True seals achieved global distribution by breaking Bergmann's rule.

Area of Science:

• Marine biology
• Evolutionary biology
• Biogeography

Background:

• True seals (phocids) exhibit a global distribution, having crossed equatorial regions multiple times.
• Warm tropical waters are often considered a barrier to marine mammal dispersal.
• Bergmann's rule suggests larger body sizes in colder climates, potentially limiting equatorial crossings to smaller species.

Purpose of the Study:

• To investigate the biogeographical history of true seals (phocids).
• To determine if tropical waters and body size influenced phocid dispersal across the equator.
• To test the applicability of Bergmann's rule to phocid evolution and dispersal.

Main Methods:

• Phylogenetic analysis of phocid evolutionary history.
• Reconstruction of ancestral body size and environmental temperature ranges.
• Analysis of evolutionary rates in relation to equatorial crossings and environmental factors.

Main Results:

• Ancestral phocids were medium-sized and did not strictly adhere to Bergmann's rule.
• Phocids inhabited a wide range of environmental temperatures.
• No significant shifts in temperature- or size-related evolutionary rates were observed after equatorial dispersals.

Conclusions:

• Tropical waters have not acted as a significant constraint on phocid biogeography.
• Phocid dispersal patterns are not solely explained by body size or temperature limitations as predicted by Bergmann's rule.