Behavioural Processes, 24 (1991) 0 1991
BEPROC
143
143-151
Elsevier Science Publishers
B.V. 0376-6357/91/$03.50
00365
Discrimination
of group members by laying hens Callus domesticus R.H. Bradshaw
Animal
Behaviour
Research Group, Department of Zoology, Oxford OX1 3PS, UK *
South Parks Road,
(Accepted 22 May 1991)
Abstract Two experiments were performed to test if hens could discriminate between conspecifics in learning experiments. In experiment 1, three adult hens were trained to discriminate between a member of their own social group and a member of a neighbouring group. Only one test bird showed evidence of discrimination above conventional levels of significance. In experiment 2, three hens were trained to discriminate between two members from their own social group. All test birds showed strong evidence of discrimination above conventional levels of significance indicating that laying hens are able to discriminate between members of their own social group.
Introduction Laying hens housed in small groups form stable linear hierarchies defined by the between individuals (Schjelderup-Ebbe, 1922). asymmetry of agonistic interactions Barnard & Burk (1979) and Zayan (1987) have pointed out that the implication that some or all group members individually recognise each other when a stable dominance hierarchy is formed has been uncritically accepted in the literature. Cuhl & Ortman (1953) have shown that visual cues are important in individual recognition. Zayan (1989) has suggested that individual recognition should not be confused with recognition of familiar conspecifics as distinct from unfamiliar ones. Zayan & Lamberty (1989) have further argued that individual recognition is a more complex learning process than recognition of familiar conspecifics, a process that occurs in chicks at much earlier stages of ontogeny, and could be accounted for in terms of habituation (Zayan 1987a). The
* Present
address
(until
Cambridge CB3 8AA, UK.
1 October
1991):
Sub-Department
of Animal
Behaviour,
Madingley,
144
great majority of the well controlled experiments which claim to have demonstrated individual recognition in the domestic fowl have only given clear evidence of the ability to discriminate familiar vs strange individuals in a social context. Early studies on social preferences in laying hens (Murchison 1935a,b,c; Cuhl 1942) and more recent work designed to contribute to an understanding of welfare (Hughes 1977; Dawkins 1982) have revealed that hens possess the ability to discriminate between familiar and unfamiliar conspecifics, particularly in social contexts clearly independent of agonistic interactions (Zayan 1987b). Animals have also been shown to possess the ability to discriminate between stimulus categories when presented with slides (e.g. pigeons: Cerella, 1979; Herrnstein & Loveland 1964; Herrnstein et al 1976; Poole & Lander 1971; budgerigars: Trillmich 1976; cock bantams: Ryan 1982; monkeys: Swartz 1983; Schrier et al 1984; Yoshikubo 1985; Dasser 1987) but little attention has focused on the classification of live animals whether allospecific or conspecific. Duncan (1987) pointed out that in order to demonstrate individual recognition it would be necessary to show that a bird could discriminate two birds from its own social group. In the case of domestic fowl an adequate proof has not been established (see Zayan 1990 for evidence of individual recognition in triads of pigs). The purpose of the present study was to develop a method by which laying hens may be trained to discriminate between members of their own social group.
Method
Subjects The work was conducted on two groups of six Ross Brown laying hens (32 weeks of age at the start of the experiment) housed in two rectangular indoor pens (2.2 m x 1.1 m X 3.5 m>, maintained on wood shavings on a light/dark schedule of 13 : 11 at 17 o C. Each bird was individually marked by means of coloured rings and were unfamiliar with each other prior to introduction at 18 weeks of age. Test birds were only separated from their group for one daily session for up to 45 min, 5 to 7 days a week. All test birds were experimentally naive.
Apparatus The animals were trained in a Y maze constructed
oi 0.8 cm plywood. The stem of the Y
maze, the start arm, was separated from each of the side arms by two raisable screens, one opaque and one transparent (Fig. I). At the end of each side-arm a perspex screen separated the stimulus bird from entry into the side-arm. Immediately in front of this screen was a small feeding pot.
General
training
procedure
Six birds (the ‘test birds’) were trained to discriminate between other laying hens (the ‘stimulus birds’). A test bird was placed in the start arm of the Y maze in an adjacent room.
145
start
clrm
0.38
m
0.38m
Fig. 1. Y-maze apparatus used for experiments 1 and 2 to train test birds to discriminate stimulus birds (not drawn to scale).
