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Making sense of variation: acquired functional categories and conditional discriminations
APPLIED ANIMAL BEHAVDUR SCIENCE
ELSEVIBR
Applied Animal Behaviour
Science 54 (1997) 59-62
Making sense of variation: acquired functional categories and conditional discriminations Christine J. Nicol Department
*
ofClinical Veterinary Science, University of Bristol, Langford House, Avon, BS18 7DU, UK
Appleby is to be congratulated on presenting a thought-provoking discussion of the limitations involved in applying animal behaviour studies to the design of welfare-improved animal accommodation. He is correct to draw our attention to the difficulties of interpretation that can result when single environmental features are considered in isolation. It may well be impossible to design an ideal environment if each stimulus is considered as a separate and lone entity. However, the primary problem with this approach may not be, as Appleby argues, the enormous variation that exists in the natural or .human-designed world per se. Animals have evolved mechanisms to deal with and make sense of complex patterns of variations. The problem is rather our tendency to underestimate or overlook these abilities. Appleby argues that innate categorisation of stimuli may be rarer than suggested in the ethological literature, and he structures his article around the difficulties that may arise when animals have to respond to a host of continuously varying environmental stimuli that do not occur in such a ready-categorised form. By taking this approach he overlooks the importance of acquired categorisations. Three mechanisms are particularly important in enabling animals to live with some economy in a variable world: habituation, generalisation resulting in functional equivalence, and discrimination allowing flexible responses to complex combinations of stimuli. Habituation to irrelevant stimuli is one of the most important mechanisms animals have for coping with the complexity of the real world. Horse owners are often advised to house road-shy animals in fields adjacent to a busy road in the hope that cars and lorries will become meaningless background stimuli, equivalent to the rustling of leaves on a tree, or the movement of clouds across the sky. Habituation substantially reduces the
* Tel: +44-117-9289473;
Fax: +44-1934-853443.
016%1591/9’7/$17.00 0 1997 Elsevier Science B.V. All rights reserved. PI1 SO168-1 591(96)01206-3
60
C.J. Nicol /Applied
Animal Behaviour Science 54 (1997) 59-62
perceived variation in the world, facilitating a gross distinction between relevant and irrelevant stimuli. The disparate stimuli that remain may then be grouped into categories. Categorisation may proceed on the basis of physical similarity between stimuli. Thus, Herrnstein et al. (1976) showed that pigeons were able to classify slides into seemingly natural classes, e.g. slides that contained trees were responded to similarly by the birds possibly because, despite the use of a wide variety of different views, the slides containing trees were more physically similar to each other than they were to the random assortment of control slides. However, animals are also able to group physically disparate stimuli into functionally equivalent categories. This ability has been demonstrated experimentally. Wasserman et al. (19921, for example, trained pigeons to associate dissimilar stimuli such as slides of chairs and slides of cars, with a common response. After this initial training pigeons were subsequently trained to associate one stimulus class only (the cars, for example) with a new behavioural response. The important finding was that the pigeons performed the new behaviour not just in response to the cars, but also in response to the chairs, although they had received no training that explicitly linked chairs with the new behaviour. The pigeons treated cars and chairs as functionally equivalent due to their initial pairing with the same outcome. Once functionally equivalent categories are established, new exemplars can rapidly be incorporated. Thus, sheep do not have an innate category that allows them to recognise sugar cubes as food, but after just one tasting of this novel substance, the subsequent sight of sugar activates food-recognition neurones (Kendrick and Baldwin, 1986). This suggests that complex interactions between innate and acquired categorisations are possible. Further flexibility and economy of response can be achieved by the ability to make conditional discriminations. Animals often behave in different ways when presented with the same stimulus. Thus, domestic cockerels may or may not give an alarm call in response to an overhead predator. Evans and Marler (1991) were able to show that this apparent variation in behavioural response depended on the presence or absence of a receptive audience. Cockerels are far more likely to give alarm calls when female chickens are present, than when they are absent. Female domestic hens show variable levels of tid-bitting behaviour in the presence of their chicks. A closer examination of this behaviour reveals