. In 2000, lizards were found inactive in 66.4 6 2.05%
(mean 6 SE) of the 59 6 1.9 (mean 6 SE) observations made
per lizard. To assess the relationship between core areas and
refuge sites, we calculated the percentage of inactive locations
overlapped by each range core. If range cores were located
independently of shelter sites, then we would predict that
the 20% core, for example, should on average have overlapped
with 20% of inactive locations. In fact, successive core areas
overlapped a higher proportion of inactive locations than expected at random (one-way t test: p , .001 for all tests, Figure
5). In 2001, 2002, and 2003 the same patterns were observed,
with a higher proportion of inactive locations within core areas
than expected at random. Lizards were rarely found inactive in
Figure 5
Percentage (mean 6 SE) of all locations of inactive lizards overlapped by each range core in 2000. Dashed line indicates expected use if
refuges used when inactive are randomly distributed over the total
range. Results of one-sample t tests comparing actual percentage
overlapped with random use for each range core. ***p .001. n ¼ 29.
Kerr and Bull
•
Exclusive core areas in overlapping ranges
385
Table 1
OEC home range (iterative, a ¼ 0.001) mean (6SE) total percentage overlap and mean (6SE) number
of home ranges overlapping per lizard within and between sexes for each year (2000–2003)
Sex overlapping (mean 6 SE)
Male
Female
Year
Sex
Total percentage
overlap
Number of home
ranges overlapping
Total percentage
overlap
Number of home
ranges overlapping
2000
Male
Female
20.4 (4.56)
62.9 (9.53)
2.0 (0.24)
2.5 (0.40)
43.1 (7.13)
7.82 (4.35)
1.7 (0.20)
1.2 (0.27)
2001
Male
Female
11.6 (4.21)
59.3 (9.29)
0.7 (0.21)
1.9 (0.31)
36.4 (7.04)
2.0 (1.42)
1.1 (0.17)
0.2 (0.12)
2002
Male
Female
14.7 (3.03)
33.3 (6.86)
1.6 (0.27)
2.6 (0.31)
19.6 (3.93)
11.9 (3.47)
1.6 (0.21)
1.1 (0.25)
2003
Male
Female
18.6 (5.21)
55.2 (8.41)
2.0 (0.34)
2.6 (0.40)
39.7 (8.78)
17.5 (6.98)
1.6 (0.25)
1.1 (0.31)
the sally zones, suggesting that core areas were strongly linked
to refuge sites.
Home range overlap
Total area of overlap among iterative OEC home ranges (Table 1)
showed a significant three-way interaction effect among year,
sex, and sex overlapping and a significant two-way interaction
between sex and sex overlapping (Supplementary Appendix
Table 3). Mean total male-male overlap was higher in 2000 than
2001, but low total intrasexual overlap for both male-male and
female-female home ranges were otherwise similar in extent.
Intrasexual overlap was markedly lower than male overlap of
female home ranges, in all years. Intersexual home range overlap was extensive, but male overlap of female home ranges was
consistently higher than female overlap of male ranges.
Changes in the relative extent of female overlap of male ranges
among years meant that in 2001, male-male overlap was less
than female overlap of male home ranges, but these did not
differ in other years. In 2000 and 2001, female overlap of male
ranges was greater than female overlap of other female ranges,
but these did not differ in 2002 and 2003. There were no significant interaction effects between year and sex or year and
sex overlapping.
Home range and pairing
While sleepy lizards are mostly monogamous, on some occasions, some lizards of each sex are observed with additional
partners. To determine whether home range overlap was associated with reproductive behavior, for each sex we compared
average home range area in each year (2000–2003), with the
average proportion of that sex observed with more than one
partner in that year. For males, there was a significant positive
correlation between the mean OEC home range area (HR)
and the proportion observed with more than one female partner (LP) (males: F1,2 ¼ 26.85, r ¼ .965, p ¼ .035, n ¼ 4, LP ¼
32.0 3 HR 32.5) but not for females with more than one
male partner (females: F1,2 ¼ 0.76, r ¼ .525, p ¼ .475. n ¼ 4,
Supplementary Appendix Figure 1). In years where more
males were found paired with multiple females, the mean male
home range size was larger.
