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In most social interactions, an animal has to determine whether the other animal belongs to its own species. This perception may be visual and may involve several cognitive processes such as discrimination and categorization. Perceptual categorization is likely to be involved in species characterized by a great phenotypic diversity. As a consequence of intensive artificial selection, domestic dogs, Canis familiaris, present the largest phenotypic diversity among domestic mammals. The goal of our study was to determine whether dogs can discriminate any type of dog from other species and can group all dogs whatever their phenotypes within the same category. Nine pet dogs were successfully trained through instrumental conditioning using a clicker and food rewards to choose a rewarded image, S+, out of two images displayed on computer screens. The generalization step consisted in the presentation of a large sample of paired images of heads of dogs from different breeds and cross-breeds with those of other mammal species, included humans. A reversal phase followed the generalization step. Each of the nine subjects was able to group all the images of dogs within the same category. Thus, the dogs have the capacity of species discrimination despite their great phenotypic variability, based only on visual images of heads.
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ORIGINAL PAPER
Visual discrimination of species in dogs (Canis familiaris)
Dominique Autier-De
´
rian
Bertrand L. Deputte
Karine Chalvet-Monfray
Marjorie Coulon
Luc Mounier
Received: 17 February 2012 / Revised: 1 January 2013 / Accepted: 14 January 2013
Ó Springer-Verlag Berlin Heidelberg 2013
Abstract In most social interactions, an animal has to
determine whether the other animal belongs to its own
species. This perception may be visual and may involve
several cognitive processes such as discrimination and
categorization. Perceptual categorization is likely to be
involved in species characterized by a great phenotypic
diversity. As a consequence of intensive artificial selection,
domestic dogs, Canis familiaris, present the largest phe-
notypic diversity among domestic mammals. The goal of
our study was to determine whether dogs can discriminate
any type of dog from other species and can group all dogs
whatever their phenotypes within the same category. Nine
pet dogs were successfully trained through instrumental
conditioning using a clicker and food rewards to choose a
rewarded image, S?, out of two images displayed on
computer screens. The generalization step consisted in the
presentation of a large sample of paired images of heads of
dogs from different breeds and cross-breeds with those of
other mammal species, included humans. A reversal phase
followed the generalization step. Each of the nine subjects
was able to group all the images of dogs within the same
category. Thus, the dogs have the capacity of species dis-
crimination despite their great phenotypic variability,
based only on visual images of heads.
Keywords Species discrimination Categorization
2D images Dogs
Introduction
Social life relates to associations of individuals from the
same species (Campan and Scapini 2002; Tinbergen 1953).
These associations require a recognition that can be at the
level of a species, a specific group, a particular social
category or a particular individual (Gheusi et al. 1994). The
sensory modalities involved in recognition processes vary
among species. In mammals, olfaction, audition and vision
are involved to a greater or lesser extent depending of the
species (Porter 1987). Nevertheless, the discrimination still
seems to be possible using only one sense, when the animal
is deprived of others as demonstrated in sheep (Ovis aries:
Porter et al. 1997; Ligout and Porter 2004; Ligout et al.
2004). So, numerous studies have demonstrated the ability
of animals to discriminate conspecifics from visual cues
alone. Examples include rhesus macaques (Macaca mulatta):
D. Autier-De
´
rian B. L. Deputte
LEEC, Universite
´
Paris 13, Av. Jean-Baptiste Cle
´
ment,
93430 Villetaneuse, France
D. Autier-De
´
rian (&) K. Chalvet-Monfray L. Mounier
Universite
´
de Lyon, VetAgro Sup Campus ve
´
te
´
rinaire de Lyon,
69280 Marcy L’Etoile, France
e-mail: autier.derian@free.fr
D. Autier-De
´
rian B. L. Deputte
G.Re.C.C.C. Ecole Nationale Ve
´
te
´
rinaire d’Alfort,
94704 Maisons-Alfort, France
B. L. Deputte
Ecole Nationale Ve
´
te
´
rinaire d’Alfort,
94704 Maisons-Alfort, France
K. Chalvet-Monfray
INRA, UR 346 Epide
´
miologie Animale,
63122 Saint-Gene
`
s-Champanelle, France
M. Coulon L. Mounier
INRA, UMR 1213 Herbivores,
63122 Saint-Gene
`
s-Champanelle, France
M. Coulon
Clermont Universite
´
, VetAgro Sup, UMR Herbivores,
BP 10448, 63000 Clermont-Ferrand, France
123
Anim Cogn
DOI 10.1007/s10071-013-0600-8
Pascalis and Bachevalier (1998); Fujita (1987), and others
macaques (Macaca fuscata, M. radiata, M. nemestrina):
Fujita (1987), in sheep (Ovis aries): Kendrick et al. (1995,
1996), in heifers (Bos Taurus): Coulon et al. (2007), in
dogs (Canis familiaris): Racca et al. (2010); Somppi et al.
