Popular Science Monthly/Volume 59/May 1901/The Frog as Parent
| THE FROG AS PARENT. |
By Professor E. A. ANDREWS,
JOHNS HOPKINS UNIVERSITY.
IN the life of a common frog or toad we seem to find none ef that altruistic solicitude for the welfare of the helpless younger members of society, that we so fondly attribute as guide in many of our own actions. In clamorous spring reunions these cold-blooded creatures deposit their eggs in the water and go their way in search of food—not knowing whether some or many of the eggs will run a normal course through tadpole or pollywog states to tailless adults, or fall a prey to hungry ducks, or more insatiable naturalists in search of 'material' for study.
The naturalists' belief that these Amphibia are closely akin to fish in many ways is borne out in their breeding habits; for, like fish, they have more or less complex 'instincts' that lead the males and females together at the laying season, and then, like fish, they separate till the next period of egg-laying. The eggs, discharged in the water, are fertilized outside the body, and undergo a process of cleavage or cell-multiplication, thus gradually differentiating into active larvæ or tadpole? without any care from the parents. The tadpole leads its own independent fishlike life for months or years, till—if not destroyed—the critical period of transformation arrives. This passed, the young frog or toad has only its instincts to guide it in learning the new life and nothing to learn from its parents—unless perchance they may be near enough to endeavor to swallow it alive.
Yet even here we might fancy some thought for the morrow of the species—the eggs are generally laid in the right place—according to the kind of frog or toad—to have enough water and not too many enemies for the young, while the protecting jelly mass about the eggs is often rather carefully fastened to plants or sticks, thus keeping them near the surface of the water and in optimum conditions for hatching.
But this is not clear until we see some of the extremes to which such prevision for the next generation is carried in certain members of this group. Just as amongst fish there are a few with most remarkable habits—the male stickleback watching and protecting the eggs in his carefully made nest—so, if we look far enough, we find frogs and toads that show most exemplary solicitude for the young. In Europe, Asia, Africa and in South America such curious life-histories are more or less common.
In most such cases the peculiar habit of the parent seems to be associated with an unusual character and development of the eggs. In the common species that have many small eggs, these are left by the parent to develop slowly in the water, where they gradually assume a froglike character. Whereas species having few and large eggs protect these in some manner until they rapidly turn into frogs with little or none of the aquatic youth we are so used to regard as a sine qua non for a frog.
We need go no further than the island of Jamaica for examples of the protected eggs. For there, where everything has to compete strenuously for light and air, the trees themselves support dense populations of plants and these harbor animals of various sorts—amongst them, frogs. One kind of frog in Jamaica lays its eggs in the water that accumulates at the bases of the leaves of Bromelias growing high up on tree trunks, and here the tadpoles have their brief existence.
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| Fig. 1. | Fig. 2. | |
Another frog abandons even this semblance of aquatic life, laying a few very large eggs under stones and 'trash' on the ground, where they may even be many miles distant from the water—and the young develop into small frogs that hatch from the egg without having known what it is to be a tadpole. Stevenson's fable admits of literal application here.
'Be ashamed of yourself,' said the frog. 'When I was a tadpole I had no tail.' 'Just what I thought,' said the tadpole; 'you never were a tadpole.'
In this frog, however, there is within the egg a stage when the young is active and has a tadpole form, lacking chiefly the medium in which to express its tadpole possibilities. Not being able to swim with its tail, it yet puts this to good use, for it is richly supplied with blood vessels and can serve as a breathing organ.
In the Solomon Islands there is another frog (Rana opisthodon) which lays large eggs, 6-10 nun. thick, on moist ground and not in the water; and the young remain within the egg until they are complete frogs in shape. While passing through so much of their life-history within the egg shell, the young manage to breathe, not by their tails, but by special folds of skin that grow out on either side of the belly, in rows, as shown in Fig. 1. The young frog also makes a temporary contrivance for breaking through the egg shell, something like the horn on the beak of a hatching chick or the protuberance used by many reptiles for the same purpose. This peculiar little organ in the frog is shown in Fig. 1 and again in Fig. 2, where it adds a decided retroussé element to a not too intellectual countenance.