Exclusive core areas
To simplify interpretation of analyses to determine if there
were exclusive core areas within lizard ranges, we first discuss
the 2000 results (Figure 6) and then extend this discussion
to patterns observed in the remaining 3 years, 2001–2003
(Figure 7). In 2000, the average total percentage overlap
showed significant three-way interactions between range core,
sex, and sex overlapping (Sexlizo) (Table 2, Figure 6A). Mean
total overlap of male total ranges (99% edge) was the same for
both sexes, but males overlapped female total ranges to a significantly greater extent than females did. Moving in from the
total range edge, mean total overlap decreased significantly,
but the pattern differed between sexes. Male-male average
total overlap approached zero by the 50% edge core area. In
contrast, female overlap of male core areas approached an
asymptote of around 10%, into the inner cores, being significantly higher than male-male overlap, from the 90% edge into
the 20% edge. Male overlap of female ranges was significantly
higher than female overlap, from the 99% edge into the inner
core. Female overlap of female core areas approached zero by
the 80% edge.
In 2000, there were similar trends in the number of lizards
with overlapping core areas (Figure 6B). The 99% ICP edge of
each male overlapped an average 3.5 other male total ranges,
with average overlap less than 5% by the 80% cores and approaching zero by the 50% edge, indicating male-male exclusive core areas. The 99% ICP edge of each male’s total range
overlapped an average of three female ranges but plateaued
below one for inner cores. Total female overlap of male core
areas showed a gentle linear decline from the 70% edge down,
the point at which the average number of females overlapping
was approximately one.
An average of four male total ranges overlapped each female total range, higher than female-male total range overlap
(Figure 6B). For females, the number of male core areas overlapping plateaued at one, with a mean total overlap of 15%.
Total male overlap of female core areas declined rapidly to the
70% edge, but at this point the average number of males overlapping was nearly two. It was not until the 50% edge that the
mean number of males overlapping approached one, and the
mean total overlap plateaued. Where pairing occurred, both
sexes shared the inner 50% core of a range with a single partner (e.g., Figure 8). Each female’s total range overlapped an
average of two other female total ranges. Average total femalefemale overlap was less than 3.5% by the 80% edge.
In 2001 and 2002, the same patterns in total overlap were
generally observed, with variation in the extent of overlap (Figure 7), but in 2002, male overlap of male core areas showed
a markedly different pattern. Male-male total range overlap was
markedly lower than previous years and significantly lower than
Behavioral Ecology
386
Figure 6
(A) Mean (6SE) total percentage overlap by males and females of each sex at each successive 10% range core for lizards in 2000. (B) Mean
(6SE) number of male and female core areas overlapping each sex at each successive 10% range core for lizards in 2000. Overlapping sex: filled
square ¼ male, open circle ¼ female. Linear mixed effects Sidak post hoc tests: NS ¼ p . .05, *p .05, **p .01, ***p .001.
female overlap of male total range. Total percentage overlap
remained well above zero, showing no significant difference
from female overlap of male core areas, from the 90% edge
to the 20% edge core. In 2003, the three-way interaction was
not significant, but there was a strong trend in the patterns seen
in 2000 and 2001, with the extent of overlap varying (Figure 7).
Of the 6359 lizard locations in this study, male-male interactions were only observed seven times. In three cases, males
were within 1 m of each other and within 5 m of burrows, on
the edge of their home ranges. Four cases involved fighting
between males with a female present (Kerr and Bull, 2002).
DISCUSSION
suggesting that refuge sites form the basis of exclusive areas.
The location of home ranges and the multinucleate cores did
not change markedly among years. Cotton spooling data (Kerr
GD, unpublished data) showed that daily activity generally
involved movement out from one of these core nuclei, often
into the sally zone, and then return to the same or another core
nucleus. The use of sally zones varied among years, more so for
males than females, causing the total range to grow, as successive years of locations were included in range determination.
Burrow location was generally in one outer vertex of each
lizard’s home range. Often multiple lizards shared one burrow,
particularly as summer advanced and conditions became hotter and drier (Kerr GD, personal observation), but their ranges
radiated out from the burrow in different directions.
Internal range structure
In this study, we have shown complexity in the internal range
structure of the sleepy lizard. Within their total range, lizards
showed strong home range site stability across years. The
ranges were split internally (40–75% ICP cores) into an average of around six core nuclei, which varied in their use and
size. Home range area was a small component (20–33%) of the
total range area each year. The typical home range had between two and four main centers of activity, moderately dispersed across the home range, with each used to a similar
extent. Within stable home ranges, the centers of activity
shifted through the activity season and among years. The multinucleate cores of ranges were closely associated with small
clusters of refuges or individual refuge sites used by the lizards
when inactive. Many cores were linked to larger, dome-shaped
perennial bushes or sometimes logs, and usually one core was
centered on a mammal burrow. In these mid-range cores the
average number of nuclei was at a maximum. Lizard behavior
in these nuclei was strongly biased to inactivity within shelters,
Population spacing system
The results suggest that the spacing system of the sleepy lizard
is determined by different factors for each sex. During the
pairing season, males and females used space differently.
