Some problems of cerebellar physiology in migratory and sedentary birds

SOME PROBLEMS OF CEREBELLAR PHYSIOLOGY IN MIGRATORY AND SEDENTARY BIRDS BY

BRUNO SCHREIBER, TORQUATO GUALTIEROTTI

AND

DANILO MAINARDI

Department of Zoology, University of Parma, Italy . Department of Physiology, University of Milan, Italy .

Introduction The problem of bird orientation is one of the most fascinating in biology and its mechanism is still rather incompletely understood . One thing is clear, however, and that is that the basis of the orientation capacity must be sought in the perception of some aspect of the "geographic situation" which could be one or more of the following : (1) astronomical data, perceived visually (according to the theory of Matthews-Kramer) ; (2) mechanical forces resulting from the earth's rotation and consisting in variations of accelerations (according to Ising's theory) ; and lastly, (3) magnetic forces (Yeagley). Other problems concerning navigation, such as for instance exploration systems have been clearly discussed by Griffin . With the possible exception of the mechanism based on magnetic forces, for the detection of which no receptors are known, all the other factors produce sensory excitations converging on the cerebellum . The convergence of optic, acoustic, and vestibular stimuli on the cerebellum has been widely demonstrated : afferent activity of the skin and afferent and efferent connections from and to the muscle (that is the gamma system and gravity and stretch receptors) are also well known . The cerebellum therefore appears to be the main integrative centre for relations with the environment taking place through any system of telereceptors (sight, hearing and sensitivity to accelerations) . Undoubtedly a certain number of facts is well explained by the theory that orientation is based upon vision, both through recognition of familiar landmarks and through sun navigation . This theory is in general the one for which the most evidence is available, both regarding sun navigation (Matthews-Kramer) and star navigation (Sauer). However, this theory is unsatisfactory from other points of view, in that it requires an extremely precise and reliable mechanism for time measurement, the existence of which has not been directly demonstrated, in order to make adequate use of observations upon sun and star movements . In fact, sun occlusion experiments

and experiments on the subjection of birds to irregular light/dark sequences, designed to test their "internal chronometer", showed contrasting results (Matthews, 1953 ; Rawson & Rawson, Kramer, Matthews, 1955 ; Hoffman, 1958) . This criticism is not intended to deny the value of theories based upon vision, but rather to point out that such a method of orientation is probably not the only one available to the animal . In any case it seems to us very probable, on a priori considerations, that no single mechanism can be responsible for orientation . The hypothesis that orientation in birds is based or rather is partly based on geodetic forces has been our main interest (Ising, Beecher, Yeagley) . A critical summary of the evidence available on this point will be given in what follows . If we consider briefly the theoretical basis and the order of magnitude of the mechanical forces required for the geodetical theory of orientation, we note that these forces are extremely small . The difference of gravity between the equator and the poles due to the centrifugal forces produced by the rotation of the earth amounts to only 0 . 005 g . Furthermore, when an animal travels at a speed of 40 miles/hr . along the equator it will be subjected to a higher centrifugal force if it flies in the direction of the earth's rotation (WE), while the force will be reduced in the opposite direction (EW). The difference in centrifugal force in these conditions is, however, only ±5 x 10- 4 g. If an animal flies along a meridian or along a parallel of latitude it will be subjected also to Coriolis accelerations which vary with latitude, when it makes lateral or flexion extension movements of the head. Coriolis acceleration results from the combined movements to which a body is subjected when, moving at a constant speed in a circle, it is displaced radially . If 9 is the angle between the two velocity vectors, one being due to the earth's rotation and the other the added velocity v, due to the head movements in a direction perpendicular to the constant velocity, the component would be v sin e . The earth's rotational speed at the equator is w r (co =angular 42

SCHREIBER et al . : CEREBELLAR PHYSIOLOGY IN BIRDS velocity, r=radius of the earth), and at any point on the earth the linear velocity is r w cos A, where A is the latitude . Coriolis acceleration is a function of the speed of the earth's rotation and the component of the added speed as defined above and it is given by the formula : a,=2wv cos A sine .

e

0 . 00346 cm./sec .2 In the above formula w=angular velocity of the earth, r=radius of the earth, V=velocity of lateral movement of the head, s=amplitude of lateral movement of the head, A=latitude, angle between movement of the head and plane of the earth's rotation . The accelerations affecting a bird in normal flight are summarized in the following table :

e =

Earth gravitation proper Acceleration inherent in flight Earth rotation Lateral head movements Coriolis acceleration

similar homing (or migrating) and sedentary species . This is, we think, the first attempt to link the activity of a nervous centre with the orientation sense, and to study objectively comparative nervous activity in relation to migration . Experimental Technique

