Growth Rate Variation in Brown Treesnakes (Boiga irregularis): An Invasive Species of Conservation Concern

Area of Science:

• Ecology
• Herpetology
• Population Dynamics

Background:

• Somatic growth rate is crucial for understanding animal metabolism, lifespan, and reproductive maturity, impacting population dynamics and management strategies.
• The invasive Brown Treesnake (Boiga irregularis) on Guam exhibits continuous reproduction, complicating cohort analysis and growth assessment.
• Accurate growth modeling is essential for effective invasive species management.

Purpose of the Study:

• To compare the efficacy of the von Bertalanffy growth function and the Gompertz model in describing snake size trajectories.
• To model growth rates of wild Brown Treesnakes and identify factors influencing them, including individual variation, sex, and environmental conditions.
• To estimate age at sexual maturity for males and females based on growth models.

Main Methods:

• Utilized quantile regression with a regularized effect to model growth rates of 270 marked, wild Brown Treesnakes.
• Compared model fit and predictive accuracy of the von Bertalanffy and Gompertz growth functions.
• Incorporated size, sex, precipitation, and seasonality (modeled as a sinusoidal function) as predictors of growth rate.

Main Results:

• The Gompertz model provided a superior fit and more realistic asymptotic size predictions compared to the von Bertalanffy model.
• Male Brown Treesnakes exhibited 1.05–1.16× faster growth rates than females, with females reaching smaller asymptotic sizes.
• Individual and sex-related differences in size-adjusted growth rates were more significant than environmental factors like precipitation and seasonality.

Conclusions:

• The Gompertz model is recommended for analyzing snake growth data, particularly for species with continuous reproduction.
• Understanding sex-specific growth differences and individual variation is critical for accurate population modeling and management of Brown Treesnakes.
• Estimated ages at sexual maturity vary significantly, with faster-growing individuals maturing earlier, informing targeted management interventions.