Male-male
During a typical spring with adequate food and typical mating
behavior (2000, 2001, 2003) male ranges overlapped extensively with other male ranges in their sally zones, and this
overlap continued into their home ranges. However, overlap
diminished to zero in the multinucleate inner cores. Very low
levels of overlap between adjacent males were recorded from
the 80% cores inward, with cores exclusive of all other males
varying among years (40–60%). Refuge sites appear to be
a key constituent of exclusive areas in males. Evidence of scale
damage (Murray and Bull, 2004) and occasional fights between males suggests that resource defense occurs.
Kerr and Bull
•
Exclusive core areas in overlapping ranges
387
Figure 7
Mean (6SE) total percentage overlap by males and females at each successive range core of male (A) and female (B) lizards in each year
(2001–2003). Overlapping sex: filled square ¼ male, open circle ¼ female. Linear mixed effects Sidak post hoc tests: NS ¼ p . .05, *p .05,
**p .01, ***p .001. In 2003, the three-way interaction was not significant invalidating post hoc comparisons.
During the drought year of 2002, the exclusive male-male
core areas within home ranges broke down. Average overlap of
between 5 and 10% continued into the innermost cores. Little
pairing behavior was recorded in this spring, and male activity
levels were atypically low and similar to female levels (Kerr and
Bull, 2006). This contrasted with marked gender differences in
activity normally seen in spring. During the hot dry weather,
lizards sought refuge, often for extended periods, in the largest
bush shelters or in deep burrows (Kerr et al., 2003), to minimize water loss. These shelters were generally shared with multiple lizards and consequently constituted part of the core
areas of each lizard’s range in that year. This atypical early
spring behavior explains the overlap into the inner cores of
ranges. In more normal years, the same aggregations of lizards
occurred later in summer after the pairing period has finished.
In this potentially long-lived species, there would be high selective pressure to maintain a range structure that included
such key refuges. The spatial arrangement and location of
ranges may be strongly influenced by the location and density
of key refuge sites.
Where ranges overlap extensively, inadequate sampling
might have resulted in small average levels of estimated overlap into the inner cores. This effect could have been exacerbated by the lack of records for lizards located on the outer
site boundary, leading to underestimation of average overlap.
In this case, the estimated overlap among a few lizards would
Behavioral Ecology
388
Table 2
Results of linear mixed effects analyses in each year (2000–2003) of total percentage overlap among ICP ranges and range cores for each sex
and each sex overlapping (Sexlizo) at each core isopleth (99, 90, 80, 70, 60, 50, 40, 30, and 20%)
Year
2000
2001
2002
2003
Factor
df
F
p
df
F
p
df
F
p
df
F
p
S
O
C
S3O
S3C
O3C
S3O3C
1,27
1,27
8,27
1,27
8,27
8,27
8,27
1.27
15.6
23.2
83.7
1.20
11.6
10.8
.271
,.001
,.001
,.001
.337
,.001
,.001
1,23
1,23
8,23
1,23
8,23
8,23
8,23
1.75
4.25
11.5
33.4
0.687
1.59
7.93
.199
.051
,.001
,.001
.699
.184
,.001
1,40
1,40
8,40
1,40
8,40
8,40
8,40
0.246
15.6
14.9
28.0
1.32
2.34
2.20
.623
,.001
,.001
,.001
.261
.037
.048
1,20
1,16.6
8,17.9
1,16.6
8,17.9
8,18.0
8,18.0
0.138
2.25
10.7
27.4
1.75
0.694
2.25
.714
.152
,.001
,.001
.154
.693
.074
S, sex; O, Sexlizo; and C, core.
be diluted by inadequate or incomplete results for many. However, the total lack of male-male overlap in the inner cores of
their ranges alleviated concerns about such bias. This suggests
that we were detecting a population-wide trend rather than
a product of inadequate sampling.
Figure 8
Total ICP range overlap of a
long-term (22 years) pair of
sleepy lizards for spring 2000.
The 99 (in bold), 80, 60, 40,
and 20% isopleths shown for
each individual. Scale in meters.
Male ¼ filled square, female ¼
open triangle.
