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Lateralized prey-catching responses in the cane toad, Bufo marinus: analysis of complex visual stimuli
ANIMAL BEHAVIOUR, 2004, 68, 767–775 doi:10.1016/j.anbehav.2003.12.014
Lateralized prey-catching responses in the cane toad, Bufo marinus: analysis of complex visual stimuli ANDREW ROBINS & LESLEY J . ROGERS
Centre for Neuroscience and Animal Behaviour, School of Biological, Biomedical and Molecular Sciences, University of New England (Received 12 August 2003; initial acceptance 1 October 2003; final acceptance 29 December 2003; published online 25 August 2004; MS. number: 7822)
We tested the responses of Bufo marinus to prey stimuli of varying visual complexity that were moved around the toads in either a clockwise or anticlockwise direction at 1.7 revolutions/min. Predatory responses directed at prey resembling an insect were frequent when the model insect moved clockwise across the visual midline into the right visual hemifield. In contrast, the toads tended to ignore such stimuli when they moved anticlockwise across the midline into the left hemifield. No such lateralization was found when a rectangular strip moved along its longest axis was presented in a similar way. The toads also directed more responses towards the latter stimulus than towards the insect prey. Hence, the results suggest that lateralized predatory responses occur for considered decisions on whether or not to respond to complex insect-like stimuli, but not for decisions on comparatively simple stimuli. We discuss similarities between the lateralized feeding responses of B. marinus and those of avian species, as support for the hypothesis that lateralized brain function in tetrapods may have arisen from a common lateralized ancestor. 2004 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd.
Vallortigara et al. (1998) found that European common toads, Bufo bufo, European green toads, B. viridis, and South American cane toads, B. marinus, prefer to direct strikes with their tongue at prey in the right visual hemifield. The toads showed this preference when tested using an automated prey stimulus: a wormlike, live Galleria mellonella larva in the case of B. bufo and B. viridis and a live adult cricket (Acheta sp.) in the case of B. marinus. However, although B. marinus showed a preference to tongue-strike at the prey stimulus as it moved through the right hemifield, the magnitude of responsiveness and the strength of preference were not as strong as in the other two toad species (Vallortigara et al. 1998). Because the result might have been related to the difference in the physical characteristics of the prey stimuli used to test the respective species of toad, in this study we used B. marinus to test the hypothesis that differences in Correspondence and present address: A. Robins, Department of Biological and Physical Sciences, Faculty of Sciences, University of Southern Queensland, QLD 4350, Australia (email: robins@usq. edu.au). L. J. Rogers is at the Centre for Neuroscience and Animal Behaviour, School of Biological, Biomedical and Molecular Sciences, University of New England, NSW 2351, Australia. 0003–3472/03/$30.00/0
the visual complexity of the prey may influence the degree of lateralization of the predatory responses. The optic fibres in the anuran visual system decussate almost completely, so input received by either eye is processed mainly by neural circuits in the opposite side of the brain, as in birds, reptiles, fish and mammals to laterally placed eyes (Rogers 2002b). In B. marinus, 96% of the retinal fibres cross to the contralateral side of the brain, with 85% of these projecting into the contralateral optic tectum (a mesencephalic structure analogous to the mammalian superior colliculus: Wye Dvorak et al. 1992). A preference to approach and strike at prey in the right visual hemifield might, therefore, reflect preferential control by circuits in the left tectum or higher visual centres in the left forebrain (telencephalon). Such a specialization would be consistent with the general direction of lateralization observed for feeding responses in other vertebrates (Rogers 2002a). However, the right-biased strikes at prey reported by Vallortigara et al. (1998) were within the toad’s binocular visual field, which means that visual inputs would go to both sides of the brain. Toads possess a wide binocular overlap in the horizontal plane (our perimetric analysis of nine B. marinus revealed the mean angle of overlap in the horizontal plane at eye
767 2004 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd.
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level G SEM to be 57.5 G 2.2 , corresponding with the published finding of 56 : Fite 1973). This wide binocular overlap precludes a clear determination of which side of the brain may be assuming dominance for controlling the predatory response, but dominance of neural circuits on the left side of the brain seems to be the most parsimonious explanation for the right-hemifield preference. Summarizing a broad spectrum of studies of feeding behaviour in vertebrates ranging from fish to mammals, Andrew & Rogers (2002) concluded that lateralized feeding responses reveal left-hemisphere specialization for focused attention and considered responses to appropriate visual stimuli, as needed in selecting food items against a distracting background. For example, a right-eye (and left-hemisphere) preference for pecking at grains scattered among pebbles has been found in chicks, Gallus gallus domesticus (e.g. Rogers & Anson 1979; Andrew et al. 1982; ¨ ntu ¨ rku ¨n Deng & Rogers 1997), pigeons, Columba livia (Gu ¨ ntu ¨ rku ¨ n & Kesh 1987) and zebra finches, 1985; Gu Taeniopygia guttata (Alonso 1998). This form of lateralization was revealed by testing the birds monocularly: when using the right eye, they were able to discriminate food grains from pebbles of similar shape, size and colour but when using the left eye they pecked randomly at pebbles and grains. Feeding responses in the chick are thought to be controlled by the left hemisphere owing to its specialization for categorizing ‘food’ from ‘nonfood’ stimuli according to selected properties (Andrew 1991; Rogers 2000). It has not been possible to test toads on a task identical to that used for birds because the prey must move to elicit feeding responses in toads (Ingle 1976a). However, standardized methods for testing predatory responses in toads have been developed (e.g. Ewert 1970; Ewert & Ingle 1971; Ingle 1976a; Burghagen & Ewert 1982). Individual toads are contained within a transparent cylinder that is, continuously and mechanically, encircled by a ‘dummy’ prey item (so termed as it offers no reinforcement ¨ sser & Gru ¨ sserto continue predatory responses: Gru Cornehls 1976; Ingle 1976a; Burghagen & Ewert 1982; Fig. 1). The dummy prey can encircle the toad in either a clockwise or anticlockwise direction at a constant rate. When the toad is placed in the test apparatus, the dummy prey is seen to travel from left to right (clockwise) or right to left (anticlockwise) visual fields, respectively. Probably the most widely known finding from using the test apparatus is that simple stimuli are processed by the toad’s visual system into ‘worm’ or ‘antiworm’ categories (Ewert 1984). Rectangular objects moving in the direction of the longitudinal axis are responded to as ‘worm’ stimuli and elicit approach and tongue-striking behaviour (Ewert & Ingle 1971; Ewert et al. 1994). The same shape moving perpendicular to the longitudinal axis (i.e. upright) is responded to as a threatening ‘antiworm’ that elicits escape or defensive behaviour (Ingle 1976a; Ewert 1997). The tectum of the toad actively filters the continuous flow of information from the retina and directs predatory behaviour to all moving stimuli with worm-like properties (Ingle 1976b; Ewert 1997). Habituation, association, memory consolidation and other higher forms of visual processing are carried out in the toad’s telencephalon,
Figure 1. The test apparatus. The toad was contained within a transparent plastic cylinder (12 cm in diameter, 30 cm high) on a circular and immobile platform (15 cm in diameter). A freely rotating wheel beneath the platform was driven by a 240-V motor (not shown), and rotated in either an anticlockwise or clockwise direction at a constant rate of 1.7 revolutions/min. The rotating wheel supported the dummy prey (the model insect used in experiment 3 is shown; the grey arrow shows the prey moving clockwise). Dummy prey used in the study are illustrated in relative scale: (a) live cricket (mean length 25 mm: experiment 1), (b) horizontal strip (20 mm long ! 2.5 mm high: experiment 2) (c) plastic model insect (body 18 mm long: experiment 3).
which inhibits any inappropriate predatory behaviour directed by the tectum (Ewert & Finkensta¨dt 1987; Muzio et al. 1993, 1994; Ewert et al. 1994). Although the analyses of simple and comparatively complex visual stimuli are known to be carried out at different levels of the visual system of toads, the possibility that such processing may be lateralized has not been considered previously. We hypothesized that prey types of varying visual complexity would reveal corresponding differences in predatory lateralization in B. marinus. Specifically, we predicted that toads tested with the simplest dummy prey (a black horizontal rectangular strip) might show a different degree of lateralized behaviour from those tested with insect-like dummy prey (either model or live insects), since the latter requires additional processing from higher levels of the anuran visual system. We also tested whether the presence of live prey, available within the cylinder for the toads to feed on while being tested, might promote complex decision making and lateralization of predatory responses.
