PARTICIPATION OF BREAST AND LEG MUSCLES IN SHIVERING THERMOGENESIS INYOUNG TURKEYS AND GUINEA-FOWL

Turkey (Meleagris gallopavo) and guinea fowl (Numida meleagris) chicks (0-27 days posthatch) were exposed to decreasing or increasing ambien t temperatures. Root mean square electromyographic activity of musculu s pectoralis (m. pect.) and musculus iliotibialis (ill. iliot.) was re corded simultaneously with O-2 consumption and CO2 production. From bo th muscles, relative mass, water fraction and fibre type were determin ed. M. iliot. participated in shivering from hatching onwards. The rel ationship between its root mean square electromyographic activity and ambient temperature resembled that of metabolic rate and ambient tempe rature, and the shivering threshold temperature was indistinguishable from the lower critical temperature. This suggests that the leg muscle s are major contributors to shivering thermogenesis. M. pect. particip ated in shivering only at days 6-20 in turkeys and at days 6-10 in gui nea fowl. Both water fraction and histological analysis indicated that m. pect. was less developed than m. iliot. at hatching. We hypothesiz e that a minimal level of maturity is required before a muscle can par ticipate in shivering, which is probably represented by a water fracti on of about 0.85. Both species recruited the aerobic leg muscles first ; the anaerobic breast muscle was recruited only when the rate of mass -specific heat loss was high.