Chapter 1. Introduction

The survival of a newborn homeothermic animal greatly depends on how well it is able to respond to the large thermal transition and subsequent heat loss that occurs at birth. Well-developed or precocial species (Blix & Steen 1979) such as the sheep and rabbit are able to keep their body temperature high immediately after birth through the onset of effective metabolic heat production, or non-shivering thermogenesis (NST) (Heim & Hull 1966, Nedergaard et al. 1986). Altricial, ‘nest-dependent’ species such as the rat and ‘immature’ species such as the hamster are more susceptible to variations of ambient temperature and their capacity for thermoregulatory heat production by NST does not develop until during the first few days or weeks of life (Hissa 1968, Nedergaard et al. 1986). It is well established that the main site of NST in the mammalian neonates is brown adipose tissue (BAT) (Heim & Hull 1966, Cannon & Nedergaard 1985, Nedergaard et al. 1986) which plays a key role in their postnatal survival in the cold. NST in BAT is stimulated by the cold-induced release of the sympathetic nervous transmitter noradrenaline (NA) (Jansky 1973) and is supported by the activation of thyroid hormones (Bianco & Silva 1987, Trayhurn et al. 1993a). In addition to neonatal mammals, BAT plays an important role in rewarming the body at the end of hibernation, and in some adult mammals adapted to remain active in the cold (Cannon & Nedergaard 1985, Trayhurn 1993).

Specific anatomical locations of BAT have been described in different species (Afzelius 1970, Néchad 1986) that confirm the specialization of this tissue for the distribution of heat (Smith & Horwitz 1969). The specialization of BAT for thermogenesis is apparent by its highly organized cellular structure with abundant mitochondria, multilocularly dispersed fat, dense capillarization and sympathetic innervation of adipocytes (Néchad 1986, Lonèar 1991). White adipose tissue (WAT) is usually unilocular, with few mitochondria and less visible innervation and vascularization. On the basis of cell morphological features, BAT has been characterized and identified in numerous mammalian species (Néchad 1986). The capacity of BAT for thermogenesis is ultimately dependent on the presence and activity of the tissue-specific 32000 M_r-uncoupling protein, UCP (UCP₁ since 1997, cf. Boss et al. 1998) located in the inner membrane of brown adipocyte mitochondria (Cannon & Nedergaard 1985, Klaus et al. 1991). UCP₁ uncouples the chemical energy released in the oxidation of the fatty acids from ATP synthesis, thus releasing it primarily as heat (Nicholls & Locke 1984). The fatty acids are also believed to activate UCP₁ (Nicholls et al. 1986, Boss et al. 1998, Lowell & Spiegelman 2000), but the nature of this mechanism has yet not been elucidated. As UCP₁ appears only in brown adipocytes, its detection by immunological techniques has been used as a criterion for identifying BAT and differentiating it from WAT in various species, including humans (Cannon & Nedergaard 1985, Klaus et al. 1991, Trayhurn 1993). During most of its research history, BAT has been most intensively studied in the altricial rat and other laboratory species. This has led to a situation where much basic knowledge is still lacking about the presence and function of BAT in large precocial species. Lately, large ruminants have been more studied (Casteilla et al. 1987, 1989, 1994, Trayhurn et al. 1993a, b) but large wild or freely ranging terrestrial species that are exposed to cold in their natural environment have been studied only incidentally.

The reindeer (Rangifer tarandus) is a circumpolar northern species of Cervidae that exhibits advanced adaptation to extreme seasonal changes in its arctic and subarctic environments. In Fennoscandia, most reindeer are semi-domesticated and their herding constitutes an important livelihood and basis of Sami culture. Reindeer calves are born under adverse weather conditions during spring when the pastures are usually snow-covered and the ambient temperature frequently falls below 0°C. Reindeer calves are precocial (Blix & Steen 1979) and are able to follow and suckle their mothers within hours of birth. The transition from the uterus to the external thermal environment represents a large thermogradient (30-60˚C) and thus an extreme thermoregulatory challenge for a neonatal reindeer. Previous studies have shown that newborn reindeer are capable of effective thermoregulation and they have suggested the presence of BAT and a high capacity for NST (Krog et al. 1977, Hissa et al. 1981, Markussen et al. 1985, Soppela et al. 1986), as with another arctic ungulate, the muskoxen (Blix et al. 1984). However, the evidence for the presence of BAT in reindeer has been fragmentary and its significance for NST poorly established. The reindeer provides an example of a large freely ranging precocial terrestrial species for whose neonatal survival and reproduction success NST appears crucial. The nutritional condition of a newborn reindeer is also of interest as both the foetal development and calving of reindeer occur in very poor nutritional conditions. Early calf mortality in reindeer is frequently associated with poor nutrition and poor resistance to disease (Eloranta & Nieminen 1986).

