Archive file# r062600a
donated by James Vandale
Lizard Tough Guys
Allison Alberts, Ph.D.
The Vivarium, September/October 19923, 5(2):24-25, 30.
space require-ments and the distribution of key resources in the
environment, social behavior among lizards can vary from defense of
exclusive territories to the formation of dominance hierarchies. As
anyone who has watched large groups of lizards is aware, increases in
population density usually lead to more social interaction and
heightened aggression, especially among mates. In captivity, where
dispersal is not possible, dominance hierarchies tend to emerge even
in species that appear to be strictly territorial in nature
(Brattstrom, 1974). Dominance hierarchies can become established
quite quickly among captive lizards, almost always within a day, but
sometimes after only a few hours (Carpenter, 1967).
In many lizard
species, dominance hierarchies are characterized by a pecking order,
in which higher ranking individuals dominate those of lower social
rank. In other species, a single highly dominant individual
tyrannizes all others in the local vicinity. In either case, the
potential benefits of dominant social status are many. For
carnivorous species such as Komodo monitors, Varanus komodoensis,
feeding order at carcasses depends on size and social status.
Dominant male brown anoles, Anolis sagrei, are far more
successful at acquiring and defending preferred perch sites than are
subordinate males (Tokarz, 1985). Higher ranking spinytailed iguanas, Ctenosaura
sp., tend to occupy the best rocky crevice retreats, restricting
lower ranking males to lower, more marginal areas (Carothers, 1981).
Perhaps most importantly, dominant males in virtually all lizard
species are much more likely to attract females and mate successfully.
The benefits of
being a dominant lizard do not come without costs, however. Compared
to subordinates, dominant males probably consume a great deal more
energy, suffer from increased exposure to predators, and run a higher
risk of serious injury during fights. In male mountain spiny lizards, Sceloporus
jarrovi, dominance is associated with higher mortality, probably
as a result of the tremendous amount of time and energy expended in
aggressive interactions (Marler and Moore, 1988). In order to avoid
the costs of escalated interactions with dominant males, subordinate
male lizards often conspicuously advertise their low status. For
example, subordinate male anoles become darker in the presence of
dominants, and subordinate male spiny lizards adopt submissive
postures which hide their bright blue belly colors. In bearded
dragons, Pogona (= Amphibolurus) barbota,
submission is signaled by a distinctive overhand wave (Carpenter et
To begin to
understand the significance of these behaviors for herpetoculturists
wishing to maintain healthy, active colonies, it is useful to explore
some of the causes and effects of dominance hierarchies in captive
lizards. Much of the available evidence suggests that hierarchical
behavior is promoted by spatial clumping of the resources on which
lizards depend. For example, when separate rock piles are provided
for captive spiny-tailed iguanas, Ctenosaura hemilopha,
each individual successfully defends his own rock pile (Brattstrom,
1974). When the same rocks are combined into a single pile, however,
a sizebased dominance hierarchy develops. Crevice dwelling night
lizards such as the granite night lizard, Xantusia henshawi,
and the nocturnal leaftailed gecko, Phyllodactvlus xanti, are
territorial and defend cracks in rocks. When the number of available
cracks is reduced, lizards begin to form hierarchies and subordinate
individuals are forced to accept less desirable cracks (Brattstrom, 1974).
of social subordination in lizards appear to extend far beyond
restricted access to preferred sites within enclosures. Among captive
sixlined racerunners, Cnemidophorus sexlineatus, subordinate
individuals lose weight over the course of the breeding season and
may exhibit other physiological signs of social stress (Brackin,
1978). When male green anoles, Anolis carolinensis are housed
in pairs, one male invariably becomes dominant. Levels of
corticosterone, the major steroid hormone associated with stress in
lizards, are much higher in subordinate males, which are also less
advanced spermatogenically (Greenberg et al., 1984). Dominant male
anoles tend to monopolize positions that afford the best access to
heat and light, preventing subordinates from thermoregulating
normally and rendering them more vulnerable to a variety of
infectious agents (Bels, 1984).
