Mating system structure and population density in a polygynous lizard, Sauromalus obesus (= ater)

Increases in population density often are associated with a change in mating system structure in numerous taxa. Typically, male interactions are minimal in extremely low density populations. As density increases, males exhibit territoriality but if density becomes too high, the energetic cost of defending a territory will eventually outweigh the reproductive benefits associated with territoriality. Consequently, males in high density populations may abandon territoriality and adopt dominance polygyny, lekking behavior, or scramble competition. We investigated the relationship between population density and mating system structure in three populations of the chuckwalla, Sauromalus obesus (= ater), near Phoenix, Arizona. Densities in the Phoenix Mountains (2.7 chuckwallas/ha) were lower than any population previously studied. In the Santan Mountains (10.9 chuckwallas/ha), densities were similar to populations studied in the Mojave Desert, and in the South Mountains (65 chuckwallas/ha), densities were the highest yet recorded. Male mating behavior was examined by determining home range overlap and by making direct behavioral observations. Male home range size decreased with increasing population density. There was little overlap in home ranges among males in all three populations, whereas home ranges of males and females consistently overlapped, indicating that males were strictly territorial. This conclusion was supported by behavioral observations of interactions among individuals in a natural setting. The number of females wihin male territories was correlated with food resources (plants) in all three populations. Female home range size appeard to be related to food resources whereas male home ranges appeared to be related to female distribution, population density, and geology. The retention of territoriality in spite of high population densities raises new questions about the relationship between density and resource defense. Copyright 2002.