with them, 63.8% were married couples living together, 2.7% had a female householder with no husband present, and 32.3% were non-families. 27.9% of all households were made up of individuals, and 16.7% had someone living alone who was 65 years of age or older. The average household size was 1.92 and the average family size was 2.28. Among the population; 8.5% under the age of 19, 1.1% from 20 to 24, 7.5% from 25 to 44, 32.7% from 45 to 64, and those aged 65 or older represented 50.1%. The median age was 65 years. For every 100 females, there were 89.8 males. For every 100 females age 18 and over, there were 89.5 males age 18 and over. The median income for a household in the city was $97,788, and the median income for a family was $138,194. Males had a median income of $80,152 versus $45,458 for females. The per capita income for the city was $79,742. About 3.6% of families and 7.0% of the population were below the poverty line, with 21.3% of those under age 18 and 3.4% of those age 65 or over. Ecology The island's curved shrimp-like shape forms Tarpon Bay on the north side of the island. It is linked to the mainland by the Sanibel Causeway, which runs across two small manmade islets and the Intracoastal Waterway. A short bridge links Sanibel Island to Captiva Island over Blind Pass. The Bailey-Matthews Shell Museum on Sanibel is the only museum in the world dedicated entirely to the study of shells. The Gulf-side beaches are excellent on both Sanibel and Captiva, and are world-renowned for their variety of seashells, which include coquinas, scallops, whelks, sand dollars, and many other species of both shallow-water and deeper-water mollusks, primarily bivalves and gastropods. Sanibel Island is home to a significant variety of birds, including the roseate spoonbill and several nesting pairs of bald eagles. Birds can be seen on the beaches, the causeway islands, and the reserves, including J. N. "Ding" Darling National Wildlife Refuge. Common sights include pelicans, herons, egrets, and anhingas, as well as the more common birds like terns, sandpipers, and seagulls. There is a population of American alligators on Sanibel Island. A lone rare American crocodile had been seen at the Wildlife Refuge for over 30 years, but she died in 2010 of unseasonably cold winters or old age. A memorial was set