6.2. Development of shivering thermogenesis

This study is both one of the few systematic studies of the development of shivering thermogenesis in young birds and also the largest of them. The earlier studies have revealed that the first signs of shivering can be detected in hatchlings or even in late embryos in precocial birds, whereas in altricial nestlings shivering appears during later post-hatching development, (e.g. Whittow & Tazawa 1991). However, incipient shivering is difficult to observe without electromyography. Aulie and Moen (1975) did not observe visible shivering tremors in chicks of willow ptarmigan (Lagopus lagopus) before the age of two days, but this did not mean that younger chicks lack shivering thermogenesis since they were capable of increasing heat production slightly when exposed to cold. Obviously, they only lacked visible tremors. Subsequent EMG recordings from the chicks of willow ptarmigan revealed that newly-hatched chicks also produce heat by means of shivering (Aulie 1976). EMG recordings were used in the present study, and in all the species of studied this work, even the earliest measurements revealed myoelectrical activity (II). However, the beginning of shivering was apparent only when the amplitude of the EMG increased in response to cold exposure.

All three precocial galliform species of the present study utilized mainly leg muscles in shivering thermogenesis during the first post-hatching week (II). This kind of utilization is obvious, because in newly-hatched precocials most of the muscle mass is in the legs, and leg muscles are also more mature in comparison to the pectorals (Aulie 1976, Ricklefs 1979). From the second week onwards, the pectorals were also recruited for shivering (II). Especially in domestic fowl, the shivering in the pectorals remained at a low level even at three weeks of age. In this species, the importance of leg muscles in heat production is obviously greater even in adults due to the very low proportion of aerobic muscle fibres in the pectorals observed by histochemical analyses (Barnard et al. 1982, Smith et al. 1993). In the present study, the amplitude of the shivering in the pectoral muscles was apparently dependent on the adult size being larger in small species (II), thus reflecting the negative correlation between the metabolic rate and the body mass. The adult body masses for both sexes of the domestic chicken (leghorn), grey partridge and Japanese quail are approximately 670–1020g, 380–450g and 100–150 g, respectively (Ichilcik & Austin 1978, Dunning 1992). In the present study with these three galliforms species, the shivering intensity level in the breast muscles was also observed to show a positive correlation with the adult bird’s flight endurance, which depends on the level of oxidative metabolism of breast muscles.

In the altricial domestic pigeon, shivering in pectorals was present at two days of age and the pectorals had a predominant role in heat production (II). Breast muscles are also the principal site of thermogenesis from the beginning in Passeriformes (Olson 1994). In the young pigeons, there was also an evident cold-induced increase in the shivering amplitude in the leg muscles (II), which thus seem to have an assisting role in heat production during severe cold. This is an interesting observation, since generally, the role of the pectorals has been emphasized in the heat production of the altricial birds in the cost of the leg muscles. Despite the fact that EMG signs of shivering begin early in pigeons, intense thermogenesis appeared clearly later than in Galliformes (II). The appearance of shivering both in the precocials and altrial species of the present work are consistent with many earlier fragmental observations and few systematic measurements of shivering in young birds, which are now first time comprehensively reviewed (Fig. 4). Several conclusions of the ontogeny of shivering can be done from the figure. 1) Both in precocials and semiprecocials, shivering appears at a late embryo stage or soon after hatching. 2) In most precocials, the beginning of shivering in leg muscles clearly precedes the onset in the pectorals. 3) In semiprecocials, shivering in breast muscles coincides with the onset in legs, or only a few hours later. 4) In semialtricial and altricial nestlings, the beginning of shivering clearly occurs later than in precocial and semiprecocial chicks. 5) Moreover, in semialtricial and altricial development modes, the onset of shivering in the pectorals precedes the beginning of shivering in leg muscles.

In order to compare the shivering intensities of different species and of different individuals within a species, the standardization of the attachment site of the recording electrode is essential. For example, in EMG measurements with bipolar wire electrodes, the recording site has been found to result in more variation than the electrode material or its dimensions (Gans & Gorniak 1980). Similarly in the breast muscle of Japanese quail chicks in the present study, the amplitude of the shivering at the surface and intramuscular locations varied (Fig. 2): the amplitudes of the EMG increased with decreasing ambient temperature in both locations, but the surface EMGs were consistently more intense than the intramuscular ones. Moreover, in the gastrocnemius muscle, the subcutaneous electrodes on the surface of the muscle yielded more intense EMG amplitudes when placed near the distal end of the muscle than when they were placed on the middle of the muscle (personal observation). The reason for the variation of the amplitude may be due to fact that the proportion of different types of muscle fibres varies within muscle (e.g. Rosser & George 1986) as well as the distribution of muscle fibres in motor units (the fibres of some motor units may be clumped and in others distributed), so resulting in different EMG amplitudes in different locations. Moreover, the damage in muscle tissue due to implantation of the electrode inside muscle, may affect the recorded EMG intensity (c.f. Fig. 2). Therefore comparisons of shivering intensities between individuals should be made due caution and an awareness of these and other potential error sources.

Measurements in 21-day-old Japanese quails verified that shivering truly produced an increase in muscle temperature (Fig. 3A). The correlation of the temperature difference T_muscle–T_cloaca to rms voltage of the EMG was curvilinear and to oxygen consumption linear (Fig. 3B), just as also Hohtola (1982) observed in adult pigeons. Thus the present study confirms that direct muscle temperature recordings quantify shivering thermogenesis even more reliably than EMG recordings. In adult birds, the amplitude of EMG usually correlates well with the heat production or oxygen consumption (Hohtola 1982). The present study clearly showed that both in altricial and precocial hatchlings an early cold-induced increase in shivering does not necessarily cause a significant increase in heat production (II). For example, in one-day-old partridges and 2 to 4-day-old domestic pigeons, the EMG intensity increased in cold without a simultaneous measurable augmentation in heat production. Thus, even before muscles are mature enough for thermogenesis, their motoneuronal control and myoelectrical control of shivering are functional. In precocial birds, motoneuronal maturation seems to occur during the embryo stage. For example, in Japanese quail chicks, the motor end plates are stained when using the acethylcholine estherase staining method (see Karnovsky & Roots 1964) already immediately hatching both in the pectoral and leg muscles, thus indicating the mature synaptic junctions (Marjoniemi & Hohtola, unpublished). However, not only motoneurons, but also muscles have to reach a certain minimal level of maturation before thermogenesis is enabled. A water fraction of ≤ 0.85 in skeletal muscles seems to be the bottom limit for heat production in these muscles in many species (Ricklefs & Webb 1985, Dietz et al. 1997, Visser 1998). In four-week-old Pekin ducklings and three-week-old Japanese quails, all muscles both in cold-acclimated and warm-acclimated individuals were clearly below this fraction (III). Furthermore, all muscles also showed increased shivering activity in cold which was accompanied by increased heat production.