At the end of each side arm a cage was placed containing a stimulus bird. The test bird was released from the start arm 5 seconds after the removal of the opaque screen by raising the transparent screen. If the test bird ran to the positive stimulus bird then she was allowed to ingest 1 mealworm, placed in the feeding pot (so designed that neither the test nor stimulus birds could see the food placement). Food was administered on a schedule of continuous re-inforcement throughout the experiments. If the test bird ran to the negative stimulus bird then she found no food reward, and was pushed gently back to the start arm and screens were lowered. Each cycle of events as described constituted a ‘trial’. Each test bird was taught to discriminate between other individuals which were chosen randomly from the available groups (either familiar or unfamiliar depending on the experiment). The two individuals the test birds were taught to discriminate between were chosen randomly from the relevant group. Training took place 5 days a week, IO to 30 trials each afternoon. All test birds were mildly food deprived prior to training. Side cages and feeding pots were removed after each trial while the screens were in place even when replaced as before. Manipulations were conducted in the same order after each trial so that no particular sound sequence could cue the test bird as to the future position of the stimulus bird. Side cages were alternated every 15 trials so that choice could not be based on the side cage itself. The position of the stimulus birds was alternated according to a quasi-random series with the restriction that no more than three left or right choices could be made in a row, so as to prevent spatial preferences developing. If position preferences did occur (defined as six choices to one side) food was presented at the opposite side to the one preferred until chosen. The training then proceeded as before. Once a test bird had reached criterion for learning success (see below) a ‘blind test’ was conducted. During a ‘blind test’ a food reward was placed at both of the feeding pots (one at the end of each of the side arms) for 13 trials and then at neither of the feeding pots for 12 trials. The test bird would be expected to extinct the association of the positive stimulus bird with a food reward during
146
the test. Thus the blind test was designed to establish whether the birds had adopted the desired learning procedure. If the test bird continued to choose the positive stimulus bird she must have been making the discrimination in some way other than the association of the positive stimulus stimulus bird).
bird with
a food reward (e.g. by avoiding a dominant
negative
Data analysis Learning success was analysed in terms of blocks of 25 trials. The criterion for successful performance was set at p = 0.05. According to the binomial probability table, a minimum of 17 correct of 25 responses was required for a significant trial block. A minimum of two successful trial blocks in succession were required for criterion. In the case of a ‘blind test’, the 13 trials with the food reward presented at both feeding pots and the 12 trials where no food rewards were presented, were pooled; 17 correct choices out of the pooled 25 trials constituted a significant blind test.
Experiment
7
Three test birds (A, B, and C) were required to discriminate between one ‘familiar’ bird (member of own group) and one ‘unfamiliar’ bird (member of neighbouring group). A food supply of 800 g (corn and cereal> per 24 hour period was provided. Training took place for 5 consecutive days, followed by two days when training did not take place. For the first 100 trials side cages were exchanged randomly. exchanged according to the quasi-random series.
Experiment
Thereafter
side cages were
2
Three test birds (X, Y and Z> were trained according to the general training procedure to discriminate between two stimulus birds from their own social group. Training in experiment 2 differed from that previously employed in that it was conducted in a number of discrete ‘steps’, a step consisting of a discrimination between a positive and a negative ‘stimulus object’. Steps became progressively more difficult as training proceeded and may be identified according to the objects to be discriminated (see Table I>.This modification was adopted because the results of experiment 1 were somewhat inconclusive, and the training procedure was extremely labour intensive. The test bird was only transferred between steps once the criterion for learning success was reached for the preceding step. The ‘training’ element of the procedure (association of the positive stimulus bird with the food reward) was, therefore, separated from the ‘task’ (discrimination of two familiar hens). A test bird that failed to reach criterion for a particular step was discontinued from training. The birds were food deprived for four hours prior to training and, as in the previous experiment, stimulus birds were alternated according to the quasi-random series.
147 TABLE1 The discrimination
steps in experiment
2.
Step
Positive object
Negative object
1
Familiar hen (P)
Empty cage
2
Familiar hen (P)
Plant in pot
3
Familiar hen (P)
Pigeon decoy
4
Familiar hen (P)
Familiar hen (Q)
5
Familiar hen (P)
New familiar hen (R)
6
New familiar hen (S)
New familiar hen (R)
Results
Experiment
The data
1
for experiment
Bird A achieved
the
1 are summarised
discrimination
required
in Table and
2.
yielded
a significant
blind
test result.
may have been an artefact of negative bird avoidance (a light weight bird with a very small comb). Bird B did not achieve a significant discrimination ability after 625 trials. This bird died of a prolapse three days after the completion of 600 trials, and
This finding
performance discrimination
TABLE
may have reflected over 500 trials.
the onset of illness.
Bird C failed
to achieve
significant
2
Average number of correct responses per blocks of 25 trials in experiment Trial block
1.
