slower than in free-ranging toads (Gittins et al. 1980; van Gelder et al. 1986). Larger toads dis- played higher locomotory activity suggesting that the reduced ...
Journal of
J Comp Physiol A (1987) 161:715-727
Sensory, Neural,
Physiology Physiology A ~a,~,,
9 Sprinoer-Verlag 1987
Orientation behaviour of toads ( Bufo bufo) displaced from the breeding site Ulrich Sinsch* Max-Planck-Institut ffir Verhaltensphysiologie, Abteilung Mittelstaedt, D-8131 Seewiesen, Federal Republic of Germany Accepted April 27, t987
Summary. The sensory basis and spatial range of orientation to the breeding site were studied in the toad Bufo bufo, during two breeding seasons. Toads were displaced passively from their breeding pond and fitted with a tracking device to record the path of migration in individuals. The directional choice and the straightness of trails after release were used to quantify the effect of experimental treatments. In both years, control (untreated) toads headed to the breeding site with the same precision at all release sites. The initial orientation of toads blinded by opaque tape over their eyes did not differ from controls, but the return paths were not as direct. The directional choice of anosmic toads was apparently random, however, individuals followed a straight path in a chosen direction. Anosmic toads also blinded were completely disoriented, moving in cycloid trails. Bar magnets glued to the head caused an increase in dispersion of toads. However, in some individual releases a directional bias without increased dispersion was observed. Sky conditions (clear or overcast) did not influence the initial orientation or the dispersion of toads. Nevertheless, the breeding site component was significantly correlated with wind direction in relation to the breeding site. Wind blowing from the breeding site improved the initial orientation, whereas wind from the opposite direction reduced the breeding site component. The spatial range for the ability to relocate the breeding pond after displacement exceeded 3 kin, but the time taken to select the correct direction increased with the displacement distance. The results indicate that after displacement the initial orientation of B. bufo is based mainly on olfactory and magnetic cues, with visual control of straightness. * Present address: Zoologisches Institut der Universitfit Bonn, Poppelsdorfer SchloB, D-5300 Bonn 1, FRG
Introduction Many species of the Amphibia relocate their breeding site or home range following passive displacement (for review see Ferguson 1971; Able 1981; Adler/982), and some of the orientation cues used during the return journey have been identified in American toads of the genus Bufo spp. Evidence supporting the use of celestial cues has been found in B. woodhousei fowleri (Ferguson and Landreth 1966) and immature B. boreas (Tracy 1971), but homing was not impeded in blind adult B. boreas (Tracy and Dole 1969), B. americanus (Dole 1972), B. valliceps (Grubb 1970). However, the lack of vision combined with anosmia causes disorientation in B. valliceps (Grubb 1970). Anosmic B. boreas (Tracy and Dole 1969) are disoriented, but the initial orientation of anosmic B. arnericanus (Dole 1972) and B. valliceps (Grubb 1970) is not affected. Auditory cues such as conspecific breeding choruses biasses the direction of migrating B. woodhouseifowleri (Ferguson and Landreth 1966). These data demonstrate clearly that a variety of sensory information is involved in the orientation of toads. Olfactory information appears to be more important for toad species with long distance breeding migration such as B. boreas. Since the other Bufo species which have been studied live near their breeding ponds, it is of interest to analyze the orientation cues of another long distance migrating species. If a relationship exists between the spatial range of the natural breeding migration and the directional stimuli used for orientation, then the cues used by this species should be more comparable to B. boreas than other species. The common European toad B. bufo migrates over distance of 3 km for breeding (Heusser 1968). Toads of a given population return to the same breeding site in spring each year and spawn syn-
716
chronously within a period of a few days. Although extensive field studies have dealt with the breeding migration of B. bufo (Jungfer 1943; EiblEibesfeldt 1950; Heusser 1967, 1969), little is known about orientation cues. In the present study, the initial orientation behaviour of toads displaced from their breeding pond was quantified using a tracking device (as decribed in Dole 1965). Circular statistics were applied to the directional data as summarized in Batschelet (1981). The aims of this study were to; (1) identify the orientation cues of B. bufo; (2) evaluate the influence of meteorological factors on the directional choice; (3) estimate the spatial range of the relocation ability to the breeding pond. Materials and methods Study area. The study was conducted in the sub-montane region of Bavaria near Seewiesen, West Germany at altitudes between 681-718 m. The study population of B. bufo inhabited an area of about 6 km 2 mainly covered with pine forest and pasture. The breeding pond (ca. 3200 m 2 of water surface) was located in the northeastern section of this area. Toads spawned always near the northern shore of this pond.
