- Email: [email protected]
Learning of colour and position cues in domestic chicks: Males are better at position, females at colour
BEHAVI~U~~AL ELSEVIER
PROCESS
Behavioural
Processes 36 (1996) 289-296
Learning of colour and position cues in domestic chicks: Males are better at position, females at colour Giorgio Vallortigara
*
Istituto di Filosojia, Pedagogia, Didattica delle Lingue Moderne, Unioersitb di Udine, Via Antonini 8, 33100 Udine, Italy Accepted 27 July 1995
Abstract Male and female chicks were trained to discriminate between two boxes for food reinforcement. The correct box was indicated by a colour cue (red or brown) and a position cue (right or left). After learning, the colour and the position cues were dissociated: the right-left location of the two boxes was alternated between trials according to a semi-random sequence.The chicks were thus retrained to discriminate either on the basis of colour (irrespective of position) or on the basis of position (irrespective of colour). There were no sex differences, during training, with both position and colour cues. However, during re-training females performed better on the colour learning task and males performed better on the position learning task. Keywords: Visual learning; Colour discrimination Domestic chick; Gallus gallus domesticus
learning;
Position
discrimination
learning;
Attention;
Sex difference;
1. Introduction Sex differences in cognitive abilities have been described in several species of mammals (Maccoby and Jacklin, 1974; McNemar and Stone, 1932; Barrett and Ray, 1970; Gaulin and Fitzgerald, 1989). In birds, evidence for such differences is scanty. However, studies on brain lateralization in the domestic chick (Gallus gallus domesticus), provide indirect evidence that they do exist. Rashid and Andrew (1989) trained chicks to look for food hidden in a sawdust-filled tray and then dissociated distant and local spatial cues by rotating the tray through 180”. Both sexes showed a left eye advantage in orientation by topographical cues; however, left-eyed females made preferential use of conspicuous local cues, whereas left-eyed males made preferential use of distant cues provided by the general environment (Rashid, 1988).
* E-mail: [email protected] 0376-6357/96/$15.00 0 1996 Elsevier Science B.V. All rights reserved SSDI 0376-6357(95)000631
290
G. Vallortigarrr
/ Behuoioural
Prcmzsses 36 (1996) 289-296
Studies of simultaneous visual discrimination learning have revealed substantially identical abilities in the two sexes: males and females took the same number of trials and errors in both a colour discrimination task (Vallortigara, 1989) and a right-left discrimination task (Vallortigara et al., 1988; Vallortigara, 1989). Nevertheless, the fact that males and females are equally capable of learning object-specific and position-specific cues, does not rule out the possibility that the two sexes use the two cues differently. Indeed, the results of Rashid (1988) suggest that the two sexes may have a predisposition to attend to different characteristics of the environment during topographical learning. Similar predispositions could be present in visual discrimination learning as well, providing that multiple cues are available. This present study was designed to determine whether sex differences in colour and/or position learning could be observed in chicks following simultaneous visual discrimination learning of colour and position cues.
2. Materials
and methods
2.1. Subjects The subjects were 32 male and 32 female ‘Hybro’ (a local hybrid variety derived from the White Leghorn breed) chicks (Callus gallus domesticus) obtained from a commercial hatchery (Alba Allevamenti Avicoli, Ponte di Castegnero, Vicenza) when they were only a few hours old. Chicks were reared socially in same sex groups of 3-4 at a controlled temperature of 30-35°C. Food and water were available ad libitum. The rearing cages, 45 cm wide X 25 cm high X 35 cm deep, were illuminated from above by fluorescent lamps, and an artificial 14: 10-h light-dark cycle was provided. 2.2. Apparatus The experimental set up (see Fig. 1) consisted of a rectangular white-painted cage (40 X 30 X 60 cm) which was open at the top and had a slit at the bottom of one of the smaller walls through which two small boxes (5 X 6 X 4 cm) carrying visual stimuli and reinforcing food could be introduced. These boxes were made of a plastic material and had a drawer containing reinforcing food (Fig. lb). The stimulus figures (see below) were discs made of coloured cardboard and stuck onto a white background; they were inserted into the top of the boxes and when the chick pecked at the correct stimulus, the drawer containing food was pushed open by the experimenter so that food appeared just below and in front of the stimulus. A cloth (Fig. Ic) was attached above the slit in order to hide all manual operations of the experimenter from the chick. Above the cage an electric light bulb (60 W> lit the environment, and a unidirectional screen (Fig. lc) allowed the experimenter to watch the chick without being seen. There were two identical experimental cages so that simultaneous training of two animals could be carried out. 2.3. Procedure Chicks were isolated 24 h before testing (in cages identical to the rearing cages described above) in order to accustom them to the social isolation required for training. Testing started when chicks were
G. Vallortipru
291
/ Behauioural Processes 36 (1996) 289-296
b
Fig. 1. Apparatus used to train chicks. a = unidirectional screen in order to observe the chick without being seen; b = boxes containing food reinforcement; c = cloth to hide all manual operations or movements of the experimenter from the chick; d = mobile partition to coniine the chick to the far end of the apparatus during the intertrial interval.
