J Comp Physiol B DOI 10.1007/s00360-007-0149-4 123 ORIGINAL PAPER EVects of diet quality on phenotypic Xexibility of organ size and digestive function in Mongolian gerbils (Meriones unguiculatus) Quan-Sheng Liu · De-Hua Wang Received: 8 November 2006 / Revised: 24 January 2007 / Accepted: 29 January 2007  Springer-Verlag 2007 Abstract In the context of evolution and ecology, there is a trade-oV between the beneWts of processing food through a digestive system with speciWc pheno- typic attributes and the cost of maintaining and carry- ing the digestive system. In this study, we tested the hypothesis that digestive modulations at several levels can match each other to meet the energy and nutrient demands of Mongolian gerbils, a small granivorous rodent species, by acclimating them to a high-quality diet diluted with alfalfa powder. Mongolian gerbils on the diluted diet maintained metabolizable energy intake by an integrated processing response (IPR), which included increases in dry matter intake, gut capacity and rate of digesta passage after 2-weeks of acclimation. Down-regulation of hydrolytic enzyme activity in the intestinal brush-border membrane sup- ported the adaptive modulation hypothesis. The absence of up-modulation of summed enzyme hydro- lytic capacity on the diluted diet indicated that greater mass of small intestine on a high-Wbre diet is not a direct indicator of digestive or absorptive capacity. Changes in mass of vital organs and carcass suggested that the amount of energy allocated to various organs and hence physiological functions was regulated in response to diet shift. Keywords Adaptive modulation hypotheses · Integrated processing response (IPR) · Mongolian gerbils · Gastrointestinal tract · Sucrase · Maltase · Aminopeptidase-N · Metabolizable energy intake · Vital organs Introduction In the context of evolution and ecology, there is a trade-oV between the beneWts of processing food through a digestive system with speciWc phenotypic attributes (e.g. gut mass and length, the density and activity of intestinal enzyme and nutrient transporters) and the cost of maintaining and carrying the digestive system (Sibly 1981; Diamond and Karasov 1983; Toloza et al. 1991; Diamond 1991; Derting and Bogue 1993; Naya et al. 2005). For example, the digestive and absorptive capacity of the vertebrate small intestine should be matched to nutrient and energy intake through natural selection (Diamond 1991; Karasov and Hume 1997). If there were no such match, then valu- able food energy might be wasted in excreta when feeding on diets with high substrate levels, and/or the metabolic expenses of synthesizing and maintaining the molecular machinery to hydrolyze and absorb sub- strate would be wasted when feeding on diets with very Communicated by I.D. Hume. Q.-S. Liu · D.-H. Wang (&) State Key Laboratory of Integrated Management for Pest Insects and Rodents, Institute of Zoology, Chinese Academy of Sciences, 25 Beisihuan Xilu, Zhongguancun, Haidian, Beijing 100080, China e-mail: wangdh@ioz.ac.cn Q.-S. Liu Guangdong Key Laboratory of Integrated Pest Management in Agriculture, Guangdong Entomological Institute, 105 Xin’gang Xilu, Haizhu, Guangzhou 510260, China Q.-S. Liu Graduate School of the Chinese Academy of Sciences, Yuquan Lu, Beijing 100049, China