Generalproeedures. All toads which participated in the displacement experiments were captured daily between 2 pm-4 pm at their breeding site. This choice ensured that captured toads were disposed for reproduction willing to return to the breeding site and the goal was precisely known. Displacement of the toads to the release site occurred passively in closed opaque plastic containers (vol. 10 1). The release sites were never approached from the direction of the breeding site to minimize visual and olfactory en route information. At the release site, a tracking device weighing 6.5 g was fastened to the toads' back consisting of a sewing machine bobbin in a holder. The holder was tied around the waist with elastic tape and the bobbin contained about 60 m of thread (for further details see Dole 1965). Only toads weighing at least 30 g were trailed to minimize any side effects on behaviour from the additional weight. A total of 432 toads (331 males, 101 females) were used in the displacement experiments (each individual was displaced once). The effect of each experimental treatment was tested at four release sites (direction and distance relative to the breeding site: 188 ~ 155 m; 090 ~ 200 m; 024 ~ 145 m; 270 ~ 185 m). The spatial range of correct orientation behaviour to the breeding site was tested along the north/south axis at two release sites each, at distances of 150 m, 250 m, 500 m and 1000 m. In addition, 10 toads from a second population (Aschering, 3 km southeast of Seewiesen, West Germany) were displaced a distance of 3000 m from their breeding pond (direction towards Aschering breeding pond - 140~ This release site was situated 185 m east of the Seewiesen pond (direction towards Seewiesen pond = 265~ There was no direct line of sight to the breeding pond from any of the release sites. At each displacement site, 20 toads were released simultaneously, chosen randomly for each experimental treatment. There was no indication of a sex bias in the results, however, males and females were used in equal proportions. The loose end of the thread was attached to a small stake at the release point and each toad was placed in a small hole in forest soil and covered with leaves. As toads of this population migrated
U. Sinsch: Orientation of displaced Bufo bufo exclusively at night, none of the concealed animals left the release site until sunset (7.20 pm-7.42 pm). On the morning following release two measurements were made for each individual: 1) locomotory activity as judged by the metres of thread unwound during the night, 2) position of the animal in polar coordinates, i.e. azimuth angle and line of sight distance relative to the release point. To avoid confusion between toads returning to the breeding pond and those simply looking for a better hiding place, only individuals which had migrated at least 5 m of locomotory activity were included in the statistical analysis. Every set of experimental treatments was repeated once at four symmetrical release sites, always testing orientation behaviour at opposite release site simultaneously. All experiments were conducted in the pre-spawning period of 1985 (2.4-11.4) and agai n in 1986 (7.4.-24.4).
Experimental treatments. In four types of experimental treatment the role of vision, olfaction and magneto-reception were examined for the initial orientation after displacement. All treatments were applied directly after capture at the breeding site. The first group of toads (BLIND) was deprived of visual landmarks or star positions by covering the eyes with opaque adhesive tape. This tape was fixed to the skin with cyano-acrylate glue to prevent loss during the experiment. The nostrils were left uncovered. Despite this treatment toads remained nocturnal, possibly due to extra-ocular light perception (Adler 1976) or circadian rhythms. As a control, toads had the opaque tape glued to the skin without covering the eyes. A second group of toads (ANOSM) was deprived of olfaction by filling the nostrils with cyano-acrylate glue, completely covering the olfactory epithelia. The ability to smell was tested by application of amylacetate to the skin near the nostrils. Untreated toads tried to remove the fluid by wiping with the forelimbs, but anosmic toads did not show such a reaction. The third group of toads (ANOSM + BLIND) was treated by a combination of the above described procedures, depriving them of both olfaction and vision. In this case, control toads were equipped with tape as in the BLIND group. The fourth group of toads (MAG) was treated by gluing round bar magnets (diameter 8 mm, height 2 mm, mass 0.95 g, magnetic field strength ca. 75 G at 1 cm) to the head. In one sub-group magnetic north of the artificial field pointed distally (upwards, M A G N), in the second sub-group it pointed proximally (downwards, M A G S). As a control, toads were fitted with a brass bar of the same size and mass as the magnets. The influence of meteorological conditions such as the sky and the wind direction were included in the analysis of data from releases of untreated toads at the four symmetrical sites. Celestial cues for clear nights (visibility of stars at least to midnight) were distinguished from overcast nights (complete cloud cover during the night), but orientation behaviour was not analyzed on nights with changing sky conditions. Wind velocity and direction were measured at the release sites during the main period of locomotory activity (9 pm-10 pm). Further, only nights with wind strength of 1 m/s or more from a stable direction were considered in analysis. Over the whole pre-spawning period of 1986, the time course of migration for 10 toads of the Aschering population was studied after a displacement of 3000 m. The thread supply for these toads was checked every morning and replaced, if necessary. After recording nocturnal movements, all toads were collected and maintained in large aquaria for at least three weeks. Many toads moulted during the time in the aquaria, and the short and long term effects of cyano-acrylate on the skin were studied, but no moulted individuals displayed any sign of in-
U. Sinsch: Orientation of displaced Bufo bufo
717
Table 1. Mean locomotory activity of control toads and treated individuals displaced from the breeding site in 1985 and 1986 Day of release 1985 : 04.04. 05.04. 06.04. 08.04. 09.04. 10.04. 11.04. 1986 : 08.04. 09.04. 15.04. 21.04. 22.04.
Mean locomotory activity (m/d)
Tests for correlation were performed by Mardia's circular-linear rank correlation test or linear correlation analysis. All tests of circular statistics are described in Batschelet (1981).
