Pregnancy in Animals

the physiological condition of the female from the moment of fertilization until the birth of the fetus. Pregnancy among mammals (except oviparous ones) is substantially different from the similar state among so-called viviparous fish (such as sharks and skates), and reptiles (such as adders and viviparous lizards). Among mammals, the development of the fertilized egg, or zygote, begins in the oviduct, through which it advances to the uterus, where it lies freely for a while, feeding from the small reserves of yolk and secretions from the uterine glands. Implantation (the fastening and embedding of the embryo in the wall of the uterus) then takes place gradually. In the process a link is established between the developing embryo and the body of the mother by means of the placenta, and embryonic membranes form.

Pregnancy is accompanied by considerable changes in the mother’s body. A good appetite is observed at the beginning of the animal’s pregnancy, and by the middle of the pregnancy the animal becomes plump. In the second half of the pregnancy, when the fetus grows more rapidly and demands ever-greater nourishment, the animal begins to grow thin. Externally, pregnancy in animals is manifested by the cessation of sexual desire and heat and the enlargement of the mammary glands. Toward the end of pregnancy, the groins sink, the ligament of the sacrum weakens, and the vagina swells. The uterus of the pregnant female expands in volume to ten to 15 times its normal size. The length of pregnancy among various animals is given in Table 1.

The duration of pregnancy is correlated to the environmental conditions in which the organism develops after birth. Thus, among hoofed animals, which essentially live in open places, the young are born well-developed and can move independently immediately after birth. The pregnancy of such animals lasts much longer than, for example, that of predators, which generally lead secretive lives; among these animals, the young are born blind and toothless, and they need the special protection and care of the mother. Among marsupials, the young are born underdeveloped; they complete their development inside a special organ, the pouch of the mother. (Thus, pregnancy lasts only about 21 days in the opossum and 39 days in the giant kangaroo.) The young are borne in the pouch for several months, and they feed on the mother’s milk. Pregnancy is of shorter duration among rabbits, which live in holes, than among hares, which do not burrow holes; there are other, similar examples of this rule.

The length of pregnancy may vary in the same species depending on the season (it is shorter in the summer than in the winter), the sex of the embryo (female fetuses are carried for a somewhat shorter period than male fetuses), the number of fetuses (single fetuses are carried somewhat longer than twins), and so forth. In martens, sables, and certain other animals, pregnancy is of considerably longer duration because of the latent period, or diapause, a period of delayed development of the embryo at the blastocyte stage. Pregnancy may be single-fetus or multifetus, depending on the result of the maturation and ovulation of several egg cells during a single period of heat. Sometimes twins are monozygotic—that is, they are formed from a single fertilized egg, or zygote. Various mammal groups have different numbers of offspring in their litters: predators have between two and 20, rodents between two and 10 (hamsters have 18 in each of two litters per year), ungulates between one and five, proboscoids (elephants) one, pinnipeds one, and chiroptera one or two. Among marsupials, the giant kangaroo has one and the opossum has between eight and 16.