The octopus has many traits that resemble our own: binocular vision, a playful mind, and penislike erectile tissue that enlarges by altering the pressure of the fluid within. That last item came as a shock to biologists Joe Thompson, then at the University of North Carolina, and Janet Voight of the Field Museum in Chicago, who recently discovered that the male Octopus bimaculoides has an expandable “penis” of vascular tissue and collagenous connective tissue. An enlarged sex organ apparently helps male octopuses deliver more sperm to the female, a selective advantage in the mating game. But because it lacks the pigments that cover the rest of the octopus’s body, the sex organ needs to be inconspicuous most of the time to avoid attracting predators. “Octopuses and mammals do not share a recent common ancestor but have independently come up with the same solution to a common problem,” Thompson says.