For northern freely ranging mammals, winter and early spring are the most challenging periods for survival due to the poor availability of food and its low quality. The major winter diet of Fennoscandian reindeer in many areas, ground lichens (Cladina spp.), are rich in carbohydrates but poor in protein content as are most of the other winter feed sources for reindeer (Nieminen & Heiskari 1989). Several studies have shown that reindeer enter a negative energy balance when feeding on natural pastures during winter (McEwan 1968, Reimers et al. 1983, Nieminen et al. 1984). During the course of winter when the availability of feed is restricted due to hard digging conditions, the reindeer often enters a state of starvation. The use of adipose tissues has been proposed to play an important role in the wintertime survival of reindeer (Ringberg et al. 1981, Larsen et al. 1985), especially in the Svalbard reindeer that is the fattest of the subspecies of Rangifer (Pond et al. 1993). As reindeer are generally lean animals, the importance of adipose tissue as an energy source for their survival during winter has also been questioned (Tyler 1987). The roles of adipose tissues or their fatty acids other than as an energy source have attracted little attention. However, specific fatty acids, in particular polyunsaturated fatty acids (PUFAs) have important physiological and growth-related functions in the body (Innis 1991, Bruckner 1992). The changes in the fatty acid composition of adipose tissues in relation to nutritional condition in freely ranging ungulates, including reindeer, have not been studied.

There is increasing evidence from in vitro experiments that the lipolysis and release of fatty acids from triacylglycerols (TAGs) in adipose tissue is not a random process but favors long-chain and unsaturated fatty acids (Gavino & Gavino 1992, Raclot & Groscolas 1993, Raclot et al. 1995). These findings and the evidence of the selective mobilization of specific PUFAs from the adipose tissues of weight-cycled rats (Chen et al. 1995) and fasting emperor penguins (Grosclas et al. 1990) have suggested that unsaturated fatty acids may have special roles in the body during the periods of a negative energy balance. The low interest in the fatty acid composition of ruminant adipose tissues, including that of reindeer is perhaps because they are - due to the effective biohydrogenation of unsaturated fatty acids by rumen microorganisms - mainly saturated (Garton & Duncan 1971, Christie 1981). The striking exception in this respect are leg bone marrow lipids, which contain a high proportion of unsaturated fatty acids in reindeer (Meng et al. 1969, Pond et al. 1993) and other ruminants (West & Shaw 1975, Turner 1979, Christie 1981), and seem to be preserved until the last phase of undernutrition (Ransom 1965, Nieminen & Laitinen 1986, Davis et al. 1987). The deprivation of dietary PUFAs in rodents and humans are reflected as rapid reductions in their circulating levels (Chen & Cunnane 1992, Leichsenring et al. 1995). These observations motivate studies also in ruminants as their major serum lipids are enriched with PUFAs (Christie 1981). The changes in the fatty acids of bone marrow TAGs and serum lipids in ruminants appear particularly interesting as potential indicators of undernutrition.

The factors that regulate feed intake, body weight and body fat cycles in ruminants are poorly known. It has been shown that species that have clear seasonal cycles in body weight, such as the reindeer, decrease their feed intake and body weight voluntarily during winter even if they are given high quality rations without limitation (Ryg & Jacobsen 1982, Larsen et al. 1985, DelGiudice et al. 1987, Suttie & Webster 1995). The effect of a short day-length mediated by hormones such as an insulin-like growth factor (Suttie & Webster 1995) has been proposed to participate in the intrinsic regulation of seasonal feed intake and body weight in highly seasonal species. Recent studies have proposed that the secretory product of adipose tissue, leptin, may also play a role in the control of feed intake and body weight in vertebrates by providing the brain with a signal about the amount of stored adipose tissue, thereby acting as a satiety factor (Considine & Caro 1997, Trayhurn et al. 1998). The exact mechanism of the effect of leptin is not known and is being actively studied (Schwartz et al. 2000). The production of leptin is decreased during fasting and increased during overfeeding (Trayhurn et al. 1998). As reduction in feed intake and body weight during winter is common in reindeer, this species offers an opportunity to also view the regulatory aspects of the adiposity.

Physiological studies in reindeer are relatively new as the reindeer is not a conventional target of investigation and its viability depends greatly on ecological factors. Due to its large size, long reproduction cycle, and elaborate handling, only a small number of animals can be taken into experiments, and samples have to be collected over several years. However, research in a seasonal and highly adapted species such as the reindeer is important in providing a basis for estimating whether the results obtained in laboratory and domestic species can be generalized to encompass animals in natural conditions. Such studies provide information about the mechanisms underlying or regulating the adaptation of reindeer to its living environment, which can then be implemented in reindeer management. This study was undertaken in order to increase the understanding of the ecophysiological mechanisms which enable reindeer to survive neonatal cold stress and undernutrition during winter.