User:Samsara/frog life cycle

The life cycle of a frog starts with an egg. Eggs are generally laid in water, and an individual female may lay egg masses containing thousands of eggs. The eggs, however, are highly vulnerable to predation. Frogs have evolved many techniques to ensure the survival of the next generation. Most commonly, this involves synchronous reproduction. Many individuals will breed at the same time, overwhelming predators. The majority of the offspring will still die due to predation, but there is a greater chance some will survive than a laying of smaller numbers. Another way in which some species avoid the predation and pathogens eggs are exposed to in ponds is to lay eggs on leaves above the pond, with a gelatinous coating designed to retain moisture. In these species, the tadpoles drop into the water upon hatching. In some species, eggs laid out of water can detect vibrations of predaceous wasps or snakes approaching the eggs, and will hatch early to avoid being eaten. Some species, such as the Cane Toad (Bufo marinus), also possess poisonous eggs which prevent predation. While the length of the egg stage depends on the species and environmental conditions, aquatic eggs generally hatch within one week.

Egg hatch to continue life as tadpoles (occasionally known as polliwogs). Tadpoles are aquatic, lack front or hind legs, have gills for respiration and tails with fins for swimming. Tadpoles are typically herbivorous, feeding mostly on algae, including diatoms that are filtered from the water through the gills. Some species are carnivorous at the tadpole stage, eating insects, smaller tadpoles or fish. Tadpoles are highly vulnerable to predation by fish, newts, predatory diving beetles, and birds such as kingfishers. Cannibalism has also been observed among tadpoles. Poisonous tadpoles are present in many species, such as Cane Toads. The tadpole stage may be as short as a week, or tadpoles may overwinter and metamorphosize the following year in some species, such as the midwife toad Alytes obstetricans and the Common Spadefoot (Pelobates fuscus).

At the end of the tadpole stage, frogs undergo metamorphosis in which they transition into adult form. Metamorphosis involves a dramatic transformation of morphology and physiology. In metamorphosis, tadpoles develop hind and then front legs, lose thier gills, and develop lungs. Their intestines shorten as they shift from a herbivirous to a carnivorous diet. The final stage of development from froglet to adult frog involves apoptosis (programmed cell death) of the tail.

After metamorphosis, young adults may leave water and disperse into terrestrial habitats, or continue to live in the aquatic habitat as adults. Almost all species of frogs are carnivores as adults, eating invertebrates such as insects, worms and spiders. A few of the larger species may eat prey such as small mammals, fish and smaller frogs. Some frogs use their sticky tongues to catch fast-moving prey, while others capture their prey, and force it into their mouth with their hands. However, there are a very few species of frogs which primarily eat plants. Adult frogs are themselves predated by birds, large fish, snakes, otters, foxes, badgers, coatis, and other animals. Frogs are also eaten by people. Frog legs are a delicacy in China, France, and in many parts of the American South, especially Louisiana. The French custom of eating frog legs is the source of the English use of the derogatory nickname "frogs" for French people.

Once adult frogs have reached maturity, frogs will assemble at a water source such as a pond or stream to breed. The male frogs will then call to attract a mate, collectively becoming a chorus of frogs. The call is unique to the species, and will attract females of that species. Some species have satellite males who do not call but intercept females approaching one of the calling males.

The male and female frog will then undergo amplexus. This involves the male mounting the female and gripping her tightly. Fertilization is external because the egg and sperm meet outside of the body. The female releases her eggs, which the male frog covers with a sperm solution. The eggs will then swell, and form a protective coating. The eggs are typically brown or black, with a clear, gelatin-like, covering.

Most temperate species of frog reproduce in the period between late autumn to early spring. In the UK most common frog populations produce frogspawn in February although there is wide variation in timing. Water temperatures at this time of year are relatively low and typically between four and 10 degrees Celsius. Reproducing in these conditions helps the developing tadpoles because dissolved oxygen concentrations in the water are highest at cold temperatures. More importantly, reproducing early in the season ensures that appropriate food is available to the developing frogs at the right time.

Although care of offspring is poorly understood in frogs, it is estimated that up to 20% of amphibian species may care for their young in one way or another, and there is a great diversity of parental behaviours. Some species of poison dart frogs will lay eggs on the forest floor, and protect them until hatching. This protection involves guarding the eggs from predation, and keeping the eggs moist. The frog will urinate if they become too dry. After hatching, a parent (sex depending upon the species) will move them, on its back, to a water-holding bromeliad. The parent will then feed it through laying unfertilised eggs into the bromeliad, until the young have metamorphosed. Other frogs will carry the eggs and tadpoles on their hind legs or back (e.g. the midwife toads, Alytes spp.). Some frogs even protect their offspring inside their own body. The male Australian pouched frog (Assa darlingtoni) has pouches along its side in which the tadpoles will reside until metamorphosis. The female Gastric-brooding Frogs (genus Rheobatrachus) from Australia swallows its tadpoles which develop in the stomach. To do this, the Gastric-brooding Frog must stop secreting stomach acid and suppress peristalsis (contractions of the stomach). Darwin's Frog (Rhinoderma darwinii) from Chile puts the tadpoles in its vocal sac for development.


 * 1) Warkentin, K.M. 1995. Adaptive plasticity in hatching age: a response to predation risk trade-offs. Proceedings of the National Academy of Sciences 92:3507-3510.
 * 2) Crump, M.L. 1996. Parental care among the Amphibia. Advances in the Study of Behavior 25:109-144.