Abstract Do free-ranging baboons avoid traveling to- wards the sun? Sun avoidance, in addition to resource and predator locations, may influence troop movement and non-random use of the home range. This paper in- vestigates how sun avoidance, as measured by facial ex- posure to sunlight, influences directional choices. It hy- pothesizes that baboons should avoid the sun in the hot, dry season and show indifference to it in the cool, lush season. This paper also hypothesizes that baboons em- ploy sun-avoidance behaviors more while they forage or travel to resting sites than when they travel to foraging sites or engage in active social behaviors; lastly this pa- per hypothesizes that sun altitude, temperature, humidity, and cloud cover influence sun-avoidance behavior. Us- ing focal-animal techniques on 21 males from free-rang- ing baboon troops, I collected locational data, accurate to within 1.6 m, over 15 months. I calculated the difference between baboon bearings and the sun’s azimuth in angu- lar degrees. Both linear and circular statistics indicate that baboons put significantly (P<0.01) more than 90° between their bearing and the sun’s azimuth under cer- tain conditions. Contrary to hypotheses based on the det- rimental effects of insolation, baboons in the cool, lush season avoid the sun, while baboons in the hot, dry sea- son do not. In the lush season, the extent to which ba- boons avoid the sun does not depend on their other be- haviors. Dry-season baboons demonstrate stronger sun avoidance while resting than when engaged in other be- haviors. Finally, in the dry season, temperature drives sun avoidance; humidity drives it in the lush season. Key words Sun avoidance · Papio ssp · Seasonal variations · Temperature · Humidity Introduction This study investigates the relationship between direc- tional choices made by free-ranging yellow baboons (Papio cynocephalus cynocephalus) and sun avoidance, as measured by facial exposure to the sun. Mammalian faces (more than sides, chests or backs) are susceptible to heat stress. The facial morphology of many mammals, including baboons, hosts a selective cooling system for the brain (Caputa et al. 1996). Baboon faces may be par- ticularly susceptible to heat absorption because they are black, hairless and prognathic. Baboons attempting to re- duce the effects of heatstress should reduce their facial exposure to the sun. Environmental factors, including humidity, sun alti- tude, and shade, may influence the degree to which ba- boons benefit from avoiding the sun. Humidity exacer- bates the effects of heat stress (Blackshaw and Blackshaw 1994). Blazejczyk et al. (1993) find that temperature best predicts solar heat load in humans, but some studies show that the thermohygrometric index (THI) can replace tem- perature and humidity in regression models (Armstrong 1994; Tselepidaki et al. 1995; and many others). The sun’s altitude may influence heat stress since, as the sun’s alti- tude changes, the proportion of the body area receiving di- rect sunlight changes (Blazejczyk et al. 1993). Shade ame- liorates some negative effects of heat stress (Armstrong 1994). Cloudy days may provide some of the benefits of shade. The degree to which these environmental factors influence baboon sun-avoidance behavior is examined. The degree to which baboons avoid the sun may depend on their other behaviors. While copulating or engaging in aggressive behaviors, the location of a potential mate or foe may be more important than sun avoidance. Baboons often forage for the majority of the daylight hours. If they ignore the sun while engaged in foraging, they stand to ex- pose themselves to high levels of heat stress. If a baboon has an equal chance of encountering a food regardless of his bearing, he reduces heat stress effects if he chooses bearings taking him away from the sun. The same can be said of baboons traveling to resting spots. Sharon T. Pochron ( ✉ ) Department of Anthropology, State University of New York at Stony Brook, Stony Brook NY 11794, USA e-mail: spochron@ms.cc.sunysb.edu Tel.: +1-631-632-1568 Int J Biometeorol (2000) 44:141–147 © ISB 2000 ORIGINAL ARTICLE Sharon T. Pochron Sun avoidance in the yellow baboons (Papio cynocephalus cynocephalus) of Ruaha National Park, Tanzania. Variations with season, behavior and weather Received: 10 September 1999 / Revised: 30 March 2000/Accepted: 31 March 2000