Home ranges of horned lizards (Phrynosoma): circumscribed and exclusive?

Two aspects of the use of space were studied for two species of horned lizard: (1) Do they restrict their movements to limited areas or do they wander more or less randomly? (2) Is overlap between home ranges reduced below what it would be if home ranges were placed at random with respect to one another?Forty-nine individuals were tagged with radiotransmitters or radioactive tags and their movements compared to a computer-generated random model. It was found that horned lizards typically occupy areas much smaller than they would if they moved randomly, that is, they occupy a limited home range. "Home ranges" calculated from location points generated by computer randomizations share several characteristics with the limited home ranges of lizards not moving randomly. Randomly derived location points can be surrounded by a minimum convex polygon, or characterized by a probability density function or utilization distribution, and lead to a percent-change in area vs. capture number curve very similar to that shown by lizards not moving randomly. Therefore, none of these techniques distringuishes between home ranges that result from nonrandom use of space and those that result from a randomly moving animal. Previous studies have failed to draw the distinction between these two types of home range. This distinction should be recognized, however, because the underlying processes differ so dramatically.Observed home range overlap was compared with overlap that would be expected if home ranges were located at random with respect to one another. In one of the years tested, reduced overlap was detected at a marginally significant level. This pattern becomes stronger if only overlap with conspecifics is considered. This strongly suggests that home range overlap is reduced between horned lizards. Contrary to expectation, overlap between individuals of opposite sex tended to be less than overlap between individuals of the same sex.