8 days old. The chicks were deprived of food during the night of the day before testing. Food was provided again only after the end of the test session. Chicks were presented with two boxes simultaneously; each box had either a red or a brown coloured disc on its lid (25 mm diam.). The positive (reinforced) box could thus be indicated by the colour of its disc (red or brown) or by its position (right or left). There were 8 males and 8 females for each combination of the two cues indicating the positive box (i.e. red on the right, red on the left, brown on the right, brown on the left). At first, the positive box was placed in the test cage with the drawer open, and the chick was allowed to make a few pecks at the food. The drawer was then closed, and a few crumbs of food were put on the lid of the box. When the chick pecked at the crumbs, and therefore on the disc on the lid, the drawer was opened. Once the chick had eaten all the crumbs on the lid, it readily continued to peck at the disc of the box in order to have the food drawer opened again. The chick was then gently pushed back into the other half of the cage with a moveable cardboard partition that separated the two halves of the cage (see Fig. Id) and was kept there for 15 s. The cardboard partition was then lifted, and the chick was allowed to peck at the boxes for food reinforcement. When the chick pecked at the positive box, the drawer was opened and the chick was reinforced by allowing it access to the food for approximately 10 s. After this, the chick was again pushed back into the other half of the cage with the moveable partition, and was kept there for 15 s (inter-trial interval). Then the partition was lifted and the chick was again allowed to choose between the two boxes. If the chick pecked at the negative box, it was quickly pushed back into the other half of the cage with the mobile partition and, after 15 s, it was subjected to a further trial. The learning criterion used was 10 consecutive correct trials. Trials and errors to criterion were recorded. Re-training started 24 h after training (day 9). The animals were randomly assigned to two experimental groups. In both groups the right-left position of the two boxes was alternated in the
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Processes 36 (1996) 289-296
various trials according to a semi-random sequence (Fellows, 1967). In one group (16 males and 16 females) chicks were retrained to peck at the box located on the same position learnt during the training phase irrespective of the colour of its disc (position learning). In the other group (16 males and 16 females), chicks were retrained to peck at the box with the disc of the same colour of the training phase irrespective of its right/left position (colour learning). As during training, the learning criterion used during re-training was ten consecutive correct trials. The numbers of trials and errors to reach this criterion were recorded. However, errors during re-training were divided into two categories. These were errors on the cue reinforced during training and errors on the cue which had not been reinforced during training. Thus, for example, during position re-training if a chick had been trained to peck on the red disk in the left-hand position and was then re-trained to peck at the box in the left-hand position with alternation of the position of the red and brown disks between trials, it could make two types of errors: (i) a peck on the red disk (the cue reinforced during training) when this was in the right-hand position, or (ii) a peck on the brown disk (the cue which had not been reinforced during training) when this was in the right-hand position. Errors on the cue reinforced during training were designated as ‘cue errors’ and errors on the cue which had not been reinforced during training were designated as ‘no cue errors’. Data for the entire experiment were collected in four successive replications (n = 16) with balanced colour, position and sex conditions. 2.4. Statistical analyses Data for trials and errors during training were analyzed by a factorial analysis of variance (ANOVA) with sex (males, females), colour (red, brown) and position (right, left) as main effects. Data for trials and errors during re-training were analyzed by factorial ANOVA with task (colour learning, position learning) and sex (males, females) as main effects. Data for cue and no cue type of errors during re-training were analyzed by repeated measures MANOVA, with task (colour learning, position learning) and sex (males, females) as between-subjects factors and cue (cue errors, no cue errors) as the within-subjects factor.
