Feeding the newborn: comparative problems in man and animals.

The human problems that we will hear about later in this meeting concern feeding babies of unusual weights; first, babies that weigh too little at birth and secondly, babies that weigh too much after birth because they have been over-fed. Somewhat similar situations occur in animals, either in their natural circumstances, or as a result of experimental procedures. I shall describe examples of these in relation to the problems of feeding the human baby. A low birth weight may be due simply to prematurity: the foetus has grown normally but it has been born too soon. On the other hand, it may be due to slow growth in the uterus so that the baby is small for its gestational age. Such a baby may be born prematurely or it may be born at full term. If it is born prematurely it has the double handicap of immaturity and small size for its gestational age. As far as I know, no animal born after only 70% of its full gestation period has survived, yet human babies are now being reared when they have been born after only 28 weeks’ gestation instead of 40, and weighing only about I kg, or less than 30y0 of the average wTeight at full term. The small size in itself presents difficulties, but the major problem about prematurity is immaturity. Although such extreme prematurity as this, with survival, does not occur in the animal kingdom, extreme immaturity does. Many of the rodents, mice, rats and rabbits, for example, have young that are immature and helpless at birth, with eyes and ears still closed, and the same applies to carnivores, dogs, cats, lions, tigers and foxes. All these species have a number of young at one time. The mother makes a nest for them before they are born, and warmth is essential for their survival. They depend upon the warmth of the mother’s body and huddling with the rest of the litter to maintain a body temperature sufficiently high to enable their metabolic processes to proceed efficiently. They have short limbs and short tails which make for a small surface area, and this helps to prevent heat loss. These immature newborn animals also require a constant supply of food. The mouse and rat spend about 80% of their time suckling the newborn litters, and in cats and other carnivores the mother rarely leaves her young for the first few days, even to feed herself. T h e newborn rabbit, which has only one feed per d, is in effect being constantly fed because its large stomach acts as a reservoir; it empties slowly so that there is a continuous supply of nutrients being passed through the pylorus into the small intestine where they are digested and absorbed. This, and the generous supply of brown adipose tissue with which the newborn rabbit is endowed, enables it to survive in spite of the intermittent way in which it is fed, and the lack of maternal warmth and care. I n the