METHODS We used 59 mature B. marinus toads in a series of experiments of lateralized visual processing. Obtained from the Atherton tablelands in northern Australia by a professional collector (Peter Kraus), the toads were group housed in
ROBINS & ROGERS: LATERALIZED PREDATORY RESPONSES
two large tanks at the University of New England, Australia. Both housing tanks were constructed from rust-resistant metal sheeting (80 ! 80 cm and 58 cm high), inside which the environment was enriched with areas for swimming, burrowing, basking and hiding (Robins 1997). Twice each day, water was showered automatically into the housing tanks. The housing room conditions reproduced the tropical/subtropical environment preferred by B. marinus (Duellman & Trueb 1986), with the room temperature maintained between 23 and 27 C and the relative humidity between 70 and 85%. Lighting was provided in a 12:12 h photoperiod, using ultraviolet fluorescent tubes. We fed the toads at least three times a week on mealworms, compost worms, or crickets. Nearly half (26) the toads used in this study had been held in captivity for at least a year in the housing conditions described, since they were part of a long-term study of visual and motor lateralization in anuran species. The procedures for both housing and testing the toads were approved by the University of New England’s Animal Ethics Committee. The design specifications for the test apparatus used in the following experiments were based on previous research conducted with B. marinus (Wachowitz & Ewert 1996; Fig. 1). In each of three separate experiments using the apparatus, the dummy prey (live cricket, horizontal black strip, or black plastic model insect; Fig. 1) was moved around the toad at 1.7 revolutions/min, or 10 of rotation/s. At this angular velocity, B. marinus responds (by turning) more consistently and with less habituation than when tested with dummy prey moving at other speeds (Wachowitz & Ewert 1996). The dummy prey was randomly assigned to rotate about the toad in either a clockwise or anticlockwise direction. The direction of movement of the dummy prey was not changed during a trial, and was set in motion before the toad was placed in the cylinder. Trials commenced immediately after the toad was placed in the test cylinder. A larger white cylinder (40 cm in diameter) contained the entire apparatus and prevented the toad from attending to extraneous stimuli. Even illumination was provided with a 60-W incandescent lamp, supplementing the fluorescent room lighting. The experiments were recorded from directly overhead with a video camera. The videotapes were analysed using frame-by-frame playback. The predatory motor acts of particular interest to this study were the turning and striking responses. We scored ‘turns’ as rotational movements of the toad’s body leftwards or rightwards to bring the prey closer to the centre of the binocular field. Predatory turns could be distinguished from other types of turns (avoidance or search behaviour) because they involved lowering the head to the level of the prey, were often repeated in pursuit of the prey stimulus, and were often accompanied by tongue striking. Tongue-strikes (hereafter referred to as ‘strikes’) were scored as the ballistic projection of the tongue at the prey, most often to the left or right of the visual midline. Strikes could also involve rapid body movement (a lunge) as part of an uninterrupted motor sequence. Here we considered the turning and striking data together as a single ‘predatory response’ measure, as used by
other authors (e.g. Burghagen & Ewert 1982; Ewert et al. 1994). We counted turns and strikes at the dummy prey in real time from the overhead videotape, using software that also calculated latency and duration (Etholog version 2.2.5, E. B. Ottoni, Department of Experimental Psychology, University of Sa˜o Paulo, Brazil). In rare cases (!5%) in which midline strikes were observed, each strike was reanalysed in a separate viewing, using frame-by-frame analysis of the videotape. Such strikes were scored as ‘left’ or ‘right’ according to the location of the prey stimulus (with regard to the midline) on the frame at which the strike was initiated. We refer to the preference to respond to prey located either in the left or right visual hemifield, regardless of the direction (clockwise or anticlockwise) in which the prey travelled about the toad, as ‘overall lateralization’. We use the term ‘direction lateralization’ to refer to lateralized predatory responses for prey in either visual hemifield, but only for toads responding to dummy prey moving in either the clockwise or the anticlockwise direction, and not both directions. Wilcoxon signed-ranks tests (two-tailed, adjusted for ties) were used to compare the differences in the number of predatory responses directed at prey in the left and right visual hemifields, considering the treatment groups together (overall lateralization) and then separately (direction lateralization). Mann–Whitney U tests (two-tailed, adjusted for ties) were used to compare the number of behavioural events or their latencies (i.e. number of turns, strikes, latency to initiate predatory responses) between the clockwise and anticlockwise treatment groups (Zar 1996). Of the toads tested in experiment 2, 18 had been tested previously in experiment 1. Fifteen toads from experiment 2 were retested in experiment 3. Owing to the random assignation of anticlockwise or clockwise test conditions, the differences in procedures in each experiment, and the time between experiments (6 weeks separated experiments 1 and 2, 4 weeks separated experiments 2 and 3), we considered the effect of retesting in the toads to be negligible. In experiment 1, two groups of 13 toads were tested with the dummy prey moving in either a clockwise or anticlockwise direction. The dummy prey was a live cricket (2–2.5 cm long) tethered to a vertical wire arm (4 cm high) with white cotton thread (not shown in Fig. 1). The movement of the tethered cricket could not be automated fully as, in addition to walking naturally, the cricket could periodically leap or be dragged short distances along the test platform. A fresh cricket was used as the dummy prey after every four trials to reduce the effects of fatigue. Another live and freely moving cricket was contained within the cylinder at the start of the trial, and served to facilitate the toads’ predatory responses to the dummy prey (e.g. Ingle 1976a). Injured and uneaten prey were fed to toads already returned to their home tanks after testing. Although the trials in this experiment were conducted over 5 min, they were terminated if the toad managed to escape from the apparatus, by climbing. We used 19 toads in experiment 2, randomly divided into groups of 9 and 10 for anticlockwise and clockwise presentations of the dummy prey, respectively. The
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ANIMAL BEHAVIOUR, 68, 4
dummy prey used was a small rectangular strip of black card (20 mm long ! 2.5 mm high; Fig. 1). This ‘configuration’ of the prey stimulus, moving in the direction of its longitudinal axis in a ‘worm-like’ manner, consistently elicited predator responses in other research using toads (e.g. Burghagen & Ewert 1982; Ewert et al. 1994; Wachowitz & Ewert 1996). We included the horizontal black strip in the study as the simplest prey model possible (two-dimensional and featureless). No live prey was introduced inside the cylinder to promote predatory responsiveness. Trials were run for 5 min. We prevented the toad escaping by lightly moistening the inner walls of the test cylinder with water. For experiment 3, we used 47 toads randomly divided into ‘clockwise’ and ‘anticlockwise’ groups of 23 and 24, respectively. A black plastic model of an insect (body 18 mm long and 6 mm wide, with legs splayed to 20 mm wide; Fig. 1) was used as the prey stimulus. The insect had configurational elements similar to both the tethered cricket (similar length, six appendages) and the black strip (colour, moved at a regular and constant rate) used in the previous experiments. When in motion, the insect was oriented with the head and body segments in the direction appropriate to natural forward movement. In contrast to experiments 1 and 2, trials in experiment 3 were run over 10 min. The first half (5 min) of the trial was run to match the conditions used in the horizontal strip experiment (experiment 2), with no live prey serving for predatory facilitation. After 5 min and as the movement of the dummy prey continued unchanged, we dropped five live mealworm prey (Tenebrio molitor larvae) into the cylinder containing the toad. We added live prey for two reasons. The main purpose was to determine what role, if any, the availability of consumable prey had on lateralized predatory responses directed at the dummy prey. The second was to examine how general predatory choices (between the worm-like live prey and dummy insect prey) may differ in toads tested with the dummy prey moving in either clockwise or anticlockwise directions. In this respect the paradigm differed from the tethered cricket experiment (experiment 1), in which both the consumable prey and the dummy prey were the same prey type. Each trial continued for the full 10 min with predatory behaviour directed towards both the live prey and dummy prey being scored. As in experiment 2, a light film of water sprayed on the inside walls of the test cylinder prevented the toads escaping.
predatory responses to be directed at the dummy prey within either left or right visual hemifield (right hemifield: XGSEM ¼ 2:7G0:7 responses; left hemifield: 1.3 G 0.4 responses; Wilcoxon signed-ranks test: Z Z 1.71, N Z 26, P Z 0.087). However, toads in the clockwise group directed significantly more responses at the dummy prey than did toads in the anticlockwise group (Mann–Whitney U test: U Z 122.50, N1 Z N2 Z 13, P Z 0.048; Fig. 2). This direction lateralization related to significantly more responses directed to dummy prey passing the visual midline (and into the right visual hemifield) by toads in the clockwise group, than responses to the dummy prey passing the visual midline and into the left visual hemisphere in the anticlockwise group (Mann–Whitney U test: U Z 122.50, N1 Z N2 Z 13, P Z 0.046; Fig. 2, postmidline). In contrast, there was no difference between toads in the two groups for the number of responses directed at dummy prey approaching the visual midline (Mann–Whitney U test: U Z 87.00, N1 Z N2 Z 13, P Z 0.89; Fig. 2, premidline). In the clockwise group, significantly more responses were directed at the dummy prey in the postmidline area (right hemifield) than in the premidline area (left hemifield; Wilcoxon signed-ranks test: T Z 64.5, N Z 13, P Z 0.005). However, the number of responses directed at the dummy prey as it moved from the premidline to postmidline areas of the visual field was not significantly different in the anticlockwise group (T Z 13.5, N Z 13, P Z 0.10; Fig. 2).
Experiment 2 Between 8 and 17 times more predatory responses were directed at the dummy prey in experiment 2 than in experiment 1 (cf. scores in Figs 2, 3). However, no overall
Mean number of responses
770
Premidline NS 8
Experiment 1 There was no significant difference between the trial durations (time before escaping the test cylinder) for toads in either treatment group (clockwise group: XGSEM ¼ 232:23G16:5 s; anticlockwise group: 246.62 G 14.7 s; Mann–Whitney U test: U Z 95.5, N1 Z N2 Z 13, P Z 0.57). With the results from the clockwise and anticlockwise groups combined, there was no overall preference for the
Postmidline * *
6 5 4
NS
3 2 1 0
RESULTS
*
7
L+R Responses
L R Clockwise
L R Anticlockwise
Figure 2. Mean C SEM numbers of predatory responses directed at a tethered cricket (dummy prey) moving in a clockwise or anticlockwise direction. Results are from two groups of 13 toads tested in either clockwise (,) or anticlockwise (-) directions. ‘L’ and ‘R’ indicate left and right hemifields, respectively. The total number of predatory responses is represented by ‘L C R Responses’, combining those elicited from the dummy prey in both left and right hemifields. See text for details. *P ! 0.05 (Mann–Whitney U tests for premidline and postmidline comparisons between groups, Wilcoxon signed-ranks tests for left and right-hemifield comparisons within groups).
ROBINS & ROGERS: LATERALIZED PREDATORY RESPONSES
Mean number of responses
Premidline NS
70
Postmidline NS
NS *
*
L R Clockwise
L R Anticlockwise
60 50 40 30 20 10 0
L+R Responses
responses; Wilcoxon signed-ranks test: Z Z 1.07, N Z 47, P Z 0.29). When considering the general responsiveness of toads from either treatment group in the first half of the trials, no significant difference was found between the number of responses directed to the dummy prey moving in either the clockwise or anticlockwise direction. None the less, there was a nonsignificant trend for fewer prey-catching responses when the dummy insect moved anticlockwise than when it moved clockwise (Mann–Whitney U test: U Z 339.50, N1 Z 23, N2 Z 24, P Z 0.13; Fig. 4a). The numbers of responses elicited by the dummy prey moving either clockwise or anticlockwise were not significantly different in either the postmidline area (U Z 337.50, N1 Z 23, N2 Z 24, P Z 0.18; Fig. 4a) or
Figure 3. Mean C SEM numbers of predatory responses made by toads towards a horizontal black strip (dummy prey) moving in a clockwise or anticlockwise direction. Results are from 10 toads tested in the clockwise (,) and nine in the anticlockwise (-) directions. ‘L’ and ‘R’ indicate left and right hemifields, respectively. The total number of predatory responses is represented by ‘L C R Responses’, combining those elicited from the dummy prey in both left and right hemifields. See text for details. *P ! 0.05 (Mann– Whitney U tests for premidline and postmidline comparisons between groups, Wilcoxon signed-ranks tests for left and righthemifield comparisons within groups).
