PRIMATES, 40(3): 499-508, July 1999 499 Establishing the Presence of a Body Temperature Rhythm in Chimpanzees (Pan troglodytes) Using a Tympanic Membrane Thermometer L. A. FOWLER, Emory University and Georgia State University W. D. HOPKINS, Emory University and Berry College H. E. ALBERS,R. D. MORRIS, Georgia State University and C. W. HYATT, Emory University ABSTRACT. Seven chimpanzees (Pan troglodytes) were trained to present their ears so that a tympanic membrane thermometer could be inserted. Temperatures were collected from both ears of each subject every 3 hours for 72 consecutive hours. The presence of a body temperature rhythm, well documented in other mammals, was established. Each ear demonstrated its own rhythm, but the rhythms in both ears gen- erally mirrored each other. Similarities in the temperature rhythms of cagemates were found. These data are the first evidence of a body temperature rhythm in chimpanzees, and they represent a non-invasive method of measuring the 24-hr rhythms in both human and non-human primates. Key Words: Body temperature rhythm; Circadian rhythm; Chimpanzee (Pan troglodytes); Tympanic membrane. INTRODUCTION Endogenous rhythms have been shown to exist throughout most known species, from single- celled organisms to humans. These rhythms have attracted a great deal of study because although they can be entrained by environmental cues, endogenous rhythms will persist in the absence of external stimuli. Circadian rhythms are those endogenous rhythms whose cycle is approximately 24-hours. One example of a circadian rhythm, well documented in mammals from rats to humans, is the body temperature rhythm. In humans, the body temperature rhythm is at its lowest point in the early morning hours, gradually increasing throughout the day, and peaking at about 2,000 hr (TAPP & NATELSON, 1989). Temperature rhythm studies have been conducted with many non-human primate species, including the tree shrew (Tupaia belangeri) (REFINETTI & MENAKER, 1992) and the rhesus monkey (Macaca mulatta) (HONNEBIER et al., 1992), but no studies have been published which identify a body temperature rhythm in chimpanzees. Non-human primates are typically accepted as the best model to use for studying the organi- zation and function of the human brain (MOORE-EDE et al., 1992), and as chimpanzees are so closely related to humans, they are the logical choice to use in comparative studies. Since per- formance and behavior in humans appear to be related to many circadian rhythms, including the body temperature rhythm (MILLERet al., 1992; MONK & MOLINE, !988), it is important to con- sider the effect of the body temperature rhythm on what is being observed when studying behavior and performance in chimpanzees. This study utilizes tympanic membrane thermome- try to establish the presence of a body temperature rhythm in chimpanzees.