We might place frogs in three groups: those that are simply layers, those that are nest-makers, and those that are nurses.[1]
As a nest builder we may reckon the Cystignathus mystaceus of Brazil. This frog makes a hole about as big as a teacup under stones or decayed logs near enough to puddles of water to be covered by water when the pond rises after heavy rains. In this nest the yellow eggs are laid in a mass of thick, white foam, very like beaten white of egg in appearance. The eggs hatch into tailed tadpoles, and, when the water rises over the nest, these young swim off like our common tadpoles. They differ, however, in being able to overtide dry seasons. When the pond dries up and common tadpoles would die, these peculiar creatures gather under boards or logs and there keep moist—apparently by the aid of an unusual amount of material secreted by their skins. Evidently the habits of this frog are nicely adjusted to the climatic conditions under which it lives.
A tree-frog of West Africa (Chiromantis rufescens) lays its eggs in leaves of trees in a mass of foamy material that, on drying, hardens on the outside, but becomes liquid within, and so lets the tadpoles swim about for a while till a rain washes them off the tree into the water. While living the short part of its tadpole stage in the nest made by the mother for it, the tadpole has gills such as our common tadpoles breathe with, as well as a tail to swim with. In this aerial existence the young have the protection not only of the surrounding foam, but often of leaves that are sometimes stuck to it. This perfecting of the nest by the use of leaves to envelop the foam mass becomes the rule in two sorts of frog in Brazil (Phyllomedusa Jheringii and Hyla nebulosa). The former puts its big white eggs into a mass 40-50 mm. long and 15-20 wide, enveloped by two or three leaves, sometimes willow tree leaves, in such a way that they make a tight case open at one end. The young seem to be so fitted for this peculiar {{hws|wet-ham|wet-hammock} wet-hammock existence that it is doubtful if they need to fall off into the water; at all events, some tadpoles of Hyla nebulosa taken out of this nest and put into water, died in a few hours from lack of breath, being unable to live without the peculiar air supply they were used to. However, the larvae of another frog (Phyllomedusa hypochondrialis) are set loose from the nest when the rain softens it and do fall into the water to continue their tadpole life. This species occurs in Paraguay, and makes use of the leaves of trees near the water. The male rides on the back of the female, while both bend in the edges of a single leaf till it makes a funnel. In this the eggs are laid and fertilized. The jelly surrounding them holds the leaf in place till the tadpoles hatch and are ready for the rain to forward them to their new destination.
In Japan there is also a nest-making frog (Rhacoporus Schlegeli) which is said to lay its eggs sometimes amongst leaves on bushes or trees. However, its usual habit is to make a nest in the ground as indicated in Fig. 3. Awakening from their winter sleep, the frogs crawl along the edges of rice fields and swamps and dig out holes above the water level. The female carries the much smaller male, and both become buried in a hole 6-9 cm. wide and 10-15 cm. above the surface of the water. This nest cavity is smoothed inside by the movements of the female and is then, in the night, supplied with a ball of white matter full of air-bubbles. This is tough and elastic and 6-7 cm. thick. This mass is to supply moisture and air for the young. It emerges from the cloaca along with the eggs, and is then kneaded thoroughly by remarkable movements of the feet of the female. Stretching out and closing the toes, she mixes the sticky mass with air and breaks the big bubbles up into smaller and smaller foam. Similar movements of the feet of the male drive the mass backward and leave the eggs more free for the fertilization that follows. When the eggs have been fertilized and provided with a protecting and aerating mass, the parents break out from the nest and take up their life amongst the trees. The foam mass meanwhile gradually becomes liquid, and flowing out through the hole the parents left on leaving the nest, carries the young into the outside water.