3. Results and discussion The results of training are shown in Table 1. The ANOVA did not reveal any statistically significant main or interaction effects (sex: trials: F(l,56) = 0.036, errors F(1,56) = 0.399; colour: trials: F(1,56) = 0.194, errors: F( 1,561 = 0.005; position: trials: F( 1,56) = 0.194, errors: F( 1,561 =
Table 1 Mean ($- S.E.M.) trials and errors to reach learning criterion during training with addition of position and colour cues Females
Males
Trials Errors
Red
Brown
Red
Brown
Right
Left
Right
Left
Right
Left
Right
Left
17.0+2.3 2.8 + 0.9
16.1 -3.2 1.9+ 1.1
16.4+ 3.6 2.4+ 1.4
12.9 + 2.2 1.4kO.9
14.0* 2.8 1.85 1.3
17.7 5 3.6 3.1 _c 1.3
17.5k3.4 2.9+ 1.2
14.6* 2.5 2.6+ 1.4
G. Vallortigara / Behauiourul Processes 36 (1996) 289-296
293
-1
MEAN
Fig. 2. Mean ( f S.E.M.) trials and errors to reach learning criterion during training in male and female chicks.
0.056); sex X colour: trials F( 1,56) = 0.286, errors F( 1,56) = 0.224; sex X position: trials F( 1,56) = 0.437, errors F( 1,56) = 0.898; colour X position: trials F(1,56) = 1.356, errors: F(1,56) = 0.306; sex X colour X position: trials F(1,56) = 0.254, errors F(1,56) = 0.224.) Data for the two colours and the two positions were combined and shown in Fig. 2. Results for re-training trials and errors are shown in Fig. 3a,b respectively. The ANOVA’s revealed that the main effects of task (trials: F(1,60) = 0.016; errors: F(1,60) = 2.656) and sex (trials: F(1,60) = 0.001; errors: F(l,60) = 0.266) were not significant. However, in both ANOVA’s the 80
MEAN TRIALS 60
Fig. 3. Mean ( f S.E.M.) trials
(a) and errors (b) to reach learning criterion during re-training on colour or on position.
294
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Fig. 4. Mean ( f S.E.M.) errors during colour and position learning as a function of the previously learnt position and colour. ‘Cue’ indicates errors on the colour (for position learning) or on the position (for colour learning) that had been reinforced during previous training. ‘No cue’ indicates errors on the colour or on the position that had not been reinforced during previous training.
task X sex interaction was significant (trials: F(1,60) = 6.394, P = 0.013; errors: F(1,60) = 5.562, P = 0.020). Males showed significant differences between the two tasks in number of trials to criterion (F( 1,30) = 4.228, P = 0.046) but not in number of errors (F( 1,30) = 0.374), whereas females showed significant differences in both trials (F(l,30) = 4.578, P = 0.038) and errors (F(1,30) = 6.190, P = 0.017). Thus, males performed slightly better in the position learning task, whereas females performed clearly better in the colour learning task (see Fig. 3). The influences of colour or position cues during training performance were then investigated by comparing ‘cue’ and ‘no cue’ type of errors during re-training. Paired individual scores for ‘cue’ and ‘no cue’ were subjected to a repeated measures Anova which served to indicate the extent to which the position (or colour) learnt during training interfered with colour (or position) learning during re-training. These data are shown in Fig. 4. The ANOVA revealed a highly significant effect of the factor ‘cue’ (F(1,60) = 30.776, P < 0.011) and a significant task x sex interaction (F(1,60) = 5.426, P = 0.021). Interestingly, there were also a task X cue, and a task X sex X cue interaction (respectively, F(1,60) = 6.667, P = 0.011; F(1,60) = 6.916, P = 0.010). A separate analysis for males revealed a significant main effect of cue (F(1,30) = 13.134, P = 0.001) without any significant main effect of task (F(1,30) = 0.329 n.s.) or task X cue interaction (F(1,30) = 0.0001). A separate analysis for females, in contrast, revealed an highly significant task X cue interaction (F(1,30) = 13.055, P = 0.001). Further analyses in females revealed no effect of cue type of errors in the colour task (F( 1,15) = 0.352 n.s.) and a significant effect of cue type in the position test (F(1,15) = 20.468, P < 0.001). These results suggest that males remembered colour and position cues equally well, whereas females remembered colour cues better than position cues.