Female-female
Across all years, irrespective of the extent of pairing behavior,
the mean female-female overlap among total ranges was low,
and each female overlapped the range of only one other female on average. There was no evident change in the pattern
Kerr and Bull
•
Exclusive core areas in overlapping ranges
of female-female overlap during the drought year. Female total
ranges appear to be dispersed relatively evenly across the landscape, and interaction among females in the sally zones was
probably limited. Similarly, home range overlap between females was close to zero in most years. In one year when home
ranges overlapped more extensively, the 75–85% cores approached zero overlap, indicating exclusive multinucleate inner
cores. No agonistic female-female interactions were observed
over the 4 years.
Intersexual
Lizards of the opposite sex with home ranges overlapping
were generally found paired at least once during the pairing
period. As predicted, both sexes in a pair shared the inner
50% core of a range (e.g., Figure 8). Pairs sometimes shared
the same refuge sites overnight but also used nearby shelters,
relocating each other the next morning (Kerr GD, unpublished data). Outside of the central core areas, but within
the home range, male lizards shared space with other females
as well as their principal partner. More males overlapped each
female’s total range, and to a significantly greater extent, than
females overlapped male total ranges. Also, more males encroached to a greater extent on female sally zones than females did onto male sally zones. This pattern continued into
the outer regions of ranges.
Varying strategies to maximize reproductive success can result in marked differences in social behavior between adult
males and females (Baird et al., 2003). Female home range
size is often related to food resources (Kwiatkowski and
Sullivan, 2002), with reproductive success limited more by
energetic costs of reproduction than by access to males (Baird
et al., 2003). In contrast, adult males in polygynous systems
should maintain ranges that maximize access to fertile females
(Baird et al., 2003). For males, increasing range size may increase access to females, but there may be costs from increased
encounters with other males (Aragón et al., 2001b). Thus, in
polygynous species, males generally have larger home ranges
than females, whereas monogamous species show little difference in range size between sexes (Pereira et al., 2002; Stamps,
1977). During years when male sleepy lizard home ranges increased in size and the extent of overlap among home ranges
increased, there was a concomitant increase in the proportion
of males found with more than one female partner.
Defended resources
The extended drought of 2002 resulted in a marked reduction,
during the spring activity season, in the size of both the mean
total range and mean home range area compared with other
years. This is a pattern typically seen in summer (Dubas and
Bull, 1992). In 2002, the average distance covered each day and
the duration of activity were less than half of those recorded in
the 2003 spring (Kerr and Bull, 2006). This marked reduction in activity and smaller range area were probably due to
low food and water availability in the spring. That year also
had very low levels of pairing (Kerr and Bull, 2004a). These
drought-induced changes in behavior are linked with reduced
use of the sally zone and smaller total range size. An opposite,
inverse relationship between food availability and home range
size has been predicted by general theory (Hixon, 1980) and
observed in a range of taxa (Adams, 2001). If food was being
defended or was a limiting resource for the sleepy lizard, then
ranges might be expected to increase in size during a drought
year. This did not occur. Similar patterns have been found for
widely foraging lizards that are either nonterritorial or defend
only specific sites within their home range (Eifler, 1996). Reduction in range size under conditions of low food and water
availability appears adaptive in ectotherms. Extended inactivity
389
at low body temperature can significantly reduce the daily
energy requirement (Bennett and Nagy, 1977; Christian
et al., 1995) and rate of water loss (Crowley, 1987; Heatwole,
1977). As such, long-term inactivity within refuges by the sleepy
lizard may be an adaptive response during drought, if these
refuges provide a relatively cool microclimate.
Two distinct patterns of refuge use are evident in the sleepy
lizard. During the pairing period prior to mating, refuge use
is linked with both thermoregulatory behavior (Kerr and Bull,
2004b; Kerr et al., 2003) and cohabitation with a mate. Over
this period, males maintain intrasexual exclusive core areas,
and refuges and/or mates may be defended resources. After
mating, during the hotter drier period of the summer or during drought, a few key refuges are shared by multiple lizards
for extended periods. These refuge sites are a key component
of the home range but are not a defended resource under
these conditions.
Territoriality and agonistic interactions
Are fights or agonistic interactions always required to define
territoriality? The long-lived sleepy lizard has stable home
range structure and can discriminate individuals through chemosensory cues (Bull and Lindle, 2002; Bull et al., 1993a, 1994,
1999; How and Bull, 2002; Main and Bull, 1996). If familiar
neighbors are recognized and avoided, the potentially high
costs of agonistic interactions may be reduced. The ability to
discriminate among familiar and unfamiliar individuals
through chemosensory cues can influence the spatial relationships between male lizards in the field (Aragón et al., 2001a).
