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Abstract

To cope with highly stochastic and/or heterogeneous environmental conditions, animals must balance energy resource allocation across physiological processes. The digestive tract and brain exhibit structural variations under strong developmental and selective pressures that vary across environmental gradients both between and within species. Here, we measured body mass, body length, digestive tract segments lengths (stomach, cecum, small intestine, large intestine), brain weight, and relative brain size (scale mass index, SMI residuals) for 67 Chinese pygmy dormice (Typhlomys daloushanensis, 29 females and 38 males), an ancient, small, arboreal rodent species with echolocation abilities, captured at 37 sites between 414 and 1757 m. From Standardized Major Axis (SMA) regressions, we found no significant relationship between body size (SMI) and elevation. Notably, neither digestive tract segment lengths and their residuals nor brain weight (absolute and residual) correlated significantly with elevation in either sex, and slopes did not differ between males and females, providing no support for the Digestive Tract Theory (DTT) or the Cognitive Buffer Hypothesis (CBH). However, brain–digestive tract relationships exhibited sex-specific differences, partially supporting the Expensive Tissue Hypothesis (ETH), suggesting that energy allocation to high-cost organs is sex-dependent. Males maintained relatively heavier brain weight under comparable digestive tract lengths, likely to meet cognitive demands associated with mate competition, territorial defense, and exploratory behavior, whereas females tended to prioritize digestive tract investment to meet the energetic demands of gestation and lactation. Thus, classical predictions of body size–environment relationships may be overridden by sex-specific physiological trade-offs in specialized species, providing new insights into how small mammals may adapt to rapid environmental change.