Significance
Control toads
Treated toads
11.5 23.4 27.9 40.6 25.5 38.6 35.6 15.2 14.2
10.0 26.0 29.4 29.7 26.3 28.9 37.0 14.5 9.0
(Blind) (Blind) (Mag N) (Ano + Bli) (Anosm) (Anosm) (Mag S) (Mag S) (Mag N)
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) Fig. 2A,B. Initial orientation o f control a n d blinded toads. A D i s t r i b u t i o n o f t o a d s at 4 sites one d a y after release. Filled s y m b o l s represent controls (dots 1985, triangles 1986), o p e n s y m b o l s are treated individuals. M e a n vectors o f the releases are s h o w n within t h e circles (? = 50 m equals radius). T h e m e a n l o c o m o t o r y activity o f the t o a d s d u r i n g each release is s u m m a r i z e d in Table I. B S t a n d a r d i z e d m e a n vectors o f the individual releases with respect to the breeding site ( f = 1 equals radius). T h e second order m e a n o f these vectors c o r r e s p o n d s to the centres o f the 9 5 % confidence ellipses (numerical values in Table 3). T h e difference C O N T R / B L I N D is n o t significant ( P > 0 . 0 5 ) . T h e m e a n s t r a i g h t n e s s o f m i g r a t i o n is significantly greater in the controls (P < 0.01)
720
U. Sinsch: Orientation of displaced Bufo bufo
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B Fig. 3A, B. Initial orientation of control and anosmic toads. Presentation of the data as in Fig. 2. A Distribution of toads at 4 sites, one day after release. B Standardized mean vectors of individual releases with respect to the breeding site and their second order means (nurnerical values in Table 3). The difference C O N T R / A N O S M is significant (P < 0.01). The mean straightness of migration is not significantly different between the two groups (P > 0.05)
affect the straightness of migration or locomotory activity (P>0.05), but strongly influenced the initial orientation towards the breeding site (Figs. 5, 6). In both groups treated this way, 4 out of 7 releases had no specific direction because they dispersed, whereas all 14 control releases approached the breeding site directly. In three releases toads oriented to the left of the breeding site direction, if south of the artificial magnetic field pointed distally (upwards). If north of the artificial field pointed distally, in two releases the toads headed towards the breeding site and in one release to the right of the correct direction. In both experi-
mental groups the second order means of the controls differed significantly from the treated toads, (MAG S, P0.05). B Histograms of the standardized mean vector length f of individual releases. Triangles indicate median values of the distributions which are not significantly different (P>0.05)
tial disorientation and reduced locomotory activity. It is important to consider whether these effects are due to anosmia itself or some non-specific reactions. Injection of formalin into the nostrils of B. valliceps (Grubb 1970) or the use of cyano-acrylate glue in B. spinulosus (Sinsch, in prep.) does not modify orientation towards the breeding site or locomotory activity, and both species are short range migrators, However, B. boreas, rendered anosmic by section of the olfactory nerve, are disoriented, compared to the normal orientation behaviour of sham-operated individuals (Tracy and Dole 1969). These results suggest that disorientation in B. bufo is likely to be caused specifically by anosmia, but anosmic B. bufo are still capable of returning successfully to the breeding pond (Heusser 1969). A possible explanation for these apparently opposite results may be due to the methods of monitoring different parts of the return journey. In the present study initial orientation represents exclusively the first part of the journey, whereas the recapture rate (Heusser 1969) is an integrated measure of the complete return path. Disturbance of the initial orientation does not imply complete inability to return to the breeding site, rather that the initial part of the return journey normally de-
U. Sinsch: Orientation of displaced BuJo bufo
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Fig. 8A, B. Initial orientation of control toads towards the breeding site under varying wind conditions. Symbols as in Fig. 1. A Influence of wind direction on the mean breeding site component Ob~of the individual releases. The circular correlation is not significant between the parameters (P>0.05). B Influence of relative wind direction on the mean breeding site component. 0 ~ wind blowing from breeding site to release site; 180 ~, opposing wind direction. Linear correlation between parameters is significant ( P < 0.001)
pends on olfactory cues. The recapture of anosmic toads at the breeding pond suggests that there are alternative cues which can replace olfaction for the remainder of the return journey. Therefore, both sets of results indicate that B. bufo uses a multisensory mode of orientation, as in many amphibian species (Adler 1982). Olfaction is not absolutely essential for homing, but olfactory cues for orientation during the initial part of the return journey appears to be rather important for B. bufo and B. boreas which migrate over large distances. There is a question of whether toad species which live near the breeding ponds rely on olfactory cues ? In B. valliceps, visual cues can supplant olfactory cues, but this species is disoriented only when both cues are eliminated (Grubb 1970); similar results have been found in B. americanus (Dole 1972). So there is a difference between the modes of orientation for toad species using short and long distance migration on the basis of the hierarchical position of
Fig. 9. Directional choice of control toads displaced at various distances along the N/S axis. Each dot represents bearing of a single toad; arrow within the circle (F= 1 equals radius) is the mean vector of distribution. Numerical values of mean direction (a), vector length ( ~ and breeding site component (E) are also given. Levels of statistical significance: P > 0.05 (--), P
