Insular lizards, birds, and mammals in high-density populations often exhibit reduced situation-specific aggression toward conspecifics. This aggressive behavior can be expressed in the form of (1) reduced territory sizes, (2) increased territory overlap with neighbors, (3) acceptance of subordinates on the territory, (4) reduced aggressiveness to certain classes of conspecifics, or (5) abandonment of territorial defense. These behavioral traits can be explained by two nonexclusive hypotheses. The resource hypothesis suggests that territorial behavior is primarily adjusted to resource densities, and that resources are more abundant on islands than on the mainland (e.g., because of a lack of competing species). The defense hypothesis suggests that, in addition to any effects of resources, the costs of defense against both territorial intruders and contenders for vacant territories are higher on islands. Recent theoretical and empirical studies indicate that these behavioral changes can occur as a result of elevated defense costs, independent of resource densities. Reduced predation, more benign climates, and an absence of habitat sinks on islands would all tend to increase the density of potential intruders and contenders, and hence the costs of defense for owners of insular territories. The two hypotheses differ in their predictions about the rates of biomass production (growth or reproduction) for holders of insular territories. Reproductive and growth data from insular-mainland pairs indicate the importance of elevated defense costs, and also suggest that many insular vertebrates reallocate their breeding resources so as to produce young that are more competitive. The suite of ecological and behavioral traits exhibited by insular territorial vertebrates can best be explained by three factors operating in concert: higher available resource densities, higher defense costs, and (sometimes) a reallocation of resources to produce young that are more competitive.
A model of vertebrate nata dispersal is developed and compared to observed patterns of dispersal distances for a variety of animals. The model, which has the same basic formulation as Waser's competition model of dispersal, is based on assumptions of the divisibility of the habitat into discrete units capable of supporting a resident, and of a constant probability of a disperser stopping as it crosses each of these units. This probability is decomposed into the probability of settling and the probability of dying. Comparisons to observed data indicated that the model is adequate to describe the pattern of dispersal distances observed in many vertebrates. This model has advantages over other treatments in that it is simple to manipulate mathematically, and it has parameters that are relatively easy to estimate from field data and are directly relevant to the behavior, ecology, and demography of dispersing animals. In addition, the theoretical distribution is in a form that makes statistical tests of the fit to observed data straightforward. Comparisons of parameter values and deviations from the geometric pattern between groups within taxa indicated that males and females may follow different patterns of dispersal. Comparisons among taxa indicated that mammals and birds show consistent differences in dispersal distributions. These results are relevant to discussions of natal philopatry, inbreeding avoidance, and proximate mechanisms of dispersal.
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