PHRAWGS

a book by Karen Ching

FROGS FROGS

ALL ABOUT FROGS THAT YOU NEED TO KNOW for David Mok Ping Wei

EVOLU TiON A fossilized frog from the Czech Republic, possibly Palaeobatrachus gigas

The

origins

and

evolutionary

molecular

phylogenetic

analysis conducted about the same

main groups of amphibians are

time concluded that lissamphibians

hotly

molecular

first appeared about 330 million

on

rDNA

years

from

2005

temnospondyl-origin hypothesis is

suggests that salamanders and

more credible than other theories.

caecilians

The

debated.

phylogeny analysis

A

based dating are

more

closely

ago

and

neobatrachians

that

seemed

the

to

related to each other than they

have originated in Africa/India, the

are to frogs and the divergence

salamanders in East Asia and the

of the three groups took place in

caecilians in tropical Pangaea.

the Paleozoic or early Mesozoic before

the

breakup

supercontinent

of

Pangaea

the and

soon after their divergence from the lobe-finned fishes. This would help account for the relative scarcity of amphibian fossils from the period before the groups split.

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Another

relationships between the three

ORPHOLOGY PHYSiOLOG

Frogs have no tail, except as larvae, and most have long hind legs, elongated ankle bones, webbed toes, no claws, large eyes, and a smooth or warty skin. They have short vertebral columns, with no more than 10 free vertebrae and fused tailbones (urostyle or coccyx).

Frogs range in size from Paedophryne amauensis of Papua New Guinea that is 7.7 mm (0.30 in) in snout–to–vent length to the up to 32 cm (13 in) and 3.25 kg (7.2 lb) goliath frog (Conraua goliath) of central Africa.

There are prehistoric, extinct species that reached even larger sizes

A bullfrog skeleton, showing elongated limb bones and extra joints. Red marks indicate bones which have been substantially elongated in frogs and joints which have become mobile. Blue indicates joints and bones which have not been modified or only somewhat elongated.

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Different species of frog use a number of methods of moving around including jumping, running, walking, swimming, burrowing, climbing and gliding.

JUMPING

BURROWING

Frogs are generally recognized as

Some frogs haVve become adapted

exceptional jumpers and, relative to

for burrowing and a life underground.

their size, the best jumpers of all vertebrates.

WALKING AND RUNNING Frogs

in

the

SWIMMING

families

Rhinophrynidae,

and

Bufonidae,

Microhylidae

adaptations

that

improve

their

have short back legs and tend to walk

swimming abilities. The hind limbs are

rather than jump.

heavily muscled and strong.

CLIMBING

GLIDING

Tree frogs live high in the canopy,

During the evolutionary history of

where they scramble around on the

frogs, several different groups have

branches,

independently taken to the air.

twigs,

and

leaves,

sometimes never coming down to earth.

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Frogs that live in or visit water have

LOCOMOTiON

04 02

L L L

2

Main types of reproduction occur in

frogs, prolonged breeding and explosive breeding. In the former, adopted by the majority of species, adult frogs at certain times of year assemble at a pond, lake or stream to breed.

Many frogs return to the bodies of water in which they developed as larvae. This often results

in

annual

thousands

of

migrations

individuals.

In

involving explosive

breeders, mature adult frogs arrive at breeding sites in response to certain trigger factors such as rainfall occurring in an arid area.

In

these

frogs,

mating

and

spawning take place promptly and the speed of larval growth

is

rapid

in

order to make use of the ephemeral pools before they dry up.

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Life cycle of the green frog (Rana clamitans).

REPRODUCTiON

LiFE CYCLE LiFE CYCLE LiFE CYCLE Frogspawn

Eggs / frogspawn

Frogs may lay their in eggs as clumps, surface films, strings, or individually. Around half of

species deposit eggs in water, others lay eggs

in vegetation, on the ground or in excavations.

The tiny yellow-striped pygmy eleuth

(Eleutherodactylus limbatus) lays eggs singly, burying them in moist soil. The smoky jungle frog (Leptodactylus pentadactylus) makes a

nest of foam in a hollow. The eggs hatch when the nest is flooded, or the tadpoles may complete their development in the foam if flooding does not occur. The red-eyed treefrog (Agalychnis callidryas) deposits its eggs on a leaf above a pool and when they hatch, the Frogspawn development

larvae fall into the water below.