that tid-bitting is increased if the chicks are feeding on food that the hen has learned is unpalatable (Nicol and Pope, 1996). The group size preferences of domestic hens depend crucially on the amount of floor area available and the social status of the hen that is choosing (Lindberg and Nicol, 1996). I would therefore argue that much of the apparent variation in behavioural response discussed by Appleby may, in fact, depend on the ability of animals to make conditional discriminations of the type “if in presence of A + B, do X, if in presence of A - B, do Y”. The variable nature of animal response may be further exaggerated by not distinguishing between response strength and response probability. In Appleby’s Fig. 2, for example, the Y-axis is simply labelled ‘Response’. However, an animal could respond reliably (i.e. 100% occurrence) to a given stimulus with variable response strength, or it could respond unreliably (i.e. on some occasions but not others) but always with a certain response strength. In the first example, response variation may be due to differences in internal motivation, in the second example the animal might be making
C. .I. Nicol/
Applied Animal Behaviour Science 54 (1997) 59-62
61
conditional decisions based on the presence or absence of a discriminative stimulus. By specifying more clearly the nature of response variation we may be able to discover patterns in apparent randomness. Apart from the issue of stimulus variation, Appleby’s main concern is that “so few stimuli have a discernable optimum”, resulting in an infinite series of relative choices. The fact that animals sometimes respond to environmental stimuli with rules of thumb, e.g. choose the most enclosed site available, could be seen as yet another mechanism to cope with environmental variation. But it is not clear whether it is correct that very few stimuli possess optimal values. Presumably, hens do not really choose the most enclosed nest site available if only because, at some point, the nest becomes physically impossible to enter. The optimal value may then be the most enclosed nest that can be accessed by the hen. But is this the nest we should try to provide in commercial systems? Probably not. Perhaps we should attempt to define the features of the minimum acceptable nest, and then determine whether choices between stimuli that exceed this minium level are purely relative, or whether they reflect increasing degrees of value. Would we discern differences in nest or substrate choice between naive and experienced hens, for example? Hens with experience of basic models may not necessarily subsequently prefer a de-luxe version. Familiarity may be of great importance in shaping subsequent preferences and experienced hens may come to prefer basic models. It would be even more interesting to look at differences in the responses of modestly provisioned and ‘spoiled’ hens. Would hens that had been provided with the most enclosed nests subsequently find more basic models unacceptable or would they, in the absence of enclosed versions, simply make a renewed relative choice? There is no convincing evidence that hens’ responses for food are influenced by prior expectations. Hens show no evidence of ‘contrast’ effects, i.e. their running speed in an alleyway is determined by the magnitude of the current food reinforcement, not by their previous reinforcement history. Hens thus show little evidence of ‘elation’ if they encounter more food than would be expected, and little evidence of ‘depression’ if they encounter less (Petherick et al., 1990). This suggests that absolute, rather than relative, values are attached to food. Nests may or may not be perceived in a similar manner, but experimental data on this point would be fascinating. Information of this kind would help us to decide whether the nest that reaches the physical limit of enclosure is truly a luxury, and not one of the bare necessities of life.
References Evans, C.S. and Marler, P., 1991. On video images as social stimuli in birds: audience Anim. Behav., 41: 17-26. Herrnstein, R.J., Loveland, D.H. and Cable, C., 1976. Natural concepts in pigeons. Behav. Proc., 2: 285-302. Kendrick, K.M. and Baldwin, B.A., 1986. The activity of neurones in the lateral incerta of the sheep responding to the sight or approach of food is modified by reflects food preferences. Brain Res., 375: 320-328.
effects on alarm calling. J. Exp. Psychol.
Anim.
hypothalamus and zona learning and satiety and
62
C.J. Nicol/ Applied Animal Behaviour Science 54 (1997) 59-62
Lindberg, A.C. and Nicol, C.J., 1996. Space and density effects on group size preferences in laying hens. Br. Poult. Sci., 37: 709-721. Nicol, C.J. and Pope, S.J., 1996. The maternal feeding display of domestic hens is sensitive to perceived chick error. Anim. Behav., 52: 767-774. Petherick, KC., Watson, G.M. and Duncan, I.J.H., 1990. Failure of domestic fowl to show contrast in learning: the implications for welfare. Appl. Anim. Behav. Sci., 28: 265-272. Wasserman, E.A., Devolder, C.L. and Coppage, D.J., 1992. Nonsimilarity base conceptualization in pigeons. Psychol. Sci., 3: 314-379.
