Size matters: micro-evolution in Polynesian rats highlights body size changes as initial stage in evolution.

Area of Science:

• Evolutionary Biology
• Zoology
• Ecology

Background:

• Microevolutionary patterns in introduced species offer insights into vertebrate evolution.
• The Polynesian rat (Rattus exulans) has a vast global distribution and long history of introduction, making it ideal for evolutionary studies.

Purpose of the Study:

• To investigate the impact of long-term insular isolation and geographic factors on the skull shape variation of introduced Polynesian rats.
• To analyze skull shape allometry and sexual dimorphism in Rattus exulans populations across diverse island and mainland environments.

Main Methods:

• Geometric morphometrics applied to 59 landmarks from dorsal, lateral, and ventral skull views of 513 specimens.
• Multivariate analyses including principal components analysis (PCA), multivariate regressions, and canonical variates analysis (CVA).

Main Results:

• Polynesian rats exhibit cranial evolutionary allometry (CREA), with larger skulls having proportionally longer snouts and a more tubular shape.
• Sexual dimorphism observed, with males possessing larger, more tubular skulls compared to females.
• Greater skull shape disparity in 'exulans-only' regions (islands without native mammals) compared to areas with native mammals.

Conclusions:

• Insularity and geographic variables significantly influence skull shape evolution in introduced Polynesian rats.
• Ecological release in mammal-poor island environments may promote greater diversification in skull morphology.
• Skull shape variation in Rattus exulans provides a model for understanding evolutionary processes in isolated populations.