Behavioral Ecology Vol. 11 No. 6: 587-590
© 2000 International Society for Behavioral Ecology
Ownership influences the outcome of male-male contests in the scincid lizard, Niveoscincus microlepidotus
a The University of Sydney, School of Biological Sciences, Zoology Building AO8, N.S.W. 2006, Australia b The University of Gothenburg, Medicinaregatan 18, SW 413 90 Gothenburg, Sweden
Address correspondence to M. Olsson at the University of Gothenburg, Medicinaregatan 18, SW 413 90 Gothenburg, Sweden. E-mail: mats.olsson{at}zool.gu.se .
Received 27 August 1999; revised 25 November 1999; accepted 30 January 2000.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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Male snow skinks (Niveoscincus microlepidotus) in the Tasmanian highlands have broadly overlapping home ranges, and fight vigorously (often with substantial damage to one or both participants) upon encountering another adult male. We observed 32 male-male contests, involving at least 49 different males, during a five-year field study near the summit of Mount Wellington. Bouts involving similar-sized lizards typically continued for longer than bouts involving a greater size disparity between the combatants. Resident males won 72% of all bouts, despite a lack of any significant difference between residents and intruders in body sizes, relative head sizes or body condition. Thus, prior residency of a site appears to be the major determinant of success in male-male rivalry.
Key words: male-male contest, ownership, lizard, Niveoscincus microlepidotus.
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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Comparative evidence suggests that agonistic interactions among males favor the evolution of traits such as large body size, weaponry and the ability to make tactical fighting decisions (Andersson, 1994
;
Bradbury and Vehrencamp, 1998;
Darwin, 1871;
Dugatkin and Reeve, 1998). This
result receives strong support from studies of behavioral ecology, which show
that asymmetries in such traits often determine the outcome of animal
conflicts (Andersson, 1994).
Even secondary traits that covary with the primary determinant of fighting
ability may be used by a potential combatant to estimate his rival's fighting
ability, and on this basis decide whether to pursue the contest
(Bradbury and Vehrencamp, 1998;
Enquist and Leimar, 1983,
1987).
Agonistic interactions are, however, not always resolved by straightforward
asymmetries in traits important for physical fighting. Instead, differences in
the value of the contested resource to the participating combatants may form
the basis for tactical contest decisions. For example, starved trout
(Salmo trutta) are more competitive in contests over food than are
fish fed to satiation (Johnsson and
Björnsson, 1994). Thus, motivation
to engage in risky fighting behavior may increase as an individual's resource
levels decline. Furthermore, previous investments into a resource, such as
time for courtship, mate guarding and investment into spermatozoa and seminal
fluids, may also motivate an animal to engage in contests to a higher level of
escalation, such as in sand lizards (Lacerta agilis;
Olsson, 1994). The value of
the resource is also set by the frequency with which it is encountered. When
resources are commonplace, selection for simple rules of thumbs for how to
resolve conflicts may be the result. A classical example of this is the
"owner wins" tactics in interactions between thermoregulating
speckle-wood butterflies for a "place in the sun"
(Davies, 1978, but see also
Wickman and Wiklund,
1983).
Prior to statistical analysis becoming commonplace in biology, Rand
(1967) performed a detailed,
graphically based exploratory data analysis of fighting behavior in male
Anolis lineatopus. He came to the conclusion that males incorporate
information about both residency and size of opponents when deciding whether
to escalate or withdraw in contests (Rand,
1967). Other studies suggest that lizard contests may be settled
by residency (Leuck, 1995;
McMann, 1993, in captive
populations), asymmetries in body size
(Olsson and Madsen, 1998),
more rarely in age (Olsson and Shine,
1996), and in several taxa by differences in nuptial coloration
between the contestants (Olsson,
1994; Olsson and Madsen,
1998; Thompson and Moore,
1991; review in Bradbury and
Vehrencamp, 1998). However, to the best of our knowledge there are
no published analyses where data are detailed enough to admit robust tests of
effects of ownership in escalated contests in a natural population of a
reptile species (Olsson and Madsen,
1998). Indeed, there have been relatively few analyses of effects
of ownership where confounding variables (e.g., body size) are controlled for
in any animal species in the wild
(Bradbury and Vehrencamp, 1998;
Riechert 1998,
Table 1, p. 68).
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In the present study of the Tasmanian snow skink (Niveoscincus
microlepidotus), we analyze data on agonistic interactions between
free-ranging, unmanipulated male lizards. The study system that we have used
(snow skinks in the Tasmanian highlands) facilitates the investigation of this
topic. Snow skinks are not strictly territorial and frequently cross into
other males' home ranges in their searches for prey and partners (Olsson and
Shine, unpublished data), which leads to conflicts between males (and
sometimes between females). Furthermore, snow skinks tend to be relatively
long-lived (up to 12-13 years, Hudson, 1996) and, hence, the acquisition of a
resource may incur a long-term benefit in several respects, such as
opportunities for thermoregulation, shelter, and the acquisition of partners.
