Seasonal variation in brown adipose tissue mass and lipid droplet size of Sorex minutus, the pygmy shrew; The relationship between morphology and metabolic rate

Abstract

1. 1. Metabolic rate, brown adipose tissue (BAT) mass and BAT lipid droplet size, were measured in winter-, summer- and spring-acclimatized Pygmy shrews, Sorex minutus. 2. 2. BAT mass changed significantly with season both in relative and absolute terms. Winter-acclimatized shrews had the smallest (135 ± 10 mg) and spring-acclimatized shrews had the heaviest BAT depots (310 ± 28 mg). Relative BAT mass varied between 2–7% depending on season. Median BAT lipid droplet size (μm 2) was smallest in winter-acclimatized (106 μm 2) and largest in spring-acclimatized shrews (326 μm 2). 3. 3. Resting metabolic rate and thermoregulatory thermogenesis were significantly positively related to BAT mass. There was a significant inverse relationship between mass independent metabolic rate and lipid droplet size in all seasonally acclimatized shrews. 4. 4. Winter-acclimatization of BAT in S. minutus is characterised by a decrease in BAT mass signifying a marked utilisation of lipid stores and by a reduction in BAT lipid droplet size, signifying an increase in thermogenic activity.