Suppose a pigeon flies NS, making lateral movements of the head of amplitude 7°, at a speed of 20 cm ./sec ., 2 cm. being the distance to the labyrinths from the centre of the head, then Coriolis acceleration in the most favourable conditions may be calculated as amounting to 3 . 46 x l0-3 cm/sec., namely 3 x l0-8g. At the 45th parallel it will be reduced to 2 .45 x l0-6 cm ./sect. with a difference of 1 x 10-'g . for a distance of about 5,000 Km . Orientation with an exactness of 1 + Km . would require, when based on Coriolis acceleration alone, a sensitivity of the sensory apparatus greater than 10-9g. w 2r± cos A V sin = 3 .4±1 . 615

a' -}-2w

43

1 g about 5 g 0 .005 g 1-5 g 0 . 000003 g

As will be discussed more fully later, the main difficulty is the smallness of the stimuli required by the theory that Coriolis forces are perceptible to the animal . In conclusion one may say that none of the three possible mechanisms of orientation is to be considered as conclusively proved to be the only one in existence or as totally inadequate to explain part at least of the orientation capacity . In spite of the above considerations, we have tried a new approach starting ;from the assumption that, it being a fact that the bird actually directs itself in space, whatever the directing capacity is, it should be linked with cerebellar function, as the cerebellum is the main integrating centre for long range perception . We have therefore looked for differences in the cerebellar response to acceleration between

As a subject for study, we have chosen first the domestic pigeon (Columba livia) because some races of this species are provided with a very high capacity of orientation (e .g . the "homing pigeons") while other domestic breeding and rock varieties are today incapable of orientation. The choice of varieties of the same species with regard to this characteristic, which was probably primitively present in these birds and must have been lost by them under domestication, is important for the purpose of genetic experimentation. We have later studied two species of doves belonging to the same genus, one of which is migratory (Streptopelia turtur) and the other sedentary (S. risoria) . Between these species there are no natural hybrids and this facilitates the isolation of the two ecological types, in contrast with the problem of the pigeon for which, on account of their facility of hybridization, we must consider an entire range of possibilities in physiologic responses of individuals chosen from the natural rock population . During the experiment the animal is firmly attached to a rotating table : two electrodes consisting of silver wires insulated by solid bakelite to the point where only a fraction of a millimetre remains exposed, are inserted into the skull bone, opposite the projection of the sides of the cerebellar vermen (see Fig . I a and b) . In this position the bone is spongy and the electrodes remain solidly fixed in the diploe . At first the table was rotated by means of a release spring and a tooth-stop, giving a rotation angle of about 315°. Later on, motion was obtained by means of an electric motor provided with brake and friction so that various speeds of rotation could be obtained (Fig . 2) . Two accelerometers fixed at opposite extremities of the table allowed us to record simultaneously the tangential starting and stopping accelerations a s well as the centripetal acceleration acting during the rotation . The following values are recorded : for a radius of 26 . 5 cm. the centripetal acceleration is 0 . 02 g, the tangential starting and stopping acceleration is 2 g . Such tangential acceleration is not very important as the same results

44

ANIMAL BEHAVIOUR, X, 1-2

were obtained when the rotation was initiated slowly, reducing the tangential acceleration to a fraction of g. As standard experimental procedure we have settled on rotation by motor for 6 seconds at a velocity of one revolution per second . The impulses picked up by the electrodes were passed via silver and copper rotating brushes to a high discrimination amplifier provided with low frequency filters, and from this to the cathoderay oscilloscope . Results The electrocerebellogram at "rest" consists of biphasic oscillations at a frequency of 130 to 300 per second and an amplitude of about 30 microvolts . At the beginning of stimulation and during the starting tangential acceleration, there is a very great increase in impulse amplitude, up to 100-150 microvolts and the same thing happens during deceleration. During the rotation period, in which there is centripetal acceleration, an increase of amplitude is also recorded proportional to this acceleration . This response varies from subject to subject . While this response is alike for "non-homing pigeons" and "homing pigeons", an important and conspicuous difference appears when the rotation stops : in the non-homing pigeons the return to normal takes place after a fraction of a second ; in the homing pigeons, after an interval of about 1 second during which the electrocerebellogram appears .with normal amplitude, a series of after-discharges is recorded, consisting of a series of about 5-20 spindle-like oscillations, the duration of each spindle being roughly * to I second, the amplitude 3 to 6 times the normal, and the frequency 2 to 4 per second . This is well shown in Fig . 3 . After subjection to rotation the Turtle Dove (Streptopelia turtur), a migratory bird, and the Ring Dove (S. risoria), a sedentary bird, showed the same differences as appeared between homing and non-homing pigeons, (Fig . 3) . At this point a technical and physiological problem must be considered : the recording of cerebellar potentials may be confused by muscle action potentials due to movements of the neck muscles which occur during rotatory stimulation . There is in fact a "nucal nystagmus" (Mowrer, Van Eyck) together with a "quivering" of the muscles involved in the nystagmus . In order to test whether the muscle action potentials affect the cerebellar electrodes we have recorded simultaneously nucal muscle potentials and the