Experiment 3 The data for the first half of the trials (i.e. with the dummy insect and without mealworms present inside the cylinder) were analysed first. No significant overall preference (clockwise and anticlockwise group data combined) was found for responding to the dummy prey in either left or right visual hemifield (right hemifield: XGSEM ¼ 7:6G2:9 responses; left hemifield: 3.5 G 1.5
Postmidline NS
Mean number of responses
hemifield bias was found when the results were combined from both clockwise and anticlockwise groups (right hemi field: XGSEM ¼ 24:9G9:4 responses; left hemifield: 21.3 G 8.8 responses; Wilcoxon signed-ranks test: Z Z 0.34, N Z 19, P Z 0.73). Furthermore, there was no significant difference between the total number of responses made by toads in the clockwise and anticlockwise groups (Mann–Whitney U test: U Z 51.50, N1 Z 10, N2 Z 9, P Z 0.59; Fig. 3). The general predatory responsiveness towards the horizontal strip by toads tested in clockwise or anticlockwise conditions was essentially ‘mirrorimaged’. The dummy prey elicited similar numbers of responses as it passed the visual midline into either left or right hemifield (Mann–Whitney U test: U Z 51.50, N1 Z 10, N2 Z 9, P Z 0.59), or approached the midline from either left or right hemifield (U Z 45.00, N1 Z 10, N2 Z 9, P O 0.99). The number of strikes and turns towards the dummy prey as it moved from the premidline to the postmidline area of the visual field was significantly elevated in both clockwise and anticlockwise groups (Wilcoxon signed-ranks test: clockwise: T Z 45.0, N Z 10, P Z 0.008; anticlockwise: T Z 21, N Z 9, P Z 0.03).
Premidline NS
(a)
25 22.5 20 17.5 15 12.5 10 7.5 5 2.5 0
NS
*
*
L+R Responses
L R Clockwise
(b)
25 22.5 20 17.5 15 12.5 10 7.5 5 2.5 0
L R Anticlockwise
Premidline ** Postmidline **
* **
NS
L+R Responses
L R Clockwise
L R Anticlockwise
Figure 4. Mean C SEM numbers of turns and strikes made by toads during two halves of a 10-min trial, responding to a plastic insect (dummy prey) moving in a clockwise or anticlockwise direction. Results are from 23 toads in the clockwise (,) condition and 24 toads in the anticlockwise (-) condition. (a) First and (b) second halves of the trial, indicating 5-min periods before and after live prey ( five mealworms) were available for the toads to consume. ‘L’ and ‘R’ indicate left and right hemifields, respectively. The total number of predatory responses is represented by ‘L C R Responses’, combining those elicited from the dummy prey in both left and right hemifields. See text for details. *P ! 0.05; **P ! 0.005 (Mann– Whitney U tests for premidline and postmidline comparisons between groups, Wilcoxon signed-ranks tests for left and righthemifield comparisons within groups).
771
772
ANIMAL BEHAVIOUR, 68, 4
premidline area (U Z 327.00, N1 Z 23, N2 Z 24, P Z 0.15; Fig. 4a). The number of responses made towards the dummy prey moving from premidline to postmidline areas in either the clockwise or anticlockwise directions was significantly higher for the postmidline area than for the premidline area (Wilcoxon signed-ranks tests: clockwise: Z Z 2.93, N Z 23, P Z 0.003; anticlockwise: Z Z 2.52, N Z 24, P Z 0.012; Fig. 4a). Nonsignificant trends seen in the first half of the trials of experiment 3 were significant in the second half (i.e. when mealworms were presented inside the cylinder). Toads in the clockwise group directed overall significantly more responses at the dummy prey than toads in the anticlockwise group (Mann–Whitney U test: U Z 410.50, N1 Z 23, N2 Z 24, P Z 0.001; Fig. 4b). With the results from the clockwise and anticlockwise groups combined, a significant preference was found for toads to direct predatory responses to dummy prey located within the right visual hemifield (right hemifield: XGSEM ¼ 8:00G2:6 responses; left hemifield: 1.9 G 0.7 responses; Wilcoxon signed-ranks test: Z Z 2.54, N Z 47, P Z 0.011). Significant differences were found between the number of predatory responses directed at the dummy prey: toads in the clockwise group significantly outperformed toads in the anticlockwise group in both postmidline (Mann–Whitney U test: U Z 399.50, N1 Z 23, N2 Z 24, P Z 0.002) and premidline (U Z 424.00, N1 Z 23, N2 Z 24, P Z 0.0002; Fig. 4b) areas. Significantly more responses were directed at the dummy prey in the postmidline area (right hemifield) than in the premidline area (left hemifield) in the clockwise group (Wilcoxon signed-ranks test: Z Z 3.11, N Z 23, P Z 0.002). The number of responses directed at the dummy prey as it moved from the premidline to the postmidline areas of the visual field was not quite significantly different in the anticlockwise group (Wilcoxon signed-ranks test: Z Z 1.83, N Z 24, P Z 0.07; Fig. 4b). The time of introduction and corresponding responsiveness to the mealworms did not differ between toads with the dummy prey moving in a clockwise or anticlockwise direction (Table 1). The latency to consume the mealworms ( feeding delay) and the duration over which mealworms were actively preyed upon (time between first and last mealworms eaten, or feeding duration; Table 1) was also similar for the clockwise and anticlockwise groups. This occurred even though all five mealworms were not always eaten, some escaping predation by remaining
motionless. There was no difference between the number of mealworms eaten by toads in groups with the dummy prey moving clockwise or anticlockwise (Table 1). We calculated the rate of switching of attention between the mealworms inside the cylinder and the dummy prey moving outside the cylinder, during the second half of experiment 3. The number of predatory responses that each toad directed at the dummy prey during the feeding period when mealworms were present was divided by the number of mealworms eaten. Sixteen toads in the clockwise group consumed mealworms, making 2.22 G 0.86 (XGSEM) turns or strikes at the dummy prey for every mealworm eaten. In the anticlockwise group, 12 toads consumed mealworms and made 0.34 G 0.18 turns or strikes at the dummy prey for each mealworm eaten. The amount of switching between the mealworm prey and the dummy prey was significantly higher in the clockwise group than the anticlockwise group (Mann–Whitney U test: U Z 142.50, N1 Z 23, N2 Z 24, P Z 0.02). DISCUSSION Bufo marinus displayed lateralized feeding responses directed towards a cricket and a model insect. The toads were not lateralized when the stimulus was a horizontal ‘worm-like’ strip, although, or possibly because, the strip was a strong releaser of predatory responses. This result suggests that both eye systems of the toad respond to certain basic or ‘key’ aspects of prey stimuli. A bias for predatory responses mediated by the right eye (left side of the brain) existed in conditions that required the toad to make considered decisions based on complex visual cues. The toads were presented with dummy prey moving continuously and in a fixed clockwise or anticlockwise direction. For each of the dummy prey types tested, toads showed a marked tendency to delay responding to the stimulus until after it had crossed the visual midline and into either left or right hemifield. This behaviour of orienting towards automated prey retreating from, rather than advancing towards, the visual midline is well documented (e.g. Ingle 1976a). We have found no satisfactory explanation in the literature as to why this unusual trait occurs. It is possible that such behaviour may be related to the ‘ambush’ strategy of predation adopted by many amphibian species (e.g. Roth et al. 1998). The artificiality of the dummy prey movement
Table 1. Results of Mann–Whitney U tests (two-tailed) comparing parameters associated with the introduction of live prey ( five mealworms) Corrected for ties Variable Mealworms introduced (s) Feeding delay (s) Feeding duration (s) Mealworms eaten
Clockwise
Anticlockwise
U
Z
P
298.70G2.22 42.63G12.35 87.81G17.62 3.09G0.46
299.83G2.95 57.58G22.90 67.17G18.95 2.04G0.46
291.50 98.50 126.50 349.50
0.33 0.12 1.42 1.66
0.74 0.91 0.16 0.10
Means are given G SEM. The time at which the mealworms were introduced, the latency to begin feeding, the number of mealworms eaten and the feeding duration are compared between clockwise and anticlockwise groups, relating to the direction of movement of the dummy prey (N Z 23 clockwise, N Z 24 anticlockwise).