Bird A
B
C
I-
2
13
12.5
16
3-
4
13
13.5
15.5
5-
6
18 *
13.5
13
7-
8
14.5
15
11
9-10
19 *
12.5
15
II-12
15.5
9
14
13-14
19.5 *
13.5
13
15-16
15.5
15
14.5
17-18
15
14.5
16
19-20
18 *
16.5
21-22
18.5 *b -
11
13.5 _
23-24 25-26
13 12
* A significant trial block occurred (I 7 correct of 25 responses); b signifies a significant blind test was conducted.
148 TABLE
3
Average number of correct responses per blocks of 25 trials in experiment Trial
Birds
blocks Step 1 I- 2 3-
2.
X
Y
Z
17
17.5 *
18 *
4
18 *
Step 2 5- 6
18 *
19 *
18 *
Step 3 7- 8
18 *
19.5 *
18 *
Step 4 9-10
15
14.5
18
11-12
16
16
16
13-14
21.5 *
16.5
19.5 *
15-16
_
18 *
Step 5 17-18
22 *1’
17
Step 6 19-20
18 *h
_
18.5 *’ _
* A significant trial block occurred (17 correct of 25 responses); conducted;
Experiment The
2
data for experiment
All three Birds
test
birds
2 are summarised
reached
criterion
X and Z were able to discriminate
Y achieved hen with
the discrimination
a ‘new familiar’
new discrimination
stimulus
200
but failed
training.
After
the
between trials.
bird (Step
(Step
bird
hen was
X was able to discriminate
stimulus
hen, and yielded
stimuli.
The
members
a second
conclusively of their
own
this
significant show
After blind
a significant
stimulus
3.
familiar
that
four
were
from
training.
blind
test result
new
4). Bird stimulus
Y was unable
to make
and was discontinued
from
laying
(Step
Z was able to make the
a ‘new
blind test result the
trials
of the negative
Bird Bird
with
bird
of the experiment.
less clear cut. Bird X made the
test result.
replaced
steps
hens after 150
replacement
5>, results
a significant
to yield
positive
results
in Table
for each of the first
and was discontinued
discrimination
between
after
and yielded
the new discrimination
6)
’ signifies a significant blind test was
f signifies the blind test was failed.
hens
the with
familiar’ ‘new
from
positive
familiar’
hen
negative
these two individuals
were
able
to
as
discriminate
social group.
Discussion The since
results the
documented 1982)
and
in experiment
ability
1 seem to have been an artefact of a poor training
to discriminate
in choice in sequences
tests
between
(Murchison
of paired
familiar 1935a,b,c;
contests
(Zayan
and
unfamiliar
Guhl
1942;
1987a).
This
procedure
conspecifics Hughes
has
1977;
suggestion
been
Dawkins
was further
149
supported by the results in experiment
2. After modifications to the training procedure a
significantly high proportion of correct choices was recorded for all test birds, indicating the ability to discriminate between members of their own social group. Despite early attempts to investigate the ability of domestic fowl to discriminate between members of their own social group (Murchison 1935b; Guhl 1942), there have not been any recent studies. This is surprising considering the quantity of literature devoted to investigating the mechanisms underlying the formation and maintenance of the dominance hierarchy in fowl, and the commercial importance such an understanding may have to the poultry industry. Studies on other species are few. Ryan (1982) reported that cock bantams were able to discriminate between conspecifics presented by means of coloured slides, and suggested the birds formed concepts of the birds to be discriminated but these results have been heavily criticised (see Dasser 1987). In a series of learning experiments designed to investigate mate choice, Trillmich (1976) revealed the discriminatory abilities of budgerigars. These experiments revealed that budgerigars were able to discriminate between both live animals and slides of conspecifics. One of these birds was able to transfer its discrimination from slides to the live birds but not from live birds to slides. Most other studies have concentrated exclusively on the use of slides. This is surprising since the extent to which an animal relates a photographic representation of a familiar conspecific to the real object is unclear (e.g. Premack 1976; Premack & Premack 1983). Dasser (I 987) was able to show long-tailed macaques apparently recognised the identity of group members presented by means of coloured slides in learning experiments. After training on a minimum of examples of slides of group members, the test birds correctly identified in unique transfer trials novel views of the stimulus animal used in training. More recent work has shown that video recognition is possible in some species (Clark & Uetz 1990; Evans & Marler 1991). There is no doubt that in the present study the test birds were discriminating between the stimulus birds presented. The precise mechanism the birds were using to make the discriminations is less clear. Test bird X continued to choose the positive bird during the blind test and carried out a series of complex transfer tests. Since we would expect the bird to extinct the association of the positive stimulus bird with a food reward during the blind test, this bird was making the discriminations in some way other than the association of the positive stimulus bird with a food reward (e.g. by avoiding a negative stimulus bird). Test bird Z failed to choose the positive bird under blind test conditions (due to the extinction of the bird’s learned association of the positive stimulus bird with the food reward) indicating the bird had learned the required procedure during training.
