It's easy to spot a committed pair of gibbons or swans: Both halves of the couple frequently hang out together. But good luck finding a twosome in a
school of 2500 Xenotilapia rotundiventralis cichlid fish, a 50-millimeter-long, plankton-eating species that swims the tropical waters of Lake
Tanganyika in Zambia. Now, however, researchers say they've hit upon out a way to figure out who's in a steady relationship with whom. The trick has to
do with the fact that females brood young in their mouths, then transfer young to the mouths of males. By studying the DNA of adults and young, the
researchers determined that the female cichlids are probably passing the kids off to their dads. That means, the researchers report online today in Biology Letters, that males and females form the first known "invisible" pair bond—well, invisible to us
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