ROBINS & ROGERS: LATERALIZED PREDATORY RESPONSES
was none the less essential for unmasking lateralized visual processing in the toads. Direction lateralization (i.e. significant right-hemifield preferences for dummy prey moving clockwise) was associated with the introduction of freely obtainable prey. The lateralization was revealed in experiment 1 (tethered cricket) and in the second half of experiment 3 (plastic insect). In the first half of experiment 3 there was a nonsignificant tendency for right-hemifield preference for clockwise-moving dummy prey, which may suggest that the availability of live prey facilitates an underlying lateralization for dummy prey stimuli. In addition to the direction of movement of the dummy prey, its appearance is also important in revealing lateralized visual processing in toads. Table 2 summarizes the relative ‘attractiveness’ of the various test stimuli used, based on the total number of predatory responses elicited by both clockwise and anticlockwise groups. Direction lateralization was found in the experiments that elicited the fewest predatory responses (i.e. experiment 1 and the second half of experiment 3). There are two possible factors linking the comparatively low numbers of striking responses with the finding of direction lateralization. First, stimuli with simple features may be processed in the visual system of the toad at a faster rate than relatively more complex stimuli. The horizontal strip elicited a total number of strikes greater than that of the other three experiments combined, where comparatively more complex, insect-like stimuli were used (Table 2). Predatory responses directed at the black strip (experiment 2) were 8–15 times more frequent than those directed at the tethered cricket (experiment 1), and 2–4 times more frequent than those directed at the plastic insect (in both halves of the trial in experiment 3). It is possible, therefore, that features of the insect-like dummy prey forced the toads to make a considered choice on whether or not to strike. This is quite likely, given that the diet of B. marinus consists mostly of insects and other small invertebrates, requiring them to make detailed choices between edible and noxious prey of similar appearance. Thus, toads may make use of the physical Table 2. Summary of the results for toads striking at different test stimuli moving in clockwise and anticlockwise directions, listed in ascending order of total elicited strikes (clockwise and anticlockwise groups combined) Dummy prey
Total
Clockwise
Anticlockwise
Cricket 4.12G0.87 5.69G1.42 2.54G0.86 (experiment 1) Insect 9.94G2.80 18.09G5.07 2.13G1.34 (experiment 3, second half) Insect 11.04G3.24 15.61G5.67 6.67G3.14 (experiment 3, first half) Horizontal strip 46.16G10.71 48.40G15.16 43.67G16.0 (experiment 2) Means are given G SEM. The numbers of striking responses at the dummy prey in experiment 3 (plastic insect) are presented as first and second halves (without and with live mealworm prey, respectively).
features of the prey such as markings and leg lengths, and possibly compare them with a memory of ‘favourable’ or ‘unfavourable’ prey characteristics. None the less, it is at the level at which such decision making occurs that lateralized predatory responses are revealed in B. marinus. The higher visual functions of learning and memory have been ascribed to the telencephalon of toads and frogs (Muzio et al. 1993, 1994; Ewert et al. 1994; Ewert 1997). Possibly, a region of the left telencephalon of toads performs lateralized functions in a manner similar to that found in the Wulst region of the forebrain in chicks (reviewed in Andrew 1991; Bradshaw & Rogers 1993; Deng & Rogers 1997). As in the chick and other avian species mentioned in the Introduction, neural circuits on the left side of the brain of B. marinus may be able to categorize and recognize ‘prey’ using a range of criteria (e.g. body segmentation, presence of legs or other appendages, markings and coloration), and thus be involved in decisions to perform prey-catching responses. Matching neural circuits in the right side of the brain of B. marinus appear to ignore stimuli with complex features. Predatory responses to simple and very attractive stimuli (i.e. the horizontal strip) are not biased to one hemifield, possibly because these stimuli are processed at a lower level (in the tectum) in circuits with equivalent functions on both sides of the brain. The close relation between predatory responses directed towards the horizontal strip and the visual processes carried out in the tectum of toads and other amphibians has been studied comprehensively (e.g. Burghagen & Ewert 1982; Ewert 1984, 1997; Roth et al. 1998). We conclude from the results of experiment 2 that visual processing of simple stimuli, carried out in the left and right optic tecta (and possibly other visual centres of the toad), is not functionally lateralized. The different specializations of the left and right sides of the toad brain, as described, may help explain why nonsignificant tendencies for prey dummies in the right hemifield ( first half of the trial in experiment 3) become significant after the inclusion of live prey (experiment 1, second half of the trial in experiment 3). The availability of live prey may also have facilitated lateralized predatory responses in toads indirectly. Increased numbers of predatory responses directed towards test stimuli after the toad is stimulated with olfactory cues, or fed with other prey, is a facilitative process thought to be driven by the telencephalon (Ingle 1976a). This is plausible, given that areas of the telencephalon are involved with associative processing (Muzio et al. 1993, 1994). Therefore, the experience of feeding on live prey may have ‘activated’ the left and right sides of the telencephalon to states that reveal their lateralized processing for insect prey moving regularly in clockwise and anticlockwise directions. It is also notable from experiment 3 that toads directed significantly more predatory responses to the clockwise-moving, rather than the anticlockwise-moving, dummy prey during the brief period that mealworms were being consumed. Also, in contrast to the ‘switching’ of attention between the mealworms and the dummy prey by toads in the clockwise condition, toads in the anticlockwise condition demonstrated selective inattention to the dummy prey.
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We have shown that feature analysis of prey is more sophisticated than has been previously considered. Using the results drawn from neuroethological studies of anuran vision, we have speculated on the levels of the B. marinus visual system that may be lateralized. Our finding of leftbrain specialization for control of predatory responses in B. marinus supports that of previous research (Vallortigara et al. 1998) and corresponds with that found in the decision-related feeding responses of higher vertebrates. This result is of particular interest given the anurans’ separate evolutionary trajectory, unique life cycle (from egg to larva and adult forms), and possession of a visual system (from the retina to mid-, hind- and forebrain levels) organized very differently to those of other vertebrates (e.g. Gaillard 1990). Our results have contributed to the known range of similarities found between the lateralized behaviours of anurans and those of other vertebrates. Briefly, motor preferences (‘handedness’) have been identified in a range of adult toads (B. marinus, B. bufo and B. viridis: Bisazza et al. 1996, 1997; summarized in Rogers 2002a) and have also been determined in the turning preferences of tadpoles across a range of species (summarized in Rogers 2002b; Wassersug & Yamashita 2002). Tadpoles of Litoria latopalmata also show lateralized visual responses to an overhead predator model, by preferentially directing escape turns to the right (Rogers 2002b). This suggests that the tadpoles may be more responsive to sudden movement observed with the left eye than with the right eye. Adult bufonids (B. marinus, B. bufo and B. viridis) are also more reactive to a predator model viewed with their left eye than the right eye (Lippolis et al. 2002). These examples of lateralization are consistent with the specialization of the right hemisphere (left eye) of chicks to detect changes in the immediate surroundings (Rogers & Anson 1979). Another example of visual lateralization that is consistent between anurans and other vertebrates is the control of agonistic behaviour. Agonistic responses are preferentially mediated by the left eye in adult B. marinus and B. bufo (Robins et al. 1998; Vallortigara et al. 1998), lizards (Anolis sp.: Deckel 1995), domestic chicks (Rogers et al. 1985) and gelada baboons, Theropithecus gelada (Casperd & Dunbar 1996). The growing evidence of brain lateralization in anurans suggests that, despite the considerable differences between vertebrate classes in the general structure of the brain and visual pathways (Striedter 1997), specializations in the left and right sides of anuran, reptilian, avian and mammalian brain have been conserved through evolution.
Acknowledgments The experiments with B. marinus formed part of A.R.’s research towards his Ph.D. at the University of New England. L. J. Rogers gratefully acknowledges funding from the Australian Research Council (a Special Investigator Award).
References Alonso, Y. 1998. Lateralization of visual guided behaviour during feeding in zebra finches (Taeniopygia guttata). Behavioural Processes, 43, 257–263. Andrew, R. J. 1991. The nature of behavioural lateralization in the chick. In: Neural and Behavioural Plasticity: The Use of the Chick as a Model (Ed. by R. J. Andrew), pp. 536–554. Oxford: Oxford University Press. Andrew, R. J. & Rogers, L. J. 2002. The nature of lateralization in tetrapods. In: Comparative Vertebrate Lateralization (Ed. by L. J. Rogers & R. J. Andrew), pp. 94–125. Cambridge: Cambridge University Press. Andrew, R. J., Mench, J. & Rainey, C. 1982. Right-left asymmetry of response to visual stimuli in the domestic chick. In: Analysis of Visual Behavior (Ed. by D. J. Ingle, M. A. Goodale & R. J. W. Mansfield), pp. 197–209. Cambridge, Massachusetts: MIT Press. Bisazza, A., Cantalupo, C., Robins, A., Rogers, L. J. & Vallortigara, G. 1996. Right-pawedness in toads. Nature, 379, 408. Bisazza, A., Cantalupo, C., Robins, A., Rogers, L. J. & Vallortigara, G. 1997. Pawedness and motor asymmetries in toads. Laterality, 2, 49–64. Bradshaw, J. L. & Rogers, L. J. 1993. The Evolution of Lateral Asymmetries, Language, Tool Use, and Intellect. New York: Academic Press. Burghagen, H. & Ewert, J.-P. 1982. Question of ‘‘head preference’’ in response to worm-like dummies during prey-capture of toads, Bufo bufo. Behavioural Processes, 7, 295–306. Casperd, L. M. & Dunbar, R. I. M. 1996. Asymmetries in the visual processing of emotional cues during agonistic interactions by gelada baboons. Behavioral Processes, 37, 57–65. Deckel, A. W. 1995. Laterality of aggressive responses in Anolis. Journal of Experimental Zoology, 272, 194–200. Deng, C. & Rogers, L. J. 1997. Differential contributions of the two visual pathways to functional lateralization in chicks. Behavioural Brain Research, 87, 173–182. Duellman, W. E. & Trueb, L. 1986. Biology of Amphibians. New York: McGraw-Hill. Ewert, J.-P. 1970. Neural mechanisms of prey-catching and avoidance behavior in the toad (Bufo bufo L.). Brain, Behavior and Evolution, 3, 36–56. Ewert, J.-P. 1984. Tectal mechanisms that underlie prey-catching and avoidance behaviors in toads. In: Comparative Neurology of the Optic Tectum (Ed. by H. Vanegas), pp. 247–416. New York: Plenum. Ewert, J.-P. 1997. Neural correlates of key stimulus and releasing mechanism: a case study and two concepts. Trends in Neurosciences, 20, 332–339. Ewert, J.-P. & Finkensta¨dt, T. 1987. Modulation of tectal functions by prosencephalic loops in amphibians. Behavioral and Brain Sciences, 10, 122–123. Ewert, J.-P. & Ingle, D. J. 1971. Excitatory effects following habituation of prey-catching activity in frogs and toads. Journal of Comparative and Physiological Psychology, 77, 369–374. Ewert, J.-P., Dinges, A. W. & Finkensta¨dt, T. 1994. Speciesuniversal stimulus responses, modified through conditioning, reappear after telencephalic lesions in toads. Naturwissenschaften, 81, 317–320. Fite, K. V. 1973. The visual fields of the frog and toad: a comparative study. Behavioral Biology, 9, 707–718. Gaillard, F. 1990. Visual units in the central nervous system of the frog. Comparative Biochemistry and Physiology, A, 96, 357–371. ¨ sser, O.-J. & Gru ¨ sser-Cornehls, U. 1976. Neurophysiology of Gru the anuran visual system. In: Frog Neurobiology (Ed. by R. Llina´s & U. Precht), pp. 297–385. Berlin: Springer-Verlag.
