Reproduction primarily involves either laying eggs or giving birth to live young. These two distinct methods represent successful evolutionary pathways, each with unique mechanisms for offspring development and survival. The choice between these reproductive modes shapes an animal’s life cycle, from parental investment to the environment in which the young develop.
Animals That Lay Eggs
The reproductive strategy of laying eggs is known as oviparity, where offspring develop inside an egg laid outside the mother’s body. The embryo within the egg receives nourishment primarily from the yolk, a nutrient-rich substance stored within the egg itself. This feeding is termed lecithotrophy.
Eggs exhibit a wide range of forms, adapted to different environments. Birds, for instance, lay hard-shelled eggs that provide protection and are permeable to gases. Reptiles, such as snakes and turtles, produce eggs with leathery shells. Amphibians like frogs and salamanders lay soft, gelatinous eggs that require a moist environment. Fish, including salmon, release eggs into the water for external fertilization.
Insects also lay eggs. Among mammals, only a small group known as monotremes are oviparous; these include the platypus and four species of echidnas. Their eggs contain a large amount of yolk. An advantage of oviparity is that the mother does not carry the developing offspring internally, which can reduce the metabolic strain on her. This allows for the production of a larger number of offspring in a single reproductive cycle compared to live birth.
Animals That Give Live Birth
Viviparity is a reproductive strategy where offspring develop inside the mother’s body and are born alive. In viviparous animals, the embryo receives direct nourishment from the mother. This nourishment is provided through the placenta, which facilitates the exchange of nutrients, oxygen, and waste between the mother and the developing young. The placenta allows for a protected and stable developmental environment for the offspring compared to external eggs.
Viviparity is diverse, with several variations. Placental mammals, including humans, dogs, whales, and elephants, carry young to a relatively advanced stage of development within the uterus before birth. Marsupials, such as kangaroos and opossums, give birth to very immature young after a short gestation period. These underdeveloped offspring then continue their development within a specialized pouch on the mother’s body, attaching to a teat for nourishment.
Ovoviviparity is another form, where eggs hatch inside the mother’s body, and the young are born live. In these cases, the embryos primarily rely on the yolk from their egg for sustenance, not a direct maternal connection like a placenta. Examples of ovoviviparous animals include some sharks, rays, certain snakes (like rattlesnakes), and a few fish species. This form offers increased protection for the developing young, as they are shielded within the mother’s body until more developed.