potentials from the cerebellar electrodes on a double beam oscillograph . Fig . 4 shows several recordings made in this way under different stimulating conditions, from which it is concluded that there is no electric diffusion of muscle potentials to frontal electrodes and that nucal and cerebellar recordings are independent although parallel responses are sometimes obtained . Moreover, cerebellar spindle-like "after potentials" can be recorded in deeply curarized animals. The potential observed may therefore be considered a true expression of cerebellar activity . Another question which has engaged our attention is the "constancy" in the type of response of particular individuals . Subjects which have given positive reactions to start with have always subsequently produced identical responses, and the same may be said for negative subjects and for those we have classified temporarily as "doubtful" . This is exceedingly important from a genetical point of view, indicating that the type of cerebellar response to rotatory stimulation is a "constant and personal" characteristic . In conclusion we can therefore affirm : (1) The cerebellar response to rotatory stimulations of homing and non-homing pigeons has been shown to differ . (2) The same difference appears between migratory and sedentary doves. (3) The response is typical and constant for every individual . (4) The response is specific and not influenced by interference from simultaneous muscular activity. The difference consists in the presence or absence of what we have called "Rhythmic spindle-shaped after-discharges", whose physiological significance we shall not discuss for the moment. The frequencies of these types of responses in the different races used, and in natural populations are summarized in Table I . From this point of view the experimental animals may be subdivided into the following categories : (1) homing pigeons, (2) rock pigeons (considered as "non-homing"), (3) selected breeds of pigeons that presumably are non-homing, (4) migratory doves, (5) sedentary doves . We must here make a few observations regarding the homogeneity of these groups . While the homing pigeons are all of a breed carefully selected over a long period of time for certain characteristics, particularly the capacity for

SCHREIBER et al. : CEREBELLAR PHYSIOLOGY IN BIRDS PLATE V

(A) (B) Fig . 1 . (a) Homing pigeon fixed on the rotatory table, with silver electrodes inserted into the spongy bone. (b) Radiography of a pigeon in same condition .

Fig . 2 . The rotatory table . Anirn . Behav., 10, 1-2

ANIMAL BEHAVIOUR, X, 1-2 PLATE VI

Fig. 3 . Records of pigeons and doves : A, B, C-nonhoming or sedentary birds ; D . E . F-homing or migratory birds.

SCHREIBER et al . : CEREBELLAR PHYSIOLOGY IN BIRDS PLATE VII

Fig . 4 . Joint of cerebellar (a) and muscular (b) activity : (1) apparent identity ; (II) after discharges without muscle response ; (III) "nucal quivering" not registering with cerebellar electrodes ; (IV) absence of after discharges and movements at the end of rotation, (V) mechanically provoked muscle movements that do not disturb cerebellar registration ; (VI) another case of complete "asincronia ."

N

ATl vs

gO IDTl UL

Fig . 5 . Typical patterns of a negative, a positive and a doubtful records of pigeons .

Anim . Behav ., 10, 1-2

SCHREIBER et al. : CEREBELLAR PHYSIOLOGY IN BIRDS

45

Table I. Frequencies of "After Discharges" in Some Races of Pigeons (C. livia) and in Two Species of Doves (S. turtur and S. risoria) . After discharges positive (+)

After discharges negative (-)

Doubtful (±) (supposed hybrid)

Total No .

%

No .

%

No .

