Most amphibians lay their eggs in fresh water or on land, but others are viviparous, meaning the babies grow within the mother’s body and are born live.
Frogs are among the most prolific breeders and can lay anywhere from 1 egg to 25,000, while salamanders lay a few dozen. Some frogs and salamanders defend their eggs and may carry eggs or tadpoles on their backs. For species which practice internal fertilization, females guard the eggs. For eggs that are externally fertilized, males guard and defend the eggs within their territory. In fact, paternal care is most common in fish and amphibians and not mammals!
Main reasons why frogs lay eggs in water
- Moisture: Frog eggs require moisture or they will dry up and die. Frogs lay their eggs coated in a jelly-like glycoprotein. The jelly helps keep the eggs from drying out, but must maintain contact with a moisture source. Frogs can lay their eggs on pond vegetation, floating on the water’s surface, or on the pond bottom. Many frogs lay their eggs in vernal pools, which are large, temporary puddles formed by spring rains.
- Tadpoles: Adult frogs can survive on land and in water, but tadpoles can survive in water only. They have gills for underwater breathing and tails for swimming. If tadpoles hatched on land, they would not be able to breath or move around.
- Protection: Bird eggs have hard shells that protect the embryos from trauma. Because frog eggs are soft and moist, they require cushioning to protect the developing tadpoles. Water provides this cushioning.
- Guarding: A few frog species, such as Darwin’s Frog, watch over their eggs and guard them from predators. Darwin’s Frog is primarily aquatic and would not be able to guard eggs laid on land away from the water.
Case study: Darwin’s Frog
Male Darwin’s frogs engage in a behavior called mouth-brooding, where they take their own fertilized eggs into their mouth just before they hatch. They’re not eating them. Instead they store them in the vocal sacs that they otherwise use to make mating calls to attract females. There the tadpoles hatch and develop in complete safety from fish, dragonfly larvae and all the other aquatic predators that normally threaten them (unless of course daddy gets eaten himself), surviving on the nutrients of the yolk that their mother’s egg provided them.
When the tadpoles fully complete metamorphosis inside their father’s vocal sac, he opens his mouth and “barfs” them out into the world, miniature versions of himself ready to fend for themselves.
Watch this video to see the tadpoles writhing underneath the skin of these daddy frogs just before they reach full development and he barfs them out to begin life on their own. You can’t help but watch this and be fascinated at how odd and wonderful the natural world is!
[youtube=http://www.youtube.com/watch?feature=player_embedded&v=HCxv6AbBwJI]
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Glossary
Viviparous – an animal that gives birth to live young. Contrast with Oviparous and Ovoviviparous
Metamorphosis – a biological process by which an animal physically develops after birth or hatching, involving a conspicuous and relatively abrupt change in the animal’s body structure through cell growth and differentiation
Reference
http://www.sciencedaily.com/releases/2012/09/120910142632.htm
http://www.worldanimalfoundation.net/f/TreeFrog.pdf
http://animals.pawnation.com/main-reason-frogs-lay-eggs-water-4238.html
http://blogs.discovery.com/animal_oddities/2011/08/frog-daddy-barfs-out-babies.html
