Terrestrial Birth and Body Size Tune UCP1 Functionality in Seals

Abstract

The molecular evolution of the mammalian heater protein UCP1 is a powerful biomarker to understand thermoregulatory strategies during species radiation into extreme climates, such as aquatic life with high thermal conductivity. While fully aquatic mammals lost UCP1, most semi-aquatic seals display intact UCP1 genes, apart from large elephant seals. Here, we show that UCP1 thermogenic activity of the small-bodied harbor seal is equally potent compared to terrestrial orthologues, emphasizing its importance for neonatal survival on land. In contrast, elephant seal UCP1 does not display thermogenic activity, not even when translating a repaired or a recently highlighted truncated version. Thus, the thermogenic benefits for neonatal survival during terrestrial birth in semi-aquatic pinnipeds maintained evolutionary selection pressure on UCP1 function, and were only outweighed by extreme body sizes among elephant seals, fully eliminating UCP1-dependent thermogenesis.