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¨ ntu ¨ rku ¨ n, O. 1985. Lateralization of visually guided behavior in Gu pigeons. Physiology and Behavior, 34, 575–577. ¨ ntu ¨ rku ¨ n, O. & Kesh, S. 1987. Visual lateralization during feeding Gu in pigeons. Behavioral Neuroscience, 101, 433–435. Ingle, D. J. 1976a. Behavioral correlates of central visual function in anurans. In: Frog Neurobiology (Ed. by R. Llina´s & U. Precht), pp. 435–451. Berlin: Springer-Verlag. Ingle, D. J. 1976b. The visual system. In: Physiology of the Amphibians (Ed. by B. Lofts), pp. 421–441. New York: Academic Press. Lippolis, G., Bisazza, A., Rogers, L. J. & Vallortigara, G. 2002. Lateralization of predator avoidance in three species of toads. Laterality, 7, 163–183. Muzio, R. N., Segura, E. T. & Papini, M. R. 1993. Effects of lesions in the medial pallium on instrumental learning in the toad (Bufo arenarum). Physiology and Behavior, 54, 185–188. Muzio, R. N., Segura, E. T. & Papini, M. R. 1994. Learning under partial reinforcement in the toad (Bufo arenarum): effects of lesions in the medial pallium. Behavioral and Neural Biology, 61, 36–46. Robins, A. 1997. An improved method of housing laboratory frogs and toads, using a miniature ecosystem. ANZCCART News (Australian and New Zealand Council for the Care of Animals in Research and Teaching), 10, 4–5. Robins, A., Lippolis, G., Bisazza, A., Vallortigara, G. & Rogers, L. J. 1998. Lateralized agonistic responses and hindlimb use in toads. Animal Behaviour, 56, 875–881. Rogers, L. J. 2000. Evolution of hemispheric specialization: advantages and disadvantages. Brain and Language, 73, 236–253. Rogers, L. J. 2002a. Lateralization in vertebrates: its early evolution, general patterns and development. In: Advances in the Study
of Behavior. Vol. 3 (Ed. by P. J. B. Slater, J. Rosenblatt, C. Snowdon & T. Roper), pp. 107–162. San Diego: Academic Press. Rogers, L. J. 2002b. Lateralised brain function in anurans: comparison to lateralisation in other vertebrates. Laterality, 7, 219–239. Rogers, L. J. & Anson, J. M. 1979. Lateralisation of function in the chicken forebrain. Pharmacology, Biochemistry and Behavior, 10, 679–686. Rogers, L. J., Zappia, J. V. & Bullock, S. P. 1985. Testosterone and eye–brain asymmetry for copulation in chickens. Experientia, 41, 1447–1449. Roth, G., Dicke, U. & Wiggers, W. 1998. Vision. In: Amphibian Biology. Vol. 3: Sensory Perception (Ed. by H. Heatwole & E. M. Dawley), pp. 783–877. Chipping-Norton: Surrey Beatty. Striedter, G. F. 1997. The telencephalon of tetrapods in evolution. Brain, Behavior and Evolution, 49, 179–213. Vallortigara, G., Rogers, L. J., Bisazza, A., Lippolis, G. & Robins, A. 1998. Complementary right and left hemifield use for predatory and agonistic behaviour in toads. Neuroreport, 9, 3341–3344. Wachowitz, S. & Ewert, J.-P. 1996. A key by which the toad’s visual system gets access to the domain of prey. Physiology and Behavior, 60, 877–887. Wassersug, R. J. & Yamashita, M. 2002. Assessing and interpreting lateralised behaviours in anuran larvae. Laterality, 7, 241–260. Wye Dvorak, J., Straznicky, C. & Toth, P. 1992. Retinal projections in the cane toad, Bufo marinus. Brain, Behavior and Evolution, 39, 208–218. Zar, J. H. 1996. Biostatistical Analysis. Upper Saddle River, New Jersey: Prentice-Hall.
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Lateralized prey-catching responses in the cane toad, Bufo marinus: analysis of complex visual stimuli ANDREW ROBINS & LESLEY J . ROGERS
Centre for Neuroscience and Animal Behaviour, School of Biological, Biomedical and Molecular Sciences, University of New England (Received 12 August 2003; initial acceptance 1 October 2003; final acceptance 29 December 2003; published online 25 August 2004; MS. number: 7822)
We tested the responses of Bufo marinus to prey stimuli of varying visual complexity that were moved around the toads in either a clockwise or anticlockwise direction at 1.7 revolutions/min. Predatory responses directed at prey resembling an insect were frequent when the model insect moved clockwise across the visual midline into the right visual hemifield. In contrast, the toads tended to ignore such stimuli when they moved anticlockwise across the midline into the left hemifield. No such lateralization was found when a rectangular strip moved along its longest axis was presented in a similar way. The toads also directed more responses towards the latter stimulus than towards the insect prey. Hence, the results suggest that lateralized predatory responses occur for considered decisions on whether or not to respond to complex insect-like stimuli, but not for decisions on comparatively simple stimuli. We discuss similarities between the lateralized feeding responses of B. marinus and those of avian species, as support for the hypothesis that lateralized brain function in tetrapods may have arisen from a common lateralized ancestor. 2004 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd.
Vallortigara et al. (1998) found that European common toads, Bufo bufo, European green toads, B. viridis, and South American cane toads, B. marinus, prefer to direct strikes with their tongue at prey in the right visual hemifield. The toads showed this preference when tested using an automated prey stimulus: a wormlike, live Galleria mellonella larva in the case of B. bufo and B. viridis and a live adult cricket (Acheta sp.) in the case of B. marinus. However, although B. marinus showed a preference to tongue-strike at the prey stimulus as it moved through the right hemifield, the magnitude of responsiveness and the strength of preference were not as strong as in the other two toad species (Vallortigara et al. 1998). Because the result might have been related to the difference in the physical characteristics of the prey stimuli used to test the respective species of toad, in this study we used B. marinus to test the hypothesis that differences in Correspondence and present address: A. Robins, Department of Biological and Physical Sciences, Faculty of Sciences, University of Southern Queensland, QLD 4350, Australia (email: robins@usq. edu.au). L. J. Rogers is at the Centre for Neuroscience and Animal Behaviour, School of Biological, Biomedical and Molecular Sciences, University of New England, NSW 2351, Australia. 0003–3472/03/$30.00/0
the visual complexity of the prey may influence the degree of lateralization of the predatory responses. The optic fibres in the anuran visual system decussate almost completely, so input received by either eye is processed mainly by neural circuits in the opposite side of the brain, as in birds, reptiles, fish and mammals to laterally placed eyes (Rogers 2002b). In B. marinus, 96% of the retinal fibres cross to the contralateral side of the brain, with 85% of these projecting into the contralateral optic tectum (a mesencephalic structure analogous to the mammalian superior colliculus: Wye Dvorak et al. 1992). A preference to approach and strike at prey in the right visual hemifield might, therefore, reflect preferential control by circuits in the left tectum or higher visual centres in the left forebrain (telencephalon). Such a specialization would be consistent with the general direction of lateralization observed for feeding responses in other vertebrates (Rogers 2002a). However, the right-biased strikes at prey reported by Vallortigara et al. (1998) were within the toad’s binocular visual field, which means that visual inputs would go to both sides of the brain. Toads possess a wide binocular overlap in the horizontal plane (our perimetric analysis of nine B. marinus revealed the mean angle of overlap in the horizontal plane at eye
767 2004 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd.
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level G SEM to be 57.5 G 2.2 , corresponding with the published finding of 56 : Fite 1973). This wide binocular overlap precludes a clear determination of which side of the brain may be assuming dominance for controlling the predatory response, but dominance of neural circuits on the left side of the brain seems to be the most parsimonious explanation for the right-hemifield preference. Summarizing a broad spectrum of studies of feeding behaviour in vertebrates ranging from fish to mammals, Andrew & Rogers (2002) concluded that lateralized feeding responses reveal left-hemisphere specialization for focused attention and considered responses to appropriate visual stimuli, as needed in selecting food items against a distracting background. For example, a right-eye (and left-hemisphere) preference for pecking at grains scattered among pebbles has been found in chicks, Gallus gallus domesticus (e.g. Rogers & Anson 1979; Andrew et al. 1982; ¨ ntu ¨ rku ¨n Deng & Rogers 1997), pigeons, Columba livia (Gu ¨ ntu ¨ rku ¨ n & Kesh 1987) and zebra finches, 1985; Gu Taeniopygia guttata (Alonso 1998). This form of lateralization was revealed by testing the birds monocularly: when using the right eye, they were able to discriminate food grains from pebbles of similar shape, size and colour but when using the left eye they pecked randomly at pebbles and grains. Feeding responses in the chick are thought to be controlled by the left hemisphere owing to its specialization for categorizing ‘food’ from ‘nonfood’ stimuli according to selected properties (Andrew 1991; Rogers 2000). It has not been possible to test toads on a task identical to that used for birds because the prey must move to elicit feeding responses in toads (Ingle 1976a). However, standardized methods for testing predatory responses in toads have been developed (e.g. Ewert 1970; Ewert & Ingle 1971; Ingle 1976a; Burghagen & Ewert 1982). Individual toads are contained within a transparent cylinder that is, continuously and mechanically, encircled by a ‘dummy’ prey item (so termed as it offers no reinforcement ¨ sser & Gru ¨ sserto continue predatory responses: Gru Cornehls 1976; Ingle 1976a; Burghagen & Ewert 1982; Fig. 1). The dummy prey can encircle the toad in either a clockwise or anticlockwise direction at a constant rate. When the toad is placed in the test apparatus, the dummy prey is seen to travel from left to right (clockwise) or right to left (anticlockwise) visual fields, respectively. Probably the most widely known finding from using the test apparatus is that simple stimuli are processed by the toad’s visual system into ‘worm’ or ‘antiworm’ categories (Ewert 1984). Rectangular objects moving in the direction of the longitudinal axis are responded to as ‘worm’ stimuli and elicit approach and tongue-striking behaviour (Ewert & Ingle 1971; Ewert et al. 1994). The same shape moving perpendicular to the longitudinal axis (i.e. upright) is responded to as a threatening ‘antiworm’ that elicits escape or defensive behaviour (Ingle 1976a; Ewert 1997). The tectum of the toad actively filters the continuous flow of information from the retina and directs predatory behaviour to all moving stimuli with worm-like properties (Ingle 1976b; Ewert 1997). Habituation, association, memory consolidation and other higher forms of visual processing are carried out in the toad’s telencephalon,
Figure 1. The test apparatus. The toad was contained within a transparent plastic cylinder (12 cm in diameter, 30 cm high) on a circular and immobile platform (15 cm in diameter). A freely rotating wheel beneath the platform was driven by a 240-V motor (not shown), and rotated in either an anticlockwise or clockwise direction at a constant rate of 1.7 revolutions/min. The rotating wheel supported the dummy prey (the model insect used in experiment 3 is shown; the grey arrow shows the prey moving clockwise). Dummy prey used in the study are illustrated in relative scale: (a) live cricket (mean length 25 mm: experiment 1), (b) horizontal strip (20 mm long ! 2.5 mm high: experiment 2) (c) plastic model insect (body 18 mm long: experiment 3).