Bird Y failed to
reach criterion at step 5 and no blind test was conducted. Thus the results clearly show the test birds are able to discriminate between group members but in only one case was this due to the adoption of the desired learning procedure. The birds may have been responding to one or two learnt features (e.g. body colour) to make the discriminations. Body colour is subject to variation depending on the orientation of the bird and it seems unlikely that any one feature or combination of features was sufficiently defined throughout the training. It seems more likely that the test birds were responding to a number of features. The results of the present experiments are in line with Cuhl & Ortman’s assertion that laying hens individually recognise each other. They do demonstrate that individual recognition is the mechanism underlying the maintenance of the dominance hierarchy. Because laying hens possess
(1953) original not, however, formation and the ability to
150 discriminate
Conclusion A training between
procedure
members
discriminations of the two. the ability
was developed
of
their
is unclear: The
results
to form
own
they could of their
laying
group.
The
be using visual
add substantial
concepts
whereby
social
support
own
hens
were
precise
or behavioural
to the hypothesis
group
trained
cues
to discriminate
used
to
make
the
cues or a combination that laying
hens
possess
members.
Acknowledgements This
research
Studentship, Studentship
(Linacre
am grateful criticism
was supported
a Meyricke
College,
to Marian
throughout
by a Ministry
Graduate
(Jesus
Fisheries
College,
Oxford),
and Food
Research
and an A.].
Hosier
Oxford).
Stamp
the
of Agriculture
Scholarship Dawkins
study.
Matthew
her considerable Sullivan
read
encouragement
earlier
draft
and helpful
versions
of
the
manuscript.
References Barnard, C.J. & Burk, T. (1979) J. Theor. Cerella,
Dominance hierarchies and the evolution of “Individual
Recognition”.
Biol. 81: 65-73.
J. (1979)
Visual classes and natural categories in the pigeon. J. Exp. Psych.:
Hum.
Pert Perf.
5: 68-77. Clark, D.L. (Araneae
& Uetz, G.W.
(1990)
Video image recognition by the jumping spider, Maevia inclemens
: Salticidae). Anim. Behav. 40: 884-890.
Dasser, V. (1987)
Slides of group members as representatives of the real animals (Macaca
mulatta).
Ethology 76: 65-73. Dawkins,
M.S. (1982)
Elusive concept of preferred group size in domestic hens. Appl. Anim. Ethol. 8:
365-375. Duncan, I.T.J. (1987)
Social preference and cognition:
the Domestic Fowl (Zayan,
R. & Duncan, I.J.H.,
Evans, C.S. & Marler, P. (1991) on alarm calling. Anim. Guhl,
A.M.
(1942)
In: Cognitive Aspects of Social Behaviour in
Eds.), Elsevier,
New York,
p. 24-33.
On the use of video images as social stimuli in birds: audience effects
Behav. 41 : 17-26.
Social discrimination
in small flocks of the common domestic fowl.
J. Comp.
Psych. 34: 127-148. Guhl, A.M. & Ortman,
L.L. (1953)
Visual patterns in the recognition of individuals amongst chickens.
Condor 55: 287-298. Herrnstein,
R.J. & Loveland,
D.H.
(1964)
Complex
visual concepts in the pigeon. Science 146:
549-551. Herrnstein,
R.J., Loveland, D.H.
& Cable, C. (1976)
Natural concept in pigeons. J. Exp. Psych. Anim.
Behav. Proc. 2: 2855302. Hughes,
B.O. (1977)
Murchison,
C. (1935a)
Selection of group size by individual laying hens. Br. Poult. Sci. 18: 9-18. The experimental
measurement
of a social hierarchy
in Callus domesticus.
I.
The direct identification and direct measurement of social Reflex No. 1 and Social Reflex No. 2. J. Gen. Psych. 12: 3-39.
151 Murchison,
C. (193513) The experimental
The identification and inferential means of social discrimination. Murchison,
C. (1935c)
measurement of a social hierarchy in Gal/us domesticus.
J. Sot. Psych. 6: 3-30.
The experimental
measurement of a social hierarchy in Callus domesticus.
The direct and inferential measurement of Social Reflex No. 3. J. Gen. Psych. 46: Poole, J. & Lander, D.G. (1971) Premack, D. (1976)
The Mind of an Ape. W.W.
