Bonobos hang out in the treetops in large groups of 60 to 100 individuals but they travel across the ground in smaller groups when they move from roost to roost. On a recent trip to Zaire, Savage-Rumbaugh noticed that bonobos had carefully broken off plants of the same species just before and after two trails crossed, apparently as a way of marking which fork to take for groups following after. Intrigued, she beganto search the forest for more markings. She found 96 instances of plants that had clearly been broken off by bonobos. All but 7 of these served as some sort of trail marking, such as pointing which trail to take or which direction to travel, or showing the way through wet areas where no trail was visible.
Apes undoubtedly show the clearest evidence of conscious thought among nonhuman animals.
Similar intelligence might be much harder to recognise in, say, dolphins, simply because they are so different from ourselves. Humans and some apes use their hands to fashion tools, a sign of intelligence. Even so, Reiss sees clear glimpses of an active intelligence. For example, the dolphins she studies at Marine World Africa, USA, in Vallejo, California, have learned to carry rocks around by sucking them onto their blowholes. The animals also blow bubble rings, just as humans blowsmoke rings from cigarettes, and then play with the rings as they rise to the surface.