which inhibits any inappropriate predatory behaviour directed by the tectum (Ewert & Finkensta¨dt 1987; Muzio et al. 1993, 1994; Ewert et al. 1994). Although the analyses of simple and comparatively complex visual stimuli are known to be carried out at different levels of the visual system of toads, the possibility that such processing may be lateralized has not been considered previously. We hypothesized that prey types of varying visual complexity would reveal corresponding differences in predatory lateralization in B. marinus. Specifically, we predicted that toads tested with the simplest dummy prey (a black horizontal rectangular strip) might show a different degree of lateralized behaviour from those tested with insect-like dummy prey (either model or live insects), since the latter requires additional processing from higher levels of the anuran visual system. We also tested whether the presence of live prey, available within the cylinder for the toads to feed on while being tested, might promote complex decision making and lateralization of predatory responses.
METHODS We used 59 mature B. marinus toads in a series of experiments of lateralized visual processing. Obtained from the Atherton tablelands in northern Australia by a professional collector (Peter Kraus), the toads were group housed in
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two large tanks at the University of New England, Australia. Both housing tanks were constructed from rust-resistant metal sheeting (80 ! 80 cm and 58 cm high), inside which the environment was enriched with areas for swimming, burrowing, basking and hiding (Robins 1997). Twice each day, water was showered automatically into the housing tanks. The housing room conditions reproduced the tropical/subtropical environment preferred by B. marinus (Duellman & Trueb 1986), with the room temperature maintained between 23 and 27 C and the relative humidity between 70 and 85%. Lighting was provided in a 12:12 h photoperiod, using ultraviolet fluorescent tubes. We fed the toads at least three times a week on mealworms, compost worms, or crickets. Nearly half (26) the toads used in this study had been held in captivity for at least a year in the housing conditions described, since they were part of a long-term study of visual and motor lateralization in anuran species. The procedures for both housing and testing the toads were approved by the University of New England’s Animal Ethics Committee. The design specifications for the test apparatus used in the following experiments were based on previous research conducted with B. marinus (Wachowitz & Ewert 1996; Fig. 1). In each of three separate experiments using the apparatus, the dummy prey (live cricket, horizontal black strip, or black plastic model insect; Fig. 1) was moved around the toad at 1.7 revolutions/min, or 10 of rotation/s. At this angular velocity, B. marinus responds (by turning) more consistently and with less habituation than when tested with dummy prey moving at other speeds (Wachowitz & Ewert 1996). The dummy prey was randomly assigned to rotate about the toad in either a clockwise or anticlockwise direction. The direction of movement of the dummy prey was not changed during a trial, and was set in motion before the toad was placed in the cylinder. Trials commenced immediately after the toad was placed in the test cylinder. A larger white cylinder (40 cm in diameter) contained the entire apparatus and prevented the toad from attending to extraneous stimuli. Even illumination was provided with a 60-W incandescent lamp, supplementing the fluorescent room lighting. The experiments were recorded from directly overhead with a video camera. The videotapes were analysed using frame-by-frame playback. The predatory motor acts of particular interest to this study were the turning and striking responses. We scored ‘turns’ as rotational movements of the toad’s body leftwards or rightwards to bring the prey closer to the centre of the binocular field. Predatory turns could be distinguished from other types of turns (avoidance or search behaviour) because they involved lowering the head to the level of the prey, were often repeated in pursuit of the prey stimulus, and were often accompanied by tongue striking. Tongue-strikes (hereafter referred to as ‘strikes’) were scored as the ballistic projection of the tongue at the prey, most often to the left or right of the visual midline. Strikes could also involve rapid body movement (a lunge) as part of an uninterrupted motor sequence. Here we considered the turning and striking data together as a single ‘predatory response’ measure, as used by
other authors (e.g. Burghagen & Ewert 1982; Ewert et al. 1994). We counted turns and strikes at the dummy prey in real time from the overhead videotape, using software that also calculated latency and duration (Etholog version 2.2.5, E. B. Ottoni, Department of Experimental Psychology, University of Sa˜o Paulo, Brazil). In rare cases (!5%) in which midline strikes were observed, each strike was reanalysed in a separate viewing, using frame-by-frame analysis of the videotape. Such strikes were scored as ‘left’ or ‘right’ according to the location of the prey stimulus (with regard to the midline) on the frame at which the strike was initiated. We refer to the preference to respond to prey located either in the left or right visual hemifield, regardless of the direction (clockwise or anticlockwise) in which the prey travelled about the toad, as ‘overall lateralization’. We use the term ‘direction lateralization’ to refer to lateralized predatory responses for prey in either visual hemifield, but only for toads responding to dummy prey moving in either the clockwise or the anticlockwise direction, and not both directions. Wilcoxon signed-ranks tests (two-tailed, adjusted for ties) were used to compare the differences in the number of predatory responses directed at prey in the left and right visual hemifields, considering the treatment groups together (overall lateralization) and then separately (direction lateralization). Mann–Whitney U tests (two-tailed, adjusted for ties) were used to compare the number of behavioural events or their latencies (i.e. number of turns, strikes, latency to initiate predatory responses) between the clockwise and anticlockwise treatment groups (Zar 1996). Of the toads tested in experiment 2, 18 had been tested previously in experiment 1. Fifteen toads from experiment 2 were retested in experiment 3. Owing to the random assignation of anticlockwise or clockwise test conditions, the differences in procedures in each experiment, and the time between experiments (6 weeks separated experiments 1 and 2, 4 weeks separated experiments 2 and 3), we considered the effect of retesting in the toads to be negligible. In experiment 1, two groups of 13 toads were tested with the dummy prey moving in either a clockwise or anticlockwise direction. The dummy prey was a live cricket (2–2.5 cm long) tethered to a vertical wire arm (4 cm high) with white cotton thread (not shown in Fig. 1). The movement of the tethered cricket could not be automated fully as, in addition to walking naturally, the cricket could periodically leap or be dragged short distances along the test platform. A fresh cricket was used as the dummy prey after every four trials to reduce the effects of fatigue. Another live and freely moving cricket was contained within the cylinder at the start of the trial, and served to facilitate the toads’ predatory responses to the dummy prey (e.g. Ingle 1976a). Injured and uneaten prey were fed to toads already returned to their home tanks after testing. Although the trials in this experiment were conducted over 5 min, they were terminated if the toad managed to escape from the apparatus, by climbing. We used 19 toads in experiment 2, randomly divided into groups of 9 and 10 for anticlockwise and clockwise presentations of the dummy prey, respectively. The
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dummy prey used was a small rectangular strip of black card (20 mm long ! 2.5 mm high; Fig. 1). This ‘configuration’ of the prey stimulus, moving in the direction of its longitudinal axis in a ‘worm-like’ manner, consistently elicited predator responses in other research using toads (e.g. Burghagen & Ewert 1982; Ewert et al. 1994; Wachowitz & Ewert 1996). We included the horizontal black strip in the study as the simplest prey model possible (two-dimensional and featureless). No live prey was introduced inside the cylinder to promote predatory responsiveness. Trials were run for 5 min. We prevented the toad escaping by lightly moistening the inner walls of the test cylinder with water. For experiment 3, we used 47 toads randomly divided into ‘clockwise’ and ‘anticlockwise’ groups of 23 and 24, respectively. A black plastic model of an insect (body 18 mm long and 6 mm wide, with legs splayed to 20 mm wide; Fig. 1) was used as the prey stimulus. The insect had configurational elements similar to both the tethered cricket (similar length, six appendages) and the black strip (colour, moved at a regular and constant rate) used in the previous experiments. When in motion, the insect was oriented with the head and body segments in the direction appropriate to natural forward movement. In contrast to experiments 1 and 2, trials in experiment 3 were run over 10 min. The first half (5 min) of the trial was run to match the conditions used in the horizontal strip experiment (experiment 2), with no live prey serving for predatory facilitation. After 5 min and as the movement of the dummy prey continued unchanged, we dropped five live mealworm prey (Tenebrio molitor larvae) into the cylinder containing the toad. We added live prey for two reasons. The main purpose was to determine what role, if any, the availability of consumable prey had on lateralized predatory responses directed at the dummy prey. The second was to examine how general predatory choices (between the worm-like live prey and dummy insect prey) may differ in toads tested with the dummy prey moving in either clockwise or anticlockwise directions. In this respect the paradigm differed from the tethered cricket experiment (experiment 1), in which both the consumable prey and the dummy prey were the same prey type. Each trial continued for the full 10 min with predatory behaviour directed towards both the live prey and dummy prey being scored. As in experiment 2, a light film of water sprayed on the inside walls of the test cylinder prevented the toads escaping.
predatory responses to be directed at the dummy prey within either left or right visual hemifield (right hemifield: XGSEM ¼ 2:7G0:7 responses; left hemifield: 1.3 G 0.4 responses; Wilcoxon signed-ranks test: Z Z 1.71, N Z 26, P Z 0.087). However, toads in the clockwise group directed significantly more responses at the dummy prey than did toads in the anticlockwise group (Mann–Whitney U test: U Z 122.50, N1 Z N2 Z 13, P Z 0.048; Fig. 2). This direction lateralization related to significantly more responses directed to dummy prey passing the visual midline (and into the right visual hemifield) by toads in the clockwise group, than responses to the dummy prey passing the visual midline and into the left visual hemisphere in the anticlockwise group (Mann–Whitney U test: U Z 122.50, N1 Z N2 Z 13, P Z 0.046; Fig. 2, postmidline). In contrast, there was no difference between toads in the two groups for the number of responses directed at dummy prey approaching the visual midline (Mann–Whitney U test: U Z 87.00, N1 Z N2 Z 13, P Z 0.89; Fig. 2, premidline). In the clockwise group, significantly more responses were directed at the dummy prey in the postmidline area (right hemifield) than in the premidline area (left hemifield; Wilcoxon signed-ranks test: T Z 64.5, N Z 13, P Z 0.005). However, the number of responses directed at the dummy prey as it moved from the premidline to postmidline areas of the visual field was not significantly different in the anticlockwise group (T Z 13.5, N Z 13, P Z 0.10; Fig. 2).
