Two-and-a-half years ago, researchers in Colombia stumbled upon two white spider monkeys—male siblings. Andres Link, a primatologist at Universidad de Los Andes in Bogotá, Colombia, and colleagues have been monitoring them ever since, and have been able to videotape the primates as they lounge on branches.

The brothers are likely leucistic—possessing white- or light-colored fur, but with some color elsewhere—rather than albino, because they still have black eyes. Albino animals lack any pigment.

But their unusual color may be a sign of inbreeding in this isolated population, Link says.   And that doesn't bode well for their future. Inbred populations tend to be more vulnerable to changes in habitat or climate than genetically diverse groups. Link and colleagues are waiting on tests