Hillsdale.
Norton & Co., New York.
(I982) Concept formation and individual recognition in the domestic chicken (Callus
Behav. Anal. Lett. 2: 213-220.
gal/us).
Schelderup-Ebbe, Schrier,
III.
76-102.
The pigeon’s concept of pigeon. Psych. Sci. 25: 157-158.
Intelligence in Ape and Man. Lawrence Erlbaum Association,
Premack, D. & Premack, A.J. (1983) Ryan, C.M.E.
II.
measurement of Social Reflex No. 1 and Social Reflex No. 2 by
A.M.,
monkeys:
T. (1922)
Angarella,
Beitrage zur Sozialpsychologie R. & Povar, M.L.
Concept human,
monkeys
(1984)
des Haushuhns.
Studies
Z. Psychol. 88: 225-252.
of concept formation
and letter A. J. Exp.
Psychol.
Anim.
by stumptailed
Behav.
Proc.
10:
564-584. Swartz,
K.B. (1983)
Psychobiol. Trillmich,
Species discrimination
in infant pigtail macaques with pictorial stimuli.
Develop.
16: 219-231.
F. (1976)
Learning experiments
on individual
recognition in budgerigars. Z. Tierpsychol.
41: 372-395. Yoshikubo,
S. (1985)
mulatta).
Species discrimination
and concept formation
by rhesus
monkeys
(Macaca
Primates 26: 285-299.
Zayan, R. (1987a)
Recognition of individuals indicated by aggression and dominance in pairs of fowl.
In: Cognitive Aspects of Social Behaviour Eds.), Elsevier,
in the Domestic
Fowl (Zayan,
R. & Duncan,
I. J.H.,
New York, p. 321-438.
Zayan, R. (198713) Recognition of individuals indicated by activity independent of agonistic context in laying hens: In: Cognitive Aspects of Social Behaviour in the Domestic Fowl (Zayan, R. & Duncan, I.J.H., Zayan,
Eds.), Elsevier, R. & Lamberty,
New York, p. 439-492. Y. (1989)
Experimental
learning or habituation? In: Ethnoexperimental R.J.,
Brain,
P.F.,
Netherlands, Zayan,
R. (1990)
Blanchard,
D.C.
evidence of social recognition
in domestic fowl:
Approaches to the Study of Behaviour (Blanchard,
& Parmigiani,
S.,
Eds.),
Kluwer
Academic Publishers,
The
p. 691-702. The effect of social recognition upon aggression and corticosteroid
triads of piglets. In: Social Stress Academic Publishers,
in Domestic Animals
The Netherlands,
p. 157-199.
(Zayan,
R. & Dantzer,
responses
R. Eds.),
in
Kluwer
BEPROC
143
143-151
Elsevier Science Publishers
B.V. 0376-6357/91/$03.50
00365
Discrimination
of group members by laying hens Callus domesticus R.H. Bradshaw
Animal
Behaviour
Research Group, Department of Zoology, Oxford OX1 3PS, UK *
South Parks Road,
(Accepted 22 May 1991)
Abstract Two experiments were performed to test if hens could discriminate between conspecifics in learning experiments. In experiment 1, three adult hens were trained to discriminate between a member of their own social group and a member of a neighbouring group. Only one test bird showed evidence of discrimination above conventional levels of significance. In experiment 2, three hens were trained to discriminate between two members from their own social group. All test birds showed strong evidence of discrimination above conventional levels of significance indicating that laying hens are able to discriminate between members of their own social group.
Introduction Laying hens housed in small groups form stable linear hierarchies defined by the between individuals (Schjelderup-Ebbe, 1922). asymmetry of agonistic interactions Barnard & Burk (1979) and Zayan (1987) have pointed out that the implication that some or all group members individually recognise each other when a stable dominance hierarchy is formed has been uncritically accepted in the literature. Cuhl & Ortman (1953) have shown that visual cues are important in individual recognition. Zayan (1989) has suggested that individual recognition should not be confused with recognition of familiar conspecifics as distinct from unfamiliar ones. Zayan & Lamberty (1989) have further argued that individual recognition is a more complex learning process than recognition of familiar conspecifics, a process that occurs in chicks at much earlier stages of ontogeny, and could be accounted for in terms of habituation (Zayan 1987a). The
* Present
address
(until
Cambridge CB3 8AA, UK.
