The term "oviparous" simply means that instead of the young growing inside the mother, the baby is fertilized and forms inside of a shell outside of the body. Oviparous animals range from spiders and scorpions, all the way up to your feathered bird pal or fish in your tank. You might even be surprised to learn that some egg-laying creatures don't have scales, shells or feathers at all -- some actually have fur.
Some birds lay only one or two fertilized eggs, while others lay several -- the number of eggs is the "clutch." Generally birds who lay one or two eggs, like the little tinamou, don't survive for long in the wild. These birds spend a lot of time caring for their young to help the species stay alive. In contrast, birds who lay 10 or more eggs, including the Mandarin duck, have a higher survival rate, lessening their need to stay with their babies as long.
Amphibians and Reptiles
Frogs, newts and salamanders are all amphibians -- they can live in the water and on land, but usually lay eggs in the water because they're shell-less and need moisture. A few species, including the Ranitomeya imitator frog, actually give birth to live babies after the fertilized egg develops in the oviduct. Reptiles, such as lizards, crocodiles, alligators, turtles and snakes, can be either land-dwelling or water-living creatures that lay eggs in soil or water. Due to the high risk of losing eggs to predators, which lowers the species survival rate, amphibians and reptiles tend to lay numerous eggs, sometimes having a clutch size of several hundred.
All fish lay their eggs, or roe, in the water, but they don't have arms, legs or wings to nest and incubate their eggs. Fish either let a stream of eggs out while swimming, lay some on a plant where they stick in place, or release eggs into a small burrowed hole. Ideally, the male fish releases sperm to fertilize these eggs externally. Some fish, like the Bagrid catfish or most cichlids, hold the eggs in their mouth after fertilization, to protect their unborn young from predators. Because of the reduced species survival rate, fish can lay hundreds or thousands of eggs at a time.
Creepy Crawly Critters
Many of those arthropod critters with exoskeletons, or hard outer shells, are oviparous. Spiders, scorpions, cockroaches, centipedes, lobsters and crabs are just some of the millions of varieties of arthropods that lay eggs -- up to several hundred at a time. Some lay eggs that were fertilized during sexual intercourse, while others lay unfertilized eggs that still require sperm. Other critters, such as the blue crab, release as many as 8 million eggs into an external sac that gets fertilized while the mother is carrying them around.
It's very rare for warm-blooded mammals to lay eggs. Only five such animals exist, including the duck-billed platypus and four different types of spiny anteaters in the echidna family. These fur-covered egg-laying creatures, known as "monotremes," are so scarce, they're only found in Australia and parts of Africa. Monotremes are different from other oviparous animals because they still produce milk for their young, just like other mammals and humans. Usually these creatures lay just one or two fertilized eggs.
- Encyclopaedia Britannica: Provisions for the Developing Embryo
- University of Michigan Museum of Zoology: Arthropoda
- University of California Museum of Paleontology: Monotremes
- The Regents of the University of California: Amphibian Facts
- Dictionary.com: Oviparous
- Merck Manuals: Breeding and Reproduction of Fish
- University of California San Diego: Why Do Some Bird Species Lay Only One Egg? UC San Diego Study Offers Some Answers
- Public Broadcasting Service: The Life of Birds
- National Geographic: Platypus Ornithorhynchus anatinus
- University of Wisconsin Madison: Amphibian Reproduction
- Martin Lauricella/iStock/Getty Images