Changes in Plasma Testosterone and Aggressive Behavior in Male Green Iguanas (Iguana iguana) Following Orchidectomy
Many male green iguanas (Iguana iguana) demonstrate aggressive behavior towards their owners
and conspecifics following the onset of sexual maturity. The aggression is particularly severe during the natural
breeding season (late November-February) in northern Florida. This aggressive behavior often involves vicious attacks
with biting and tail whipping. Veterinarians have attempted to mediate this aggressive behavior with castration.
A 10-mo study was conducted in order to investigate whether castration reduces aggressive behavior
and the mean circulating plasma testosterone concentration as well as to determine if there is a temporal relationship
between castration and breeding season in the amelioration of aggression. Sixteen clinically normal green iguanas, (based
on physical examination, complete blood count and plasma biochemistry analysis) ranging in weight from 0.660-3.62 kg
(mean = 1.6 kg) and snout to vent length from 26.0-44.5 cm (mean = 32.3 cm), were obtained from various sources. The
iguanas were separated into three size classes: Small (< 1 kg), medium (> 1kg < 2 kg) and large (> 2 kg).
Animals from each size class were randomly placed into one of three groups. Group 1 (five animals) were castrated in
September, 6 wk prior to the onset of the natural breeding season. Group 2 (five animals) were castrated in December,
during the breeding season, after the onset of aggressive behavior. Group 3 (six animals) served as the control group
which had anesthesia induced and maintained, and had a sham operation performed (three animals prior to breeding season
and three animals during breeding season) consisting of creating an abdominal incision closed in a manner similar to that
of the castration groups.
Data collection consisted of aggression scores, plasma testosterone concentration and femoral pore
diameter measurement. Aggression scores were obtained 2-3 times/wk for the duration of the study. Aggression scores were
determined by placing a large (66 cm x 33 cm) mirror in each iguana's cage and counting the number of open mouth contacts
to the mirror in a 5-min period. Baseline aggression scores were obtained for 3 wk prior to any surgical manipulation.
Blood was collected from the ventral coccygeal vein, on days that were not aggression score days, every 2 wk for
determination of plasma testosterone levels using a radio-immuno assay validated for iguana plasma. Two baseline blood
samples were collected prior to any surgical intervention.
The resulting data were analyzed with a repeated measures analysis of variance (ANOVA) using SAS
General Modeling Procedure. A borderline statistically significant difference in aggression was seen among groups
indicating that a difference among groups occurred over time and this difference was seen during the breeding season.
Comparison of group 1 and its own control group during the breeding season (December-January) detected a statistically
significant difference in aggression, while comparison between group 2 and its control group was not significantly
different. However, there was a trend towards reduction in aggressive behavior between the groups.
Plasma testosterone concentration from group 1 showed no statistically significant change over time
while group 2 showed a significant decline over the course of the study. No significant difference in plasma testosterone
concentration was detected between group 1, group 2 and their respective controls during the breeding season.
The data support that castration before the breeding season reduces aggression in male green iguanas
and that the testosterone concentration remains low in castrated iguanas even during the breeding season.