Threshold for shivering in aerobic and anaerobic muscles in bantam cocks and incubating hens

Electromyographic activity (EMG) from the musculus pectoralis (breast muscle), m. iliotibialis (thigh muscle) and m. gastrocnemius (leg muscle), cloacal temperature (Tb) and O2 consumption were measured in bantam cocks (Gallus domesticus) exposed to different ambient temperatures (Ta). The same parameters were measured in bantam hens incubating eggs artificially thermoregulated to 40 degrees and 25 degrees C (Te). EMG activity appeared in thigh and leg muscles at Ta below 32 degrees C (Tsh). This temperature probably represents the thermoneutral temperature (TNT) of the cock. EMG activity in breast muscles appeared at Ta below 20 degrees C, or 4 degrees C below the lower critical temperature (Tc). All muscles were quiet when the hen incubated 40 degrees C egg at Ta = Tsh. When Te was abruptly changed to 25 degrees C, EMG activity in the iliotibialis muscle appeared 3 min before the activity in the pectoralis muscle. Tb dropped from 41.2 degrees to 40.6 degrees C in 14 min. When Te was returned to 40 degrees C, the EMG activity in the pectoralis muscle disappeared almost at once, while the iliotibialis muscle was active until Tb returned to normal. Aerobic muscles seem to be responsible for shivering thermogenesis between Tc and Tsh, while anaerobic muscles are recruited at lower Ta or when the heat loss during incubation becomes severe.