1 October
1991):
Sub-Department
of Animal
Behaviour,
Madingley,
144
great majority of the well controlled experiments which claim to have demonstrated individual recognition in the domestic fowl have only given clear evidence of the ability to discriminate familiar vs strange individuals in a social context. Early studies on social preferences in laying hens (Murchison 1935a,b,c; Cuhl 1942) and more recent work designed to contribute to an understanding of welfare (Hughes 1977; Dawkins 1982) have revealed that hens possess the ability to discriminate between familiar and unfamiliar conspecifics, particularly in social contexts clearly independent of agonistic interactions (Zayan 1987b). Animals have also been shown to possess the ability to discriminate between stimulus categories when presented with slides (e.g. pigeons: Cerella, 1979; Herrnstein & Loveland 1964; Herrnstein et al 1976; Poole & Lander 1971; budgerigars: Trillmich 1976; cock bantams: Ryan 1982; monkeys: Swartz 1983; Schrier et al 1984; Yoshikubo 1985; Dasser 1987) but little attention has focused on the classification of live animals whether allospecific or conspecific. Duncan (1987) pointed out that in order to demonstrate individual recognition it would be necessary to show that a bird could discriminate two birds from its own social group. In the case of domestic fowl an adequate proof has not been established (see Zayan 1990 for evidence of individual recognition in triads of pigs). The purpose of the present study was to develop a method by which laying hens may be trained to discriminate between members of their own social group.
Method
Subjects The work was conducted on two groups of six Ross Brown laying hens (32 weeks of age at the start of the experiment) housed in two rectangular indoor pens (2.2 m x 1.1 m X 3.5 m>, maintained on wood shavings on a light/dark schedule of 13 : 11 at 17 o C. Each bird was individually marked by means of coloured rings and were unfamiliar with each other prior to introduction at 18 weeks of age. Test birds were only separated from their group for one daily session for up to 45 min, 5 to 7 days a week. All test birds were experimentally naive.
Apparatus The animals were trained in a Y maze constructed
oi 0.8 cm plywood. The stem of the Y
maze, the start arm, was separated from each of the side arms by two raisable screens, one opaque and one transparent (Fig. I). At the end of each side-arm a perspex screen separated the stimulus bird from entry into the side-arm. Immediately in front of this screen was a small feeding pot.
General
training
procedure
Six birds (the ‘test birds’) were trained to discriminate between other laying hens (the ‘stimulus birds’). A test bird was placed in the start arm of the Y maze in an adjacent room.
145
start
clrm
0.38
m
0.38m
Fig. 1. Y-maze apparatus used for experiments 1 and 2 to train test birds to discriminate stimulus birds (not drawn to scale).
At the end of each side arm a cage was placed containing a stimulus bird. The test bird was released from the start arm 5 seconds after the removal of the opaque screen by raising the transparent screen. If the test bird ran to the positive stimulus bird then she was allowed to ingest 1 mealworm, placed in the feeding pot (so designed that neither the test nor stimulus birds could see the food placement). Food was administered on a schedule of continuous re-inforcement throughout the experiments. If the test bird ran to the negative stimulus bird then she found no food reward, and was pushed gently back to the start arm and screens were lowered. Each cycle of events as described constituted a ‘trial’. Each test bird was taught to discriminate between other individuals which were chosen randomly from the available groups (either familiar or unfamiliar depending on the experiment). The two individuals the test birds were taught to discriminate between were chosen randomly from the relevant group. Training took place 5 days a week, IO to 30 trials each afternoon. All test birds were mildly food deprived prior to training. Side cages and feeding pots were removed after each trial while the screens were in place even when replaced as before. Manipulations were conducted in the same order after each trial so that no particular sound sequence could cue the test bird as to the future position of the stimulus bird. Side cages were alternated every 15 trials so that choice could not be based on the side cage itself. The position of the stimulus birds was alternated according to a quasi-random series with the restriction that no more than three left or right choices could be made in a row, so as to prevent spatial preferences developing. If position preferences did occur (defined as six choices to one side) food was presented at the opposite side to the one preferred until chosen. The training then proceeded as before. Once a test bird had reached criterion for learning success (see below) a ‘blind test’ was conducted. During a ‘blind test’ a food reward was placed at both of the feeding pots (one at the end of each of the side arms) for 13 trials and then at neither of the feeding pots for 12 trials. The test bird would be expected to extinct the association of the positive stimulus bird with a food reward during
146
the test. Thus the blind test was designed to establish whether the birds had adopted the desired learning procedure. If the test bird continued to choose the positive stimulus bird she must have been making the discrimination in some way other than the association of the positive stimulus stimulus bird).
bird with
a food reward (e.g. by avoiding a dominant
negative
Data analysis Learning success was analysed in terms of blocks of 25 trials. The criterion for successful performance was set at p = 0.05. According to the binomial probability table, a minimum of 17 correct of 25 responses was required for a significant trial block. A minimum of two successful trial blocks in succession were required for criterion. In the case of a ‘blind test’, the 13 trials with the food reward presented at both feeding pots and the 12 trials where no food rewards were presented, were pooled; 17 correct choices out of the pooled 25 trials constituted a significant blind test.