(2012), in birds (Melopsittacus undulatus): Brown and
Dooling (1992), and in invertebrates, paper wasps (Polistes
fuscatus): Tibbetts (2002).
In humans, faces seem to have a special informational
value for individual recognition. In fact, face discrimina-
tion has been shown to be more efficient and specific
compared to non-face object discrimination (Farah et al.
1998). This specific face processing has also been sug-
gested by the results of brain studies using neuroimaging,
single-cell and MEA experiments in temporal cortex
(Kendrick et al. 2001a; Perrett and Mistlin 1990; Perrett
et al. 1982, 1988; Pinsk et al. 2009; Tate et al. 2006; Tsao
et al. 2006). Face discrimination is configurally sensitive,
since the presentation of an upside-down face decreases
discrimination performance. This so-called ‘inversion
effect’ has been shown in humans (Yin 1969), in chim-
panzees (Parr et al. 1998) and in sheep (Kendrick et al.
1996; Peirce et al. 2000). The inversion effect seems less
obvious in some others species such as dogs (Racca et al.
2010) or in macaques, where studies showed contrasting
results (Bruce 1982; Parr and Heintz 2008; Perrett et al.
1988). For the investigation of face discrimination, 2D
pictures seem to be appropriate stimuli (for review, see
Leopold and Rhodes 2010; Tate et al. 2006).
In some species, individuals recognize more easily or
are more attracted by images of individuals belonging to
their own species, than those belonging to another species.
This so-called ‘species-specific effect’ has been shown in
macaques (Fujita 1993; Fujita and Watanabe 1995; Pas-
calis and Bachevalier 1998; Dahl et al. 2009), chimpanzees
(Hattori et al. 2010) and humans (Pascalis and Bachevalier
1998; Dahl et al. 2009; Hattori et al. 2010). Moreover, it
has been demonstrated that below the species level, indi-
viduals showed less difficulty to discriminate individuals of
their own breed (e.g., in heifers; Coulon et al. 2009)orof
their own race (e.g., in humans; Malpass and Kravitz 1969;
Meissner and Brigham 2001; Young et al. 2009) rather than
of others. Those preferences (for the own species or own
breed/race) may be linked with sex (Fujita and Watanabe
1995) or with cognitive development (Pascalis et al. 2002).
Moreover, cross-fostering experiences in sheep and goat
have pointed to the primacy of the species providing
maternal care upon other social or interspecific interactions
(Kendrick et al. 1998, 2001b).
Ultimately, these studies compared behavioral responses
of subjects confronted to images of conspecifics compared
to images of a limited number of species, and generally
with a limited number of instances of each category. For
example, in one of the most exhaustive studies in non-
human primates (Dufour et al. 2006), subjects were tested
with only four categories of faces (humans, own species
and two species of the same genus), with 10 instances per
face category. This is also the case in studies involving
domestic animals. Species discrimination has been con-
firmed only for a few categories: in sheep, only with
humans, dogs and unfamiliar sheep breeds (Kendrick et al.
1995); in heifers with sheep (5 pictures), horses (3 pic-
tures), goat (1 picture) and dog (1 picture; Coulon et al.
2007). In dogs, the only two studies highlighting their
ability to discriminate 2D images of their own species
among other species used a procedure involving visual
preferential looking between dogs and humans, with 24
pictures per category, humans and dogs, for the first study
(Racca et al. 2010) and a similar number for the second one
(Somppi et al. 2012).
Because of the small number of stimuli used, these
studies could not take into account morphological species-
specific diversity. In fact, there is more morphological
diversity among breeds in domestic species compared to
wild species (Hemmer 1990). The largest morphological
variety among all animal species is found in domestic dogs,
Canis familiaris (Wayne and Ostrander 2007). There are
considerable variations between breeds in size, weight
(from less than one kg -Chihuahua- to 100 kg—Mastiff-),
color, hair length and texture (stiff or curly), form and
position of ears (upright or drooping) and tail, independent
profile, etc. (Denis 2007). According to the American
Kennel Club and the International Cynologic Federation
(FCI), 400–500 breeds of dogs are now registered, some of
which are nearly identical morphologically, while others
differ to a very large degree (cf. Miklo
´
si 2007 pp 109).