Our own research
on green iguanas, Iguana iguana, at the San Diego Zoo has
taught us much about how dominance hierarchies are established and
maintained in this species. Among adult green iguanas inhabiting a
large outdoor exhibit, we found that high social rank was associated
with elevated levels of testosterone, highly developed jaw
musculature, and a greater incidence of headbob display (Pratt et
al., 1992). Dominant males won more aggressive encounters and spent
more time patrolling a centrally located rock where most of the
females preferred to bask. In addition, top ranking males possessed
greatly enlarged femoral glands compared to subordinate males
(Alberts et al., 1992). Presumably, dominant males that produce more
secretions can scent mark their home ranges more effectively and as a
result may be better able to attract females and defend them from
male green iguanas, dominance relationships are established
surprisingly early in development. We investigated the effect of
limited resources on the establishment of dominance hierarchies by
housing a group of ten day old hatchlings such that the available
heat resources were restricted to only 10% of the perches in the
enclosure (Phillips et al., 1993). A second group was housed in a
similar enclosure, but with the available heat spread out more evenly
over 50% of the perch sites. In the group with limited heat
resources, a dominance hierarchy developed among the males within the
first month of life. Approximately a third of the males monopolized
all of the heated perches, denying access to the remaining
individuals. By the end of the study, the socially dominant males had
grown significantly larger than their subordinate counterparts. In
the group without limited heat resources, all males grew at
approximately the same rate. Interestingly, hatchling females were
not excluded from heat resources by either males or by each other,
and their growth rates remained unaffected by the distribution of
heat resources within the enclosure.
It appears that
the presence of dominant adults can profoundly affect the growth and
health of juvenile green iguanas. For one year, a group of juvenile
males at the zoo was exposed to the sight of an adult female, a
second group was exposed to both the sight and smell of an adult
female, a third group was exposed to the sight of an adult male, and
a fourth group was exposed to both the sight and smell of an adult
male. Juvenile males continuously exposed to chemical and visual
signals from adult males showed signs of chronic stress, including
reduced growth rates, lower testosterone levels, higher
corticosterone levels, and fewer headbob displays. Although the sight
of an adult male alone was sufficient to induce some of these
effects, visual and chemical signals appeared to act in concert to
influence the behavior and physiology of juveniles.
Although there is
still much to learn, these and other studies provide clues about the
basic factors that contribute to the formation of dominance
hierarchies in captive groups of lizards. Of primary importance
appears to be the distribution of essential resources, including
perch sites, basking sites, heat sources, and retreats. When
resources are clustered rather than uniformly distributed throughout
the enclosure, dominance hierarchies will almost always result. For
those herpetoculturists interested in observing varied and sometimes
vigorous social interactions, this may not be an undesirable outcome.
However, if successful captive breeding is the ultimate goal, it may
be preferable to house lizards in pairs or single male groups to
avoid the inevitable stress associated with male competition. In
particular, the presence of adult males, even when there is no
physical contact, can impede the normal developmental process in
young males and should be avoided when rapid sexual maturation is
desired. It is clear that the more we can learn about dominance
interactions and how they affect reproductive processes in lizards,
the better equipped we will be to interpret their behavior and manage
them successfully in captivity.
N.C. Pratt, and L.A. Phillips. 1992. Seasonal productivity of lizard
femoral glands: relationship to social dominance and androgen levels. Physiol.
Bels, V.L. 1984.
Ethological problems of anoline lizards in captivity. In Acta
Zool. Path. Antverp. V.L Bels and P. Van Den Sande (eds), 78:85-100.
1978. The relation of rank to physiological state in Cnemidophorus
sexlineatus dominance hierarchies. Herpetologica 34:185-191.
1974. The evolution of reptitian social behavior. Am. Zool. 14:35-49.
1967. Aggression and social structure in iguanid lizards. In Lizard
Ecology: A Symposium, W.W. Milstead (ed.), pp. 87-105.
University of Missouri Press, Columbia.
LA. Badham, and B. Kimble. 1970. Behavior patterns of three species
of Amphibolorus (Agamidae). Copeia 1970:497-505.
1981. Dominance and competition in an herbivorous lizard. Behav. Ecol.
Greenberg, N., T.
Chen, and D. Crews. 1984. Social status, gonadal state, and the
adrenal stress response in the lizard Anolis carolinensis. Horm.
Marler, C.A. and
M.C. Moore. 1988. Evolutionary costs of aggression revealed by
testosterone manipulations in freeliving male lizards. Behav.
Ecol. Sociobiol. 23:21-26.
J.A.,A.C. Alberts, and N.C. Pratt. 1993. Differential resource use,
growth, and the ontageny of social relationships in the green iguana. Physiol.
Pratt, N.C., A.C.
Alberts, K.G. FultanMedler, and LA. Phillips. 1992. Behavioral,
physiological, and morphological components of dominance and mate
attraction in male green iguanas. Zoo Biol. 11:153-163.
1985. Body size as a factor determining dominance in staged agonistic
encounters between male brown anoles (Anolis sagrei). Anim. Behav.
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