Forty-five years ago I first set foot in Kasoje, a treasure trove for chimpanzee research set in the Mahale Mountains and bordered by Lake Tanganyika. I started out as a graduate student at the age of 24. Time has passed since then, and it still passes; most of today’s active researchers were born after my research began. When I was an undergraduate student, I was interested in the exploration of terra incognita such as the Amazon, Borneo, Sumatra, or Africa. It was the early 1960s, only 15 years after the end of the Second World War, and Japan had not yet achieved a level of foreign exchange that allowed ordinary people to travel abroad. At that time, three books appeared on gorilla expeditions, written by Kinji Imanishi, Junichiro Itani, and Masao Kawai. These books not only filled me with interest in the great apes but also made me realise that if I studied chimpanzees, I could go to Africa! Although I was a zoology student, I only vaguely imagined before reading these books that I might study the ecology of animals. Fortunately, the graduate course of Physical Anthropology in the Zoology department, with Imanishi as the first professor, was established in 1962, as if it were prepared just for me. Therefore, I was happy to enter the course with my colleagues, Takayoshi Kano and Kohsei Izawa. I thought I would study chimpanzees for three years or so and then change my research target to human beings and their traditional lives as hunter-gatherers.
Once I began to study chimpanzees, I soon realised they were not creatures that could be understood in a few years of study. We encountered new discoveries in behaviour just as surely as a new year arrives and the old year departs. Every year I found new dietary items, new behavioural patterns, new personalities, and new relationships. Individuals who appeared by birth or immigration have fascinated me by their doing something new. My first observations of chimpanzees encountering an unexpected animal were recorded for a crocodile in 1983, a lion in 1989, and a freshly dead leopard carcass in 1999. Similarly, cannibalism and ostracism were first observed more than two decades after the start of my research. The apes’ behaviour is so rich in variety that, no matter how many years I observe them, I will never grow bored. Every year a new student comes to Mahale and surprises me by reporting a behavioural pattern that I have never seen before. Such discoveries happen because of the flexible behaviour of chimpanzees and the wonderfully diverse environment of Mahale, which is rich in biodiversity and landscape variation. This is why I have continued to conduct research for so long.