Baby Seal

When compared to terrestrial mammals (cow, sheep, pigs, humans), California sea lions (Zalophus californianus), Northern elephant seals (Mirounga angusturistris), Ringed seals (Phoca hispida), and Harbor seals (Phoca vitulina) had lower levels of the anionic surfactant phospholipids (phosphatidyldlycerol and phosphatidylinositol). Although the role of PG and PI are not clear, they appear to interact with hydrophobic surfactant proteins SP‐B and SP‐C to prevent the irreversible removal of the phospholipids from the surfactant film during alveolar compression and collapse for rapid reinsertion upon reexpansion. There is a also greater percentage of short fatty acid phosphatidylcholine, the primary phospholipid contributor to surfactant function, in pinnipeds than in terrestrial mammal. Since fluidity of the surfactant mixture is also dependent on fatty acid tail length where length leads to increased surfactant fluidity, the change in tail length may indicate a surfactant system adaptation in response to the selection for an effective anti‐adhesive surfactant in collapsed lungs of diving pinnipeds.
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.