4. General discussion Since previous work using the same experimental paradigm has not revealed sex differences in position or in colour discrimination learning (see Introduction), the present data cannot be interpreted
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in terms of superior position learning abilities in males and superior colour learning abilities in females. Rather, it seems more likely that the two sexes have similar learning abilities, but have a predisposition to pay attention to different aspects of the environment. It seems that when both object-specific cues and position-specific cues are presented simultaneously, the two sexes encode both types of cues but tend to use them differently when the cues are subsequently dissociated. Male chicks seem to pay more attention to position and females more attention to colour. In the present study, there seemed to be sex differences in the degree of specificity with which cues were encoded. During re-training males seemed to remember the characteristics of one or other cue in a similar fashion though perhaps slightly better cues related to position; whereas females remember the specific colour on which they had been trained but did not remember (or use) previously learnt position cues. Although this is the first report of a sex difference in visual discrimination learning, studies of stabilization of attention, by testosterone, have produced pertinent results (Andrew, 1991). Sex differences appeared in a food search task where two familiar types of food differing in colour were scattered over a test floor (Andrew, 1972). Each chick began searching with a run of choice of one of the two colours. Untreated males commonly shifted, after varying periods, to equally consistent selection of the other colour. Testosterone postponed or prevented this shift in males. Females, in contrast, chose only one type of colour throughout the test; this held for both testosterone-treated as well as for controls. Also, positions where food had been found were remembered by testosteronetreated males, whereas females did not show any persistence of search in particular positions (Rogers and Andrew, 1989). Thus, males appeared to be more affected by position than females which seemed to maintain a stable strategy of search on the basis of a particular colour. Speculations on the adaptive function of such a sex difference are possible, though perhaps of limited value. McBride and Foenander (1962) and McBride et al. (1969) have reported that, in natural populations of feral animals, single dominant cocks maintain and patrol a large territory within which a number of females live. Sex differences in behaviour that are appropriate to such a social structure may be present in chicks, with males showing a predisposition to attend to spatial cues. Predispositions to attend to object-specific cues irrespective of their spatial locations could perhaps be associated with stronger social and affiliative behaviour in females (Workman and Andrew, 1989; Vallortigara et al., 1990), being object-specific cues such as visual characteristics of the head and neck regions crucial for species and individual recognition (Vallortigara and Andrew, 1994).
Acknowledgements I wish to thank Maria da1 Cin for help in training the animals, Mario Zanforlin for providing facilities in the Comparative Psychology Laboratory at the University of Padua where the experimental work was carried out, Lucia Regolin and two anonymous reviewers for thoughtful comments on the manuscript.
References Andrew, R.J., 1972. Changes in search behaviour Anim. Behav., 20: 741-7.50.
in male and female chicks, following
different
doses of testosterone.
