bbc.co.uk navigation

Red-eared turtle hatching out of its egg

Egg layer

Oviparous animals lay eggs, inside which the young then develop before hatching occurs. Birds, reptiles, amphibians, fish, invertebrates and even some mammals (the monotremes) all lay eggs but they're of very different size and construction. The patterns on bird eggs are produced by glands right at the last moment as the egg is laid. For terrestrial species that lay their eggs on land, a waterproof shell prevents the contents from drying out.

Watch video clips from past programmes (30 clips)

In order to see this content you need to have an up-to-date version of Flash installed and Javascript turned on.

View all 30 video clips

Egg size range

An illustration showing the range of egg sizes

A comparison of egg size

Animals with this behaviour

Mammals

Birds

Reptiles

Amphibians

Cartilaginous fish

Ray-finned fishes

Lampreys

Cephalopods

Insects

Arachnids

Crabs, shrimp and krill

Merostomata

Snails and slugs

Starfish

Polychaete worms

Sea urchins

Trilobites

About

Oviparous animals are animals that lay eggs, with little or no other embryonic development within the mother. This is the reproductive method of most fish, amphibians, reptiles, all birds, the monotremes, and most insects, some molluscs and arachnids.

With more scientific rigor, five modes of reproduction can be differentiated based on relations between zygote and parents:

  • Ovuliparity : fecundation is external (in arthropods and fishes, most of frogs)
  • Oviparity : fecundation is internal, the female lays zygotes as eggs with important vitellus (typically birds)
  • Ovo-viviparity : or oviparity with retention of zygotes in the female’s body or in the male’s body, but there are no trophic interactions between zygote and parents. (Anguis fragilis is an example of ovo-viviparity.) In the sea horse, zygotes are retained in the male’s ventral "marsupium". In the frog Rhinoderma darwinii, the zygotes developed in the vocal sac. In the frog Rheobatrachus, zygotes developed in the stomach.
  • Histotrophic viviparity : the zygotes developed in the female’s oviducts, but find their nutriments by oophagy or adelphophagy (intra-uterine cannibalism in some sharks or in the black salamander Salamandra atra).
  • Hemotrophic viviparity : nutriments are provided by the female, often through placenta. In the frog Gastrotheca ovifera, embryos are fed by the mother through specialized gills. The lizard Pseudomoia pagenstecheri and most mammals exhibit a hemotrophic viviparity.

Land-dwelling animals that lay eggs, often protected by a shell, such as reptiles and insects, do so after having completed the process of internal fertilization. Water-dwelling animals, such as fish and amphibians, lay their eggs before fertilization, and the male lays its sperm on top of the newly laid eggs in a process called external fertilization.

Almost all non-oviparous fish, amphibians and reptiles are ovoviviparous, i.e. the eggs are hatched inside the mother's body (or, in case of the sea horse inside the father's). The true opposite of oviparity is placental viviparity, employed by almost all mammals (the exceptions being marsupials and monotremes).

There are only five known species of oviparous mammals (monotremes): four species of Echidna and the Platypus.

Read more at Wikipedia

This entry is from Wikipedia, the user-contributed encyclopedia. If you find the content in the 'About' section factually incorrect, defamatory or highly offensive you can edit this article at Wikipedia. For more information on our use of Wikipedia please read our FAQ.

Behaviours

Other Reproductive strategy behaviours

BBC News about Egg layer

Video collections

Take a trip through the natural world with our themed collections of video clips from the natural history archive.

  • Timelapse photography: speeding up life Timelapse photography: speeding up life

    Some of the most memorable sequences in natural history result from timelapse photography, an astonishing filming technique that opens our eyes to a whole new world.

  • Attenborough's frozen planet Attenborough's frozen planet

    Frozen Planet is Sir David Attenborough's latest exploration into the remote and isolated polar environments.

  • Baby Animals Baby Animals

    With Ooh's and Ahh's galore this video clip collection celebrates a world of adorable animal babies.

  • Seaside spectacular Seaside spectacular

    When it comes to summer holidays, there's no better place than the seaside and if you know where to look you'll be surprised at the wildlife you can find.

  • Garden wildlife Garden wildlife

    From badgers to butterflies and frogs to foxes, garden wildlife is both varied and surprising.

  • The wildlife of Life The wildlife of Life

    In autumn 2009, a major new series brought us life as we've never seen it before.

  • David Attenborough's Madagascar David Attenborough's Madagascar

    Like nowhere else on Earth, the mystery and magic of Madagascar leaves a vivid impression on all those who visit, and none more so than David Attenborough.

  • George's marvellous minibeasts George's marvellous minibeasts

    A video collection featuring bugs and insects in amazing close up selected by insect expert and TV presenter George McGavin, with Goliath spiders, killer centipedes, ants and moths.

  • David Attenborough's favourite moments David Attenborough's favourite moments

    Watch the most memorable moments from an incredible career watching wildlife, chosen by Sir David from the BBC archive.

More …

BBC © 2012 The BBC is not responsible for the content of external sites. Read more.

This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.