Behavioral, physiological, and morphological components of dominance and mate attraction in male green iguanas

Zoo Biology 11:153-163 (1992)

Behavioral, Physiological, and Morphological Components of Dominance and Mate Attraction in Male Green Iguanas Nancy C. Pratt, Allison C. Alberts, Kathryn G. Fulton-Medler, and John A. Phillips

Center for Reproduction of Endangered Species, Zoological Society of San Diego, San Diego, California

This study investigated the morphological, physiological, and behavioral components of social dominance important for mate attraction in male green iguanas (Iguana iguana). A group of 9 male and 1 1 female adult green iguanas was studied in a large semi-natural enclosure during one reproductive season (Octo- ber-January). Four of the nine males never initiated aggressive encounters; the other five were observed to display aggressively toward each other and were ranked in a linear dominance hierarchy. Head size was the most important factor influencing fighting success. Head size and display frequency were positively correlated with plasma testosterone levels. Dominance rank directly influenced ability to monopolize areas containing resources used by females. The quality of a male’s home range, measured as his access to a large basking rock in the enclosure, was related to the proportion of potential mates found within his home range. One male greatly surpassed the others in his ability to defend a home range of high quality and attract potential mates. These data suggest that physiological and morphological factors, through their influence on social behavior, may ultimately affect male reproductive fitness. 0 1992 Wiley-Liss, Inc.

Key words: social dominance, Iguana iguana, male mating strategy, mate attraction

INTRODUCTION

Iguanid lizards are particularly well-suited for investigating the effects of environmental and hormonal cues on agonistic and reproductive behavior. Their physiology, behavior, and natural history have been well-documented and they

Received for publication September 4, 1991; revision accepted February 7, 1992. Address reprint requests to N.C. Pratt, C.R.E.S., Zoological Society of San Diego, P.O. Box 551, San Diego, CA 92112-0551.

0 1992 Wiley-Liss, Inc.

154 Pratt et al.

exhibit their natural behavioral repertoire in captivity [Crews, 1978; Phillips, 19901. The green iguana (Iguana iguana), a large, long-lived, arboreal iguanine native to neotropical lowland forests, displays a broad range of social behaviors, especially during the breeding season [Dugan, 1982a,b]. In the wild, this species exhibits an extensive courtship period for several weeks prior to pair formation and at least two additional weeks prior to copulation [Dugan, 1982bl. Males may mate with more than one female (females are moderately asynchronous), and territory ownership is crucial to male reproductive success, because nearly all courtship and copulation occur within defended territories. Females usually mate with only one large male, although they may encounter several potential mates in a given breeding season [Dugan, 1982b3.

In the wild, male green iguanas display three generalized mating strategies [Dugan, 1982b; Rodda, 19921. Typically, large, dominant males defend mating temtories (one or more tall trees), attract most of the reproductive females, and perform most (90%) of the successful copulations. Medium-sized males remain on the periphery of mating territories held by larger males, and may take over a territory if a dominant male dies or becomes unable to defend its temtory. Small males appear and behave like females, enter the dominant males’ temtories unnoticed, and attempt to mate opportunistically through forced copulation. Small males may have greater mating success than medium-sized males; they attempt copulations significantly more often [Dugan, 1982bl. The physiological and morphological components underlying these male mating strategies have not been assessed in natural populations.

Green iguanas communicate using stereotyped visual displays. The species- specific signature display is used to advertise spatial position, especially as an individual moves into a new area [Dugan, 1982al. A second display type, the rotary head-nod, is probably an active display of aggression [Distel and Veazey, 19821. During male-male aggressive encounters, displays may be accompanied by lateral body compression, dewlap extension, tail slapping, biting, and hissing [see Distel and Veazey, 1982; Dugan, 1982al. Display rates in other species of lizards have been positively correlated with circulating androgen levels [Marler and Moore, 19881 and negatively correlated with glucocorticoid levels [Tokarz, 19871. Green iguanas probably also communicate through chemical signals [Burghardt et al., 19861. Exocrine glands in the femoral region exude secretions through pores. The size of the femoral pores is much larger in males than in females [Rodda, 19911, and secretory activity is seasonal, related to hierarchical rank among males, and may be under androgenic control [Alberts et al., 19921.

