On Friday my focal chimp was Frodo, the ex-alpha male of the Kasekela community. My field assistant and I had been following him for the past few hours after finally discovering him in the southern portion of the park. He was alone and traveling in frequent bursts through a dense patch of machaca (terrible vine tangles). Pausing periodically to feed on a few leaves or ants, he eventually emerged from the vines. At last able to walk on two legs again, I found myself bypassing my field assistant, who is typically in the lead. Frodo hadn’t been feeding on anything substantial for the past few hours and I could feel that he would soon arrive at a more impressive food patch, the type of event my research focuses on. After a few minutes of walking along the trail, Frodo paused and looked behind him. There sat Freud, Frodo’s older brother.
The two males walked along for a while, feeding independently of one another at various small patches of grass or leaves. Then, at 4:18 PM, Frodo casually climbed a tree as Freud fed in a nearby area. This msongati tree didn’t look all that impressive to me and I relaxed a bit as Frodo fed quietly above. Shortly thereafter, Freud started to approach and peered into the leave above. At 4:20 he too began to climb and unexpectedly began the build-up of a pant-hoot (FD panthoot) which transitioned into a bout of very quiet, yet distinctive rough-grunts- the calls that I study. They were so quiet that I was having trouble recording them from just below the relatively short tree. As Frodo continued to feed in silence, Freud produced food calls on and off for as long as 19 minutes after he had arrived. At 4:41 a chorus of pant-hoots was heard from a nearby party of chimpanzees (Chorus). Frodo and Freud immediately responded with pant-hoots of their own and then the excitement escalated. Suddenly, both Freud AND Frodo were making food-calls and a little louder than before (Freud and Frodo rough-grunts). At 4:45, just before the new group arrived, Frodo climbed down the tree and entered another one a little farther away. Suddenly there was crashing through the undergrowth as Ferdinand, the current alpha male, performed a dominance display and dashed up Frodo’s tree. As he paused at the top of the trunk, Frodo gave loud submissive pant-barks before continuing to feed and produce food calls. Now his calls were the loudest they had been so far (Frodo rough-grunts) and more calls could be heard from chimps feeding in Freud’s tree. Soon, six more chimps arrived and joined in a chorus of food calls, screams and panthoots as they fed on tiny little flowers, the cause of all of the excitement. After only a few minutes, Ferdinand moved off, and one by one the others did too. Frodo lingered a bit, now feeding silently. Just after 5:15 he headed towards the others, breaking off into a new group of himself, Freud, and the young male, Sampson.
I was exhilarated. This is what I have come to Gombe National Park to study. Most people believe that these calls inform other group members about an attractive food source and that different sounding calls let them know about different kinds of food. They also think that this calling behavior is altruistic, meaning that the caller suffers a cost (less food) in order to benefit others (tell them about food). I can easily see where this impression comes from. Indeed, Frodo and Freud produced food calls and other chimps did arrive and feed in the trees. However, when it was just the two males, it was Freud, the second chimp to arrive, who gave the food calls. At this point Frodo was the only one who could hear them and he already knew about the food (he was eating it!). Furthermore, the calls got progressively louder the closer the distant group came, peaking when the other chimps were already beginning to arrive and feed in the trees. If these calls function to inform others of the food, I would expect them to be the loudest when others were farthest away. Lastly, the sound of the calls changed throughout the course of the feeding bout, even though the food stayed the same, something I would not expect if different sounding calls referred to different types of food. I suspect that chimpanzee rough-grunts do something other than inform group members about the presence of food, and that is what I have come to investigate.