The territorial defense hypothesis and the ecology of insular vertebrates

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.

Rainfall, activity and social behaviour in the lizard, Anolis aeneus

The effects of drought periods on the behaviour of a tropical lizard (Anolis aeneus) were measured. The number of lizards active at study sites and display frequencies varied inversely with days since rainfall. Animals that were artificially watered during a drought drank and showed an immediate recovery to prior, non-drought display levels. Rainfall records indicate that drought periods of the duration studied here are frequent in the natural habitat.

Variation and Stereotypy in the Displays of Anolis Aeneus (Sauria: Iguanidae)

Abstract

The reactions of a territorial male lizard (Anolis aeneus) to an intruder were filmed and analyzed. Five displays were given to the intruder: Fanbob, Multibob, Jerkbob, Gorged-throat and Fan. Fanbob consists of the species-specific signature bob and dewlap extension and is the normal display given as a challenge to intruders. Multibob, another bob pattern, occurs when a degree of anxiety is present. It closely resembles the Courtship Bob, so the male is in effect giving an appeasement display while courting a female. Jerkbob is used as an introductory display preceding Fanbob during territorial advertisement. Gorged-throat is a threat display, and Fan may be used instead of Fanbob when inhibitory factors prevent Fanbob from being expressed. Fanbob was analyzed to test whether it fulfills the criteria of the Fixed Action Pattern. The cadence of the signature bob was individual-specific and stereotyped, but other components of Fanbob varied greatly. Among these were presence and absence of introductory movements (related to intruder distance), degree of sideflattening and degree of dewlap extension (related to resident aggressiveness), and height of head during the display. Height and dewlap extension, while necessary and integral components of Fanbob, varied independently, and were correlated with different sets of stimulus and response variables. Fanbob was thus shown to fit none of the criteria of a fixed action pattern.