Experiment 2 Between 8 and 17 times more predatory responses were directed at the dummy prey in experiment 2 than in experiment 1 (cf. scores in Figs 2, 3). However, no overall
Mean number of responses
770
Premidline NS 8
Experiment 1 There was no significant difference between the trial durations (time before escaping the test cylinder) for toads in either treatment group (clockwise group: XGSEM ¼ 232:23G16:5 s; anticlockwise group: 246.62 G 14.7 s; Mann–Whitney U test: U Z 95.5, N1 Z N2 Z 13, P Z 0.57). With the results from the clockwise and anticlockwise groups combined, there was no overall preference for the
Postmidline * *
6 5 4
NS
3 2 1 0
RESULTS
*
7
L+R Responses
L R Clockwise
L R Anticlockwise
Figure 2. Mean C SEM numbers of predatory responses directed at a tethered cricket (dummy prey) moving in a clockwise or anticlockwise direction. Results are from two groups of 13 toads tested in either clockwise (,) or anticlockwise (-) directions. ‘L’ and ‘R’ indicate left and right hemifields, respectively. The total number of predatory responses is represented by ‘L C R Responses’, combining those elicited from the dummy prey in both left and right hemifields. See text for details. *P ! 0.05 (Mann–Whitney U tests for premidline and postmidline comparisons between groups, Wilcoxon signed-ranks tests for left and right-hemifield comparisons within groups).
ROBINS & ROGERS: LATERALIZED PREDATORY RESPONSES
Mean number of responses
Premidline NS
70
Postmidline NS
NS *
*
L R Clockwise
L R Anticlockwise
60 50 40 30 20 10 0
L+R Responses
responses; Wilcoxon signed-ranks test: Z Z 1.07, N Z 47, P Z 0.29). When considering the general responsiveness of toads from either treatment group in the first half of the trials, no significant difference was found between the number of responses directed to the dummy prey moving in either the clockwise or anticlockwise direction. None the less, there was a nonsignificant trend for fewer prey-catching responses when the dummy insect moved anticlockwise than when it moved clockwise (Mann–Whitney U test: U Z 339.50, N1 Z 23, N2 Z 24, P Z 0.13; Fig. 4a). The numbers of responses elicited by the dummy prey moving either clockwise or anticlockwise were not significantly different in either the postmidline area (U Z 337.50, N1 Z 23, N2 Z 24, P Z 0.18; Fig. 4a) or
Figure 3. Mean C SEM numbers of predatory responses made by toads towards a horizontal black strip (dummy prey) moving in a clockwise or anticlockwise direction. Results are from 10 toads tested in the clockwise (,) and nine in the anticlockwise (-) directions. ‘L’ and ‘R’ indicate left and right hemifields, respectively. The total number of predatory responses is represented by ‘L C R Responses’, combining those elicited from the dummy prey in both left and right hemifields. See text for details. *P ! 0.05 (Mann– Whitney U tests for premidline and postmidline comparisons between groups, Wilcoxon signed-ranks tests for left and righthemifield comparisons within groups).
Experiment 3 The data for the first half of the trials (i.e. with the dummy insect and without mealworms present inside the cylinder) were analysed first. No significant overall preference (clockwise and anticlockwise group data combined) was found for responding to the dummy prey in either left or right visual hemifield (right hemifield: XGSEM ¼ 7:6G2:9 responses; left hemifield: 3.5 G 1.5
Postmidline NS
Mean number of responses
hemifield bias was found when the results were combined from both clockwise and anticlockwise groups (right hemi field: XGSEM ¼ 24:9G9:4 responses; left hemifield: 21.3 G 8.8 responses; Wilcoxon signed-ranks test: Z Z 0.34, N Z 19, P Z 0.73). Furthermore, there was no significant difference between the total number of responses made by toads in the clockwise and anticlockwise groups (Mann–Whitney U test: U Z 51.50, N1 Z 10, N2 Z 9, P Z 0.59; Fig. 3). The general predatory responsiveness towards the horizontal strip by toads tested in clockwise or anticlockwise conditions was essentially ‘mirrorimaged’. The dummy prey elicited similar numbers of responses as it passed the visual midline into either left or right hemifield (Mann–Whitney U test: U Z 51.50, N1 Z 10, N2 Z 9, P Z 0.59), or approached the midline from either left or right hemifield (U Z 45.00, N1 Z 10, N2 Z 9, P O 0.99). The number of strikes and turns towards the dummy prey as it moved from the premidline to the postmidline area of the visual field was significantly elevated in both clockwise and anticlockwise groups (Wilcoxon signed-ranks test: clockwise: T Z 45.0, N Z 10, P Z 0.008; anticlockwise: T Z 21, N Z 9, P Z 0.03).
Premidline NS
(a)
25 22.5 20 17.5 15 12.5 10 7.5 5 2.5 0
NS
*
*
L+R Responses
L R Clockwise
(b)
25 22.5 20 17.5 15 12.5 10 7.5 5 2.5 0
L R Anticlockwise
Premidline ** Postmidline **
* **
NS
L+R Responses
L R Clockwise
L R Anticlockwise
Figure 4. Mean C SEM numbers of turns and strikes made by toads during two halves of a 10-min trial, responding to a plastic insect (dummy prey) moving in a clockwise or anticlockwise direction. Results are from 23 toads in the clockwise (,) condition and 24 toads in the anticlockwise (-) condition. (a) First and (b) second halves of the trial, indicating 5-min periods before and after live prey ( five mealworms) were available for the toads to consume. ‘L’ and ‘R’ indicate left and right hemifields, respectively. The total number of predatory responses is represented by ‘L C R Responses’, combining those elicited from the dummy prey in both left and right hemifields. See text for details. *P ! 0.05; **P ! 0.005 (Mann– Whitney U tests for premidline and postmidline comparisons between groups, Wilcoxon signed-ranks tests for left and righthemifield comparisons within groups).
771
772
ANIMAL BEHAVIOUR, 68, 4
premidline area (U Z 327.00, N1 Z 23, N2 Z 24, P Z 0.15; Fig. 4a). The number of responses made towards the dummy prey moving from premidline to postmidline areas in either the clockwise or anticlockwise directions was significantly higher for the postmidline area than for the premidline area (Wilcoxon signed-ranks tests: clockwise: Z Z 2.93, N Z 23, P Z 0.003; anticlockwise: Z Z 2.52, N Z 24, P Z 0.012; Fig. 4a). Nonsignificant trends seen in the first half of the trials of experiment 3 were significant in the second half (i.e. when mealworms were presented inside the cylinder). Toads in the clockwise group directed overall significantly more responses at the dummy prey than toads in the anticlockwise group (Mann–Whitney U test: U Z 410.50, N1 Z 23, N2 Z 24, P Z 0.001; Fig. 4b). With the results from the clockwise and anticlockwise groups combined, a significant preference was found for toads to direct predatory responses to dummy prey located within the right visual hemifield (right hemifield: XGSEM ¼ 8:00G2:6 responses; left hemifield: 1.9 G 0.7 responses; Wilcoxon signed-ranks test: Z Z 2.54, N Z 47, P Z 0.011). Significant differences were found between the number of predatory responses directed at the dummy prey: toads in the clockwise group significantly outperformed toads in the anticlockwise group in both postmidline (Mann–Whitney U test: U Z 399.50, N1 Z 23, N2 Z 24, P Z 0.002) and premidline (U Z 424.00, N1 Z 23, N2 Z 24, P Z 0.0002; Fig. 4b) areas. Significantly more responses were directed at the dummy prey in the postmidline area (right hemifield) than in the premidline area (left hemifield) in the clockwise group (Wilcoxon signed-ranks test: Z Z 3.11, N Z 23, P Z 0.002). The number of responses directed at the dummy prey as it moved from the premidline to the postmidline areas of the visual field was not quite significantly different in the anticlockwise group (Wilcoxon signed-ranks test: Z Z 1.83, N Z 24, P Z 0.07; Fig. 4b). The time of introduction and corresponding responsiveness to the mealworms did not differ between toads with the dummy prey moving in a clockwise or anticlockwise direction (Table 1). The latency to consume the mealworms ( feeding delay) and the duration over which mealworms were actively preyed upon (time between first and last mealworms eaten, or feeding duration; Table 1) was also similar for the clockwise and anticlockwise groups. This occurred even though all five mealworms were not always eaten, some escaping predation by remaining
motionless. There was no difference between the number of mealworms eaten by toads in groups with the dummy prey moving clockwise or anticlockwise (Table 1). We calculated the rate of switching of attention between the mealworms inside the cylinder and the dummy prey moving outside the cylinder, during the second half of experiment 3. The number of predatory responses that each toad directed at the dummy prey during the feeding period when mealworms were present was divided by the number of mealworms eaten. Sixteen toads in the clockwise group consumed mealworms, making 2.22 G 0.86 (XGSEM) turns or strikes at the dummy prey for every mealworm eaten. In the anticlockwise group, 12 toads consumed mealworms and made 0.34 G 0.18 turns or strikes at the dummy prey for each mealworm eaten. The amount of switching between the mealworm prey and the dummy prey was significantly higher in the clockwise group than the anticlockwise group (Mann–Whitney U test: U Z 142.50, N1 Z 23, N2 Z 24, P Z 0.02). DISCUSSION Bufo marinus displayed lateralized feeding responses directed towards a cricket and a model insect. The toads were not lateralized when the stimulus was a horizontal ‘worm-like’ strip, although, or possibly because, the strip was a strong releaser of predatory responses. This result suggests that both eye systems of the toad respond to certain basic or ‘key’ aspects of prey stimuli. A bias for predatory responses mediated by the right eye (left side of the brain) existed in conditions that required the toad to make considered decisions based on complex visual cues. The toads were presented with dummy prey moving continuously and in a fixed clockwise or anticlockwise direction. For each of the dummy prey types tested, toads showed a marked tendency to delay responding to the stimulus until after it had crossed the visual midline and into either left or right hemifield. This behaviour of orienting towards automated prey retreating from, rather than advancing towards, the visual midline is well documented (e.g. Ingle 1976a). We have found no satisfactory explanation in the literature as to why this unusual trait occurs. It is possible that such behaviour may be related to the ‘ambush’ strategy of predation adopted by many amphibian species (e.g. Roth et al. 1998). The artificiality of the dummy prey movement
Table 1. Results of Mann–Whitney U tests (two-tailed) comparing parameters associated with the introduction of live prey ( five mealworms) Corrected for ties Variable Mealworms introduced (s) Feeding delay (s) Feeding duration (s) Mealworms eaten
Clockwise
Anticlockwise
U
Z
P
298.70G2.22 42.63G12.35 87.81G17.62 3.09G0.46
299.83G2.95 57.58G22.90 67.17G18.95 2.04G0.46
291.50 98.50 126.50 349.50
0.33 0.12 1.42 1.66
0.74 0.91 0.16 0.10
Means are given G SEM. The time at which the mealworms were introduced, the latency to begin feeding, the number of mealworms eaten and the feeding duration are compared between clockwise and anticlockwise groups, relating to the direction of movement of the dummy prey (N Z 23 clockwise, N Z 24 anticlockwise).
