MATERNAL PROCESSES IN THE COLD‐ADAPTATION OF MICE

SUMMARY1 Both laboratory and wild house mice, Mus musculus , given bedding, can breed in captivity in an environment kept at – 3°C. The nest temperature when a young litter is present then fluctuates widely. In a typical laboratory (at 21°C) the temperature of the nest is both higher and more constant. 2 The ovaries of pregnant mice breeding at – 3°C have more corpora lute a than controls at 21°C. This is not an index of a higher ovulation rate, but is evidently due to the presence of corpora lutea from a pievious ovulation. 3 In the absence of concurrent lactation, weights and numbers of foetuses at the sixteenth day of gestation are little affected by cold; but in both environments foetal weight diminishes with increasing size of litter. This is a systemic effect: foetal weight is hardly if at all influenced by the number of other foetuses in the same uterine horn. 4 Cold delays the onset of breeding and lengthens the interval between litters. Mean litter sizes are usually lower than in the warm environment, mainly through absence of large litters. 5 The body weights of laboratory mice are usually lower at – 3°C than 21°C at all ages from 3 weeks. This does not, however, apply to strain C57BL, which never stores much fat in adipose tissue. Wild mice bred at – 3°C are heavier than controls at 21°C, possibly because only the heavier individuals survive in early life. 6 F1 hybrids produced by crossing two inbred strains breed better and more consistently than the parent strains at both temperatures; but the effect of heterozygosis is much greater in the cold environment. 7 Food intake changes little during pregnancy, but rises greatly during the first 10 days of lactation at both temperatures. 8 At 21°C, body weight, excluding the weight of the litter, increases only slightly during pregnancy; but the weights of the heart and liver are greatly increased. The weight of the stomach also rises; the small intestine lengthens, but becomes lighter. During lactation the liver becomes still heavier, and the small intestine more than restores its loss of weight. The kidneys also become heavier. At – 3°C similar changes occur, but the heart is heavier at all stages of the reproductive cycle than it is at 21°C. The kidneys, too, are consistently heavier in the cold, and so is the small intestine. By contrast, the liver of pregnant or lactating females at – 3°C is no heavier than in the warm environment. 9 Pregnancy entails an increase in the absolute amount of nitrogen in the body, in both environments; but females at – 3°C have less nitrogen and collagen than controls. Pregnancy does not alter body fat at either temperature, but lactation is accompanied by some loss. At birth, mice born in the cold environment have more than twice as much body fat as controls. 10 When mice are bred for their full reproductive span, the effect of a cold environment depends markedly on genotype. Mice of strain A2G/Tb eventually produce as many young in the cold environment as in the warm, but take longer to do so; C57BL/Tb produce fewer young, Wild mice produce fewer litters at – 3°C, and have a much higher nestling mortality. Most of the mortality is due to loss of whole litters. 11 The preceding statements apply to mice of the first two or three generations in a cold environment, There are further effects of breeding for many generations in the cold. Wild mice bred for ten generations lose fewer litters in later than in earlier generations. After ten generations, some wild mice were moved from –3 to 21°C. Their reproductive performance was then much superior to that of controls which had been kept at 21°C throughout. The transferred mice were also quicker than the controls to make a nest of paper. 12 Genetically heterogeneous laboratory mice, after twelve generations in the cold, were similarly returned to the warm environment. Their offspring were heavier than controls; but there was no superiority in reproductive performance. 13 A2G/Tb mice kept at –3°C, though highly inbred, also improved in reproductive performance over a number of generations: in particular, their infant mortality declined. This was probably not due to a genetical change, but to a cumulative maternal effect. 14 Maternal performance was studied by cross‐fostering young at birth between these ‘Eskimo’ mice, ‘immigrant’ mice of the first or second generation reared in the cold, and controls at 21°C. There was some evidence of an effect of true parentage, regardless of foster parentage, on body weight: the young of the Eskimo mice tended to be heavier than the others. There was also evidence that this influence persisted into a second generation. Mortality among the fostered young was influenced only by true parentage, not by foster parentage or environmental temperature. Some of the fostered mice were mated. Again, among their young, mortality in the nest was not affected by environmental temperature; but those whose true ancestry was Eskimo displayed a lower mortality than the others. 15 If a young mammal is given special treatment (such as exposure outside the nest), the treatment may influence, not only the individual treated, but also the behaviour of the parents; and the altered parental behaviour may in turn affect the development of the young. Enhanced parental attention in the nest has been directly observed after young have been exposed to cold or other treatment. It can probably accelerate maturation, and improve reproductive performance by lowering mortality among the young of the treated mice. Hence the direct effects of treatment in infancy can never be distinguished with certainty from indirect effects through changed parental behaviour, unless the experimental animals are reared artificially. 16 A comprehensive theory of ‘stress’, that is, of the response of a species to an environmental change for the worse, requires that attention should be paid to the following: (i) the effects of physiological (ontogenetic) adaptation to one ‘stressor’, such as cold, on response to another, such as infection; (ii) the ways in which conditions of rearing, especially early exposure to mildly adverse conditions such as lower temperature, influence later physiological, reproductive and behaviour al performance; (iii) the relationships of the above with the adaptive changes of pregnancy and lactation.