This study was designed to investigate parameters associated with male dominance and mate attraction in captive green iguanas that are difficult to assess in natural populations. Specifically, the investigators aimed to examine the morphological, physiological, and behavioral characteristics associated with social dominance and its potential influence on male mating strategy. Because it is an important factor in territory ownership and access to reproductive females, it was hypothesized that social dominance in males would be associated with high levels of testosterone and/or low levels of corticosterone, large body size, and aggressive behavior. Although small sample size limits the conclusions that can be drawn from this study, relationships between body size, hormone levels, and aggressive behavior are discussed in the context of the natural history of the species.

Mate Attraction in the Green Iguana 155

MATERIALS AND METHODS Subjects

A captive breeding group of 9 male and 11 female adult green iguanas (>5 years of age) housed in a large outdoor enclosure (264 m2) was studied during one breeding season over a period of 4 months at the San Diego Zoo (32"N). Although the density of iguanas kept at the zoo varies by 2-10 times the density found in natural populations [see Dugan, 1982b], normal reproductive patterns and social interactions are exhibited. These lizards were wild-caught as hatchlings in Central and South America, and were housed together as adults at the zoo for more than 2 years before the study began. The enclosure included four small trees, several small bushes, a lily pond, and a large synthetic rock (20 m3). Because green iguanas are heterothermic, they must spend a significant amount of time each day absorbing heat energy. The rock was south-facing and was the site within the enclosure where lizards could bask most efficiently. Trees in the enclosure were protected to prevent iguanas from climbing. Animals were maintained on a standard zoo diet of fresh fruits and vege- tables, supplemented with commercial vitamins and minerals.

Variables Measured From October through January, the breeding season for wild iguanas, several

morphological and physiological parameters were measured weekly, and behavior was observed 3 dayslweek for 2 hr at mid-day. At the outset of the study, body mass, snout-vent length (SVL), and head width were recorded for each individual. Elec- tronic calipers (Fowler Ultra-Cal 11, Newton, MA) were used to measure the diam- eters of the two most proximal femoral pores on each leg. The number of attacks (lunges or bites), tail slaps, and supplantations were recorded on a score sheet by an observer for each male-male aggressive encounter. An individual was determined to win an aggressive encounter if it forced the other to retreat at least 5 m. Rates of signature displays and rotary head-nods were also recorded for each individual. Four of the nine males never initiated aggressive encounters with other males. The other five males were observed to display aggressively toward each other and were ranked in a linear dominance hierarchy based on the outcomes of 130 observed interactions during the 4-month breeding season. Ordinal dominance ranks were determined by a computer program that created a win-loss matrix of paired encounters which mini- mized reversals (the number of times that a lower-ranking individual defeated one of higher rank) [Boyd and Silk, 19831. Cardinal ranks were not determined because of the extreme linearity of the hierarchy.

Data on the location of individuals were recorded on a map of the enclosure every 5 min during the observation periods in order to establish home ranges of the males and to assess the spatial preferences of the females. Home range sizes were mapped by a probabilistic method estimating the area which encompassed 95% of each lizard's total utilization of space [Anderson, 19821. Home range quality was determined by calculating the percentage of days on which a particular home range map included part of the large basking rock in the enclosure. Female proximity (the percentage of females associated with certain males) was determined by taking the mean of the number of females within 2 m of each male divided by the total number of females present in the enclosure on each day.

156 Pratt et al.

Blood samples were collected for analysis of testosterone (T), corticosterone (B), and 17P-estradiol (E,). B is the major adrenal glucocorticoid in lizards [Sandor et al., 19761, and E, accounts for 60-98% of measurable estrogens in female iguanas [Phillips et al., 19871. All samples were taken weekly between 0800 and 1000 hr to control for circadian rhythmicity. Blood samples were also collected monthly for 8 months past the end of the breeding season, so that annual steroid profiles could be generated. This species is large enough to allow repeated blood sampling, and blood is obtained from the caudal vein without difficulty [Esra et al., 19751. Since these animals had been handled regularly by keepers and veterinarians for at least 2 years prior to this study, steroid responses to handling were probably minimal.

Radioimmunoassays Blood was collected into EDTA-treated evacuated collection tubes and centri-

fuged at 2,000 rpm for 15 min at 4°C. The plasma was then stored at -20°C until assayed. Radioimmunoassays for T, B, and E, were performed by the methods described in Lance and Lauren [1984]. The antibodies used in the assays were obtained from ICN Biomedicals, Inc. (Costa Mesa, CA). Each male sample was assayed in 0.05 ml duplicate aliquots and each female sample in 0.1 ml duplicate aliquots. The intra- and inter-assay coefficients of variation for T were 2.8% and 14.4%, respectively, calculated from duplicate determinations for three assays (sensitivity = 10 pg/tube). The intra- and inter-assay coefficients of variation for B were 3 .O% and 12.0%, respectively, calculated from duplicate determinations for five assays (sensitivity = 25 pg/tube). The intra- and inter-assay coefficients of variation for E, were 7.0% and 9.3% respectively, calculated from duplicate determinations for four assays (sensitivity = 5 pg/tube).

