Изображения страниц
PDF
EPUB

passu with one another (correlated variation). In the former the individuals possess extreme and transitional stages transitional stages of different degrees, in the latter they are either all extreme or all transitional stages.

7. Each varietal type has a restricted geographical distribution, without overlapping; and the more nearly related types occupy adjacent areas. Compared with other South African tortoises, it is found that each species of the genus Homopus has a restricted distribution, with characters showing little variation, while T. pardalis and T. angulata are widely distributed, also with slight variability.

8. Among the ten described species of the geometrica-group, only three distinct types of combination are recognizable : oculifera, geometrica, and tentoria. These may for the present be considered as sufficiently separated from any transitional forms to be given specific rank, and tentoria presents at least three well marked varieties or sub-species : tentoria (sensu strictu), verreau xii, and trimeni. Four of the other described species smithii, fiskii, seimundi, boettgeri) are not sufficiently distinct from the fluctuations of tentoria to warrant even varietal recognition, 'while strauchi may be a variety of geometrica.

By J. E. DUERDEN, Ph.D., A.R.C.S. Professor of Zoology, Rhodes University College, Grahamstown.

Justly or unjustly the ostrich has become proverbial for stupidity. Both in literature and conversation the bird is frequently referred to as representing the extreme of foolishness from its supposed habit of hiding its head when alarmed, and leaving the rest of its body exposed to observation and danger. It must be understood that in applying such terms to an animal's behaviour we are, without warrant, presupposing consciousness and passing the same judgments upon it that we would upon the actions of an intelligent human being. Perhaps an enquiry into the actual habits of the ostrich and their underlying significance may serve to shed some light upon the justification or otherwise for the doubtful mentality usually accorded the bird. One generally discovers there is some foundation of fact in the habits of creatures or persons which have become bywords, though it is often found that in the first and superficial impressions the true significance of the phenomenon has been missed.

All ostrich breeders are aware that if ostrich chicks, while only a few days or weeks old, are suddenly startled they will at once crouch down, as if dead, and remain thus for some time. The action is much more likely to occur if the chicks have been hatched and reared out on the veld, away from human influence. An actual personal experience will best illustrate what happens. Driving over an ostrich farm one day a pair of ostriches were come upon, surrounded by a dozen or so chicks about a week old ; the chicks had been hatched out on the open, and no personal attention given them. Dismounting from the cart, a sudden, clamorous rush was made towards the group, when the parent birds at once ran away and the chicks scattered and disappeared so suddenly and so completely that for the moment one knew not which way to turn in order to follow them. After remaining quiet for a few moments, and getting the eye accustomed to the surroundings, it was found that several chicks were lying quite near, so that one might almost have stepped

Their bodies were prone upon the ground, with the neck and head stretched out resting upon the surface. semblance of the mottled black and brown bodies of the chicks to the rocky ground and scant vegetation was so perfect that it was with much difficulty the various individuals were discovered.

The recumbent chicks allowed themselves to be approached and picked up. when they were found to be quite limp and motionless, appearing altogether lifeless, and only recovered after being conveyed for some distance.

This behaviour on the part of the ostrich chick is a clear and striking instance of death-feigning or death-shamming. In this state the animal appears and acts as if dead, and, thinking of it in terms of human actions, we explain it as if consciously assumed for the purpose of deceiving its enemies. The same instinct is also well developed in many birds (rails, bustards), mammals (opossum, jackals, foxes), and especially in insects.

upon them.

The reAfter a pro

From the behaviour of the ostrich chicks it may be assumed that on rushing towards them the first instinct to assert itself was that of flight, and the birds scattered. Then extreme fright or terror supervened, and brought about a general nervous collapse of the chick, as a result of which it flopped down. From this condition of nervous collapse it only slowly recovered. In insects, as, for instance, in many beetles, it is often found that a sudden touch will bring about the same response, though it is very questionable as to how far what we know as fear is here concerned. It seems to be little more than a reflex reaction which has proved itself useful to the animal's ancestors, and thereby been preserved and more perfectly developed by natural selection.

Death-feigning in the ostrich chick has evidently a greater significance than that of merely giving to the animal the appearance of being dead and inert, in the same manner as in an insect. The principle of protective resemblance is also involved. Every ostrich farmer knows the extreme difficulty of finding a brood of ostrich chicks once they scatter in the bush and crouch down. longed search he will secure only a few of those immediately near him, and then return for a second attempt, when the birds have again gathered round the parents. The peculiar black and brown striping of the neck, found only in chicks, and the mottled black and brown natal plumage harmonize most closely with the surroundings when the chicks are recumbent upon the ground.

The chicks appear to get beyond the instinct of death-shamming very early. The ones just mentioned never again displayed the reaction after being brought home and reared by hand ; and it seems to be rarely resorted to by incubated chicks artificially reared. Under some circumstances, however, very young birds, say to one month old, will fall flat from almost any object or noise which suddenly startles them, and this applies to tame chicks as well as to wild or nearly wild ones. Also as the chicks get older the collapse is not so complete nor does it continue for so long a period as at first ; for frequently after a bird has crouched down and one makes towards it, the creature will spring up and make off, perhaps to drop a second time if hard pressed. The experience of farmers is that by the time the chicks are a month or more old they resort more freely to running when alarmed, and only crouch when hard pressed or very suddenly startled.