Diversity and changes in morphotypes are still increasing
with hybridization and the proliferation of dogs associated
with their short duration of gestation.
Considering this amazing morphological diversity
among dogs, visual species recognition may represent a
true cognitive challenge. Kerswell et al. (2010) showed that
changes in morphological features could affect communi-
cation in young dogs. For example, a shorter snout seems
to increase the frequencies with which some social signals
are sent or elicited from other dogs; Relationships with eye
cover and coat length were also found. Several studies have
shown that visual cues are significant for domestic canids
in interactions between conspecifics and heterospecifics
(Gaunet and Deputte
2011; Hare and Tomasello 1999;
Range et al. 2007a; Vira
´
nyi et al. 2004, 2008). With the
constraint of its large morphological diversity, companion
dogs are an interesting model for studying visual species
discrimination with 2D images of faces.
Behavioral responses of domestic dogs confronted to
still 2D pictures have been investigated under various
Anim Cogn
123
procedures. On the one hand, procedures have explored
preferential looking with black and white pictures of dogs,
humans and objects (Racca et al. 2010) or with color pic-
tures of dogs, humans, children’s toys or alphabetic char-
acters (Somppi et al. 2012). On the other hand, operant
conditioning procedures have explored dogs’ choices when
confronted with color pictures of dogs and landscapes
(Range et al. 2007b) or confronted with black and white
images of humans faces, blank or smiling (Nagasawa et al.
2011). A further paper (Adachi et al. 2007) has examined
whether dogs have a cross-modal (acoustic and visual)
representation of human individuals. These studies have
shown that the particularity of a dog’s visual system does
not prevent them being sensitive to still pictures. Compared
with that of human beings (Kanwisher and Yovel 2006)or
non-human primates (Tate et al. 2006; Tsao et al. 2006),
for which an efficient system for the processing of its own
species faces was demonstrated, the canine visual system
could be considered inferior: in degree of binocular overlap
(Sherman and Wilson 1975), in color perception (Jacobs
et al. 1993), in brightness discrimination (Pretterer et al.
2004), in accommodative range and in visual acuity
(Neuhaus and Regenfuss 1967; Miller 2008). In other
ways, such as the capacity to be sensitive to rapid move-
ments (Coile et al. 1989), the canine visual systems may
outperform the human visual system.
In short, dogs display a very efficient visual communi-
cation system toward conspecifics and also to human
beings. We assume that this communication could not work
without efficient discrimination of conspecifics. Moreover,
the dog is subjected to significant constraints such as a
large morphologic diversity and the fact that some dog
morphotypes may impair visual communication. Therefore,
the present study aims at exploring whether dogs using
only visual cues are able to discriminate various morpho-
types of conspecifics not only from humans but also from
other animal species. To address this issue, we used a great
variety of pictures of faces from various dog breeds and
cross-breeds, to be discriminated from a variety of pictures
of faces from humans and other animal species, domestic
and wild.
Methods
Animals
Nine adult domestic dogs (Canis familiaris), five females
and four males, were used as subjects (Fig. 1). Two dogs
were pure-bred (one Labrador, one Border collie), seven
were cross-breeds (Fig. 1). None had the same morphotype
in terms of form, color, marking, hair length, type of ears,
that is, upright or drooping (Fig. 1). At the beginning of the
experiment, the dogs were between 2 and 5 years old
(means 2.4), and their height at eye level was approxi-
mately the same as that of the stimuli. Our dog subjects
were pets owned by students at the National Veterinary
School of Lyon, France, VetAgro Sup. They were fed twice
a day on a normal diet. They all had extensive experience
of visual interspecific and intraspecific interactions before
the study. They all stayed in Vet School kennels during the
day and were at their owner’s home by evening. Before the
beginning of our study, all the dogs had acquired basic
obedience training. Moreover, prior to the study, all the
subjects were submitted to an ophthalmological examina-
tion and to a behavioral evaluation in the clinics of
VetAgro-Sup, to make sure that they had no visual or
behavioral disorders.