ELSEVIBR
Applied Animal Behaviour
Science 54 (1997) 59-62
Making sense of variation: acquired functional categories and conditional discriminations Christine J. Nicol Department
*
ofClinical Veterinary Science, University of Bristol, Langford House, Avon, BS18 7DU, UK
Appleby is to be congratulated on presenting a thought-provoking discussion of the limitations involved in applying animal behaviour studies to the design of welfare-improved animal accommodation. He is correct to draw our attention to the difficulties of interpretation that can result when single environmental features are considered in isolation. It may well be impossible to design an ideal environment if each stimulus is considered as a separate and lone entity. However, the primary problem with this approach may not be, as Appleby argues, the enormous variation that exists in the natural or .human-designed world per se. Animals have evolved mechanisms to deal with and make sense of complex patterns of variations. The problem is rather our tendency to underestimate or overlook these abilities. Appleby argues that innate categorisation of stimuli may be rarer than suggested in the ethological literature, and he structures his article around the difficulties that may arise when animals have to respond to a host of continuously varying environmental stimuli that do not occur in such a ready-categorised form. By taking this approach he overlooks the importance of acquired categorisations. Three mechanisms are particularly important in enabling animals to live with some economy in a variable world: habituation, generalisation resulting in functional equivalence, and discrimination allowing flexible responses to complex combinations of stimuli. Habituation to irrelevant stimuli is one of the most important mechanisms animals have for coping with the complexity of the real world. Horse owners are often advised to house road-shy animals in fields adjacent to a busy road in the hope that cars and lorries will become meaningless background stimuli, equivalent to the rustling of leaves on a tree, or the movement of clouds across the sky. Habituation substantially reduces the
* Tel: +44-117-9289473;
Fax: +44-1934-853443.
016%1591/9’7/$17.00 0 1997 Elsevier Science B.V. All rights reserved. PI1 SO168-1 591(96)01206-3
60
C.J. Nicol /Applied
Animal Behaviour Science 54 (1997) 59-62
perceived variation in the world, facilitating a gross distinction between relevant and irrelevant stimuli. The disparate stimuli that remain may then be grouped into categories. Categorisation may proceed on the basis of physical similarity between stimuli. Thus, Herrnstein et al. (1976) showed that pigeons were able to classify slides into seemingly natural classes, e.g. slides that contained trees were responded to similarly by the birds possibly because, despite the use of a wide variety of different views, the slides containing trees were more physically similar to each other than they were to the random assortment of control slides. However, animals are also able to group physically disparate stimuli into functionally equivalent categories. This ability has been demonstrated experimentally. Wasserman et al. (19921, for example, trained pigeons to associate dissimilar stimuli such as slides of chairs and slides of cars, with a common response. After this initial training pigeons were subsequently trained to associate one stimulus class only (the cars, for example) with a new behavioural response. The important finding was that the pigeons performed the new behaviour not just in response to the cars, but also in response to the chairs, although they had received no training that explicitly linked chairs with the new behaviour. The pigeons treated cars and chairs as functionally equivalent due to their initial pairing with the same outcome. Once functionally equivalent categories are established, new exemplars can rapidly be incorporated. Thus, sheep do not have an innate category that allows them to recognise sugar cubes as food, but after just one tasting of this novel substance, the subsequent sight of sugar activates food-recognition neurones (Kendrick and Baldwin, 1986). This suggests that complex interactions between innate and acquired categorisations are possible. Further flexibility and economy of response can be achieved by the ability to make conditional discriminations. Animals often behave in different ways when presented with the same stimulus. Thus, domestic cockerels may or may not give an alarm call in response to an overhead predator. Evans and Marler (1991) were able to show that this apparent variation in behavioural response depended on the presence or absence of a receptive audience. Cockerels are far more likely to give alarm calls when female chickens are present, than when they are absent. Female domestic hens show variable levels of tid-bitting behaviour in the presence of their chicks. A closer examination of this behaviour reveals that tid-bitting is increased if the chicks are feeding on food that the hen has