Mating takes place and eggs are laid in a foam nest inside the burrow

Frogs that live in or visit water have adaptations that improve their swimming abilities. The hind limbs are heavily muscled and strong. The webbing between the toes of the hind feet increases the area of the foot and helps propel the frog powerfully through the water. Members of the family Pipidae are wholly aquatic and show the most marked specialization. They have inflexible vertebral columns, flattened, streamlined bodies, lateral line systems, and powerful hind limbs with large webbed feet.[89] Tadpoles mostly have large tailfins which provide thrust when the tail is moved from side to side.[90]

Burrowing

Some frogs have become adapted for burrowing and a life underground. They tend to have rounded bodies, short limbs, small heads with bulging eyes and hind feet adapted for excavation. An extreme example of this is the Purple frog (Nasikabatrachus sahyadrensis) from southern India which feeds on termites and spends almost its whole life underground. It emerges briefly during the monsoon to mate and breed in temporary pools. It has a tiny head with a pointed snout and a plump rounded body. Because of this fossorial existence it was first described in 2003, being new to the scientific community at that time, although previously known to local people
The spadefoot toads of North America are also adapted to underground life. The plains spadefoot toad (Spea bombifrons) is typical and has a flap of keratinised bone attached to one of the metatarsals of the hind foot which it uses to dig itself backwards into the ground. As it digs, the toad wriggles its hips from side to side in order to sink into the loose soil. It has a shallow burrow in the summer from which it emerges at night to forage. In winter it digs much deeper and has been recorded at a depth of 4.5 metres (15 ft).[92] The tunnel is filled with soil and the toad hibernates in a small chamber at the end. During this time, urea accumulates in its tissues and water is drawn in from the surrounding damp soil by osmosis to supply the toad's needs.[92] Spadefoot toads are "explosive breeders", all emerging from their burrows at the same time and converging on temporary pools, attracted to one of these by the calling of the first male to find a suitable breeding location.[93]

The burrowing frogs of Australia have a rather different lifestyle. The western spotted frog (Heleioporus albopunctatus) digs a burrow beside a river or in the bed of an ephemeral stream and regularly emerges to forage. Mating takes place and eggs are laid in a foam nest inside the burrow. The eggs partially develop there but do not hatch out until they are submerged following heavy rainfall. The tadpoles then swim out into the open water and rapidly complete their development.[94] Madagascan burrowing frogs are less fossorial and mostly bury themselves in leaf litter. One of these is the green burrowing frog (Scaphiophryne marmorata) which has a flattened head with a short snout and well developed metatarsal tubercles on its hind feet to help with excavation. It also has greatly enlarged terminal discs on its fore feet that help it to clamber around in bushes.[95] It breeds in temporary pools that form after rains