Very different is the nest made by another tree-frog in Brazil, The quaint, beaver-like activities of this creature (Hyla faber) are described by one who observed them in his own garden. On a moonlight night one may see a slight movement of the water as if something were moving under it. Then a little mud rises, shoved up by a tree-frog—but only the hands of this are visible. It dives down and again brings up mud;
gradually the accumulated mud seems a little wall, and eventually it rises 10 cm. above the water and extends as a ring a foot in diameter. Such atolls of mud are shown in Fig. 4. They form little circular dikes of mud elevated above the general expanse of water. The making of them is by no means a matter of chance. The frog—and it is the female that labors, in full enjoyment of her 'rights,' while the male rides passively—uses her hands to compress and to smooth the inside of the wall in a most remarkable way. The top also of the miniature fortress is carefully manipulated, the frog crawling out of the water to make all the structure solid and smooth, all but the outer escarpment, which is left rough.
The frog makes the bottom of the crater-like cavity, under water, smooth by gliding along it on its belly, and also by spreading out its hands over it. During all this work the frogs make no sound, though near at hand isolated males may be heard calling their mates.
In this circular nest the spawn is deposited, but not till four or five days, as a rule, after the nest is completed; there is a period of rest between the nest-making (which in one case required two successive nights) and the egg-laying. The young hatch out as tadpoles and continue to live in the narrow circle of home till the wall be broken down by time and weather to set them free in the world.
It is the habit of all these nest-makers to put their eggs into more or less imperfect nests and then go off and leave them to their fate, the nest, no doubt, increasing the chances of the young passing safely through the earliest stages of infancy—which we all know is a critical period for man or fish. Some other frogs have quite a different habit; carrying their eggs about with them, and so giving them the benefit, if not of actual defense against enemies, at least of passive protection, in that the eggs thus have the same conditions of moisture and concealment which they, the adults, need and obtain for themselves. Such frogs we have called 'nurses.'
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| Fig. 5. | Fig. 6. | |
Of these nurses the most often mentioned is the so-called 'obstetrical toad' found in Switzerland, France and western parts of Germany. As shown in Fig. 5, the eggs are carried about attached to a band, which is wrapped about the legs of its parent, but the parent who thus carries the offspring about in the moist grass as it seeks food in the evening is not, as one might expect, the mother, but the father toad.
When the eggs are laid and fertilized, the male takes up his burden and carries it till the young are ready to hatch, when he goes to the water and lets the brood escape into the proper element. What leads him there at the right time remains still to be found out. How he comes to assume this care is also not clear; according to some accounts the male, when upon the back of the female, seizes the egg-string first with the right, then with the left foot, and wraps it in figure of eight loops about its own legs. Others say that the male sits behind the female, with its back turned toward her, and as the eggs emerge, the male seizes them between his ankles and, throwing himself now on his back and now on his belly, turns over and over until the egg-string is all drawn out and wrapped about his legs; hence the name Alytes obstetricans. Having taken the eggs, however this may be done, the frog emits a clear musical sound and goes hunting its own food, thinking, we may suppose, as little of its offspring as do our own common toads and frogs. The young require a long time to bring them to the hatching stage, and have also an unusually large and peculiar gill on each side.
The tree-frogs of the tropics furnish examples of egg-carrying habit as well as of nest-building. Thus in Ceylon a tree-frog (Rhacophorus reticulatus) carries its eggs in a cake-shaped cluster of about twenty firmly fixed to its belly, as indicated in Fig. 6. This time it is not the male, but the female, that aids the coming race by giving it transportation and protection. Probably, however, it is the male and not the female of a frog of the Seychell Islands (Anthroleptes Seychellensis) that carries about its young on its back. This is a most complex case, for the eggs are laid upon moist earth or rotten logs and kept moist by the body of the frog until they hatch; they have large tails.
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| Fig. 7. | Fig. 8. | |
and even the beginning of hind legs. Then the youngsters get up on the back of their parent and stick there till their development is complete. The peculiar little tadpoles have great power of adhesion, and can stick to the sides of a glass dish as they do to their parent's back (Fig. 7). Another such case occurs in Trinidad and in Venezuela. When the ponds dry up the young tadpoles of the frog Phyllohates trinitates, which are as yet without legs, though they have tails, stick themselves firmly to the back of the male frog (whether it is their father or the father of some other tadpoles does not appear), and in this way are carried 'pick-a-back' to some larger pond. Similar habits have been observed in the frogs Dendrobates trivittatus and Dendrohates braccatus.