Simple rules such as body size differences, residence status,
and the ability to recognize individual conspecifics may help
to reduce or avoid costs of fights and may help to stabilize the
social system (Lopéz and Martı́n, 2001). Occasional agonistic
interactions between male sleepy lizards have been recorded
(Bull and Pamula, 1996; Kerr and Bull, 2002), and the extent of
damage to males can be severe (Kerr and Bull, 2002). Records
of scale damage indicate that interactions are more common
among unpaired males (perhaps floaters?) (Murray and Bull,
2004) rather than paired males. The dear enemy phenomenon
(Temeles, 1994) may play a role in maintaining sleepy lizard
range structure based on the relative threat presented by
neighbors versus strangers. During the pairing period, ‘‘mate
guarding’’ may be sufficient to deter neighbors (Kaufmann,
1983), with male sleepy lizards in established overlapping
ranges, passively avoiding each other in their daily movements.
Social structure
The internal structure of sleepy lizard ranges and changes
in the extent of overlap within and between years allow us to
propose a social structure of sleepy lizard communities. Female sleepy lizards maintained total ranges that mostly excluded other female lizards and maintained virtually
nonoverlapping home ranges. Total ranges of males overlapped extensively with other males, but they maintained intrasexually exclusive multinucleate core areas within their home
ranges. Internal home range structure and overlap during
spring reflected the socially monogamous mating system normally seen in this species, but intersexual overlap of the outer
regions of the home ranges and sally zones showed patterns
consistent with opportunistic polygynous associations. Males
overlapped with females, more than females overlapped with
males, perhaps because males used their sally zones more. One
interpretation of this is that males may venture occasionally
to monitor the reproductive and behavioral status of neighboring females. Range structure in the monogamous lizard
390
Uta stansburiana has similar patterns of overlap between sexes,
but both sexes were intrasexually territorial (Tinkle et al., 1962).
With higher resource availability, males may opportunistically seek extrapair matings or females may allow them. Previous studies of this population have recorded extrapair
paternity in 14% of offspring from 19% of litters (Bull et al.,
1998). These levels are very close to the average frequency of
extrapair offspring among socially monogamous bird species
(Griffith et al., 2002). Because of range overlap among male
sleepy lizards, mate guarding may play an important role. Bull
et al. (1998) suggested that females of polygynous males might
be more likely to experience extrapair fertilization. During
the pairing period, females were recorded with male partners
on an average 36% of observations (Bull et al., 1998). Males
move more than females (Kerr and Bull, 2006) and, in years
with higher range overlap, are more likely to associate with multiple female partners. This suggests that a monogamous male
mating strategy may be modified in more productive years.
Sleepy lizards provide little or no parental care (Bull, 2000;
Bull and Baghurst, 1998). In this regard, the pattern of monogamy closely resembles that observed in some precocial birds
(e.g., Anatidae and Tetraonidae) (Wittenberger and Tilson,
1980), where the advantages of monogamy are limited to premating behaviors. Bull (2000) described a range of benefits to
female sleepy lizards resulting from monogamy and speculated
that intersexual conflict led to females coercing males into
prolonged partnerships prior to mating. We now suggest that
variation in levels of genetic monogamy in the sleepy lizard
result from between-year shifts in the benefits for females
from male association, which vary with environmental quality
(Gowaty, 1996). Both reproduction and subsequent offspring
survival over their first winter may be influenced by female
sleepy lizard foraging success in the spring (Bull et al.,
1993b). Females may benefit from pair bonding through male
vigilance (Bull and Pamula, 1998), reducing predation risk or
male harassment. Thus, monogamy may be favored more in
years when spring food resources are low, and females are able
to increase their foraging efficiency as a consequence of male
vigilance during pairing.
This study provides an example of how social structure and
population spacing systems can be inferred from detailed
range analysis. Explorations into the organization of populations with social structures outside exclusive territories and
further into the territory-overlapping home range continuum
are underrepresented in the home range debate.
SUPPLEMENTARY MATERIAL
Supplementary Appendix Figure 1 and Tables 1–3 are available at http://www.beheco.oxfordjournals.org/.
We would like to thank Ron and Leona Clark for allowing us access
to their land and use of the homestead at Bundey Bore Station. Karin
Hegetschweiler, Zeta Steen, Des Sinnott, and Ron Reid helped with
field observations. We thank two anonymous reviewers for their valuable comments on the manuscript. The study was conducted according to the guidelines of the Flinders University Animal Welfare
Committee in compliance with the Australian Code of Practice for
the use of animals for scientific purposes. The Australian Research
Council and the Nature Foundation SA Inc. funded this research.
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