Additionally, most of the agonistic interactions between male snow skinks
occur immediately after they emerge from hibernation, about 3 months before
the mating season (Olsson, Birkhead, and
Shine, 1999). Thus, males defend actual sites per se, rather than,
for example, guarding receptive females.
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METHODS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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Snow skinks (Niveoscincus microlepidotus) are small (to 70 mm snout-vent length, 5 g) viviparous scincid lizards that are restricted to high-elevation rocky habitats on the island of Tasmania, southeast of the Australian mainland. We studied a population of this species at 1270 m elevation on Mt. Wellington, ca. 15 km S Hobart, for five years (1993-1997). The population was monitored from the time the lizards emerged from hibernation (in September/October) until they returned to their winter quarters in April the following year. The study site was characterized by rocky dolerite outcrops interspersed with hardy vegetation and the honey Richea bush, Richea scoparia, that produces nectar on which the lizards forage throughout the flowering season (ca. December-February, depending on climatic conditions; Olsson M and Shine R, in preparation).
A more detailed description of the methods we used has been published
elsewhere (Olsson and Shine,
1998 and Olsson et
al., 1999) and we, therefore, only give a short summary here.
Snow skinks have no sexually dimorphic conspicuous coloration (to the human
observer) and, hence, the cues to a rival's intentions and fighting ability
come solely from his body size and behavioral displays. We only quantified
morphological traits relevant for a male's fighting ability, such as body size
and head dimensions. Upon emergence from hibernation the lizards were caught
by hand or by noosing. We then measured snout-vent length (heretoforth SVL)
and total length to the nearest mm, and head length and head width to the
nearest 0.01 mm, and weighed each lizard to the nearest 0.01 g before release.
Residual scores from the general linear regression of ln mass versus
snout-vent length were used as our index of body condition in further
analyses. The lizards were marked by placing an oval tape marker (TESA tape,
Germany) on their back with an individual number. The temporary tape marker
usually remained on a lizard's back until its subsequent skin-shedding.
Lizard contest behavior was monitored by watching males interact naturally in the wild. Our observations of male movements indicate that males of this species are not strictly territorial; instead, they have overlapping home ranges. Nonetheless, each male's home range typically contained one or more sites (used for shelter or basking) in which we regularly observed this animal, and no other male. Thus, we could unambiguously assign each male as either a "resident" or an "intruder" at the time of the contest. We then monitored the duration of the interaction (using a wristwatch) and identified which male won the battle. All battles had clear victors, with the winner chasing the loser out of the disputed area.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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We observed 32 interactions involving 49 males for which we could determine identity during the study period. For some contests, we failed to capture one of the interacting males. Thus, the sample sizes for morphological traits in Table 1 differ slightly between the categories. When interactions were separated by more than 2 days, we assumed that contest observations were independent of male identity. Therefore, in a few cases we have used more than one data point per individual male (mean = 1.1 observations/individual, SD = 0.46, range 1 to 3). Three males occurred both as intruders and residents in different contests. Excluding these males did not alter the results in any significant way and they have therefore been left in for the analyses reported on in the present article. Furthermore, no males were "floaters" or were transients migrating through the study site.
Male contests varied considerably in the degree of escalation and duration. Most interactions were resolved by displays, wherein each male turned sideways towards the rival with dorso-lateral distension (median display time equaled one s, mean = 63. 0 s ± 244.0, N = 29). The combat was resolved without physical contact in most of these cases. However, in other contests males fought viciously for up to 20 min, with severe injuries to at least one of the contestants. In several cases, males lost toes and in one instance, an entire foot was torn off. Thus, some fights entail high costs to the interacting males.
Resident males were no larger in bodily dimensions than were intruding
males (see Table 1). This
result accords well with our preliminary observations that geographic areas
were not exclusively monopolized by large dominant males, as is the case in
many other lizard species (reviewed by
Olsson and Madsen, 1998).
Instead, most interactions between males occurred as a consequence of one
animal wandering into another male's home range during foraging bouts and/or
searches for partners. Thus, effects of ownership were not confounded by a
consistently larger body size of the resident male (see
Table 1). We also used paired
t tests to analyze data for encounters where we captured both
participants, to provide a more direct comparison between the individuals
involved in each combat bout. There was still no significant difference
between intruders and resident males in body dimensions (e.g., for SVL, mean
difference = 0.4 mm, t =.50, p =.62; body mass difference
averaged 0.02 g, t =.13, p =.89).
The data in Table 1 were analyzed by t-tests, strongly suggesting that there was no consistent difference in body size (SVL, mass), relative head length or body condition (mass relative to length) between "winners" and "losers" of male-male contests. These analyses were supported by paired t-tests, yielding similar results: the average differences between residents and intruders were 1.26 mm in SVL (± 0.84, SE), 0.22 g in body mass (± 0.16, SE), 0.12 mm in head length (± 0.18, SE), 0.09 mm in head width (± 0.17, SE) and 0.02 in body condition (± 0.09, SE). None of these differences in body dimensions between residents and intruders deviated significantly from zero (0.15 < p <.87; N = 23). Instead, residency determined contest outcomes. Out of 32 resident males observed in contests, 23 (71.9%) were scored as winners and nine (28.1%) as losers (Figure. 1; against a null of equal probabilities of success for each type of male, chi square = 6.1, p =.02, DF = 1).