METAMORPHOSIS

Larva of the common frog Rana temporaria a day

Metamorphosis stage with deforming jaws, large eyes, and remains of gill pouch

Young frog with a stumpy tail, metamorphosis nearly complete

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At first sight, frogs seem rather defenceless because

of

their

small

size,

slow

movement, thin skin, and lack of defensive structures, such as spines, claws or teeth.

Many use camouflage to avoid detection, the skin often being spotted or streaked in neutral colours that allow a stationary frog to merge into its surroundings. Some can make prodigious leaps, often into water, that help them to evade potential attackers,

while

many

have

other

defensive adaptations and strategies.

Strawberry poison-dart frog contains numerous alkaloids which deter

The skin of many frogs contains mild toxic substances called bufotoxins to make them unpalatable to potential predators. Most toads and some frogs have large poison glands, the parotoid glands, located on the sides of their heads behind the eyes and other glands elsewhere on their bodies. These glands secrete mucus and a range of toxins that make frogs slippery to hold and distasteful or poisonous. If the noxious effect is immediate, the predator may cease its action and the frog may escape. If the effect develops more slowly, the predator may learn The mildly toxic Ranitomeya imitator.

to avoid that species in future.

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Frogs live on all the continents except

areas than in temperate regions, such as Europe.

Antarctica, but they are not

Some frogs inhabit arid areas, such as deserts, and

present on certain islands,

rely on specific adaptations to survive. Members of

especially those far away from continental land masses. Many species are isolated in restricted ranges by changes of climate or

the Australian genus Cyclorana bury themselves underground where they create a water-impervious cocoon in which to aestivate during dry periods.

Once it rains, they emerge, find a temporary pool, and breed. Egg and tadpole development is very

inhospitable territory, such

fast in comparison to those of most other frogs, so

as stretches of sea,

breeding can be completed before the pond dries

mountain ridges, deserts, forest clearance, road construction, or other man-made barriers.

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Usually, a greater diversity of frogs occurs in tropical

up.

DiSTRi

depend on water during some lifecycle stage, 1,356 (33.6%) were considered to be threatened. This is likely to be an underestimate because it excludes 1,427 species for which evidence was insufficient to assess their status.

Frog populations have declined dramatically since the 1950s. More than one-third of frog species are considered to be threatened with extinction, and more than 120 species are believed to have become extinct since the 1980s.

Among these species are the gastric-brooding frogs of Australia and the golden toad of Costa Rica. The latter is of particular concern to scientists because it inhabited the pristine Monteverde Cloud Forest Reserve and its population crashed in 1987, along with about 20 other frog species in the area. This could not be linked directly to human activities, such as deforestation, and was outside the range of normal fluctuations in population size.

Golden toad (Bufo periglenes) – last seen in 1989.

CONSERVATiON CONSERVATiON

i

In 2006, of 4,035 species of amphibians that

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French cuisses de grenouille.

Frog legs are eaten by humans in many parts of the world. Indonesia is the world's largest exporter of frog meat, exporting more than 5,000 tonnes of frog meat each year, mostly to France, Belgium and Luxembourg. Originally, they were supplied from local wild populations, but overexploitation led to a diminution in the supply. This resulted in the development of frog farming and a global trade in frogs. The main importing countries are France, Belgium, Luxembourg, and the United States, while the chief exporting nations are Indonesia and China. The annual global trade in the American bullfrog (Rana catesbeiana), mostly farmed in China, varies between 1200 and 2400 tonnes.

The mountain chicken frog, so-called as it tastes of chicken, is now endangered, in part due to human consumption, and was a major food choice of the Dominicans. Coon, possum, partridges, prairie hen, and frogs were among the fare Mark Twain recorded as part of American cuisine.

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Because frog toxins are extraordinarily diverse, they have raised the interest of biochemists as a "natural pharmacy". The alkaloid epibatidine, a painkiller 200 times more potent than morphine, is made by some species of poison dart frogs. Other chemicals isolated from the skins of frogs may offer resistance to HIV infection. Dart poisons are under active investigation for their potential as therapeutic drugs.

CULTURAL SiGNiFiCANCE

Frogs have been featured in mythology, fairy tales and popular culture. In traditional Chinese myths, the world rests on a giant

frog, who would try to shallow the moon, causing the lunar eclipse. Frogs have been featured in religion, folklore, and popular culture. The ancient Egyptians depicted the god Heqet, protector of newborns, with the head of a frog. For the Mayans, frogs represented water, crops, fertility and birth and were associated with the god Chaac. In the Bible, Moses unleashes a plague of frogs on the Egyptians. Medieval Europeans associated frogs and toads with evil and witchcraft.

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