J. Zool., Lond. (2005) 267, 323–328 C  2005 The Zoological Society of London Printed in the United Kingdom doi:10.1017/S095283690500751X Comparative seasonal acclimatization of food and energy consumption in adjacent populations of common spiny mice (Acomys cahirinus) Michael Scantlebury*, Uri Shanas, Daniel Afik and Abraham Haim Department of Biology, University of Haifa-Oranim, Tivon 36006, Israel (Accepted 7 March 2005) Abstract Animals inhabiting environments with low productivity and food availability commonly have reduced energy demands and increased digestive efficiencies. The dry matter intake (DMI), apparent digestible dry matter (ADDM), digestible efficiency (DE) and digestible energy intake (DEI) of two populations of common spiny mouse Acomys cahirinus were compared during both winter and summer under conditions of simulated water stress. Mice were captured from the north- and south-facing slopes (NFS and SFS) of the same canyon that represent mesic and xeric habitats, respectively. Measured variables were also compared between F 1 mice that had been born to either NFS or SFS mice, and raised in the laboratory. SFS mice were able to assimilate energy more efficiently than NFS mice during the summer. By comparison, NFS mice were able to assimilate more energy during the winter. During winter, NFS mice assimilated more energy at low levels of water stress, whereas SFS mice assimilated more energy at higher levels. Differences were also apparent in F 1 mice. It is therefore suggested that local climatic conditions can impose physiological adaptations that are retained in succeeding generations, creating unique meta-populations. Key words: Acomys, energy, metabolism, thermoregulation, digestibility INTRODUCTION An important aspect that might limit survival in any given habitat is the rate and efficiency at which energy can be acquired. Several environmental factors, such as food availability and food quality, are potentially of importance (Nagy, 1983; Corp, Gorman & Speakman, 1997). Animals living in ecosystems with reduced food availability, such as deserts, commonly have reduced energy demands and an increased efficiency of nutrient uptake, compared with animals from more productive environments (McNab, 1989; Haim, 1987; Mueller & Diamond, 2001). Within Mediterranean ecosystems, different ecological condi- tions can exist in localized area due to differences in exposure to solar radiation. Environments similar to ‘xeric savanna’ and ‘temperate mesic’ can be formed on south- and north-facing slopes (SFS and NFS), respectively, of valleys and canyons due to increased amounts of incident solar radiation (up to 300%) received on the SFS (Nevo, 1995). This striking difference between slopes offers a situation where closely located populations of the same species can inhabit very different habitats. The most well- known locality in recent years is the lower Nahal Oren, the * All correspondence to current address: M. Scantlebury, Mammal Research Institute, Department of Zoology and Entomology, University of Pretoria, Pretoria 0002, South Africa. E-mail: m.scantlebury@zoology.up.ac.za ‘Evolution Canyon’ (Nevo, 1995). Apart from differences in habitat due to solar radiation, seasonal differences also impose different challenges to the species inhabiting either site. During the summer, the main environmental stressors are likely to be aridity and heat, which present more of a challenge to individuals that inhabit the SFS than the NFS. In addition, as the dry summer season is long (sometimes 8 months), the vegetation is expected to have an increased particle (or osmotic) concentration due to the higher evaporation of water (Grammatikipoulos, 1999). By comparison, during the winter, the main environmental stressor is presumably low ambient temperature, which is expected to affect individuals that inhabit the NFS more than those that inhabit the SFS. The common spiny mouse Acomys cahirinus inhabits both the NFS and the SFS of the lower Nahal Oren (Blaustein, Kotler & Nevo, 1996). We have previously shown that there are a number of physiological differences between A. cahirinus originating from these two areas. For example, NFS mice have higher field metabolic rates and a greater capacity to produce heat by non-shivering thermogenesis (NST) than SFS mice (Scantlebury, Afik et al., 2002; Scantlebury, Shanas, Speakman et al., 2003). In the current study, we aim to examine whether populations of A. cahirinus originating from the NFS and SFS of Nahal Oren also differ in their seasonal ability to acquire and process energy. Also, to examine whether