Baby Seal

Surfactant Secretion
There is also environmental pressure for a pulmonary surfactant system capable of controlled surfactant secretion even under long periods of hydrostatic pressure. Alveolar epithelial type II cells, which secrete surfactant when stretched, appear to have decreased sensitivity to pressure in California sea lions compared to terrestrial mammals so that constant levels of surfactant are maintained under long periods of pressure. Furthermore, the gene for leptin, a hormone that has a role in surfactant phospholipid synthesis, was found to have nonsynonymous substitutions that are under positive selection in phocids within otherwise highly conserved regions in mammals. The sites of substitution are predicted to be on the exposed sides of leptin where the changes likely alter leptin function. The modified leptin can then affect surfactant production in phocids.