We may now consider if there is any evidence for the instinct being continued into the adult stage of the ostrich. In a general way it is found that when an ostrich is alarmed it takes to running, and even the fleetest horse is incapable of keeping pace with it so long as the pursued continues in a straight course. Once fully started, the bird probably never has recourse to the true deathshamming instinct, but will continue its course as long as its strength lasts, and only fall flat when exhausted, often never to rise again. In the days of wild ostrich hunting the birds were thus frequently driven to death with a horse. The instinct of flight, however, is not resorted to under all circumstances by adult birds. Instances are

a

easy to

known where an adult ostrich has been come upon quite suddenly, as from the top of a kopje, and the bird was apparently so startled by the apparition that it at once collapsed on the ground with its neck and head outstretched, and made no attempt to escape. The primary instinct of death-feigning was here the first to assert itself, not that of escape.

Mr. Guy A. K. Marshall has observed the habits of the wild ostrich in Mashonaland, and states that in one case the bird squatted after running for some considerable distance and dodging about among .some low bushes. In a second instance, coming suddenly over a rise in quite open country, three ostriches were seen about 400 yards away. They detected the intruder at once and dropped like stones, being then almost indistinguishable from the ant heaps among which they were feeding. Upon continuing to walk towards them, they evidently recovered from their collapse, rose up, and soon made off out of sight.

Such well-authenticated observations prove conclusively that the ostrich may retain its instinct of death-feigning into the adult state; it is not resorted to so freely as when the bird is young, but it still comes into action under certain conditions. The circumstances calling it forth seem to be sudden or intense alarm or fear. Under ordinary conditions of alarm the birds take to flight, but when suddenly alarmed, without perhaps a chance to escape, they follow an instinct which is more usual in the young. It is understand the advantage gained by this gradual change of response. While very young the chicks would scarcely be able to escape an enemy by running, and hence death-feigning is an advantage to them,

and is the usual procedure on alarm ; as they become stronger and fleeter their increased bulk would result in exposure and danger, and thus they resort almost entirely to flight.

It is manifest that the ostrich has, as it were, a choice of actions when alarmed : either it can drop down and rely upon its inertness and close resemblance to its surroundings for protection, or it may take to running and depend upon its fleetness for escape. It is more likely to follow the former while young, the latter as it grows older. It may even act differently under what seem to be similar circum-. stances. As in many animals, particularly those higher in the scale, there is an element of uncertainty as to which of several possible courses may be chosen in an emergency.

An instinct which seems very closely related to the above is sometimes displayed by brooding or nesting birds. In this parental duty the cock and hen occupy the nest alternately, the hen mostly by day and the cock by night ; to be somewhat accurate, the hen sits from 8 or 9 a.m. to about 4 p.m., and the cock from 4 p.m. to 8 or 9 a.m.

The greyish colour of the female bird usually harmonises very closely with the natural surroundings by day and the blackness of the male by night. There can be little question that these sexual differences are the results of natural selection. Under ordinary circumstances the wild or semi-wild ostrich will sit with its long neck erect as if on the look out, but immediately it espies any danger, as

a

on the approach of man, down drops its neck and head flat on the ground, and it is with difficulty the position of the nest and bird can be detected. Often when searching for their nests the farmer finds it necessary to hide behind a distant kopje or rise, and locate the birds with their heads erect by means of a field-glass.

In studying the behaviour of animals we have to guard against attributing intelligence or even consciousness to all their actions. We see, as in the present instance, ostrich chicks acting in a manner which is obviously the one best adapted to the special conditions, but, although at first sight tempted to do so, no one would think of associating intelligence with the response, much less could we accuse the creatures of intentional deceit as is implied in the term death-shamming. Death-shamming is a congenital or hereditary act on the part of ostrich chicks; it is a complex action performed without any previous experience and independent of instruction. One can never be quite certain what are the factors, conscious or otherwise, which determine any action of an ostrich without becoming an ostrich one's self, nor altogether analyze its feelings unless one has the feelings of an ostrich ; but we are probably safe in saying that the reaction is an altogether unconscious one and without any psychical attributes.

It is the exhibition of the instinct of death-feigning which has probably given rise to the stories of the ostrich hiding its head when in danger, and leaving the rest of its body exposed ; and the tendency to put human constructions upon the actions of the lower animals has engendered the idea of stupidity as associated with the act. a state of nature the act would be unquestionably useful both to the chick and the adult bird as a protection from its enemies. To the early ostrich hunters it must have seemed the height of stupidity,, looked at from a human aspect, for such a large bird when alarmed to suddenly flop down before him limp and motionless, and with time the act has become a byword. But when we realise that this same instinct is undoubtedly useful under natural conditions we see that it has a deep significance, and can understand how in the past it may have been encouraged by natural selection.

To say that the ostrich hides its head and then believes itself safe is only an anthropomorphic way of interpreting "he behaviour of the bird. It is probably an unconscious and involuntary act on the part of the creature just as much as is the collapse of a highly-sensitive person upon sudden fear. The hiding of the head is not an essential part of the act of death-feigning, but results from the well-known fact that the ostrich habitually crouches with its neck and head resting on the ground, thereby assuming the closest resemblance to its surroundings. The stupidity lies in our attempt at an explanation of the act, and not in the bird itself.

« ПредыдущаяПродолжить »