Energy requirements and food intake of pregnant females are about 17-32% higher than non-reproducing females, and yet only 10-20% of this additional energy is retained as new tissue by the developing uterus. The rest of the energy is lost as heat, slowing down the growth rate and thus lengthening the gestation period. A slower fetal growth rate may be advantageous in an environment with limited dietary protein or minerals, especially in the case of such animals as the fruit-eating or leaf-eating primate.
In females, the fetus represents 80% of the energy retained by the uterus. Most of the increase in mass in the mother occurs after 50-60% of the gestation period has elapsed. The water content of the developing fetus also decreases while the fat, protein, and mineral content increase during gestation. The mammalian newborn, or neonate, averages 12.5 ± 2.3% protein and 2.7 ± 0.8% ash at birth. Neonatal fat, water, and energy content vary between different species. For instance, neonatal seals, guinea pigs, and humans contain 4-8 times more fat than other mammals, whose content averages 2.1 ± 1.0%. The fat reserves of the guinea pig are broken down just a few days after its birth, while those of the seal are retained because of the cold environment and the short milk production period.
Because of the very low fat content of neonatal mammals, their high metabolism, and frequently, poor insulation, the chances of survival are only a few hours to days without care from the mother.
After giving birth, the production of milk by the mammalian female bridges the dietary gap between the passive, completely dependent fetus to the weaned and more or less nutritionally independent juvenile. Milk production, or lactation, enables the young mammal to continue its growth in an almost embryonic manner without having to remain
anatomically attached to its mother. The female, in this way, is freed from the locomotory, nutritional, and anatomical constraints of carrying the fetus.
According to Lopez and Robinson in 1994, nutrient requirements for pregnancy are moderate in comparison with the estimated nutrient requirements for maintenance. In the case of captive Atlantic bottlenose dolphin, food consumption in the females showed little increase during gestation, but was 58-97% higher during lactation than during similar periods in non-reproductive years.
For most mammals, milk production closely follows the nutrient requirements of the newborn animals. In the first few days, the requirements of the newborn may be substantially lower than the mother's potential to produce milk. As the needs of the growing animal increase, so does its requirements for milk and milk production.
Lactation itself can put an enormous nutritional burden on the female. In terms of energy expenditure, lactation is two to three times more costly than gestation, and the female's nutrient requirements may increase considerably. The total energy expenditure, including milk produced, of the lactating female is about 215% higher than her non-lactating counterpart.
The production of milk generally rises during early lactation and hits a maximal peak. This is the point where weaning takes place since the neonate has to increase its nutrient intake further by relying on nutritional sources other than milk.
The decline in milk production once the peak has been reached can last for as little as five days in mice to many months in large ungulates. While it seems to make evolutionary sense for larger species to have longer lactation periods, there are many exceptions to the evolutionary constraints. The hooded seal, for instance, has one of the shortest lactation periods despite a maternal weight averaging 395 lb (179 kg). The pup grows from 47.4 to 96.3 lb (21.5 to 43.7 kg), with 70% of the gain being fat, in just four days.
A basic rule of thumb, however, is that poorly nourished females and those nursing larger litters often reduce milk production faster than do well-nourished females.
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