Frogs and toads belong to the order Anura. Amphibians that typify this order lack tails and have long hind legs suited for jumping. The two lower bones of their legs are fused together which allows for greater propulsion. They also have fused vertebrae, called a urostyle, that give additional aid when jumping.

They lay their eggs in aquatic locations. The eggs hatch and the larvae develop in water. These larvae, called tadpoles, remain in this stage for about a year. They metamorphose into adult form by first losing their tail and gills, and then developing front and hind legs. This developmental stage is a crucial determinant in the future fitness of the adult frog. Damage during this stage can lead to defects later in life, decreased fitness, and a lower chance of survival.

• Leiopelmatidae
  • Eastern United States and the Australian islands

  • Lack tympana and have tail wagging muscles

• Ascaphidae
  • Australian islands

  • Lack vocal sacks and have direct development

• Discoglossidae
  • Europe and Africa

  • External fertilization and some parental care

• Bombinatoridae
  • Europe and Asia

  • Aquatic toads

  • Unken reflex used as a defense mechanism

• Rhinophyrynidae
  • Texas

  • Fossorial

• Pipidae
  • Africa and South America

  • Aquatic

  • Adults have lateral line system

  • Lack tongue

• Pelobatidae
  • Europe

  • Terrestrial but have aquatic larvae

• Megophryidae
  • Asia

  • Camouflage themselves in leaves on forest floors

• Heleophrynidae
  • South Africa

  • Adult toes have expanded tips

• Sooglossidae
  • Seychelles Islands

  • Tiny frogs that are fully terrestrial

• Nasikabatrachus sahyadrensis
  • India

  • One species in family

  • Purple Burrowing Frog

  • Narrow ossified head

• Myobratrachidae
  • Endemic to Australia, Tasmania and New Guinea

  • Vertical pupils

  • Gastric brooding- keep young in stomach

• Leptodactylidae
  • North, Central and South America

  • Tap fingers to lure in prey

• Dendrobatidae
  • Central and South America

  • Poison dart frogs

• Hylidae
  • Global

  • Claw shaped terminal phalanges for grasping things (trees)

  • Commonly have bright coloration on thighs seen when jumping

• Pseudidae
  • South America

  • Paradox Frogs

• Centrolenidae
  • Central and South America

  • Glass Frogs

  • Lay eggs on leaves above pools

• Rhinodermatidae
  • Tip of South America

  • Darwin's Frogs

  • Males carry tadpoles in vocal sac until they are released as froglets

• Bufonidae
  • Global with the exception of Australia

  • True Toads (terrestrial)

  • Lack teeth

  • Thick skins with warts and tubercles

• Allophrynidae
  • South America

  • One species

  • Allophryne ruthveni

  • T-shaped terminal phalanges

• Brachycephalidae
  • South America

  • Gold Frogs

  • 2 toes on front feet and 3 on hind feet

  • Lack ribs

• Microhylidae
  • North, Central and South Americas along with Africa and Asia

  • Narrow-Mouthed Frogs

  • Usually tropical and fossorial

• Hemisotidae
  • Africa

  • Shovel-Nosed Frogs

  • Lack sternum

  • Stout forelimbs for burrowing

• Anthroleptidae
  • Africa below the Sahara

  • Some larvae are carnivorous

• Ranidae
  • Global

  • Most cold tolerant species

  • Most have dorso-lateral ridges

  • Contains the largest living frogs

• Hyperoliidae
  • Africa below the Sahara

  • Claw-shaped terminal phalanges

  • Lack of fusion in the second distal carpal and tarsal bones (wrists and ankles)

• Rhacophoridae
  • Africa and Asia

  • Usually tree-dwelling

  • Flattened and ossified skulls

  • Old-World Tree Frogs

There are many other trends in tadpoles not involving a population decline. Atrazine has been shown to cause male tadpoles of African clawed frogs to become hermaphroditic with development of both male and female organs. Such feminization has been reported in many parts of the world. In a study conducted in a laboratory at Uppsala University in Sweden, more than 50% of frogs exposed to levels of estrogen-like pollutants existing in natural bodies of water in Europe and the United States became females. Tadpoles exposed even to the weakest concentration of estrogen were twice as likely to become females while almost the entire control group given the heaviest dose became female [3]. In 1992, researchers Miquel Tejedo and Ricardo Reques examined whether variation in egg size was correlated with developmental rates and metamorphic traits of natterjack toads (Bufo calamita) grown at different experimental densities. They found that at high densities tadpoles had slower growth rates, longer larval periods, and lower survival, and were smaller at metamorphosis. Their results also revealed that the length of larval period was inversely related to early tadpole growth rate for all densities [4].

