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Succulent plants are prone to overheating due to their massive organs and lack of daytime transpiration. Thus; they frequently grow in cool microenvironments. Due to their verticality, cliffs avoid sunlight at noon, providing a cool refuge during the hottest hours of the day. We test the hypothesis that species in the genus Mammillaria (Cactaceae) are associated with cliffs in hot areas. We compared the temperatures and light interception of Mammillaria crucigera experimentally placed on a cliff and on the plateau above it. Through comparative analysis of 10 species we tested if those living in warmer areas inhabit steeper slopes. We found that plants and soil on the cliff were 9.9 and 14.4 degrees C cooler than their counterparts in the plateau at noon. While the temperatures on the plateau were within the range that has been found to inhibit photosynthetic activity, those on the cliff were closer to the optimal ones for CO2 fixation in other cacti. Living on the cliff may be costly because radiation is reduced by 38.2%. A comparative analysis revealed an evolutionary trend toward living on steeper slopes when the climate is hot. This may contribute to explain why cliff-dwellers have evolved in so many succulent-plant families. (c) 2006 Elsevier Ltd. All rights reserved.