Again, a frog (Hylodes liniatus) of Dutch Guiana is found with its tadpoles attached to its back, as seen in Fig. 8. The young, from a dozen to twenty, are attached, as shown, with their heads turned towards the middle of the mother's back and do not fall off, even when she leaps rapidly away. Thus the mother, and not the father, carries the young with her. In the Brazilian tree-frog (Hyla Goeldii), it is again the mother who bears the young with her. The eggs are very large and whitish and crowded together into a rounded mass on the back of the female, as represented in Fig. 9. While all the other frogs as yet mentioned seem to have no special organ or apparatus for holding the eggs or tadpoles as they carry them about with them, this Brazilian frog is provided with a growth of skin on its back to form a wall all around the eggs so that they lie in a spoon-like depression. The knowledge of such a change in the skin of the back for egg carrying may make us more ready to receive the accounts of the Surrinam toad described by Mile. Merrain in 1705. When, in 1725, the Dutchman Ruysch described the remarkable pits for carrying young which this creature has upon its back, the account met with natural skepticism, but at the present day reiterated observations place
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| Fig. 9. | Fig. 10. | |
the main facts beyond doubt. The female of this toad (Pipa dorsigera), as seen in Fig. 10, bears its young upon its back, each in a separate case, like so many papooses.
The cases are made at the breeding season, and before that the two sexes look alike. How the eggs get into these special receptacles is still an unanswered question, though the male may be a factor in the case; at all events, when the male goes away after being some twenty-four hours on the back of the female, the spawn is found on the back of the female, where the eggs gradually sink down into circular pits, that are hollowed out in the skin, and are 10-15 mm. deep. When an egg has sunk down into one of these pits, a thick, leathery or horny roof forms over it, and thus shuts it out from the external world.
These roofs are 5-6 mm. thick, and have a dark color unlike the rest of the back. Whether they, like the rest of the chamber about the egg, are formed from the skin, or whether they may be modified remnants of the mass laid with the egg, is as yet undecided. The part of the skin of the back not taken up by these pits rises up in small papillæ, and thus each pit is closely surrounded by a ring of papillæ. From 40-114 pits are formed, and 60-70 young are developed.
Each egg thus develops inside a diminutive womb-like chamber, on the back of the mother, but as yet we do not know whether the mother supplies any nutriment to the young while it is in this protecting chamber; the arrangement of lymph and blood vessels in the skin of the mother and the organization of the young, however, raise the question whether this may not be the case. In each pit the young tadpole lies with its back toward the roof and its belly downwards. For awhile it has a large yolk-bag, or enlargement of its belly, full of nutriment and richly supplied with blood vessels; also a tail. This tail is very large, and seems to be of no use, unless it may function as a breathing organ, as in the frog of Guadaloupe, mentioned above. Another peculiarity of these favored tadpoles is that they obtain their
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| Fig. 11. | Fig. 12. | |
front legs at a very early period, before their gills grow out; and thus, for a long time, appear as four-legged creatures, with most imposing tails, as represented in Fig. 11.
In this condition they break off the roofs of their little houses and each, like a baby kangaroo from its mother's pouch, peers out into the world with goggle eyes and ready hands. For nearly three months (eighty-two days) the mother is burdened like Sindbad, till the young toads jump out of their cradles and go free to shift for themselves.
Every Surrinam toad is thus launched upon its individual voyage of contest for food as a complete but small toad, and, though it had the outward symbols of tadpole life—gills and a tail—it never lived a tadpole life, with its dangers from the drying of ponds, and the chances of being swallowed alive as so many tadpoles are. Our common tadpoles may, from one point of view, be regarded as chiefly feeding phases, like the caterpillar that eats and eats to pass through the inactive chrysalis stage to the complete butterfly life. Tadpoles are phases of the frogs' lives, when almost anything can be eaten, and when there is but little perfection of the senses as compared with the future adult stage. The experience gained as tadpole would seem to be of very little direct use to the frog when it begins its new life; it is chiefly the gain in material, the results of feeding, that makes the frog the better for its tadpole life. In the Surrinam toad this gain in size is provided for by the mother, who takes all labor off the shoulders of her offspring till they are toads like herself. Is there not an analogy between such habits and our own attitude towards the next generation? The child of the savage, or of the more unfortunate classes in 'civilized' communities, is put to getting its own livelihood at the earliest possible age; the favored child of the few is protected by the parental roof and fed with even university pabulum till nearly arrived at adult structure.