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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 (R.J. Andrew, Editor), pp. 536-554, Oxford, Oxford University Press. Barrett, R.J. and Ray, O.S., 1970. Behavior in the open field, Lashley III maze, shuttle box and Sidman avoidance as a function of strain, sex and age. Devel. Psychol., 3: 73-77. Fellows, B.J., 1967. Chance stimulus sequences for discrimination tasks. Psychol. Bull., 67: 87-92. Gaulin, S.J.C. and Fitzgerald, R.W., 1989. Sexual selection for spatial-learning abilities. Anim. Behav., 37: 322-331. Maccoby, E. and Jacklin, C.N., 1974. The Psychology of Sex Differences. Stanford, Stanford University Press. McBride, G. and Foenander, F., 1962. Territorial behaviour in flocks of domestic fowls. Nature, 194: 102. McBride, G., Parer, I.P. and Foenander, F., 1969. The social organization and behaviour of the feral domestic fowl. Anim. Behav. Monogr., 2: 127-181. McNemar, Q. and Stone, C.P., 1932. The sex difference in rats on three learning tasks. J. Comp. Psychol., 14: 171-180. Rashid, N., 1988. Lateralization of topographical learning and other abilities in the chick. D. Phil. Thesis, University of Sussex, Brighton, U.K. Rashid, N. and Andrew, R.J., 1989. Right hemisphere advantages for topographical orientation in the domestic chick. Neuropsychologia, 27: 937-948. Rogers, L.J. and Andrew, R.J., 1989. Frontal and lateral visual field use by chicks after treatment with testosterone. Anim. Behav., 38: 394-405. Vallortigara, G., 1989. Behavioral asymmetries in visual learning of young chickens. Physiol. Behav., 45: 797-800. Vallortigara, G., Cailotto, M. and Zanforlin, M., 1990. Sex differences in social reinstatement motivation of young chicks (Gallus gallus) revealed by runway tests with social and nonsocial reinforcement. J. Comp. PsychoI., 104: 361-367. Vallortigara, G., Zanforlin, M. and Cailotto, M., 1988. Right-left asymmetry in position learning of male chicks. Behav. Brain Res., 27: 189-191. Vallortigara, G. and Andrew, R.J., 1994. Differential involvement of right and left hemisphere in individual recognition in the domestic chick. Behav. Process., 33: 41-58. Workman, L. and Andrew, R.J., 1989. Simultaneous changes in behaviour and in lateralization during the development of male and female domestic chicks. Anim. Behav., 38: 596-605.
PROCESS
Behavioural
Processes 36 (1996) 289-296
Learning of colour and position cues in domestic chicks: Males are better at position, females at colour Giorgio Vallortigara
*
Istituto di Filosojia, Pedagogia, Didattica delle Lingue Moderne, Unioersitb di Udine, Via Antonini 8, 33100 Udine, Italy Accepted 27 July 1995
Abstract Male and female chicks were trained to discriminate between two boxes for food reinforcement. The correct box was indicated by a colour cue (red or brown) and a position cue (right or left). After learning, the colour and the position cues were dissociated: the right-left location of the two boxes was alternated between trials according to a semi-random sequence.The chicks were thus retrained to discriminate either on the basis of colour (irrespective of position) or on the basis of position (irrespective of colour). There were no sex differences, during training, with both position and colour cues. However, during re-training females performed better on the colour learning task and males performed better on the position learning task. Keywords: Visual learning; Colour discrimination Domestic chick; Gallus gallus domesticus
learning;
Position
discrimination
learning;
Attention;
Sex difference;
1. Introduction Sex differences in cognitive abilities have been described in several species of mammals (Maccoby and Jacklin, 1974; McNemar and Stone, 1932; Barrett and Ray, 1970; Gaulin and Fitzgerald, 1989). In birds, evidence for such differences is scanty. However, studies on brain lateralization in the domestic chick (Gallus gallus domesticus), provide indirect evidence that they do exist. Rashid and Andrew (1989) trained chicks to look for food hidden in a sawdust-filled tray and then dissociated distant and local spatial cues by rotating the tray through 180”. Both sexes showed a left eye advantage in orientation by topographical cues; however, left-eyed females made preferential use of conspicuous local cues, whereas left-eyed males made preferential use of distant cues provided by the general environment (Rashid, 1988).
* E-mail: [email protected] 0376-6357/96/$15.00 0 1996 Elsevier Science B.V. All rights reserved SSDI 0376-6357(95)000631
290
G. Vallortigarrr
/ Behuoioural
Prcmzsses 36 (1996) 289-296
Studies of simultaneous visual discrimination learning have revealed substantially identical abilities in the two sexes: males and females took the same number of trials and errors in both a colour discrimination task (Vallortigara, 1989) and a right-left discrimination task (Vallortigara et al., 1988; Vallortigara, 1989). Nevertheless, the fact that males and females are equally capable of learning object-specific and position-specific cues, does not rule out the possibility that the two sexes use the two cues differently. Indeed, the results of Rashid (1988) suggest that the two sexes may have a predisposition to attend to different characteristics of the environment during topographical learning. Similar predispositions could be present in visual discrimination learning as well, providing that multiple cues are available. This present study was designed to determine whether sex differences in colour and/or position learning could be observed in chicks following simultaneous visual discrimination learning of colour and position cues.