Statistical Analyses Mann-Whitney U tests were used to detect differences in density of females on

and off the basking rock, and differences in steroid levels between hierarchy and non-hierarchy males. Morphological and behavioral differences between hierarchy males, non-hierarchy males, and females were analyzed with Kruskal-Wallis tests followed by non-parametric pairwise multiple comparisons tests. All tests were cor- rected for unequal sample sizes and tied ranks, if necessary. A schematic flowchart was derived using stepwise multiple regressions to examine the relationships between 12 measured variables while controlling for correlations among them. In each case, 11 variables were regressed against the one remaining variable. Every variable which added significance to each regression was included in the final flowchart. A model explaining the highest proportion of variance for each variable was obtained under the assumption that physiology influenced morphology, which, in turn, affected behavior. This assumption was based on past studies of the relationships between hormone levels, body size, and growth rates and aggressive behavior and display rates in male green iguanas [Alberts et al., 1992; Phillips et al., in review; Pratt et al., in review]. Relationships among independent morphological and physiological variables were analyzed using Spearman rank correlations. Statistics were performed using Statview@ software (Brainpower, Inc., Calabassas, CA).


TABLE 1. Mean (* SE) morphological and behavioral measurements for adult green iguanas during the breeding season

Head Femoral pore Signature Rotary Group N SVL (cm) width/SVL diameter/SVL displays/hr displaydhr

Hierarchy males Non-hierarchy males 4 37.6 f 1.7 0.213 f 0.009 0.047 f 0.004 0.04 f 0.03 0.02 f 0.01 Females 11 36.6 f 0.7 0.119 f 0.002 0.021 f 0.002 0.39 f 0.13 0.03 f 0.02

*Significant difference compared with non-hierarchy males and with females. **Significant difference compared with females only.

5 41.2 f 1.4 0.268 f 0.023* 0.073 f 0.006** 12.66 * 3.99* 2.14 -t 0.88*

RESULTS

Males within the hierarchy differed significantly from non-hierarchy males or females in several morphological and behavioral variables (Table 1). The four non- hierarchy males were distinctly female-like in their body and head size, femoral pore size, and display rates. Although not discussed in this study, these males, like the females, appeared more green and less orange in color than hierarchy males [see Dugan, 1982a; F’ratt et al., in review; Swanson, 19501. The mean SVL of males in the hierarchy was larger than that of the other two groups, although the differences were not quite significant (H = 0.1; P < 0.1). Head width varied significantly among the three groups (H = 11.1; P < 0.005). Hierarchy males had wider heads than non-hierarchy males (Q = 3.4; P < 0.002) and females (Q = 3.6; P < 0.002). There was no difference between head widths of non-hierarchy males and females. Femoral pore diameter showed a similar pattern (H = 14.4; P < 0.001), but only the difference between hierarchy males and females reached significance (Q = 3.9; P < 0.001). There were differences between the three groups in rates of signature (H = 12.0; P < 0.005) and rotary (H = 10.9; P < 0.005) display. Hierarchy males signature-displayed more frequently than non-hierarchy males (Q = 3.4; P < 0.003) and females (Q = 2.4; P < 0.05), and rotary-displayed more frequently than non- hierarchy males (Q = 2.6; P < 0.05) and females (Q = 3.1; P < 0.01). There were no significant differences in rates of display between non-hierarchy males and females.

The daily mean density of female iguanas on the basking rock (0.10/m2) was significantly greater (Z = 5.8; P < 0.01) than the mean density of females positioned elsewhere in the enclosure (0.02/m2). Therefore, males that occupied home ranges on or near the rock (high home range quality) had access to more females (female proximity) than males that did not (r, = 0.95; P < 0.05). One male greatly surpassed the others in his ability to attract potential mates (Table 2). This male defended a home range which included the basking rock 2.5 times more often than that of the second-ranking male. More than 29% of the females were found within 2 m of the highest ranking male on any given day, nearly three times as many as the second- ranking male. Although he ranked at the top of the hierarchy, this male displayed much less frequently than the other hierarchy males.