Experiment
7
Three test birds (A, B, and C) were required to discriminate between one ‘familiar’ bird (member of own group) and one ‘unfamiliar’ bird (member of neighbouring group). A food supply of 800 g (corn and cereal> per 24 hour period was provided. Training took place for 5 consecutive days, followed by two days when training did not take place. For the first 100 trials side cages were exchanged randomly. exchanged according to the quasi-random series.
Experiment
Thereafter
side cages were
2
Three test birds (X, Y and Z> were trained according to the general training procedure to discriminate between two stimulus birds from their own social group. Training in experiment 2 differed from that previously employed in that it was conducted in a number of discrete ‘steps’, a step consisting of a discrimination between a positive and a negative ‘stimulus object’. Steps became progressively more difficult as training proceeded and may be identified according to the objects to be discriminated (see Table I>.This modification was adopted because the results of experiment 1 were somewhat inconclusive, and the training procedure was extremely labour intensive. The test bird was only transferred between steps once the criterion for learning success was reached for the preceding step. The ‘training’ element of the procedure (association of the positive stimulus bird with the food reward) was, therefore, separated from the ‘task’ (discrimination of two familiar hens). A test bird that failed to reach criterion for a particular step was discontinued from training. The birds were food deprived for four hours prior to training and, as in the previous experiment, stimulus birds were alternated according to the quasi-random series.
147 TABLE1 The discrimination
steps in experiment
2.
Step
Positive object
Negative object
1
Familiar hen (P)
Empty cage
2
Familiar hen (P)
Plant in pot
3
Familiar hen (P)
Pigeon decoy
4
Familiar hen (P)
Familiar hen (Q)
5
Familiar hen (P)
New familiar hen (R)
6
New familiar hen (S)
New familiar hen (R)
Results
Experiment
The data
1
for experiment
Bird A achieved
the
1 are summarised
discrimination
required
in Table and
2.
yielded
a significant
blind
test result.
may have been an artefact of negative bird avoidance (a light weight bird with a very small comb). Bird B did not achieve a significant discrimination ability after 625 trials. This bird died of a prolapse three days after the completion of 600 trials, and
This finding
performance discrimination
TABLE
may have reflected over 500 trials.
the onset of illness.
Bird C failed
to achieve
significant
2
Average number of correct responses per blocks of 25 trials in experiment Trial block
1.
Bird A
B
C
I-
2
13
12.5
16
3-
4
13
13.5
15.5
5-
6
18 *
13.5
13
7-
8
14.5
15
11
9-10
19 *
12.5
15
II-12
15.5
9
14
13-14
19.5 *
13.5
13
15-16
15.5
15
14.5
17-18
15
14.5
16
19-20
18 *
16.5
21-22
18.5 *b -
11
13.5 _
23-24 25-26
13 12
* A significant trial block occurred (I 7 correct of 25 responses); b signifies a significant blind test was conducted.
148 TABLE
3
Average number of correct responses per blocks of 25 trials in experiment Trial
Birds
blocks Step 1 I- 2 3-
2.
X
Y
Z
17
17.5 *
18 *
4
18 *
Step 2 5- 6
18 *
19 *
18 *
Step 3 7- 8
18 *
19.5 *
18 *
Step 4 9-10
15
14.5
18
11-12
16
16
16
13-14
21.5 *
16.5
19.5 *
15-16
_
18 *
Step 5 17-18
22 *1’
17
Step 6 19-20
18 *h
_
18.5 *’ _
* A significant trial block occurred (17 correct of 25 responses); conducted;
Experiment The
2
data for experiment
All three Birds
test
birds
2 are summarised
reached
criterion
X and Z were able to discriminate
Y achieved hen with
the discrimination
a ‘new familiar’
new discrimination
stimulus
200
but failed
training.
After
the
between trials.
bird (Step
(Step
bird
hen was
X was able to discriminate
stimulus
hen, and yielded
stimuli.