2. Materials
and methods
2.1. Subjects The subjects were 32 male and 32 female ‘Hybro’ (a local hybrid variety derived from the White Leghorn breed) chicks (Callus gallus domesticus) obtained from a commercial hatchery (Alba Allevamenti Avicoli, Ponte di Castegnero, Vicenza) when they were only a few hours old. Chicks were reared socially in same sex groups of 3-4 at a controlled temperature of 30-35°C. Food and water were available ad libitum. The rearing cages, 45 cm wide X 25 cm high X 35 cm deep, were illuminated from above by fluorescent lamps, and an artificial 14: 10-h light-dark cycle was provided. 2.2. Apparatus The experimental set up (see Fig. 1) consisted of a rectangular white-painted cage (40 X 30 X 60 cm) which was open at the top and had a slit at the bottom of one of the smaller walls through which two small boxes (5 X 6 X 4 cm) carrying visual stimuli and reinforcing food could be introduced. These boxes were made of a plastic material and had a drawer containing reinforcing food (Fig. lb). The stimulus figures (see below) were discs made of coloured cardboard and stuck onto a white background; they were inserted into the top of the boxes and when the chick pecked at the correct stimulus, the drawer containing food was pushed open by the experimenter so that food appeared just below and in front of the stimulus. A cloth (Fig. Ic) was attached above the slit in order to hide all manual operations of the experimenter from the chick. Above the cage an electric light bulb (60 W> lit the environment, and a unidirectional screen (Fig. lc) allowed the experimenter to watch the chick without being seen. There were two identical experimental cages so that simultaneous training of two animals could be carried out. 2.3. Procedure Chicks were isolated 24 h before testing (in cages identical to the rearing cages described above) in order to accustom them to the social isolation required for training. Testing started when chicks were
G. Vallortipru
291
/ Behauioural Processes 36 (1996) 289-296
b
Fig. 1. Apparatus used to train chicks. a = unidirectional screen in order to observe the chick without being seen; b = boxes containing food reinforcement; c = cloth to hide all manual operations or movements of the experimenter from the chick; d = mobile partition to coniine the chick to the far end of the apparatus during the intertrial interval.
8 days old. The chicks were deprived of food during the night of the day before testing. Food was provided again only after the end of the test session. Chicks were presented with two boxes simultaneously; each box had either a red or a brown coloured disc on its lid (25 mm diam.). The positive (reinforced) box could thus be indicated by the colour of its disc (red or brown) or by its position (right or left). There were 8 males and 8 females for each combination of the two cues indicating the positive box (i.e. red on the right, red on the left, brown on the right, brown on the left). At first, the positive box was placed in the test cage with the drawer open, and the chick was allowed to make a few pecks at the food. The drawer was then closed, and a few crumbs of food were put on the lid of the box. When the chick pecked at the crumbs, and therefore on the disc on the lid, the drawer was opened. Once the chick had eaten all the crumbs on the lid, it readily continued to peck at the disc of the box in order to have the food drawer opened again. The chick was then gently pushed back into the other half of the cage with a moveable cardboard partition that separated the two halves of the cage (see Fig. Id) and was kept there for 15 s. The cardboard partition was then lifted, and the chick was allowed to peck at the boxes for food reinforcement. When the chick pecked at the positive box, the drawer was opened and the chick was reinforced by allowing it access to the food for approximately 10 s. After this, the chick was again pushed back into the other half of the cage with the moveable partition, and was kept there for 15 s (inter-trial interval). Then the partition was lifted and the chick was again allowed to choose between the two boxes. If the chick pecked at the negative box, it was quickly pushed back into the other half of the cage with the mobile partition and, after 15 s, it was subjected to a further trial. The learning criterion used was 10 consecutive correct trials. Trials and errors to criterion were recorded. Re-training started 24 h after training (day 9). The animals were randomly assigned to two experimental groups. In both groups the right-left position of the two boxes was alternated in the