The stepwise multiple regression model resulting in the highest proportion of explained variances (Fig. 1) indicated that T levels were associated with relative head width, which was the most important factor accounting for variance in fighting success. Display frequency was associated positively with T and negatively with B

158 Pratt et al.

TABLE 2. Morphological, physiological, and behavioral measurements for the five hierarchy males during the breeding season

Femoral Mean Head pore Home Home

SVL T ? SE width! diameter/ Signature Rotary range range Female Rank (cm) (ng/ml) SVL SVL displaydhr displayslhr size (m2) quality proximity

1 40.0 39.5 2 7.2 ,350 ,119 7.28 1.43 11.9 41.4 29.12 2 43.5 41.9 * 4.4 ,276 ,074 23.77 4.53 50.8 16.3 9.81 3 37.6 23.3 ? 5.9 ,239 ,065 16.45 3.87 38.3 3.8 9.12 4 45.4 13.9 * 3.0 ,220 ,077 0.59 0.05 34.4 0.4 8.81 5 39.3 33.3 * 6.3 ,254 ,078 15.20 0.80 19.1 2.6 4.67

Head WidtWSVL a>

Percent Fights Won

@'

Rotary Display Rate

/ Mean T

Mean B Display Rate (->

Femoral Pore Diameter/SVL

\

Signature Display Rate

\

Fig. 1. Schematic representation of relationships between morphological, physiological, and behavioral parameters of male iguanas associated with (a) success in aggressive encounters for all males, and (b) access to females by males who were ranked in the dominance hierarchy. Stepwise multiple regressions were used to create the model (see Materials and Methods). A statistically significant ( P < 0.05) proportion of variance is accounted for by the independent variable(s) in each step. Numbers in paren- theses represent the proportion of variance explained by each variable or combination of variables.

levels, and contributed significantly to success in aggressive encounters (Fig. 1 a). Rank within the hierarchy explained most of the variance in home range quality; however, femoral pore size also explained a significant portion of this variance. Home range quality, together with signature display rate, were positively associated with the percent of reproductive females to which each male had access (Fig. lb).


m E v) m E C m

5 O N D J F M A M J J A S

Month

Fig. 2. Profile of mean monthly plasma T for all male iguanas over 1 year.

Additionally, relative head width was positively correlated with mean T level (rs = 0.9; P < 0.005).

The mean monthly plasma T profile shows that T cycled throughout the year and peaked during the breeding season (Fig. 2). Mean T levels in hierarchy males were greater than those in non-hierarchy males throughout the breeding season and the difference reached statistical significance (U = 18; P < 0.05) during the month of January (Fig. 3). Females showed normal (400-1,000 pg/ml) levels of circulating E, during the breeding season [Phillips et al., 19871. Although mating behavior was never directly observed, at least two females laid fertile clutches.

DISCUSSION

The semi-natural, captive environment of this study allowed for a controlled investigation of the morphological, physiological, and behavioral factors that poten- tially affect male dominance and mate attraction in green iguanas. The highest ranking male held a home range approximately one-third the size of those of the other males, consistent with Dugan’s [1982b] data from wild green iguanas in Panama. She found that mean home range size of large, dominant males was 0.08 ha, whereas in medium and small males it was 0.22 ha, in contrast to patterns of home range size and body size in other lizard species [Turner et al., 19691. Dugan [1982b] suggested that although the home range sizes of the dominant males in her study were small, they contained preferred basking sites and display perches. The dominant male in our study had the greatest access to the large basking rock, and therefore to potential mates, since this is where females congregated. Four other males occupied home ranges on the periphery of the rock, and displayed aggressive behaviors toward each other. The remaining four males were female-like in appearance and behavior, and

160 Pratt et al.

80 - h

1 Hierarchy Males v e Non-Hierarchy Males

* \ M

60 - x 2 s l5

- 40 - U m

m z

5

m k B

OCT NOV DEC JAN

Month

Fig. 3 . Mean monthly plasma T levels for hierarchy vs. non-hierarchy males. *Hierarchy males had significantly higher plasma T levels than non-hierarchy males in the month of January (U = 18; P < 0.05).

moved in and out of the dominant male’s home range with little interference. They were never observed to display aggressive behavior or to court females.

