Late Summer Behavior of the Lizards Sceloporus merriami and Urosaurus ornatus in the Field

Milstead, William

Late summer behavior of the sympatric iguanid lizards, Sceloporus merriami and Urosaurus ornatus, was studied in and near Big Bend National Park in southwestern Texas in the summer of 1958-1963 and 1966. Both species are strongly territorial and display toward trespassing members of their own species. No interspecific displays were witnessed between adults. The displays of U. ornatus like those of certain other iguanid lizards involve challenging pushups, lateral face-offs, and a terminal chase or fight. S. merriami has the same general pattern, but has elaborated it into a preliminary challenge, challenge, circle, and fight. The preliminary challenge consists of the challenging pushups of U. ornatus and other iguanids. The challenge is a modified pushup in which the lizard stands motionless at the peak of a pushup with its head and tail elevated and its colors fully exposed. The challenge is slowly and deliberately given in a single pushup, rather than as an extreme peak of several pushups. The circle evolved from the lateral face-off display by both lizards moving in tandem in a circle as they do pushups. The fight or chase-fight is a brief encounter in which one lizard butts the other and both fall a few feet from a boulder or canyon wall. The display in U. ornatus lasts only a few minutes and the chase or fight is the most active phase of the encounter, but in S. merriami the display may last for over 4 hours and the circle is the most active and lengthy phase of the encounter. Thus, emphasis is on the display in S. merriami, and such emphasis may have been favored by selection because it reduces risk from injury or predation and conserves energy. U. ornatus appears to be polygamous during its breeding seasons, while S. merriami appears to be monogamous. Female U. ornatus do not participate in territorial defense, but female S. merriami are almost as protective of the terroritories as the males. Variations in the intensity of the behavior pattern of S. merriami appear to be related to density. At Lajitas, Texas, density is low and displays are more intense and lengthy than they are in Big Bend National Park where density is high. Territories overlap in the Park, but not at Lajitas. Frequency of contact is presumed to lessen the intensity of behavior in the Park. Food is presumed to be more abundant in the Park than in Lajitas. Thus, one function of territoriality and territorial displays may be to space animals in relation to the available food supply.

Late Summer Behavior of the Lizards Sceloporus merriami and Urosaurus ornatus in the Field

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