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Processes 36 (1996) 289-296
various trials according to a semi-random sequence (Fellows, 1967). In one group (16 males and 16 females) chicks were retrained to peck at the box located on the same position learnt during the training phase irrespective of the colour of its disc (position learning). In the other group (16 males and 16 females), chicks were retrained to peck at the box with the disc of the same colour of the training phase irrespective of its right/left position (colour learning). As during training, the learning criterion used during re-training was ten consecutive correct trials. The numbers of trials and errors to reach this criterion were recorded. However, errors during re-training were divided into two categories. These were errors on the cue reinforced during training and errors on the cue which had not been reinforced during training. Thus, for example, during position re-training if a chick had been trained to peck on the red disk in the left-hand position and was then re-trained to peck at the box in the left-hand position with alternation of the position of the red and brown disks between trials, it could make two types of errors: (i) a peck on the red disk (the cue reinforced during training) when this was in the right-hand position, or (ii) a peck on the brown disk (the cue which had not been reinforced during training) when this was in the right-hand position. Errors on the cue reinforced during training were designated as ‘cue errors’ and errors on the cue which had not been reinforced during training were designated as ‘no cue errors’. Data for the entire experiment were collected in four successive replications (n = 16) with balanced colour, position and sex conditions. 2.4. Statistical analyses Data for trials and errors during training were analyzed by a factorial analysis of variance (ANOVA) with sex (males, females), colour (red, brown) and position (right, left) as main effects. Data for trials and errors during re-training were analyzed by factorial ANOVA with task (colour learning, position learning) and sex (males, females) as main effects. Data for cue and no cue type of errors during re-training were analyzed by repeated measures MANOVA, with task (colour learning, position learning) and sex (males, females) as between-subjects factors and cue (cue errors, no cue errors) as the within-subjects factor.
3. Results and discussion The results of training are shown in Table 1. The ANOVA did not reveal any statistically significant main or interaction effects (sex: trials: F(l,56) = 0.036, errors F(1,56) = 0.399; colour: trials: F(1,56) = 0.194, errors: F( 1,561 = 0.005; position: trials: F( 1,56) = 0.194, errors: F( 1,561 =
Table 1 Mean ($- S.E.M.) trials and errors to reach learning criterion during training with addition of position and colour cues Females
Males
Trials Errors
Red
Brown
Red
Brown
Right
Left
Right
Left
Right
Left
Right
Left
17.0+2.3 2.8 + 0.9
16.1 -3.2 1.9+ 1.1
16.4+ 3.6 2.4+ 1.4
12.9 + 2.2 1.4kO.9
14.0* 2.8 1.85 1.3
17.7 5 3.6 3.1 _c 1.3
17.5k3.4 2.9+ 1.2
14.6* 2.5 2.6+ 1.4
G. Vallortigara / Behauiourul Processes 36 (1996) 289-296
293
-1
MEAN
Fig. 2. Mean ( f S.E.M.) trials and errors to reach learning criterion during training in male and female chicks.
0.056); sex X colour: trials F( 1,56) = 0.286, errors F( 1,56) = 0.224; sex X position: trials F( 1,56) = 0.437, errors F( 1,56) = 0.898; colour X position: trials F(1,56) = 1.356, errors: F(1,56) = 0.306; sex X colour X position: trials F(1,56) = 0.254, errors F(1,56) = 0.224.) Data for the two colours and the two positions were combined and shown in Fig. 2. Results for re-training trials and errors are shown in Fig. 3a,b respectively. The ANOVA’s revealed that the main effects of task (trials: F(1,60) = 0.016; errors: F(1,60) = 2.656) and sex (trials: F(1,60) = 0.001; errors: F(l,60) = 0.266) were not significant. However, in both ANOVA’s the 80
MEAN TRIALS 60
Fig. 3. Mean ( f S.E.M.) trials
(a) and errors (b) to reach learning criterion during re-training on colour or on position.
