A group of harbor porpoises in San Francisco Bay, photographed from the Golden Gate Bridge. Harbor porpoises haven't been seen in the bay since the 1930s. Researchers believe World War II activity may have contributed to their disappearance. The Navy strung a seven-mile underwater net across the mouth of the bay to keep out enemy submarines. William Keener/Golden Gate Cetacean Research hide caption

itoggle caption William Keener/Golden Gate Cetacean Research

A Kemp's ridley sea turtle like this one traveled 4,600 miles across the Atlantic ocean in 2008. After being rehabilitated in Portugal, it is being reintroduced into its native Gulf of Mexico waters on Tuesday. US EPA via flickr hide caption

itoggle caption US EPA via flickr

Northern cardinals have higher-pitched songs, but those sounds can get garbled in cities, so they've started to sing a little lower. Dr. Scott M. Lieberman/AP hide caption

itoggle caption Dr. Scott M. Lieberman/AP

A tufted capuchin uses a stone hammer to crack open a nut in Brazil's Parnaiba Headwaters National Park. Ben Cranke/Getty Images hide caption

itoggle caption Ben Cranke/Getty Images

A chimpanzee eats a coconut at the Alamogordo Primate Facility at Holloman Air Force Base, N.M., in an undated photo from the National Institutes of Health. A plan to move chimps that had retired to the facility after being subjects of medical experiments sparked controversy and a review of research policies. AP hide caption

itoggle caption AP