Dr. Clark and his colleagues recently developed a new way to search for these genes and looked for those more likely to be broken in marine mammals than in terrestrial ones. The scientists ended up with a short list of genes that were repeatedly shut down in marine mammals.

Most were involved in smelling, which supported earlier studies showing that marine mammals have little or no sense of smell.

But at the top of the list was a gene that had nothing to do with smell, called PON1.

Wynn K. Meyer, a postdoctoral researcher at the University of Pittsburgh and co-author of the new study, said she was taken aback when she found out what the gene is best known for: a defense against some toxic chemicals.

These chemicals are called organophosphates, a class of compounds that includes certain pesticides as well as nerve agents like sarin gas.

PON1 encodes an enzyme called paraoxonase that can quickly break down organophosphates. Mice genetically engineered without paraoxonase die quickly when they’re exposed to the chemicals.

Dr. Meyer and her colleagues found that all marine mammals have broken copies of the PON1 gene, with a few exceptions: walruses, fur seals and spotted seals.

To see if the gene were truly kaput, the researchers gathered blood plasma from a range of mammal species. They then added pesticides to the plasma.