The
members
a second
conclusively of their
own
this
significant show
After blind
a significant
stimulus
3.
familiar
that
four
were
from
training.
blind
test result
new
4). Bird stimulus
Y was unable
to make
and was discontinued
from
laying
(Step
Z was able to make the
a ‘new
blind test result the
trials
of the negative
Bird Bird
with
bird
of the experiment.
less clear cut. Bird X made the
test result.
replaced
steps
hens after 150
replacement
5>, results
a significant
to yield
positive
results
in Table
for each of the first
and was discontinued
discrimination
between
after
and yielded
the new discrimination
6)
’ signifies a significant blind test was
f signifies the blind test was failed.
hens
the with
familiar’ ‘new
from
positive
familiar’
hen
negative
these two individuals
were
able
to
as
discriminate
social group.
Discussion The since
results the
documented 1982)
and
in experiment
ability
1 seem to have been an artefact of a poor training
to discriminate
in choice in sequences
tests
between
(Murchison
of paired
familiar 1935a,b,c;
contests
(Zayan
and
unfamiliar
Guhl
1942;
1987a).
This
procedure
conspecifics Hughes
has
1977;
suggestion
been
Dawkins
was further
149
supported by the results in experiment
2. After modifications to the training procedure a
significantly high proportion of correct choices was recorded for all test birds, indicating the ability to discriminate between members of their own social group. Despite early attempts to investigate the ability of domestic fowl to discriminate between members of their own social group (Murchison 1935b; Guhl 1942), there have not been any recent studies. This is surprising considering the quantity of literature devoted to investigating the mechanisms underlying the formation and maintenance of the dominance hierarchy in fowl, and the commercial importance such an understanding may have to the poultry industry. Studies on other species are few. Ryan (1982) reported that cock bantams were able to discriminate between conspecifics presented by means of coloured slides, and suggested the birds formed concepts of the birds to be discriminated but these results have been heavily criticised (see Dasser 1987). In a series of learning experiments designed to investigate mate choice, Trillmich (1976) revealed the discriminatory abilities of budgerigars. These experiments revealed that budgerigars were able to discriminate between both live animals and slides of conspecifics. One of these birds was able to transfer its discrimination from slides to the live birds but not from live birds to slides. Most other studies have concentrated exclusively on the use of slides. This is surprising since the extent to which an animal relates a photographic representation of a familiar conspecific to the real object is unclear (e.g. Premack 1976; Premack & Premack 1983). Dasser (I 987) was able to show long-tailed macaques apparently recognised the identity of group members presented by means of coloured slides in learning experiments. After training on a minimum of examples of slides of group members, the test birds correctly identified in unique transfer trials novel views of the stimulus animal used in training. More recent work has shown that video recognition is possible in some species (Clark & Uetz 1990; Evans & Marler 1991). There is no doubt that in the present study the test birds were discriminating between the stimulus birds presented. The precise mechanism the birds were using to make the discriminations is less clear. Test bird X continued to choose the positive bird during the blind test and carried out a series of complex transfer tests. Since we would expect the bird to extinct the association of the positive stimulus bird with a food reward during the blind test, this bird was making the discriminations in some way other than the association of the positive stimulus bird with a food reward (e.g. by avoiding a negative stimulus bird). Test bird Z failed to choose the positive bird under blind test conditions (due to the extinction of the bird’s learned association of the positive stimulus bird with the food reward) indicating the bird had learned the required procedure during training.
Bird Y failed to
reach criterion at step 5 and no blind test was conducted. Thus the results clearly show the test birds are able to discriminate between group members but in only one case was this due to the adoption of the desired learning procedure. The birds may have been responding to one or two learnt features (e.g. body colour) to make the discriminations. Body colour is subject to variation depending on the orientation of the bird and it seems unlikely that any one feature or combination of features was sufficiently defined throughout the training. It seems more likely that the test birds were responding to a number of features. The results of the present experiments are in line with Cuhl & Ortman’s assertion that laying hens individually recognise each other. They do demonstrate that individual recognition is the mechanism underlying the maintenance of the dominance hierarchy. Because laying hens possess
(1953) original not, however, formation and the ability to
150 discriminate
Conclusion A training between
procedure
members
discriminations of the two. the ability
was developed
of
their
is unclear: The
results
to form
own
they could of their
laying
group.
The
be using visual
add substantial
concepts
whereby
social
support
own
hens
were
precise
or behavioural
to the hypothesis
group
trained
cues
to discriminate
used
to
make
the
cues or a combination that laying
hens
possess
members.
Acknowledgements This
research
Studentship, Studentship
(Linacre
am grateful criticism
was supported
a Meyricke
College,
to Marian
throughout
by a Ministry
Graduate
(Jesus
Fisheries
College,
Oxford),
and Food
Research
and an A.].
Hosier
Oxford).
Stamp
the
of Agriculture
Scholarship Dawkins
study.
Matthew
her considerable Sullivan
read
encouragement
earlier
draft
and helpful
versions
of
the
manuscript.
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