PARENTAL CARE IN AMPHIBIA
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, Ichthyophis beddomei, 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, Boulengerula taitanus 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.
Dermophis mexicanus 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 Gegeneophis seshachari 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 Eleutherodactylus cundalli, 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.
Jamaica’s hylid frogs breed in the water-filled leaf-axils and have adapted to the harsh environments of bromeliads (i.e., low oxygen levels and limited food reserves), by producing rapidly developing eggs and by laying further eggs which are eaten by the first-born larvae. Remarkably, the eggs laid during the first few days are fertilised and later the unfertilized eggs are laid, which the larvae consume rapidly.
Mountain Chicken Frog (Leptodactylus fallax) is highly endangered and limited to Dominica, St. Kits, Martinique and a few neighbouring islands, lays eggs in foam nests underground, and the tadpoles develop without ever seeing water. The startling footage taken by some researchers shows tadpoles feeding on unfertilized eggs produced by their mother. Subsequent research has revealed that the mother uses her rear legs to re-distribute the unusual food, and perhaps to give all of her progeny a chance to feed and survive. The 25-50 tadpoles that she rears require 10,000 to 25,000 unfertilized eggs to see them through metamorphosis to adult stage.
There are some species of poisonous frogs in South America where the males transport tadpoles. The males crouch down in the leaf litter next to the hatching eggs and the tadpoles wriggle up onto the father’s back and he transports them to water. Mothers of the Jamaican cave frog species—Eleutherodactylus cundalli—carry their froglets from the cave into the rain forest. It is the only known example of females transporting froglets.
Dendrobates auratus female lays up to six eggs in a small pool of water. The eggs are encased in a gelatinous substance for protection. The mating season occurs throughout the rainy season, from mid-July to mid-September. During the two week development period, the male returns to the eggs periodically to check on them. Once the tadpoles hatch, they climb onto the males back and he carries them to a place suitable for further development, such as a lake or a stream. For the duration of this trip, the tadpoles are attached to the males back by a mucus secretion, which is soluble only in water so that there is no chance of them accidentally falling off. Once they are at their final destination, the tadpoles are on their own. They take an additional six weeks to develop into adult frogs.
Males of Rhinoderma darwinii, Darwin’s Frog, brood their developing young in their vocal sacs until they metamorphose. The Darwin’s frog males have a very unusual behaviour in the amphibian community. After the male and female mate, the female lays her eggs in moist leaf litter on the forest floor. She then hops off, leaving the male to attend to them. Dutifully he guards the eggs until a few days later when they begin to transform into tiny tadpoles, but still encased in the egg sac. The movement inside the eggs stimulates the father frog to swallow them into the modified vocal sac called gular pouch. There they remain until transformed into miniature adults, upon which he opens his throat and allows them to leave and live on their own.
The commonly named Moustache frog, Vibrissaphora ailaonica undergoes quite a transformation just before the breeding season. The males begin growing long, hair-like skin extensions from one end of their mouth to the other. They also choose a large boulder near a stream and begin building a nest underneath it. From here they will call incessantly for females, and then guard the eggs of any and all females who come and mate with them. The eggs typically take a month to hatch and tadpoles slip into the waters below. Metamorphosis will not fully occur until two years later. The microhylids, Anodenthyla, Platypelis and Plethodontohyla, which are all natives of Madagascar, deposit their eggs in the rain filled axils of plants. Male frogs then stay nearby the eggs for anywhere from 26 to 35 days; the time it takes for the different species to hatch.
The genus Alytes contains the midwife toads, such as A. cisternasii and A. obstetricans . Both species attach the fertilized eggs to their hind legs. This starts by the male wrapping them first around his ankles. Sometimes they mate up to four times, carrying up to 200 eggs upon their bodies. The father frogs then keep the eggs moist by settling into shallow puddles and pools, allowing the eggs to double in size. Midwife toads sense when their eggs are ready to hatch, and will then wade into shallow waters to allow the young tadpoles to escape into water.
Male Leptodactylid, Thoropa petropolitana also cares for the eggs in a similar way. The microhylid males Breviceps adspersus and Synapturanus salseri also do the same.
The frog genus Pristimantis lays eggs on land, which develop directly into miniature adults with no tadpole stage. These are the most widespread and commonly occurring frogs in the New World tropics. In Africa the genus Arthroleptis, known as “squeakers”, are all direct developers. There are also many other direct developing frogs on Madagascar and in Southeast Asia. Among salamanders most species of the largest family, the Plethodontidae, are direct developers.
A few species of frogs give birth to living young. Members of the African genus Nectophrynoides retain eggs in the oviduct and some nourish the young as they grow. These are born as miniature adult. One Puerto Rican species of the genus Eleutherodactylus, now thought to be extinct (E. jasperi), also retained eggs in the oviduct to give live births. Salamandra salamandra, S. atra and some related species either give birth to larvae or to completely metamorphosed juveniles.