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Game-theory suggests that when the size difference between two rivals
increases, less time will be necessary to establish which male has the higher
fighting ability and thus, who should withdraw before being injured (e.g.,
Enquist and Leimar, 1983). Our
data support this prediction: The smaller the absolute size difference between
rivals, the longer the contest (rs = -.33, p
=.05, N = 24; one-tailed test). Game theory models such as the
sequential assessment game (Enqvist and Leimar, 1983) also predict that the
longest contests will occur when the resident is marginally smaller than the
intruding rival. This prediction derives from the idea that the asymmetry in
ownership and previous investment motivates the resident to prolong the fight
and take greater risks when defending his resource. Anecdotal observations
from our study does not support this prediction. The resident lizard was
heavier than the intruder (by 3.6% and 9.9% in body mass respectively) in both
of the two longest fights that we recorded (lasting 20 min, and 9 min and 45
sec respectively).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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Our data provide a reptilian example whereby success in a male-male contest is determined not by an animal's fighting ability (as determined by size, etc.), but by ownership (or some trait relating to it). Nonetheless, although the literature contains several examples of this phenomenon in many taxa from isopods to birds, few of these are from natural populations (Riechert, 1998
;
Table 1, p. 68).
We doubt that snow skinks are unique among reptiles in showing
ownership-effects on the outcome of contests. The difficulty of observing
male-male contests of free-ranging reptiles, especially involving
unmanipulated animals, means that there are virtually no empirical data with
which to investigate the issue in a natural population (but see
Olsson 1992;
Rand, 1967; review of studies
in captivity in Olsson and Madsen,
1998). The lack of comparative evidence makes it difficult to
speculate on the conditions that favor the "owner wins" rule in
snow skink contests. Furthermore, in some of the best studied taxa there is
conflicting evidence as to what degree of ownership determines the outcome of
contests. In an English population of the speckled wood butterfly (Pararge
aegeria), the convention "owner wins" seemed to be generally
adopted (Davies, 1978). In a
Swedish study of the same species, however, removed owners were engaged in a
prolonged spiral flight on rerelease on their former territory and, although
the original territory owner eventually got his territory back, a different
convention with respect to escalation appeared to rule
(Wickman and Wiklund, 1983).
Davies suggested that in the English population, warm sunny spots were more
plentiful than in a cold Swedish spring and, hence, resource values differed
between the two sites (see discussion of this and related example in
Bradbury and Vehrencamp, 1998,
pp. 712-720). A similar argument may apply in snow skinks to a greater degree
than in many other reptiles. In their severely cold environment, a home range
with a protected basking site is likely to be crucially important to
organismal fitness. Given the very high population densities of lizards at our
study site (several hundred/ha; Olsson and Shine, unpublished data.), very few
such sites are unoccupied. Thus, a resident has much to lose from being
displaced, where the intruder has the option of simply returning to his usual
home-site. Such circumstances may generate a substantial asymmetry in the
value of the resource (the basking site) to the two participants, and thus
favor considerably enhanced motivation for the resident to escalate contests.
The enhanced probability of successful basking-site defense could even result
in an on average elevated body temperature of a resident, thereby making him a
more successful fighter (Edsman L, unpublished data, cited in
Olsson and Madsen, 1998).
An alternative hypothesis to the higher incidence of wins in resident than
in intruder males is that resident males are simply better fighters than are
intruders and that this quality is independent of male size. In several other
species, such as Midas cichlids (Cichlasoma citrinellum;
Barlow et al., 1986)
and tree lizards (Urosaurus ornatus;
Thompson and Moore, 1991),
fighting ability is strongly influenced by some innate level of aggression.
This could, for example, be set by plasma levels of testosterone,
corticosteroids, or growth hormone that we did not sample in the present
study. If lizards differ in this way, better fighters may be more likely to
defend sites against intruders. However, this hypothesis seems unlikely to
explain the snow skink data because intruders were not "homeless"
but males defending adjacent sites (i.e., being residents under such
conditions).
In conclusion, male snow skinks engage in contests that may involve a high
level of risk in the sense that entire limbs may be torn off during its
escalated phase. Our data demonstrate that fights do escalate between
residents and intruders and, hence, although resident males win more contests,
there is no strict adoption of the rule "owner wins." Although we
could not detect a size difference between intruders and resident males, we
cannot rule out the possibility that there is a difference in some other
aspect of fighting ability between the two categories of males that also
contributes to the success of resident males. However, the primary determinant
of male-male contests in lizards has been repeatedly demonstrated to be body
size (Olsson and Madsen,
1998). Because this factor can be ruled out in the present study,
there seems to be little doubt that ownership plays a significant part in
determining the outcome of contests in this species.
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ACKNOWLEDGEMENTS |
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We are grateful to the Australian Research Council and the Swedish Natural Science Research Council for financial support, to Tobbe Helin, Erik Wapstra, and Lollo Ba'k-Olsson for assistance in the field, and to the Wilkes families for encouragement and logistic support.
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