97 . 3

0

0

1

2.7

Homing pigeons

37

36

Rock pigeons of Parma

88

6

Rock pigeons of Milan Piacentini

129

45

6.8 34 .87 52 . 94

69

78 . 8

13

14 .8

51

39 . 53

33

25 . 58

14

41 . 17

2

5 .88

34

18

Turtle doves (S. turtur)

6

6

100

0

0

0

0

Ring doves (S. risoria)

7

0

0

6

85

1

15

orientation, those indicated as "non-homing" belong to two categories unselected for this character. The rock pigeons owe their origin to the fact that once they have settled in a town square, they do not return to the place of origin . Some among them are certainly wild Columba livia . The "Piacentini" race, on the other hand, is a strain bred for the market especially for its plumpness and white colour, characteristics that are antagonistic to those of homing pigeons . However, these may have contributed originally to the stock, and, in any case, selection against orientation is not practised directly . Finally, the doves, which we have chosen for their systematic affinity with pigeons, present the further advantage of being of two different species, which do not naturally hybridize, and are therefore genetically homogeneous groups from this point of view : Streptopelia turtur is a migratory species, while S. risoria is non-migratory. The results of experiments on a total of about 300 individuals have been interpreted according to the following criterion : considered as "positive" are those animals which, on rotation for 6 seconds and after a well-determined latent period (0 . 6-0 . 8 seconds) after the end of rotation, present a series of rhythmical fusiform afterdischarges of not less than 6-10 wave-trains . Considered as "negative" are the individuals in which, under the same conditions, the oscillograph records return to normal amplitude at the end of rotation ; "doubtful" those individuals which have shown small or arhythmic afterdischarges (Fig. 5) . The term doubtful refers only to discrimination between the positive and negative types, without entering for the moment into their possible physiological or genetical significance, We remark only that the few F,

hybrids between the homing and the non-homing pigeons observed by us all present this type of response . On the basis of this criterion it can be seen that homing pigeons always react in a typical and absolutely constant way : among 37 homing pigeons, 36 are typically positive, and only one doubtful, none negative . The positive response is therefore characteristic of this variety and, as the same thing is true for individuals of the wild Columba livia, the form which must presumably be considered as "ancestral", the character in question may also be considered as "primitive" . The same thing is true of the two species of doves S. turtur and S. risoria . Non-homing rock pigeons were of two different origins, one from Parma and one from Milan, which show a difference in the respective frequencies of the different types of response . In Parma the great majority is negative and only 6 . 8 per cent. clearly positive, with 15 per cent . doubtful, while in Milan the percentage of positives is notably higher, reaching 35 per cent ., while about 40 per cent . are negative and 25 per cent . doubtful. The special "Piacentini" breeding strain shows about the same proportions (53 %+, 41 %-, 6%±) . Further work will take place on natural populations of other towns to study the frequency of appearance of the after-discharges . The results obtained in doves are interesting in regard to the problem which originated our research : although the number of individuals examined is small, the presence of after-discharges in migratory species appears constant, while the response is negative in the sedentary ones (Fig. 3) . Here also the character appears in

46

ANIMAL BEHAVIOUR, X, 1-2

a few individuals of the sedentary species, although in different proportions (85%-,

15%±) . In conclusion we can affirm that in Columba livia the various stocks show different percentages of individuals which react to rotatory stimulation with after-discharges ; the percentage is highest (100%) in homing pigeons, while it is very low in the rock populations . It is interesting to note how this differential characteristic which, among the varieties of the same species (Columba livia) is of value only if considered statistically, on account of interfecundity among the various strains, shows a greater difference between the two species of doves, in parallel with their different ecological behaviour . Conclusion and Programme of Further Researches As far as physiological significance is concerned, namely detection of geodetic forces, no conclusion can yet be given : the threshold to acceleration of the cerebellar cortex seems to be extremely low . A significant direct response to acceleration showed by increased amplitude of cerebellar potential during rotation has been recorded at even 0-005 g. The threshold for afterdischarges seems to be related also to the duration of the rotation. After 10 seconds of rotation to 0 . 1 g some after-discharges can still be recorded . Moreover temporal summation with a series of rotations in quick succession might further decrease the threshold for after-discharges. We are still very far from the difference of acceleration due to geodetic forces in relationship with flight . In this case the sensitivity of the cerebellum should be of the order of 0 .000005 g, that is a thousand times higher . The fact remains, as we have demonstrated, that there is a difference in response to acceleration between subjects of Columba and Streptopelia capable of orientation and those that have lost this capacity. We must in fact consider the presence of after-discharges as primitive in these birds, the character having been lost in the course of domestication . We consider that further investigations are necessary, on other migratory and sedentary pairs of species, to confirm our observations . We are now carrying on two series of researches in the Zoological Department of Parma University (1) the study of hybrids between homing and non-homing pigeons which are negative in respect of the presence of after-discharges, and the genetic behaviour of the character itself,

