Reproduction and life history
of Amphibians

Selection of the breeding ground, appropriate climatic condition, micro-habitat characters (Physico-chemical and biological characteristics viz., relative humidity, water temperature, water pH, moisture of soil, available prey, etc of immediate surrounding of a species forms its micro-habitat), courtship, type of fertilization, mode of development and type of metamorphosis makes the amphibian life history more complex compared to other vertebrates. The fertilized eggs grow and hatch as tadpoles or larvae, which are purely aquatic.

The tadpoles grow and develop and finally metamorphose into terrestrial juvenile, leaving behind aquatic characters. This has led to important ecological and evolutionary implications on the amphibians. The individuals are exposed to two entirely different ecosystems. In their early part of life cycle, the eggs, which are anamniotic (no complex membranes around the embryo) and devoid of protective shell layer, are susceptible to desiccation, pollution and predation. Sites at which these eggs were laid, will also determine the further development. The tadpoles must feed, breath and stay in water that requires specific physical and physiological characters, defence mechanism, habitat and food spectrum, where as the adults have to adapt for terrestrial environment, leaving behind the aquatic characters and they might have dispersal difficulties due to biological restraints.

In Caecilians, fertilization is internal. They may be oviparous (egg laying) or viviparous (hatching young ones). Parental care is observed in oviparous parents.

In Salamanders, the life cycle is more complex. The fertilization may be internal or external. They have elaborate courtship display, includes, chemical, visual and tactile cues. Paedomorphosis is common among Salamanders. In this process, life history begins with aquatic eggs and they hatch release the aquatic larvae. Under favourable conditions, these larvae retaining some morphological characteristics become permanently aquatic reproducible paedomorphic adults. Some times, the larvae undergo typical development within one summer and metamorphose into a morphologically distinct but non-reproducible terrestrial juvenile called eft. Depending on the temperature and availability of food, eft stage lasts from 1-8 years. Efts for the second time undergoes transformations to grow as reproductive adults.

In anurans, the fertilization is external. Usually male frogs grasp the female frogs, with nuptial pads. This posture of male clasping the female during the fertilization of eggs is called Amplexus. Amplexus may be inguinal (male grasping the female in front of the her back legs) or axillary (male grasping the female in front of the her hands). And the life history is very typical, with aquatic tadpole stage metamorphosing into terrestrial adult stage. In somes anurans, the development is direct (where eggs hatch into juvenile adults rather undergoing tadpole stage).

Axillary amplexus in Rana curtipes