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

Abstract

True seals (phocids) have achieved a global distribution by crossing the equator multiple times in their evolutionary history. This is remarkable, as warm tropical waters are regarded as a barrier to marine mammal dispersal and—following Bergmann's rule—may have limited crossings to small‐bodied species only. Here, we show that ancestral phocids were medium sized and did not obviously follow Bergmann's rule. Instead, they ranged across a broad spectrum of environmental temperatures, without undergoing shifts in temperature‐ or size‐related evolutionary rates following dispersals across the equator. We conclude that the tropics have not constrained phocid biogeography.