(2) Research on the offspring of individuals, isolated from natural rock populations, normally considered non-homing, but which show a positive response : we want to see whether or not these individuals, bred and trained in the same manner as the homing pigeons, are able to return to their nests . If this is so, and is supported by experiments showing no return of negative subjects, our initial supposition, namely that after-discharges are correlated with the orientation mechanism would be confirmed . This research programme will be supported by a grant from the U.S .A . Navy (Contract 3148-00) . REFERENCES Beecher, W . J . (1954) . On Coriolis Force and bird navigation . Sci . Monthly, 77, 27-31 . Griffin, D . R . (1944) . The sensory basis of bird navigation . Quarterly Rev . Biol., 19, 15-31 . Griffin, D . R . (1952). Bird navigation. Biol. Rev., 27, 359-391 . Griffin, D . R. (1955) . Bird Navigation. Chap . 6 in Recent Studies on Avian Biology . Univ. Illinois Press, Urbana . Hoffmann, K . (1958) . Repetition of an experiment on bird orientation . Nature, 181, 1435-1437 . Ising, G . (1946) . Die physikalische Moglichkeit eines tierischen orientierungssinnes auf Basis der Erdrotation . Arkiv for matematic, atsronomic . fysik., 32A, 1-23 . Kramer, G. (1952) . Experiments on bird orientation . Ibis, 94, 265-285 . Kramer, G . (1955) . Ein weiterer Versuch, die Orientierung von Brieftauben durch jahreszeitliche Anderung der Sonnenhole zu beeinflussen . Gleichzeitig eine Kritik der Theorie des Versuchs . J. f Ornith ., 96, 173-185 . Kramer, G., Pratt, J . C. & St. Paul, U . (1957) . Two direction experiments with homing pigeons and their bearing on the problem of goal orientation . Amer . Nat., 93, 37-48 . Matthews, G . V . T . (1953) . Sun navigation in homing pigeons. J. exper . Biol., 30, 243-267 . Matthews, G. V. T. (1955). An investigation on the "chromometer" factor in bird navigation . J. exper . Biol., 32, 39-58 . Mowrer, C . H . (1935). The nystagmic response of the pigeon to constant angular acceleration on liminal and supraliminal intensities . J. comp. Psychol., 19, 177-193 . Rawson, K . S . & Rawson, A . M . (1955) . The orientation of homing pigeons in relation to change in sun declination . JJ. Ornith ., 96, 168-172. Sauer, F . (1957). Die Sternnenorientierung nachtlich ziehender Grasmucken (Sylvia atricapilla, horin and curruca). Zeitsch . F. Tierpsychol., 14, 29-70. Scherrington, C. S . (1947). The integrative action of the nervous system . Cambridge : University Press. Schreiber, B., Gualtierotti, T . & Mainardi, D . (1955) . Effetto di accelerazioni centripete e tangenziali sui potenziali cerebellari del piccione normale e viaggiatore . Instituto Lombardo (Rend . Sc .), 88, 860-884 .

SCHREIBER et al. : CEREBELLAR PHYSIOLOGY IN BIRDS Schreiber, B ., Gualtierotti, T. & Mainardi, D . (1956) . Risposte differenziali del piccione viaggiatore e normale alla sollecitazione rotatoria . Boll. Zoo!., 23,17-31 . Schreiber, B ., Gualtierotti, T . & Mainardi, D . (1957a). Risposte elettriche cerebellari differenziali a sollecitazioni rotatorie in tortore migranti e stazionarie . Istituto Lombardo (Rend. Sc.), 91, 664-671 . Schreiber, B ., Gualtierotti, T . & Mainardi, D . (1957b) . Attivitb cerebellare ed elettromiogramma nucale nel piccione viaggiatore . Istituto Lombardo (Rend. Sc.), B. 92, 187-190. Schreiber, B ., Gualtierotti, T . & Mainardi, D . (1958) . Some problems of cerebellar physiology in migratory and non-migratory birds . XVth Internat . Congr . Zool., Sect . XI, Paper 19 .

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Schreiber, B . & Mainardi, D. (1958) . Sulla costanza delle risposte elettrocerebellari a sollecitazioni rotatorie ripetute a distanza di tempo in Columba livia . (in press) . Van Eyck, M . V . (1956) . Etude analytique du ph6nom8ns de compensation vestibulaire apr6s labyrinthectomie unilat6rale. Acta Oto-Laring., 46, 279-284 . Yeagley, H . I. (1947) . A preliminary study of a physical basis of bird navigation . J. appl . Physics ., 18, 1035-1063 . Yeagley, H . I. (1950) . A preliminary study of a physical basis of bird navigation. Part II . J. app!. Physics., 21, 746-760. Accepted for publication 20th July, 1961 .