ROBINS & ROGERS: LATERALIZED PREDATORY RESPONSES
was none the less essential for unmasking lateralized visual processing in the toads. Direction lateralization (i.e. significant right-hemifield preferences for dummy prey moving clockwise) was associated with the introduction of freely obtainable prey. The lateralization was revealed in experiment 1 (tethered cricket) and in the second half of experiment 3 (plastic insect). In the first half of experiment 3 there was a nonsignificant tendency for right-hemifield preference for clockwise-moving dummy prey, which may suggest that the availability of live prey facilitates an underlying lateralization for dummy prey stimuli. In addition to the direction of movement of the dummy prey, its appearance is also important in revealing lateralized visual processing in toads. Table 2 summarizes the relative ‘attractiveness’ of the various test stimuli used, based on the total number of predatory responses elicited by both clockwise and anticlockwise groups. Direction lateralization was found in the experiments that elicited the fewest predatory responses (i.e. experiment 1 and the second half of experiment 3). There are two possible factors linking the comparatively low numbers of striking responses with the finding of direction lateralization. First, stimuli with simple features may be processed in the visual system of the toad at a faster rate than relatively more complex stimuli. The horizontal strip elicited a total number of strikes greater than that of the other three experiments combined, where comparatively more complex, insect-like stimuli were used (Table 2). Predatory responses directed at the black strip (experiment 2) were 8–15 times more frequent than those directed at the tethered cricket (experiment 1), and 2–4 times more frequent than those directed at the plastic insect (in both halves of the trial in experiment 3). It is possible, therefore, that features of the insect-like dummy prey forced the toads to make a considered choice on whether or not to strike. This is quite likely, given that the diet of B. marinus consists mostly of insects and other small invertebrates, requiring them to make detailed choices between edible and noxious prey of similar appearance. Thus, toads may make use of the physical Table 2. Summary of the results for toads striking at different test stimuli moving in clockwise and anticlockwise directions, listed in ascending order of total elicited strikes (clockwise and anticlockwise groups combined) Dummy prey
Total
Clockwise
Anticlockwise
Cricket 4.12G0.87 5.69G1.42 2.54G0.86 (experiment 1) Insect 9.94G2.80 18.09G5.07 2.13G1.34 (experiment 3, second half) Insect 11.04G3.24 15.61G5.67 6.67G3.14 (experiment 3, first half) Horizontal strip 46.16G10.71 48.40G15.16 43.67G16.0 (experiment 2) Means are given G SEM. The numbers of striking responses at the dummy prey in experiment 3 (plastic insect) are presented as first and second halves (without and with live mealworm prey, respectively).
features of the prey such as markings and leg lengths, and possibly compare them with a memory of ‘favourable’ or ‘unfavourable’ prey characteristics. None the less, it is at the level at which such decision making occurs that lateralized predatory responses are revealed in B. marinus. The higher visual functions of learning and memory have been ascribed to the telencephalon of toads and frogs (Muzio et al. 1993, 1994; Ewert et al. 1994; Ewert 1997). Possibly, a region of the left telencephalon of toads performs lateralized functions in a manner similar to that found in the Wulst region of the forebrain in chicks (reviewed in Andrew 1991; Bradshaw & Rogers 1993; Deng & Rogers 1997). As in the chick and other avian species mentioned in the Introduction, neural circuits on the left side of the brain of B. marinus may be able to categorize and recognize ‘prey’ using a range of criteria (e.g. body segmentation, presence of legs or other appendages, markings and coloration), and thus be involved in decisions to perform prey-catching responses. Matching neural circuits in the right side of the brain of B. marinus appear to ignore stimuli with complex features. Predatory responses to simple and very attractive stimuli (i.e. the horizontal strip) are not biased to one hemifield, possibly because these stimuli are processed at a lower level (in the tectum) in circuits with equivalent functions on both sides of the brain. The close relation between predatory responses directed towards the horizontal strip and the visual processes carried out in the tectum of toads and other amphibians has been studied comprehensively (e.g. Burghagen & Ewert 1982; Ewert 1984, 1997; Roth et al. 1998). We conclude from the results of experiment 2 that visual processing of simple stimuli, carried out in the left and right optic tecta (and possibly other visual centres of the toad), is not functionally lateralized. The different specializations of the left and right sides of the toad brain, as described, may help explain why nonsignificant tendencies for prey dummies in the right hemifield ( first half of the trial in experiment 3) become significant after the inclusion of live prey (experiment 1, second half of the trial in experiment 3). The availability of live prey may also have facilitated lateralized predatory responses in toads indirectly. Increased numbers of predatory responses directed towards test stimuli after the toad is stimulated with olfactory cues, or fed with other prey, is a facilitative process thought to be driven by the telencephalon (Ingle 1976a). This is plausible, given that areas of the telencephalon are involved with associative processing (Muzio et al. 1993, 1994). Therefore, the experience of feeding on live prey may have ‘activated’ the left and right sides of the telencephalon to states that reveal their lateralized processing for insect prey moving regularly in clockwise and anticlockwise directions. It is also notable from experiment 3 that toads directed significantly more predatory responses to the clockwise-moving, rather than the anticlockwise-moving, dummy prey during the brief period that mealworms were being consumed. Also, in contrast to the ‘switching’ of attention between the mealworms and the dummy prey by toads in the clockwise condition, toads in the anticlockwise condition demonstrated selective inattention to the dummy prey.
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We have shown that feature analysis of prey is more sophisticated than has been previously considered. Using the results drawn from neuroethological studies of anuran vision, we have speculated on the levels of the B. marinus visual system that may be lateralized. Our finding of leftbrain specialization for control of predatory responses in B. marinus supports that of previous research (Vallortigara et al. 1998) and corresponds with that found in the decision-related feeding responses of higher vertebrates. This result is of particular interest given the anurans’ separate evolutionary trajectory, unique life cycle (from egg to larva and adult forms), and possession of a visual system (from the retina to mid-, hind- and forebrain levels) organized very differently to those of other vertebrates (e.g. Gaillard 1990). Our results have contributed to the known range of similarities found between the lateralized behaviours of anurans and those of other vertebrates. Briefly, motor preferences (‘handedness’) have been identified in a range of adult toads (B. marinus, B. bufo and B. viridis: Bisazza et al. 1996, 1997; summarized in Rogers 2002a) and have also been determined in the turning preferences of tadpoles across a range of species (summarized in Rogers 2002b; Wassersug & Yamashita 2002). Tadpoles of Litoria latopalmata also show lateralized visual responses to an overhead predator model, by preferentially directing escape turns to the right (Rogers 2002b). This suggests that the tadpoles may be more responsive to sudden movement observed with the left eye than with the right eye. Adult bufonids (B. marinus, B. bufo and B. viridis) are also more reactive to a predator model viewed with their left eye than the right eye (Lippolis et al. 2002). These examples of lateralization are consistent with the specialization of the right hemisphere (left eye) of chicks to detect changes in the immediate surroundings (Rogers & Anson 1979). Another example of visual lateralization that is consistent between anurans and other vertebrates is the control of agonistic behaviour. Agonistic responses are preferentially mediated by the left eye in adult B. marinus and B. bufo (Robins et al. 1998; Vallortigara et al. 1998), lizards (Anolis sp.: Deckel 1995), domestic chicks (Rogers et al. 1985) and gelada baboons, Theropithecus gelada (Casperd & Dunbar 1996). The growing evidence of brain lateralization in anurans suggests that, despite the considerable differences between vertebrate classes in the general structure of the brain and visual pathways (Striedter 1997), specializations in the left and right sides of anuran, reptilian, avian and mammalian brain have been conserved through evolution.
Acknowledgments The experiments with B. marinus formed part of A.R.’s research towards his Ph.D. at the University of New England. L. J. Rogers gratefully acknowledges funding from the Australian Research Council (a Special Investigator Award).
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