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Cardiopulmonary organs were not affected by body size (p = 0.8) or the interaction period*sex (p = 0.2) but their lean dry mass varied with sex (Fi_>72⁼ 8.9, p < 0.01) and periods (F3.72 = 12.2, p < 0.0001). Males (68 ± 1 mg) had 9.7% heavier heart+lungs than females (62 ± 1 mg). These organs increased in mass by 14.0% between fali and midwinter (Tukey: p<0.01), then started to decline, although non-significantly (p= 0.2) at the end of winter and decreased by 17.6% between the end of winter and summer (Tukey: p < 0.001) (figurę 4.4b).

Variations in lean dry muscle mass were affected by body size (Fj, 71 — 14.0, p < 0.001), sex (Fi,71= 32.7, p<0.0001) and period (F₃i7I=11.5, p<0.0001) but not by the interaction period*sex (p = 0.1). For a given structural size, males (926 ± 11 mg) had muscles 11.2% heavier than females (833 ± 11 mg). Muscle mass increased by 10.7% between fali and midwinter (Tukey: p< 0.0001). It then tended to decline non-significantly toward the end of winter (p = 0.3) and decreased by 9.8% from the end of winter to summer (Tukey: p < 0.01), to a level indistinguishable from that recorded in the fali (Tukey: p = 0.5) (figurę 4.4c).

The lean dry mass of excretory organs varied independently from the effects of body size (p = 0.5), periods (p = 0.6) and the interaction term period*sex (p = 0.3). It was, however, affected by sex (Fi.74- 6.4, p < 0.05) with males (111 ±2 mg) having their kidneys+liver 6.5% heavier than females (104 ± 2 mg).

Variations in brain mass were independent from body size (p = 0.3) and the interaction term period*sex (p = 0.7) but were related to variation in these latter variables taken independently (sex: F|,7i= 17.0, p< 0.0001, period: F3.₇i = 3.6, p<0.05). Males (115 ± 2 mg) had a lean dry brain mass 10.6% heavier than that of females (104 ±2 g) and average brain mass was at its highest in summer with a value 10.4% and 10.5% higher than in the fali (Tukey: p < 0.05) and midwinter (Tukey: p < 0.05) respectively (figurę 4.4d).

Lean dry skin mass varied with body size (Fi_t7o“5.4, p<0.05), sex (Fi_i70=6.2, p<0.05) and period (F₃,70= 29.5, p< 0.0001) but not with the interaction period*sex (p = 0.4). Controlling for size, skin mass in males (119 ± 2 mg) was 7.2% heavier on average than in females (111 ±2 mg). Lean dry mass of the skin increased by 10.0% between fali and midwinter (Tukey: p < 0.05), remained stable until the end of winter (Tukey: p = 0.9) and then increased by a further 30.6% between end of winter and summer (Tukey: p < 0.0001) (figurę 4.4e).