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Frog Reproduction

Sexual reproduction and Life Cycle of Frogs 

Photo: Courtesy of Damon Ramsey

Sexual reproduction in frogs is remarkably diverse. It usually involves the male grabbing the female in a sexual position called amplexus. Many male frogs may develop a hard patch of skin on the back of their thumb; this is presumed to aid in grip on the female during this position. The female frog is generally larger than the male, sometimes substantially so. This is likely to be an adaptation that enables her to support him during mating, especially if she is swimming or climbing. As she releases the eggs, he fertilizes them externally. Eggs of frogs are usually laid in a moist or watery environment. These soft, jelly like eggs then hatch into mainly aquatic larvae. This stage will then usually, but not always, undergo a complete transformation into a more terrestrially adapted amphibian. 

Variations on sexual reproductive cycle

There are variations on the basic frog reproductive cycle in species found in the Australian tropical rainforest. The Stony Creek Frog Litoria leseuri actually builds a nest separate but close to the creek to mate and lay her eggs. Some frogs have developed reproductive cycles that rely less directly on water. The Barred Frogs Mixophys spp. actually lay their eggs out of the water. They are, however, near the edge, so the tadpoles are eventually washed in (Barker et al 1995). Some frogs have developed reproductive cycles that can occur even further from the water. The Microhylid family is often referred to as ‘nursery frogs’. This is because they have the unusual reproductive strategy of laying eggs that hatch directly into little frogs; they skip the usual larval stage of the tadpole, having no need for a complete aquatic environment in their moist rainforest habitat. Some species, such as Spheophryne fryi, have been observed guarding their nests of eggs (Alford 2002).

Frog calls (role in reproduction)

Photo: Courtesy of Damon Ramsey

In most frogs, a signal produced by the male has evolved that serves to attract females. This is the vocal call, and it allows for species-specific identification and location, both for the female frog and the herpetologist. These calls can be recorded and displayed on as an oscillogram, which displays signal amplitude in volts/millivolts, or a sonogram, which display frequency (pitch) (Barker et al 1995). The female frog often has hearing which is tuned into the frequencies of which the male of its own species tends to call; this eliminates confusion with other frog calls (Alford 2002). Many frogs have evolved to develop large vocal sacs. These are blown up to extraordinary proportions to make calls (pictured). Many frogs lack this feature, but may still call. Others may take advantage of the acoustics within piles of rocks or pipes to amplify their call. Many will call from the same area, their combined calling perhaps attracting females from some distance. While rain is often associated with these calls, the exact cues of many species to call and then mate are quite difficult to determine, and may be a combination of rainfall, temperature, humidity and social cues.
Script: Courtesy of Damon Ramsey BSc.(Zool) Biologist Guide

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Lake Eacham, Atherton Tablelands
Tropical North Queensland, Australia.
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