565 Million Age, in years, of the fossils of soft-bodied tubular organisms in South Australia that provide the earliest evidence of sexual reproduction. The fossils, described in 2008, are attached to the seafloor in groupings that resemble the clustered pattern of today’s sexually reproducing corals and sponges.
7 Number of different sexes of Tetrahymena thermophila, a single-celled organism found in pond water. The diversity may help maximize the chances of finding a mate: An individual can reproduce with members of any sex except its own, leading to 21 possible couplings. Other organisms achieve even headier numbers. The slime mold Physarum polycephalum exists in more than 700 mating types, though not all of them are sexually compatible.
8:1 Ratio of the length of a barnacle’s penis to the rest of its body, making it proportionally the largest phallus in the animal kingdom. Barnacles need an unusually long appendage in order to reach mates while anchored in place; a recent study found that barnacles in turbulent waters had much shorter and stouter organs than those in protected harbors, perhaps due to “a trade-off between length and maneuverability” of the penis in high waves. Among vertebrates the Argentine lake duck is the leader, with a penis that can exceed the length of the bird’s 16-inch body.
500 Number of animal species in which homosexual behavior has been documented. A 2009 analysis of same-sex animal romance notes, among other things, that half of all sexual interactions by male bottlenose dolphins occur with male partners, and male-male pairings account for up to 26 percent of bearded vulture mountings. Conversely, a four-year study in Hawaii found that 31 percent of Laysan albatross pairs there consist of two females; one such couple has been observed together for 19 years.
19,000,000 Number of new cases of sexually transmitted infections each year in the United States, including 56,000 cases of HIV, according to the Centers for Disease Control. The National Survey of Sexual Health and Behavior, published in 2010, reports that sexually active singles use a condom only one-third of the time.