But the Surrinam toad is not the only one that is built mammallike, to carry its young within its own body; the group of pouched or marsujnal frogs of Venezuela have an even stranger contrivance for this purpose. Those who are fortunate enough to have access to a copy of Professor Davenport's profusely illustrated little volume, 'Introduction to Zoology,' will find a very attractive figure of one of these 'brooding tree-frogs,' taken from a water-color painting at Harvard College. On the middle of its back, above the loins, is a very large opening leading into the interior of the animal. This is the opening of a large brood-chamber. In one frog (Nototrema oviferum) this chamber continues forward on each side as two, even larger, chambers that reach almost to the head. The middle chamber is on the back, while the side chambers extend not only over the back, but down on the side, so as almost to meet one another across the belly. All these chambers lie just beneath the skin and are not deep, but flat, though they displace the viscera.
The walls of the chambers are very thick and vascular, except near the external opening, where the wall is of the same nature as the skin. The skin, in fact, seems to have grown in to line these chambers, but has been much changed in its character in those parts of the chambers that are remote from the openings. It is not known how this big bag grows over the body, nor whether it is always there, or only developed at the breeding season. These chambers are found in the female, and in some unknown way the eggs are transferred from the ovary into the pouches. As there is no internal opening to the pouch, the eggs must be laid as usual and then put upon the back, and so into the external opening of the brood pouch. Possibly the male aids in this function.
The eggs of this species are exceedingly large, being 1 cm. in diameter, more than eight times the bulk of a common frog's egg, and are also but few in number. In one case there were only four eggs in the outer, middle, chamber and eleven others m the two side chambers—fifteen eggs in all.
The young that develop from these comparatively large eggs inside this peculiar skin-bag are remarkable enough to satisfy the ideals of so bizarre a parent. Like the young Surrinam toad, these get their front legs at a very early period, and at an early period are also found without the adhesive organs and horny jaws that seem so essential to all common tadpoles. But their chief departure from received tadpole style is the phenomenal character of the gills. These are large, bell-shaped, flower-like membranes that envelop the tadpole like a mantle and, coming between it and the walls of the mother's pouch, may serve as a means of getting oxygen and possibly food from the mother, for the gills are richly supplied with blood-vessels, and the walls of the mother's pouch are also vascular, giving the anatomical conditions for interchange such as takes place in a mammal's placenta. Each of these two gills seems to have been made by the fusing of two specialized gills, and each retains two stalks. Eventually these big gills are probably lost and replaced by inside gills, just as in common tadpoles the outside gills are always followed by inside gills. Whether the mother goes into the water and lets the young escape where they can use their gills, or whether she keeps them at home till they have lost these youthful structures and can 'come out' in budding maturity, is not known, in this case. But in Nonotrema marsupiatum and Nototrema plumbeum the young are set free into the water when they are still tadpoles.
The way in which such a capacious pocket on the back can have come about is perhaps indicated by the state of things in Nototrema pygmæum. The brood pouch is here small and slit-like, and when the young are ready to leave it they press and wriggle till the pouch is torn open, from the external opening forwards. There are only four to seven young that can come forth in this partially Cæsarian way, and their appearance seems to have been prearranged by the way in which the pouch is made. Two folds of the skin grow up to meet one another along the back, and when they fuse they leave a sort of seam, which is also the line along which the pouch ruptures to let the young out. The pouch of this tree-frog is thus to some degree intermediate between the simple cup on the back of Hyla Goeldii, not well shown in Fig. 9, and the more perfect sac of the other pouched frogs described above, and may indicate the lines along which structures and habits like those of Hyla Goeldii could have been evolved in those of Nototrema oviferum. On the other hand, the small size of this frog and the large size of its eggs make it both impossible to get the eggs into the usual external opening of the pouch and impossible for the big larvæ to escape through it; hence it may be that the pouch grows up as folds after the eggs are laid on the mother's back, and that these folds remain easy of separation to allow the young to escape, all independent of any mode of development of the pouches of other marsupial frogs. These strange skin pits and bags of the Surrinam toads and the Venezuelan frogs are both an aid to the young and an inconvenience to the parent, and it seems in keeping with our general experience that it is the female that has these special organs and the female that suffers the dangers that go with the prolonged care of the offspring. In both respects, however, with regard both to the phenomenal nature of the breeding organs and in the amount of personal sacrifice, this female is outdone by the male of a little frog of Chili.
