5.2. Development of shivering thermogenesis
In precocial birds, first signs of shivering thermogenesis appeared in leg muscles, and in altricial pigeons in breast muscles (II). A clear increase in shivering amplitude during cold exposure occurred in breast muscles of pigeons, domestic fowl, quail and partridge at 2, 7, 7 and 10 d of age, respectively. In the gastrocnemius, the equivalent ages were 4, 1–2, 1 and 1 d. In the gastrocnemius of two-day-old pigeons, the amplitude of the EMG increased due to movement activity. The earliest shivering response observed in the pigeons and partridge did not result in significant heat production. The amplitude of shivering was dependent on the adult size being higher in small species. The amplitude of shivering varied within different locations of breast muscles of Japanese quail. Fig. 2 shows the amplitudes of integrated EMGs measured from surface and intramuscular locations at four different ambient temperatures. The amplitude of the EMG increased with decreasing ambient temperature in both locations, but the surface EMGs were more intense than the intramuscular ones.
Figure 2. The intensity of shivering EMGs in surface (solid circles) and intramuscular (open circles) locations of pectoral muscles of 21-d-old Japanese quail chicks at four different ambient temperatures. N = 4–5 at each point. Vertical bars indicate SE. (Marjoniemi, unpublished).
The shivering threshold temperature was found to be dependent on the muscle studied, on the age of the bird and on cold-acclimation (II, III) but not on the nutritional state of a chick (IV). In galliform chicks, except for the Japanese quail, shivering in the gastrocnemius started at a higher temperature than in pectoralis. During postnatal development the order changed towards a higher onset temperature for the pectoralis. In Japanese quail chicks, the shivering threshold temperatures did not differ between the pectoralis and the m. gastrocnemius. In the pigeon, shivering (or muscle activity in gastrocnemius) began at 2 d of age at the same temperature in the pectoralis and the gastrocnemius and thereafter shivering was always triggered at a higher temperature in the pectoralis.
The galliform chicks shivered in bursts and only in the fowl, from the age of 14 d onwards, was regular bursting or “true bursting” visible. In the pigeons, a bursting-type of shivering was the first form of shivering observed, but continuous shivering also developed parallel with the increased heat productive capacity. The shivering pattern was not influenced by the nutritional state of the bird (IV).
It was found that the size of the bird had an effect on the shivering frequencies. The median frequency of shivering had a tendency to increase with growth and maturation. Similarly, the frequency content of the EMG was dependent on the adult size, being higher in small species.
Figure 3. A: Oxygen consumption, root mean square EMG of the pectoralis, surface temperature of the pectoral muscle and temperature of the cloaca during steadily decreasing ambient temperature (0.33°C·min-1) from 32 to -5°C in a 21-day-old Japanese quail chick. B: Temperature difference between the pectoral muscle and the cloaca first versus rms voltage of the EMG and second versus the oxygen consumption (Marjoniemi, unpublished).
Slight 2–3°C hypothermia was observed to have no appreciable effect on the frequency content of shivering (II) while fasting clearly decreased shivering frequencies (IV). Fig. 3A shows the effect of shivering thermogenesis on the surface temperature of pectoral muscle and Tcloaca during a steadily decreasing ambient temperature in a 21-day-old Japanese quail chick. The temperature gradient between the muscle and the cloaca increased with increased shivering thermogenesis. The correlation of the temperature difference between the muscle and the cloaca to the rms voltage of the EMG was curvilinear and to the oxygen consumption linear (Fig. 3B).