| PHYLUM CHORDATA |
Introduction to Parental Care:
The amphibians were the pioneers amongst the vertebrates to invade land. On coming to this new environment they had to face many hostile forces and they had to develop various ways and means to overcome the obstacles.
Naturally greatest importance was given to the perpetuation of race, so that they can ultimately win the struggle.
As a result, the reproductive, mechanism had undergone extensive modifications. Perpetuation of race can be maintained either by the process of overproduction of eggs or by caring of a small number of eggs.
Rearing or caring of the offsprings is an achievement in the trend of evolution. It will be unwise to think that the care of youngs is a mammalian monopoly.
Although it has taken a perfect shape in mammals, the phenomenon of parental care is quite well-developed in amphibians where extreme modifications in structure and behaviour are observed.
Parental care is mostly a modification in the parents to take care of the offsprings so that with a meagre number of eggs continuation of race can be maintained.
Factors affecting Parental Care:
The parental care in Amphibia seems to have developed independently in the animal kingdom. There are various ways by which the parental care is manifested in amphibians.
Selection of site:
Care of the eggs is restricted to the selection of site in certain frogs and toads. In Rhacophorus schlegli of Japan, the eggs are laid in a hole on muddy bank of river or pond.
The eggs are covered by foamy mucus to prevent desiccation. The eggs are washed out into the water of the river or ponds by the rain and development starts there.
In Gyrinophilus, the eggs are laid under the stones in stream. The eggs are then taken up on any part of the body and a secretion prevents growing of fungi over them. Eggs are laid on the under surface of leaves hanging above water in Hylodes.
In Triton and Leptodactylus the eggs are laid under the surface of leaves near the vicinity of water. In Triton, the eggs may be fixed with the aquatic weeds by glues. In hynobiid salamanders, the males remain with the eggs for fertilization and take active interest of the eggs.
Frothing of water
Some anurans just after the laying of the eggs, the surrounding water is made frothy by the wriggling movement of hind limbs, so that the eggs are prevented from desiccation and also can escape the sight of enemies. Both males and females participate in this process.
According to Mallick, Mallick and Das (1980), Polypedates (= Rhacophorus) maculatus secrets a jelly-like secretion with eggs from time to time which is beaten by the cross-wise movement of the hind- limbs of the female partner only, resulting in a formation of a small frothy mass.
The frothy mass looks like an irregular ball.
At the moment of separation of partners, the frothy mass looks white but gradually turns yellowish. The mass of froth with eggs is called foam-nest. Within few hours the outer surface hardens gradually and remains in this condition for days together.
Foam-nest forms on the leafy branches of the bushes, between the leaves overhanging water, on the grass beds in between the leaves overhanging water, on the grass beds in the vicinity of water-line and on the leaves of the floating water hyacinth .
In India, foam-nest has been recorded in R. maculatus by Ferguson (1904), Annandale (1912), Mallick et al., (1980); in Rhacophorus leucomystax by Annandale (1912) and in R. malabaricus by Ferguson (1904). Bhaduri (1932, 1953) and Okada (1928) also reported the foam-nest of Rhacophorus.
Formation of nests
After lying of eggs the parents take care of them by building nests.
Three types of nests are encountered.
In Hyla Faber, parents dig a small hole in the mud for the developing eggs and the surplus mud acts as the wall (Fig. 7.47A).
Habitat – Brazil.
In Phyllomedusa (Fig. 7.48A), a South American tree-frog, the leaf nest is built by folding the margin of the leaves. Leaf margins are glued together by cloacal secretion. It has an aperture at the base and the nest overhangs watery area.
Triton constructs shoot nest by fixing the shoots of the trees in which the eggs are deposited and the youngs are developed. The whole nest remains covered by a gelatinous secretion.
Carrying eggs over the body:
In Hyla goeldii (Fig. 7.48D), the females carry the eggs on their back in incipient brood pouches. The youngs come out as full-fledged frogs but With tail. The males of midwife toad, Alytes obstetricians show peculiar type of parental care. This type of toad is abundant in France and Italy. Several males are reported to collect round a female on land.