Although this study did not measure reproductive fitness directly, access to cycling females may estimate reproductive success [Ruby, 19841. Large, dominant male green iguanas perform 90% of all copulations in the wild, with the remaining 10% being performed by both medium-sized and small males [Dugan, 1982bl. Me- dium-sized males may simply be waiting for their turn at attaining dominance at a later time, in which case their lifetime reproductive success would increase. These males may also be able to mate opportunistically with females as they move in and out of a dominant male’s home range. Small, female-like males may never obtain the physical attributes required for dominance. However, these males may have greater survivorship because they do not experience the costs associated with high T production and metabolism and increased levels of aggression. Costs of social dominance and high T production have been demonstrated in another iguanid, Sceloporus jurrovi [Marler and Moore, 1988, 19911.

Dugan [ 1982bl found a direct positive correlation between male mating strategy and SVL. Using a quantitative model, Rodda [1992] predicted that large, territorial males have the greatest reproductive success. The present results indicate a relation- ship between social dominance and relative head width among males. Head size has been associated with male reproductive success in other lizards [Carothers, 1984; Cooper and Vitt, 1989; Vitt and Cooper, 19851 and appears to be under sexual selection. The fact that T levels were positively associated with head width supports this hypothesis and suggests that head width may be a secondary sexual characteristic in male green iguanas. Large head size may be important for advertising social dominance in a gregarious, visually oriented species such as the green iguana.


The rates of signature and rotary display were much higher among hierarchy males than among non-hierarchy males in this study. However, the highest ranking male did not display as often as the second- and third-ranking males. As the breeding season approaches in the wild, males begin to interact more, a dominance hierarchy begins to emerge, and territories are established [Rodda, 19921. During this time, territorial and peripheral males display at high rates, but small female-like males generally do not display. Once social relationships among males are established, dominant, territorial males may not need to display aggressively as much as peripheral males, who are competing directly with each other.

The regression analyses of the present study indicated that high T levels, coupled with low B levels, explained a significant proportion of the variance in the frequency of signature display. Aggressive and territorial behavior have been linked with T levels in several other lizard species. Castration results in reduced aggression in mountain spiny lizards [Moore, 19871, broad-headed skinks [Cooper et ai., 19871, and green anoles [Mason and Adkins, 19761. Moreover, aggressive behaviors in lizards are less frequent during the non-breeding season, when testicular regression results in lower T levels [Moore and Marler, 1987; Ruby, 19781. The effect of B levels on aggression and territoriality has also been documented. There is some evidence that social subordination can result in adrenal hypertrophy [Brackin, 1978; Wilhoft, 19641 and increased plasma B [Greenberg et al., 19841 in reptiles. High levels of B can, in turn, interfere with the production of T, and ultimately affect aggressive, territorial, and courtship behaviors [Lance and Elsey, 1986; Tokarz, 19871. In the present study, there was no association between mean plasma B levels and mean plasma T levels. Small sample size may have precluded direction of this correlation, or the rise in B levels may not have been chronic enough to have been measured accurately in weekly blood samples. Femoral pore diameter, together with hierarchical rank, was positively associated with home range quality. Thus, males which produce more glandular secretion with which to mark their home ranges may maintain more attractive mating territories than other males. Glandular size and secretory activity have been positively linked to T levels in this and other lizard species [Alberts et al., 1992; Billy and Crews, 1986; Chiu et al., 1975; Fergusson et al., 19851.

The mating system of the green iguana is a form of polygyny in which there is intense competition among males for mates. Female mate choice may be based on courtship quality and/or mating territory qualities, although the relative importance of each remains to be determined [Dugan and Wiewandt, 1982; Rodda, 19921. The data collected from this captive group indicate a male social system consisting of one dominant male, a peripheral group, and several female-like males, corresponding to those social classes found in wild populations by Dugan [ 1982b] and Rodda [ 19921. An individual male’s success in aggressive encounters, and therefore his rank, was affected by a combination of head size, hormones, and display frequency. Rank largely influenced the quality of male home ranges. Finally, home range quality, coupled with signature display rate, was the most important factor associated with access to females. Taken together, these results indicate that morphological, physiological, and behavioral variables play important roles in establishing and maintain- ing dominance relationships which affect variance in mating success and, ultimately, lifetime reproductive fitness in male green iguanas.

162 Pratt et al.

ACKNOWLEDGMENTS

The authors thank the veterinary, curatorial, and keeper staffs of the Zoological Society of San Diego for access to and care of the animals, V. Lance, N. Czekala- Gruber, and L. Callison for assistance with the radioimmunoassays, and J. Bradbury for use of the computer program for analysis of dominance. Funding was provided in part by NSF BNS 89-09207 and NIH 1 F 32 DC 00025-01.

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