Social influence on the use of a modifiable status signal

Abstract

Abstract. Recent studies have revealed that male tree lizards, Urosaurus ornatus , have two sexually dimorphic colour badges (throat colour and dorsal darkening) that function as status signals. In high-density populations, males may darken dorsally to signal dominance; this is a dynamic, modifiable badge. To ascertain the circumstances under which this showy trait is used, tree lizards were observed under varying social conditions in large (2 × 4 m) outdoor pens containing clusters of cinder blocks to simulate the natural environment. The three social conditions consisted of the following groups (nine or 10 replicates of each): three males with one female (MMMF), three males (MMM) and one male with two females (MFF). Behaviour and dorsal colour of the males in these groups were monitored every other day over a 28-day period using the scan-sampling technique. When females were present (MMMF and MFF), one male in each group exhibited dominance behaviour and darkened dorsally some of the time; in the absence of females (MMM), males infrequently exhibited dominance behaviour and dorsal darkening. In groups with multiple males (MMMF and MMM), dominant-subordinate relationships consistently arose only when a female was present (MMMF). The dominant male in each group displayed aggressively significantly more often when both other males and a female were present (MMMF). Thus, males use the dorsal badge to signal their dominant status and exhibit territorial behaviour most readily when females are available to defend.