Mammal Molar Size Ratios and the Inhibitory Cascade at the Intraspecific Scale.

Area of Science:

• Evolutionary biology
• Comparative anatomy
• Mammalian anatomy

Background:

• Mammalian molar crowns exhibit size covariation, often explained by the inhibitory cascade model (ICM) or molar module component (MMC) model.
• The ICM's applicability across biological scales, particularly at the intraspecific level, remains a concern, with limited testing in primates.

Purpose of the Study:

• To assess intraspecific variation in relative molar sizes of the cotton mouse (*Peromyscus gossypinus*).
• To test the ICM and MMC models at the intraspecific level.
• To develop recommendations for appropriate sampling protocols in future intraspecific studies of molar size variation across Mammalia.

Main Methods:

• Modeling the sensitivity of molar ratio estimates to sample size in cotton mice.
• Simulating the use of composite molar rows when complete rows are unavailable.
• Analyzing intraspecific variance structure and average ratio values.

Main Results:

• Intraspecific variance structure of molar ratios in *P. gossypinus* did not align with predictions of the ICM or MMC.
• Neither the ICM nor the MMC model provided a significantly better fit to observed variation.
• Standing variation in molar size ratios is consistent across mammalian samples with complete molar rows.

Conclusions:

• The ICM and MMC models require re-evaluation for intraspecific variation in molar size.
• Small sample sizes (≥2) are sufficient for estimating mean molar ratios, with composite specimens being useful.
• Large sample sizes (≥40-50) are necessary for analyzing variance structure, and composite rows are unsuitable for this purpose.