Changes in Plasma Testosterone and Aggressive Behavior in Male Green Iguanas (Iguana iguana) Following Orchidectomy
IAAAM 2000
Brad A. Lock1, DVM; R. Avery Bennett1, DVM, MS, DACVS; Timothy S. Gross2, PhD
1Zoological Medicine Service, College of Veterinary Medicine, University of Florida, Gainsville, FL, USA; 2USGS Biological Resources Division, Florida Caribbean Center, Wildlife Reproductive Toxicology, Gainesville, FL, USA

Abstract

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.

Speaker Information
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Brad A. Lock, DVM


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