A field study of the social structure, mating system, and display behavior of the green iguana

During the non-breeding season, the iguanas were sporadically active. They ate leaves, flowers, and fruits, and movements were determined in part by patterns of food availability. The iguanas maintained overlapping home ranges. The largest males tended to avoid one another; rarely did 2 large males remain in the same location for more than a few hours. Large males confined their movements to an area that was roughly 1/3 the size of the home ranges of other members of the population.

Mating activity began in the last half of October, with increasing rates of display by large males. Male-female encounter rates were high in November. By late November, large males had established mating territories containing 1 - 4 females. . Copulation occurred during the first 6 weeks of the dry season, from December through mid-January.

Male breeding strategies were related to size and the development of secondary sex characteristics. Small reproductively mature males, indistinguishable from females at a distance, moved into territories and mated opportunistically. Most medium males remained on the edge of territories, and assumed ownership of vacated territories. Large males maintained their status in a circumscribed area year-round, and thus insured priority of access to suitable mating territories in November. Large males, 38% of total males, performed significantly more copulations (90% of observed) than did small or medium males.

Females mated preferentially with large males, ignored medium males, and violently protested copulation attempts by small males. Extensive courtship with several males before females established residence in a territory, and immediately prior to copulation, suggests that courtship is an important factor in female mate choice, in cementing bonds of familiarity between mates, and in facilitating female receptivity. Females mated several times during a single breeding period, usually with a single male. The adaptive significance of male and female mating strategies is discussed.

Agonistic encounters among males were rare. Spacing was mediated primarily by visual displays. Large males displayed more frequently than other members of the population; display rates were highest between mid-October and mid-Janaury. The conspicuous nature of the headbob displays was enhanced by the intensified color of territorial males, and by the selection of highly visible display posts. Headbob displays play an important role in both territory establishment and mate acquisition.

Four headbob displays were identified, and the variability and contextual use of each were analyzed. The roll, high amplitude vertical and lateral (head rotation) head movements, was performed most frequently when the male was solitary or greater than 3 m from a female. The shudder, a series of shallow rapid vertical bobs, was performed most frequently when females were within 3 m of a male, and during male-female interactions throughout the year. The roll-shudder, rapid alternations of rolls and shudders in variable sequences, occurred during transitions between social contexts, and between sequences of rolls and sequences of shudders. The signature bob, the most frequently occurring display, was used in a wide variety of contexts; this bob punctuated all other activity.

The temporal variability of each display is related to its contextual use. The roll is less variable than the shudder or roll-shudder, but much more variable than the signature bob. The roll is a conspicuous advertisement display; it may function to call attention to the signature bob that invariably follows. The shudder is the most variable of the 4 displays. Shudder duration was significantly longer during male-female interactions. This display may communicate the male's lack of aggressiveness, and its duration appears to reflect level of arousal. The roll-shudder is intermediate between the roll and shudder in its form, variability and use. The signature bob, the most stereotyped display, encodes species identity, and possibly population and individual identity. Interindividual variability was significantly greater than intraindividual variability in 12 analyzed components of this display. The variability in form and use of the headbob displays was interpreted in terms of the problems that an iguana displaying to wide-ranging conspecifics in a forest environment must solve.