Visual stimuli
Stimuli were presented as pairs of a positive stimulus (S?)
and a negative stimulus (S-). Two sets of colored digital
head pictures were used as stimuli: one set of unfamiliar
dogs and one set of unfamiliar ‘non-dog’’ animals. The set
of dog pictures consisted of 3,000 pictures from cross-
breed and pure breed dogs illustrating the unique vari-
ability of dog morphotypes (Clutton-Brock 1996; Denis
2007; Megnin 1897
; Regodon et al. 1991)
with respect of
the four major morphotypes, wolf type, hound type, mastiff
type or greyhound type (Megnin 1897). This variability
included for instance different features of head shape, hair
length and color, position of ears (upright or drooping).
The set of ‘non-dog species’ included 3000 pictures of
domestic and wild species (e.g., cows, cats, rabbits, and
birds, reptilians, wild felines and humans; cf. Fig. 2). No
faces of wolves or foxes were included within the ‘non-
dog’ species sample. Stimuli were front, right and left
profile and views of both dog and ‘non-dog’ heads
(Fig. 2) distributed in equal proportions within each ses-
sion. The original background of all pictures was replaced
by a uniform blue color (R17G97B168: Adobe Photoshop
CS3 2007
Ò
). The blue color was chosen in light of the
dichromatic vision of the dog (Jacobs et al. 1993) and
provided the best contrast with the fur and feathers of all
stimuli (Fig. 2). The size of the different stimuli was
adjusted to cover about 70 % of the overall screen. All the
stimuli were controlled for brightness. Stimuli were pre-
sented on two 19’’ screens by DELL Intel Core 2 Duo
Ò
computer (Fig. 3), using Microsoft PowerPoint 2007
Ò
software. They produced a 36.9 cm 9 23.2 cm picture on
the screens. A summary of the stimuli is shown in Table 1.
Throughout the experiment, all the sessions, whatever the
task, were comprised of 12 pairs of stimuli. The position
(left/right) of S? and S- varied randomly from trial to
trial. During all the training tasks, the same single pair of
Anim Cogn
123
pictures was presented repeatedly to the nine subjects
(Table 1; Fig. 4). During the generalization tasks, each
stimulus, dog or ‘non-dog’’, was presented only once
(Table 1; Fig. 4): therefore, a subject never saw the same
stimulus twice. Stimuli were randomly drawn from the sets
of dog and ‘non-dog’ pictures.
Fig. 1 Dog subjects. The
subject’s name, its breed (Bc
border collie, L labrador) or
cross-breed (Cb) its sex and age
(2y2 years) are specified below
each portrait. These portraits
highlight the variety of the
subject’s phenotype
Fig. 2 Examples of stimuli used. a Dog heads displaying the variety of dog breeds (shape of head, hair length, ears and coat color). b ‘Other
species’ stimuli including humans, and domestic and wild vertebrates
Anim Cogn
123
First training task with ‘S?=a bowl of kibble’ (Task 0)
In the first training task, called Task 0, the pair of pictures
included a picture of a bowl of kibble as S? and a black
screen as S- (Table 1; Fig. 4).
Training tasks with ‘S?=dog’ (Task 1–3)
In the second training task, called Task 1, the pair of pic-
tures included the picture of the dog D1 as S? and a black
screen as S- (Table 1; Fig. 4). Then in Task 2, the pair of
pictures included the picture of the same dog D1 as S? and
a blue screen as S- (The same blue as the backgrounds of
the animal pictures; Table 1; Fig. 4) and in Task 3, the
picture of the same dog D1 as S? and that of the cow C1 as
S- (Table 1; Fig. 4).