learned is unpalatable (Nicol and Pope, 1996). The group size preferences of domestic hens depend crucially on the amount of floor area available and the social status of the hen that is choosing (Lindberg and Nicol, 1996). I would therefore argue that much of the apparent variation in behavioural response discussed by Appleby may, in fact, depend on the ability of animals to make conditional discriminations of the type “if in presence of A + B, do X, if in presence of A - B, do Y”. The variable nature of animal response may be further exaggerated by not distinguishing between response strength and response probability. In Appleby’s Fig. 2, for example, the Y-axis is simply labelled ‘Response’. However, an animal could respond reliably (i.e. 100% occurrence) to a given stimulus with variable response strength, or it could respond unreliably (i.e. on some occasions but not others) but always with a certain response strength. In the first example, response variation may be due to differences in internal motivation, in the second example the animal might be making
C. .I. Nicol/
Applied Animal Behaviour Science 54 (1997) 59-62
61
conditional decisions based on the presence or absence of a discriminative stimulus. By specifying more clearly the nature of response variation we may be able to discover patterns in apparent randomness. Apart from the issue of stimulus variation, Appleby’s main concern is that “so few stimuli have a discernable optimum”, resulting in an infinite series of relative choices. The fact that animals sometimes respond to environmental stimuli with rules of thumb, e.g. choose the most enclosed site available, could be seen as yet another mechanism to cope with environmental variation. But it is not clear whether it is correct that very few stimuli possess optimal values. Presumably, hens do not really choose the most enclosed nest site available if only because, at some point, the nest becomes physically impossible to enter. The optimal value may then be the most enclosed nest that can be accessed by the hen. But is this the nest we should try to provide in commercial systems? Probably not. Perhaps we should attempt to define the features of the minimum acceptable nest, and then determine whether choices between stimuli that exceed this minium level are purely relative, or whether they reflect increasing degrees of value. Would we discern differences in nest or substrate choice between naive and experienced hens, for example? Hens with experience of basic models may not necessarily subsequently prefer a de-luxe version. Familiarity may be of great importance in shaping subsequent preferences and experienced hens may come to prefer basic models. It would be even more interesting to look at differences in the responses of modestly provisioned and ‘spoiled’ hens. Would hens that had been provided with the most enclosed nests subsequently find more basic models unacceptable or would they, in the absence of enclosed versions, simply make a renewed relative choice? There is no convincing evidence that hens’ responses for food are influenced by prior expectations. Hens show no evidence of ‘contrast’ effects, i.e. their running speed in an alleyway is determined by the magnitude of the current food reinforcement, not by their previous reinforcement history. Hens thus show little evidence of ‘elation’ if they encounter more food than would be expected, and little evidence of ‘depression’ if they encounter less (Petherick et al., 1990). This suggests that absolute, rather than relative, values are attached to food. Nests may or may not be perceived in a similar manner, but experimental data on this point would be fascinating. Information of this kind would help us to decide whether the nest that reaches the physical limit of enclosure is truly a luxury, and not one of the bare necessities of life.
References Evans, C.S. and Marler, P., 1991. On video images as social stimuli in birds: audience Anim. Behav., 41: 17-26. Herrnstein, R.J., Loveland, D.H. and Cable, C., 1976. Natural concepts in pigeons. Behav. Proc., 2: 285-302. Kendrick, K.M. and Baldwin, B.A., 1986. The activity of neurones in the lateral incerta of the sheep responding to the sight or approach of food is modified by reflects food preferences. Brain Res., 375: 320-328.
effects on alarm calling. J. Exp. Psychol.
Anim.
hypothalamus and zona learning and satiety and
62
C.J. Nicol/ Applied Animal Behaviour Science 54 (1997) 59-62
Lindberg, A.C. and Nicol, C.J., 1996. Space and density effects on group size preferences in laying hens. Br. Poult. Sci., 37: 709-721. Nicol, C.J. and Pope, S.J., 1996. The maternal feeding display of domestic hens is sensitive to perceived chick error. Anim. Behav., 52: 767-774. Petherick, KC., Watson, G.M. and Duncan, I.J.H., 1990. Failure of domestic fowl to show contrast in learning: the implications for welfare. Appl. Anim. Behav. Sci., 28: 265-272. Wasserman, E.A., Devolder, C.L. and Coppage, D.J., 1992. Nonsimilarity base conceptualization in pigeons. Psychol. Sci., 3: 314-379.