Parental care means care of the eggs or juveniles till they reach the reproductive age. Parental care evolved to reduce the energy expenditure on reproduction, as in the absence of it animals must produce millions of eggs so that few could survive to replace the parents to ensure existence of the species. Lower animals produce excessively large number of eggs and do not exhibit parental care but higher animals such as vertebrates, show varied degree of parental care in order to reduce the energy expenditure in reproduction. Terrestrial environment being much harsher than the aquatic one, amphibians were the first vertebrates to have evolved different kinds of parental care to protect their young ones as given in the following description.
Caecilians or apoda are long, worm-like legless amphibians having about 165 species in 33 genera. Little is known about these animals, most of which are tropical or subtropical, and occur in Central and South America, Africa, and south and Southeast Asia.
Caecilians exhibit parental care. The female coils around the egg clutch and periodically rotates it, till the eggs hatch. The mother caecilian does not take any food during the parental care period. The Beddome’s Caecilian, Ichthyophisbeddomei, found in Kerala (India) is known to have 25 to 38 eggs in an egg clutch. Egg size ranges from 6 mm at the time of laying to 12 mm at the time of hatching. Eggs generally hatch in 60 to 90 days. A newly hatched larva possesses 3 pairs of external pinnate gills.
Mothers of the Kenyan caecilian, Boulengerulataitanus provide their own cast skin as a food source to their offspring. Boulengerula taitanus is a direct-developing oviparous caecilian, the skin of which is transformed in brooding females to provide a rich supply of nutrients for the developing larvae, which are equipped with a specialized dentition, which they use to peel and eat the outer layer of their mother’s modified skin.
Dermophismexicanus is a Central American salamander, whose embryos are 2 mm in diameter and feed on the egg yolk supply for only about three months of gestation before the yolk supply is exhausted. After that the mother produces a nutritious secretion from the internal oviductal glands. Foetal caecilians move around within the oviduct and have specialized dentition with which they scrape the oviduct skin in order to stimulate and ingest the mother’s nutritive secretion. The dentition is shed at birth and a different adult dentition is rapidly acquired within a few days. Foetuses also have elaborate tri-branchiate gills for respiration (Wake 2003).
Viviparity is reported in Gegeneophisseshachari in which the oviducts are highly vascularized and contain patches of thickened, layered tissues, similar to foetal gut contents. It resembles other viviparous caecilians in having foetuses that ingest thickened oviduct lining using specialized deciduous teeth. Gegeneophis is the only caecilian genus known to include oviparous and viviparous species, and G. seshachari is the smallest known viviparous caecilian. Phylogenetic analysis of mitochondrial DNA sequences supports the assignment of G. seshachari to a monophyletic genus Gegeneophis.
In general, it is hypothesized that maternal care should be found in taxa with internal fertilization, and paternal care should be found in taxa with external fertilization.
Hynobius retardatus is a slender, chocolate brown salamander having length of 18 cm. The distribution is limited to the Northern Japanese island Hokkaido. Each female attaches an egg sac on the robust branches that run horizontally touching the water surface. Sato (1990) reports that in nature the egg sacs are mainly set off about 3 cm under the water surface. Each egg sac contained between 44 & 102 eggs.
The Paghman mountain salamander (Paradactylodon mustersi) is entirely water-dwelling and is restricted to three tributaries of the Paghman stream drainage system in Afghanistan. The stream is fed by glaciers and this species seems to prefer cold, fast-flowing water below 14°C. Fertilisation of eggs in paired egg sacs is external, and these sacs are attached to the underside of rocks and are guarded by the males.
The Gorgan mountain salamander (Paradactylodon gorganensis) is restricted to a single cave and stream in the mountains of northern Iran. It lives almost entirely in water, feeding on invertebrates and breeding in a long, narrow pool within the Shir-Abad cave. During mating, the female produces paired arc-shaped, gelatinous egg sacs, each containing 35-70 eggs. The male grasps these and fertilises them externally.
Cryptobranchids are generally found living in depressions under stones in streams and rivers. Andrias occurs in central China and Japan, whereas Cryptobranchus lives in eastern North America, where mating occurs in late summer or early fall. Males prepare nests below large, submerged stones or logs. Females lay long, paired strings of several hundred eggs which are fertilized externally by the male. Males guard the eggs until they hatch in 2-3 months after egg-laying.
Some frogs carry the eggs and tadpoles on their hind legs or back (e.g. the midwife toads, Alytes spp.). Some frogs even protect their offspring inside their own bodies. The male Australian Pouched Frog (Assa darlingtoni) has pouches along the side of body in which the tadpoles reside until metamorphosis. The female Gastric-brooding Frogs, Rheobatrachus, from Australia, swallow its tadpoles, which then develop in the stomach. To do this, the Gastric-brooding Frog must stop secreting stomach acid and suppress peristalsis. Darwin’s Frog (Rhinoderma darwinii) from Chile puts the tadpoles in its vocal sac for development.
The strawberry poison-dart frog (Oophaga pumilio) is a species of poison dart frog found in Central America. Strawberry Poison Dart Frogs avoid laying eggs in ponds and streams. Instead, the eggs develop on land until the tadpoles are ready to hatch. Then the mother carries the tadpoles on her back to water-filled bromeliads (epiphytic plants) on the trees. The tadpoles complete their development in these tiny, predator-free pools, and the mother feeds them with unfertilized eggs. The males defend and water the nests and the females feed the oophagous tadpoles with their unfertilized eggs. O. pumilio tadpoles are considered obligate egg feeders as they are unable to accept any other form of nutrition. After mating, the female will lay an average of three to five eggs on a leaf or bromeliad axil. The male will then ensure that the eggs are kept hydrated by transporting water in his cloaca. After about ten days, the eggs hatch and the female transports the tadpoles on her back to some water body.
Care about the young reaches the highest degree in the case of two species of Australian toads — Southern gastric-brooding frog (Rheobatrachus silus) and Northern gastric-brooding frog (Rheobatrachus vitellinus). These species are the only ones which carry about 20 young in the stomach, during which they do not feed. The female swallows the eggs after the male fertilizes them. Seven or eight weeks afterward, fully formed froglets come out of the stomach to the mouth of the mother, sit on its tongues, and jump out from it to the water. It was found that tadpoles secrete special chemical substance — prostaglandin E2, which suppresses secretion of the acid by the mother’s stomach.
Jamaican endemic frogs exhibit some amount of parental care. All the Eleutherodactyls have direct development from heavily yolked eggs to small froglets, bypassing the tadpole stage, presumably as an adaptation to scarcity of water. But Eleutherodactyluscundalli, which breeds in the Windsor Great Cave, where the humidity is 100%, guards the egg clutch until the young hatch as tiny froglets, which then climb onto the back of the mother to be carried out of the cave.