Scientists are constantly trying to beef up the nutrient value of what we eat. From iron-fortified cereals to vitamin A rice, we have ever-better foods to choose from. But in tropical forests, the opposite is happening. Over the past 30 years, tropical forest leaves that some monkeys depend on have lost protein content and increased in fiber, researchers report in an upcoming issue of Ecology. (More fiber makes the leaves less nutritious, and monkeys, such as this red colobus (above), prefer high-protein, low-fiber ones.) A nutritional ecologist studying the diets of monkeys happened to cross paths with researchers who in the late 1970s and mid-1990s had analyzed the nutrient composition of leaves in the same Ugandan tropical rainforest where she was working. She used the same techniques on 10 tree species, sometimes sampling leaves from the same exact trees, and found that the increasing temperature and variability in rainfall brought about by climate change have taken a toll. The forest hosts some of the highest densities of primates in the world, and, based on the leaves now available, models predict the populations of leaf-eating primates may decline by 31% in the coming years.