Birth (zoology)

Animals may be born via parturition after an internal pregnancy (a gestation period). They may also be born from an egg that hatches externally. This includes eggs that are spawned and then fertilized externally by organisms such as fishes; those fertilized internally and laid in huge numbers to hatch on their own, as in snails and millipedes; or those laid in much smaller numbers and incubated by their parents, as in birds.

In all animals, except those which reproduce via asexual reproduction, male and female reproductive cells (gametes) unite to form a single cell known as a zygote. The zygote then undergoes successive cellular divisions, as well as cellular differentiation, to form a new organism. In most higher animals, individuals of a species are male or female, according to the type of reproductive cells they produce. Male reproductive cells—the sperms—are motile cells with heads containing nuclei and tails that allow them to move. Female reproductive cells—eggs or ova—are round cells many times larger than sperms. They also contain large amounts of cytoplasm located around the nucleus.

Viviparous Birth

In viviparous organisms, a fertilized egg will develop into an incompletely finished miniature or miniatures of an adult of the same species. After fertilization of an egg, the zygote enters the uterus, undergoes both cell division and differentiation and forms an embryo within the mother. In due time, parturition (birth) occurs. Most viviparous organisms are mammals. Early in gestation, the implanted dividing egg and the uterine wall become interconnected by a placenta, composed of both maternal and embryonic tissue. The placenta brings oxygen and nutrients to the embryo and carries away wastes. The transfer of nutrients uses the circulatory systems of both the mother and the embryo.

At the time of birth, hormonal changes cause the mother’s birth canal to enlarge, the muscles of the uterus to rhythmically contract, and the fetus is expelled as a newborn. The overall process can be exemplified by the female gorilla. She menstruates monthly and can mate successfully at any time of year. Her gestation period is 9.5 months, yielding one or two almost fully formed offspring. Gestation is much shorter in smaller primates, and there are variations in the difficulty of parturition related to the head-first entry of young into the world.

Some primates—including humans—must undergo major dilation of the uterine mouth (cervix) before parturition can begin. This allows the large head of the fetus to pass out of the body safely. In species such as monkeys, in which the head of the fetus is close to the size of the cervical opening, far less dilation is needed. In other placental mammals, the position of fetuses in the uterus and the fashion of birth differ.

Oviparous Birth

Many animals, including snails, insects, birds, lizards, and fish, lay eggs either before they are fertilized or before their young are completely developed. These organisms are termed oviparous. In the case of snails, most species are hermaphrodites. This means that each snail has both male and female sex organs. However, each individual snail usually mates with another snail of the same species, passing sperm to its partner and getting sperm from the partner. Fertilized eggs are then spawned into the water or laid on rocks or aquatic plants. The eggs hatch in two weeks to two months. Hatching is considered to be the time of birth of the young snails. In most cases, offspring hatch as miniature replicas of their parents.

In insects, eggs are laid in a wide variety of places. For example, grasshoppers lay eggs in the ground or on plants. When the offspring are born, they hatch as wingless grasshopper larvae called nymphs. Over several months, the nymphs undergo metamorphosis into adult locusts. In contrast, ants, wasps, and termites lay their eggs in special chambers in their nests (or colonies). Worker termites place eggs laid by a colony’s queen into hatching chambers in “nurseries.” Termites are born as wormlike larvae when eggs hatch. The larvae undergo metamorphosis into workers, soldiers, or reproductives (kings or queens) as a result of being fed varied amounts of hormones obtained from queens.

Birds lay eggs in nests located in a wide variety of locales depending on species. Adults then incubate the eggs by sitting on them. Offspring are born when they use a specialized egg tooth to break open their eggshells. In the case of lizards, the eggs are laid after they are fertilized. However, they are not cared for by parents. Large lizards such as alligators, crocodiles, and caimans lay eggs covered with hard, calcium-containing shells like those of bird eggs—reptiles and birds are distant relatives—in holes in the ground, where they hatch into offspring that look like adults. Most fish lay fertilized eggs on plants or on the bottom of the sea, lakes, or rivers and leave their offspring to hatch on their own. These offspring then develop into adults.

Ovoviviparous Birth

Ovoviviparous animals produce eggs in shells like those of the oviparous organisms, but the eggs are hatched within the body of the mother, or by expulsion from her body. There are numerous examples of ovoviviparous organisms among animals. They include some oysters, snails, and other gastropods, as well as numerous species of sharks and live-bearing tropical fish such as the guppy or swordtail. The eggs of live-bearing guppies hatch internally just before leaving the mother’s body, and the young are born alive. These young fish usually leave the body of the mother headfirst.

In all cases, the development of the egg or eggs of ovoviviparous species begins with internal fertilization of the female of the species. Then, the zygotes formed pass through many cycles of internal cell division and differentiation. Ultimately, each egg yields a miniature of the adult organism involved. However, there is no placenta formed and the zygote becomes the complete organism in processes that depend on a yolk sac for food and energy. Often, upon birth, the newborn organism has part of its yolk sac left and can survive for one or several days without eating.

Many animals display unique reproductive, pregnancy, and birth behaviors. For example, five mammals in the order Monotremata reproduce by laying eggs but feed their young milk—duck-billed platypus, eastern long-beaked echidna, western long-beaked echidna, short-beaked echidna, and Sir David's long-beaked echidna. These animals' eggs are fertilized in the mother's body, laid in a nest she builds, and incubated for seven to fourteen days before they hatch. Also unique are marsupials, born in an embryonic state around one month after conception. The tiny, underdeveloped animal must climb to its mother’s pouch and latch onto a teet, where it remains for several weeks as it develops.

Other notable behaviors include the mouthbrooding of the fish in the family Cichlidae. The male fish carries fertilized eggs in its mouth until they begin hatching. Flatworms can reproduce by splitting themselves in half, fertilizing their own eggs with their own sperm, or by having their eggs fertilized by another flatworm. In seahorses, pipefishes, and leafy sea dragons, the males give birth to the young after females leave eggs in their brood pouch to be fertilized.

Principal Terms

Gestation: The term of pregnancy

Hormone: A substance produced by one organ of a multicellular organism and carried to another organ by the blood, which helps the second organ to function

Larva: A newly hatched form of an organism that looks very different from adults of the species and must undergo metamorphosis to the adult form

Metamorphosis: The form changes in a larva that turns it into the adult form

Motile: Able to move about spontaneously

Oviparous: Born from an externally incubated egg

Parthenogenesis: A process whereby a female sex cell develops without fertilization in an organism that reproduces sexually

Uterus: The organ in which fertilized eggs develop during gestation

Viviparous: Born alive after internal gestation

Zygote: A fertilized egg

Bibliography

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