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Fig. 4. Mean ( f S.E.M.) errors during colour and position learning as a function of the previously learnt position and colour. ‘Cue’ indicates errors on the colour (for position learning) or on the position (for colour learning) that had been reinforced during previous training. ‘No cue’ indicates errors on the colour or on the position that had not been reinforced during previous training.
task X sex interaction was significant (trials: F(1,60) = 6.394, P = 0.013; errors: F(1,60) = 5.562, P = 0.020). Males showed significant differences between the two tasks in number of trials to criterion (F( 1,30) = 4.228, P = 0.046) but not in number of errors (F( 1,30) = 0.374), whereas females showed significant differences in both trials (F(l,30) = 4.578, P = 0.038) and errors (F(1,30) = 6.190, P = 0.017). Thus, males performed slightly better in the position learning task, whereas females performed clearly better in the colour learning task (see Fig. 3). The influences of colour or position cues during training performance were then investigated by comparing ‘cue’ and ‘no cue’ type of errors during re-training. Paired individual scores for ‘cue’ and ‘no cue’ were subjected to a repeated measures Anova which served to indicate the extent to which the position (or colour) learnt during training interfered with colour (or position) learning during re-training. These data are shown in Fig. 4. The ANOVA revealed a highly significant effect of the factor ‘cue’ (F(1,60) = 30.776, P < 0.011) and a significant task x sex interaction (F(1,60) = 5.426, P = 0.021). Interestingly, there were also a task X cue, and a task X sex X cue interaction (respectively, F(1,60) = 6.667, P = 0.011; F(1,60) = 6.916, P = 0.010). A separate analysis for males revealed a significant main effect of cue (F(1,30) = 13.134, P = 0.001) without any significant main effect of task (F(1,30) = 0.329 n.s.) or task X cue interaction (F(1,30) = 0.0001). A separate analysis for females, in contrast, revealed an highly significant task X cue interaction (F(1,30) = 13.055, P = 0.001). Further analyses in females revealed no effect of cue type of errors in the colour task (F( 1,15) = 0.352 n.s.) and a significant effect of cue type in the position test (F(1,15) = 20.468, P < 0.001). These results suggest that males remembered colour and position cues equally well, whereas females remembered colour cues better than position cues.
4. General discussion Since previous work using the same experimental paradigm has not revealed sex differences in position or in colour discrimination learning (see Introduction), the present data cannot be interpreted
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in terms of superior position learning abilities in males and superior colour learning abilities in females. Rather, it seems more likely that the two sexes have similar learning abilities, but have a predisposition to pay attention to different aspects of the environment. It seems that when both object-specific cues and position-specific cues are presented simultaneously, the two sexes encode both types of cues but tend to use them differently when the cues are subsequently dissociated. Male chicks seem to pay more attention to position and females more attention to colour. In the present study, there seemed to be sex differences in the degree of specificity with which cues were encoded. During re-training males seemed to remember the characteristics of one or other cue in a similar fashion though perhaps slightly better cues related to position; whereas females remember the specific colour on which they had been trained but did not remember (or use) previously learnt position cues. Although this is the first report of a sex difference in visual discrimination learning, studies of stabilization of attention, by testosterone, have produced pertinent results (Andrew, 1991). Sex differences appeared in a food search task where two familiar types of food differing in colour were scattered over a test floor (Andrew, 1972). Each chick began searching with a run of choice of one of the two colours. Untreated males commonly shifted, after varying periods, to equally consistent selection of the other colour. Testosterone postponed or prevented this shift in males. Females, in contrast, chose only one type of colour throughout the test; this held for both testosterone-treated as well as for controls. Also, positions where food had been found were remembered by testosteronetreated males, whereas females did not show any persistence of search in particular positions (Rogers and Andrew, 1989). Thus, males appeared to be more affected by position than females which seemed to maintain a stable strategy of search on the basis of a particular colour. Speculations on the adaptive function of such a sex difference are possible, though perhaps of limited value. McBride and Foenander (1962) and McBride et al. (1969) have reported that, in natural populations of feral animals, single dominant cocks maintain and patrol a large territory within which a number of females live. Sex differences in behaviour that are appropriate to such a social structure may be present in chicks, with males showing a predisposition to attend to spatial cues. Predispositions to attend to object-specific cues irrespective of their spatial locations could perhaps be associated with stronger social and affiliative behaviour in females (Workman and Andrew, 1989; Vallortigara et al., 1990), being object-specific cues such as visual characteristics of the head and neck regions crucial for species and individual recognition (Vallortigara and Andrew, 1994).
Acknowledgements I wish to thank Maria da1 Cin for help in training the animals, Mario Zanforlin for providing facilities in the Comparative Psychology Laboratory at the University of Padua where the experimental work was carried out, Lucia Regolin and two anonymous reviewers for thoughtful comments on the manuscript.
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