A recent study by Johnson et al. showed that high levels of nutrients used in farming and ranching increase parasite infections that have caused frog deformities in habitats across North America. The study showed increased levels of nitrogen and phosphorus cause increase the number of trematodes, and that the parasites form cysts in the developing limbs of tadpoles causing missing limbs, extra limbs and other severe mutations including five or six extra or even no limbs [3].

Although tadpoles typically are in aquatic habitats for longer periods than their adults, they sometimes are more difficult to find and nearly always more difficult to identify. In spite of these difficulties, some patterns in tadpole appearance are apparent. Most tadpoles are drab in coloration and pattern and even distantly-related frog species have very similar appearing tadpoles. Even though the tadpoles of many North American frogs appear very similar, they have certain morphological traits that can be used to distinguish among species, including varying mouthparts, noses, eyes, vents, and tooth row structures [2]. Hopefully, with the aid of this website, one can train their eye to notice these minute differences and become an expert in classifying the tadpoles from their area.

1. Alford, R. A. and S. J. Richards. 1999. "Global Amphibian Declines: A Problem in Applied Ecology". Annual Review of Ecology and Systematics. 30: 133-165.

2. Altig, R., R. W. McDiarmid, K. A. Nichols, and P. C. Ustach. Tadpoles of the United States and Canada: A Tutorial and Key. URL: http://www.pwrc.usgs.gov/tadpole/

3. "Decline in Amphibian Populations". Wikipedia: The Free Encyclopedia. 2009. URL: http://en.wikipedia.org/wiki/Decline_in_amphibian_populations

4. Tejedo, M. and R. Reques. 1992. "Effects of Egg Size and Density on Metamorphic Traits in Tadpoles of the Natterjack Toad". Journal of Herpetology. 26: 2: 146- 152.

There are many factors contributing to the decline in frog populations, but the most important factor is habitat loss. The expanding human population and urbanization has resulted in destruction of the wetlands where many frog species live and reproduce, making it difficult to sustain natural populations. The fragmentation of frog habitats also increases their susceptibility to diseases.

The disease chytridiomycosis, caused by the pathogenic chytrid fungus known as Batrachochytrium dendrobatidis, is responsible for recent mass die-offs in frog populations across the globe, but especially in the Americas, Spain and Australia. Not much is known about the full extent of the disease or its total impact on frog populations, however, scientists do know that the fungal infection attacks and destroys the keratin in the skin of frogs and tadpoles, and also affects the nervous system resulting in unusual behaviors. The disease is probably transferred by direct contact between frogs and tadpoles or by exposure to infected water. Certain species are more susceptible than others to die as a result of the infection.

While it is unclear from where the chytrid fungus originated, some frog species show a resistance or can tolerate low, non-lethal levels of infection. This leads to speculation that the disease may have been localized to an initial area where the frogs had developed a natural immunity, but international sale and transportation of frogs spread the disease out of its natural boundaries to populations around the world. Chytridiomycosis is not the only disease that threatens frog populations, however it is the most destructive.

Pollution is being linked to malformations and reduced reproductive success in many frog populations that reside near developed areas. Many studies have linked frequent malformations in frog populations to proximity to agricultural lands where pesticides are used. A recent study from the University of Georgia found that higher mercury content in the diet of the southern leopard frog is directly linked to mortality, malformations, growth and development problems and premature metamorphosis.

Trematode infestation has also been shown to cause limb malformations, while excess nutrients in water systems from fertilizer runoff has been linked to increased parasite populations. This demonstrates that pollutants as well as pathogens could be responsible for the birth defects. While currently little is understood about the cause or consequences of frog malformations, it raises legitimate concerns over the environmental health impact of human activities.

Other factors such as global climate change, over-exploitation, exposure to increasing UV-B radiation and competition with introduced species have also been identified as causes of frog population decline around the globe. Currently, little is understood about how these factors interact to affect frog populations since many of these threats have only recently emerged and been studied.

However, in all aspects of the environmental threat to frogs, human activity is directly or indirectly the cause of the problem, raising serious questions about future environmental sustainability. Since frogs are among the most sensitive species to environmental changes in an ecosystem, it is extremely important for the scientific community to continue research into the cause of the population decline and how we can prevent an imminent ecological disaster.

http://amphibiaweb.org/declines/diseases.html

http://www.uga.edu/aboutUGA/research-frogs.html

http://www.environment.nsw.gov.au/animals/FrogChytridFungus.htm

http://amphibiaweb.org//declines/index.html Video on frog population decline

http://info.hartwick.edu/biology/def_frogs/Introduction/Introduction.html

http://lter.limnology.wisc.edu/K_12_pub_frogs.html