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The monotypic genus Pearsonomys is one of the least studied sigmodontine genera, so far known only from its holotype. We record four new localities from the Región de Los Lagos (southern Chile) for P. annectens expanding its known distribution by approximately 217 km. Next, we present a phylogenetic analysis based on nuclear DNA sequences aimed to test the phylogenetic position of Pearsonomys. Results of this analysis corroborate that Pearsonomys belongs to the abrotrichine radiation being sister of Geoxus. Subsequently, we provide descriptions of P. annectens molars (tetralophodont pattern with reduced mesoloph/id), penis (complex type), stomach (unilocularhemiglandular), and karyotype (2n=56; FN=62). The discussion of these new data is framed according the results of the phylogenetic analysis.

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Torpor is a phenotype characterized by a controlled decline of metabolic rate and body temperature. During arousal from torpor, organs undergo rapid metabolic reactivation and rewarming to near normal levels. As torpor progress, animals show a preference for fatty acids over glucose as primary source of energy. Here, we analyzed for first time the changes in the maximal activity of key enzymes related to fatty acid (Carnitine palmitoyltransferase and β-Hydroxyacyl CoA dehydrogenase) and carbohydrate (Pyruvate kinase, Phosphofructokinase and Lactate dehydrogenase) catabolism, as well as mitochondrial oxidative capacity (Citrate synthase), in six organs of torpid, arousing and euthermic Chilean mouse-opossums (Thylamys elegans). Our results showed that activity of enzymes related to fatty acid and carbohydrate catabolism were different among torpor phases and the pattern of variation differs among tissues. In terms of lipid utilization, maximal enzymatic activities differ in tissues with high oxidative capacity such as heart, kidney, and liver. In terms of carbohydrate use, lower enzymatic activities were observed during torpor in brain and liver. Interestingly, citrate synthase activity did not differ thought torpor-arousal cycle in any tissues analyzed, suggesting no modulation of mitochondrial content in T. elegans. Overall results provide an indication that modulation of enzymes associated with carbohydrate and fatty-acid pathways is mainly oriented to limit energy expensive processes and sustain energy metabolism during transition from torpor to euthermy. Future studies are required to elucidate if physiological events observed for T. elegans are unique from other marsupials, or represents a general response in marsupials. J. Exp. Zool. 325A:41-51, 2016. © 2015 Wiley Periodicals, Inc.