This frog is not much more than an inch long, and was first found by Darwin on the voyage of the 'Beagle.' In some unknown way the large eggs get into the mouth of the male and are carried a long time inside a huge sac that opens only into the front part of the mouth. In this pouch the young develop their legs and small tails. It is probable that they remain protected within the male till they are complete lung-breathing frogs and then get out of his mouth and escape. Why he does not eat them is a question that might naturally occur to one knowing only our common frogs.
The brood-sac of this male extends over the throat and belly, back to the loins and up on each side nearly to the backbone. The eggs and young that are found in it are from five to fifteen in number, and lie scattered about in the capacious chamber. The general anatomical relations of this sac are shown in the rude diagram. Fig. 12. This represents some of the organs that would be seen on cutting the frog into halves, lengthwise. The brain and spinal cord along the back are shown in black. Below this are the mouth, stomach and intestines. On the floor of the mouth is an elevated region, the tongue; behind this is the opening to an irregular cavity, one of the lungs. In front of the tongue is the opening to a very large sac, the brood-sac, in which the eggs are represented as large balls.
A bag of this size necessarily causes the skin of the throat to bulge out and also presses upon the internal organs. It is found that even the bones of the shoulder girdle and chest are modified in connection with this remarkable organ, and that the stomach, liver and intestines may be pressed out of place, so that feeding must be difficult. In some cases the digestive organs are said to be so impaired as to be of no use, while in other cases the brood-sac is of much less extent and would seem not to interfere seriously with feeding and digestion.
The brood-sac lies free under the skin, except in certain regions of the throat, where it is fastened to the skin. The lining of this sac is a continuation of the lining of the mouth, in fact, the sac is but an enormous side pouch from the mouth. In looking for any similar organ in common frogs, we find the single or paired resonance chambers that open into the mouth and serve to give volume to the voice. These chambers are specially large in the male and most used in the breeding season. This Venezuelan frog, Rhinoderma Darwinii, seems to have only an enormous development of this resonance chamber converted to the singular purpose of harboring the young. As far as yet known, this frog leads the van of progress from the selfishness of the common frog to the altruism of the few; but there may remain to be discovered in tropical regions other frogs with structures and habits even more advantageous to the race and inconvenient to the individual.
Granting that all the above accounts of the breeding habits of frogs are reliable, they yet leave many details unknown. Very much remains to be found out before we can know the complete life histories of these remarkable creatures. Till we have a fuller knowledge, any attempt at explanation of the breeding habits and structures of these frogs would seem to be necessarily of a provisional nature.
Though it is difficult to describe these frogs without ascribing to them a far-seeing intelligence, the zoologist of to-day knows little ground for such assumption. Still less does he see in such examples of protection for the good of the race any direct acts of the Deity. He commonly interprets them as the results of the working of natural selection; and granting the potency of this means of evolution, the application to the above life-histories seems not difficult.
But it has been said that science is not concerned with the why, but only with the question, 'What is it precisely that does happen'" Shall we not work for more complete knowledge of the facts in the hope of a clearer view of the interconnection of organisms and environment?
May not our present attempts to understand such problems seem, in the future, as unscientific as does at present the fancy of the Japanese poet, who, centuries since, wrote:
"With hands resting on the ground, reverentially you repeat your poem, O Frog."
- ↑ The remarkable life histories not in the first group have recently been brought together by R. Wiedersheim, from whose papers in the 'Biologisches Centralblatt' most of the facts and illustrations used in this article are taken.