Generalization tasks with ‘S?=dog’ (Task 4–6)
In the generalization tasks, we used dog pictures that were
as varied as possible. Within each session, the 12 dog
pictures included 3 pictures of each of the four major
morphotypes of dog (wolf, hound, mastiff and greyhound
types). In Task 4, pairs of dog pictures as S? and cow
pictures as S- were presented to the subjects (Table 1;
Fig. 4). Then in Task 5, we introduced species other than
cows in the pairs ‘dog/non-dog’’. A session of 12 trials was
then constituted with 12 dog pictures as S? versus 12
‘non-dog’ pictures as S-, divided into 6 cows and 6
species different than dogs and cows (Table 1; Fig. 4). In
Task 6, the diversity in ‘‘non-dog’ stimuli was even larger,
Fig. 3 Apparatus. a, b The dog sits in front of the experimenter, on a line between the 2 screens. c When hearing an order, the dog expressed his
choice by going to a given screen and putting his paw in front of the chosen image
Table 1 Experimental procedure and presentation stimuli
Type of task Task Stimulus pictures in each session
(12 trials/session)
Training 0 A single pair of a bowl (S?) versus a
black screen (S-)
Training 1 A single pair of the dog D1 (S?) versus
a black screen (S-)
Training 2 A single pair of the dog D1 (S?) versus
a blue screen (S-)
Training 3 A single pair of the dog D1 (S?) versus
the cow C1 (S-)
Generalization 4 A set of 12 dogs (S?) versus 12 cows
(S-)
Generalization 5 A set of 12 dogs (S?) versus 6 cows and
6 others species (S-)
Generalization 6 A set of 12 dogs (S?) versus 12 others
species (S-)
Reversal reward
contingencies
Training 7 A single pair of the cow C2 (S?) versus
the dog D2 (S-)
Training 8 A single pair of the cow C2 (S?) versus
the dog D2 (S-)
Generalization 9 A set of 12 cows (S?) versus 12 dogs
(S-)
Generalization 10 A set of 12 others species (S?) versus
12 dogs (S-)
In training tasks, a same single pair of pictures was used in a different
random side -left or right- each trial. In generalization tasks, 12 dif-
ferent pairs of pictures were used in each session of 12 trials. In
generalization tasks, a given picture was never used twice as a
stimulus, so the subjects never saw twice a same stimulus, dog or
‘non-dog’
Anim Cogn
123
as sessions included 12 dog pictures as S? versus 12 ‘‘non-
dog’ pictures as S2 including different species other than
dogs, all different from each other (Table 1; Fig. 4).
Training tasks in reversal reward contingencies
with ‘S?=non-dog species’ (Task 7–8)
After Task 6, the stimulus-reward contingency was
reversed, so that S? was a ‘non-dog’ picture and S- was
a dog picture. As in the previous training tasks, in Task 7,
the same single pair was used as stimuli, that is, the picture
of the cow C2 as S? and the blue screen as S- (Table 1;
Fig. 4). Then in Task 8, the single pair used was the picture
of the same cow C2 as S? and the picture of the dog D2 as
S- (Table 1; Fig. 4).
Generalization tasks in reversal reward contingencies
with ‘S?=non-dog species’ (Task 9–10)
In Task 9, sessions included 12 different pairs of different
stimuli, 12 cow pictures as S? versus 12 dog pictures as
S- (Table 1; Fig. 4). In Task 10, sessions included 12
different pairs, 12 pictures of ‘non-dog’ species as S?, all
species being different from each other, and 12 dog pic-
tures as S- (Table 1; Fig. 4).
Apparatus
The apparatus was installed in a windowless room of
3m9 5 m. The two computer screens presenting stimuli
were inserted in a wooden panel (Fig. 3). The dog could
see the two pictures placed at its eye level when assuming a
relaxed posture with its neck in the horizontal position. The
stimuli were presented at a distance of 1.9 m from the
sitting area where subjects made their choice. This distance
of 1.9 m was chosen in light of data on the visual acuity of
dogs (Neuhaus and Regenfuss 1967) and was validated by
preliminary testing. A partition of 0.9 m length was set
perpendicularly to the panel between the two screens
(Fig. 3a, b); when the dog was in front of one screen, it was
unable to see the other screen (Fig. 3c).
Procedure
Subjects were tested using a simultaneous discrimination
paradigm. The experimental protocol was adapted from
previous experiments in cows (Rybarczyk et al. 2001;
Coulon et al. 2007, 2009,
2010), sheep (Ferreira et al.
2004) and dogs (Pretterer et al. 2004). Each trial consisted
of a choice between two stimuli, one of them (S?) being
rewarded while the other (S-) was not. Prior to conducting
the experiments (3–5 weeks), the dogs were trained using
an operant positive conditioning method with a clicker
followed by a food reward. The experimenter first taught
them to sit and stay motionless on the ‘sitting area’
(Fig. 3); then to move toward one of the stimuli after
hearing a verbal order (namely ‘image’’) given by the
experimenter; then to put the paw on the tablet in front of
the stimulus chosen (Fig. 3c). The pair of stimuli used
during this initial shaping was a bowl full of kibble (S?)
Fig. 4 Procedure with the 11 discrimination tasks from 0 to 10.