Of the males, one becomes successful to grasp round the waist of the female. The male in this position massages and lubricates the cloacal region of the female, whereupon the females discharge the eggs. The eggs are fertilized during the expulsion of eggs by the sperms of the grasping male. After fertilization, the eggs are wrapped round the back of the thigh and he withdraws himself into a hole near the pond.
When the eggs are ready to hatch, the male carries them to nearest water where the larvae come out. Development and metamorphosis take place in water (Fig.
7.47G). Phyllobates (Fig. 7.48C) an inhabitant of western Colombia, transport the tadpoles, carrying on the back of the females to the nearest water body. In Desmognathus, the females carry the eggs (Fig. 7.47D) and live in underground hole.
In Pipa pipa, the eggs are carried by females on the back. Similar phenomenon is observed in Cryptobatrachus evansi (Fig. 7.44C). The soft spongy skin on the dorsal side sinks into small pockets into which eggs are lodged (Fig. 7.47E1). The same phenomenon is also observed in Pipa dorsigera where the eggs are deposited in pits on the back of females.
During breeding season, the dorsal skin of the females becomes soft, spongy and gelatinous. The male place the eggs on the back of the female where each egg sinks into a small pit. An operculum covers the pit. It is developed out of the remnant of the egg envelop and integumentary secretion. Development is completed inside the cutaneous pit. The partitions between the pits become highly vascularized.
The developing larva attains a vascular tail (Fig. 7.47E2). Physiological exchange of materials between the embryonic and maternal tissues is claimed to occur in this animal. The larva fails to develop gills and the tadpole larva comes out 80 days after the deposition of the egg.
In Nototrema (= Gastrotheca) pygmaeum (Fig. 7.47F), only a few eggs develop in the brood pouch bears a small slit-like opening in the posterior side. In Nototrema marsupiata, the youngs hatch out as the tadpoles while in N. oviferum the metamorphosis is fully completed inside the brood pouch.
Carrying larva from one place to other:
In Arthroleptis, the larvae are attached to the males and are carried from one pond to the other. At the time of danger they are kept inside the buccal cavity.
Placement of eggs in safest part:
In oviparous caecilians as exemplified by Ichthyophis the body remains coiled round the egg mass to guard them until hatching (7.40). But in ovoviviparous caecilian, Ceotrypetes the eggs are yolky and migrate to the last part of the oviducts. When the yolk is exhausted and the external gills are atrophied, the embryos, about 25 mm long, hatch and remain inside the oviducts till the embryos become 75 mm in length.
The developing youngs get nutrition from the uterine ‘milk’. Metabolic exchanges also occur between the vascular maternal and foetal tissues. In Rhinoderma darwini (Fig. 7.48E), the eggs are swallowed by the males and are placed inside the inflated vocal sac. The eggs may remain there up-till hatching or even up to the completion of metamorphosis.
Extreme modification is observed in Salamandra atra and S. maculosa. The eggs are placed inside the uterine cavity where entire tadpole hood is completed. Two eggs are laid at a time. The larvae remain attached with the uterine wall by membrane which functions physiologically in the manner of a primitive placenta.
The broad and vascular tail also helps in metabolic exchanges.
The foregoing description reveals some of the ways by which parental care is effected in different amphibians. The question now is whether this phenomenon of parental care in amphibians has any parallelism with that observed in mammals.
Experimental evidences on this particular issue are meagre to establish the actual controlling mechanism. It is certain that the care of youngs in amphibians evolved probably in response to their habit and the environment they live.
Two alternate explanations can be forwarded.
- The care of the youngs by the parents is largely behavioural and mechanical in nature which may be influenced by the hormonal actions.
The care of the youngs is either instinctive or due to the reflex action developing during the breeding season. This contention seems more convincing. The care of eggs or youngs is largely, if not purely, instinctive in nature rather than anything else.