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 al., 1970).

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).

The consequences 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 rival males.

Among juvenile 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.