Examples of pair of stimuli used during trials are presented in pictures
below. From Task 1 to 6, the stimulus rewarded S? is a dog. From
Task 7 to 10, which corresponds to the reversal learning of phase 1,
the stimulus rewarded is another species than dog, that is, ‘not a
dog’’. The stimulus rewarded S? is highlighted with a line below the
pictures. Tasks are divided into training and generalization. Training
tasks 0, 1, 2, 3, 7, 8: subjects discriminate between the same single
pair of stimuli as showed in the pictures. Generalization tasks 4, 5, 6,
9, 10: subjects discriminate between pairs of unknown stimuli
Anim Cogn
123
versus a uniformly black screen (S-). The position (left/
right) of the picture with the bowl of kibble varied ran-
domly from trial to trial. The criterion for passing this
shaping period was that the dog without any assistance
immediately returned to the ‘sitting area’ and retook the
sitting position after the order ‘place!/here!’ was given by
the experimenter behind him, sitting motionless before
hearing the order ‘image!’ given by the same experi-
menter, and then in a delay of less than 10 s gets up to put
his paw in front of the chosen stimulus.
The experimenter wore dark glasses and stood motion-
less, arms by his sides, 1 m behind the dog. The experi-
menter gave the order ‘place’ to the dog. The
experimenter looked at his feet and changed the stimuli
with a remote control. He then gave the order ‘image’’ in a
neutral tone. The experimenter raised his eyes only when
the dog ran toward a picture and made his response. Then,
the experimenter activated the clicker if the dog’s choice
was correct and dropped a food reward behind him. After
making an incorrect choice, the dog simply returned to the
‘sitting area’ where he resumed his sitting position, facing
the screens (Fig. 3). Thus, great care was taken to avoid
visual, tactile, or acoustical cues that could inform the
subject about the location of the rewarded stimulus. Before
the experiment, the experimenter was trained to stay
motionless by means of a video under observation by two
other experimenters. The whole experiment was video
recorded and then checked for possible cueing of the dogs.
All sessions were the same for the nine dog subjects. The
criterion for a subject to pass from a given task to the fol-
lowing one was set at 10 correct trials out of 12, for two
consecutive sessions. These thresholds were chosen to
considerably decrease the probability of passing a session
by chance. The probability of getting at least 10 successful
trials out of 12 trials by chance, when the probability for one
trial is 0.5, was P = 0.019. The probability of obtaining two
consecutive successful sessions was 0.019
2
= 0.00037.
When the subject succeeded with a task, it was moved on to
the next one.
The training and generalization tasks included discrim-
ination tasks of increasing complexity (Table 1). Without
this progressive complexity, the subjects lost their moti-
vation for the experiments (personal observation in pilot
studies). They laid in front of the screens showing distress
(moaning, intention to leave the room, yawns, scrapings,
e.g., Beerda et al. 1997).
One to four consecutive sessions of 12 trials were given
in the morning, depending on the dog’s motivation. There
were at least 24 h between daily blocks of sessions. At the
beginning of the test, a dog was taken from the kennel and
led to the experiment room after a relaxing walk. The
owner was not present in the experimental room unlike
other studies using dogs (Range et al. 2007b; Racca et al.
2010; Nagasawa et al. 2011; Somppi et al. 2012); the
attention of the dogs was considerably reduced in all pre-
liminary tests when the owner was present, even if he or
she was hidden from the dog’s view.
Data analysis and statistical tests
For each trial, we recorded the success or the failure of the
dog to choose S?. For each task, we recorded the number
of sessions that each subject needed to reach the criterion.
The number of sessions constituted the main variable. The
data analysis was carried out using R 2.13.1 (R Develop-
ment Core Team 2010).
Since the same dogs were tested repeatedly in different
tasks, data were not independent, so we took into account
the individual dog as a ‘random effect’ in the analysis.
The comparison of the number of sessions to reach the
criterion between tasks for all subjects was analyzed by
means of a generalized linear mixed model using lme4
package version 0.999375-35 for R (Bates and Maechler
2010). This model aimed at explaining the variation of the
number of sessions that the dogs required for a given task.
Using this model, it was possible to predict the expected
mean value of the number of sessions for each task. Task
3 was chosen as a reference because it was the first task
where the subject had to discriminate between two pic-
tures of animals. Since the number of sessions to criterion
was a count, we used a Poisson regression; since the
observations were made on the same dogs, the effect of
individual was taken into account as a random effect
(Ogura 2011). In order to find the most relevant model for
describing the average number of sessions required for a
task, we used the minimal Akaike Information Criterion
(AIC) method (Akaike 1973; Ogura 2011). The most rel-
evant model was obtained for the Poisson regression
model with dogs as random effect on the intercept and
‘tasks as a factor’ for a fixed effect. The normality of the
distribution of the residuals was assessed by graphic rep-
resentation and the Shapiro test. We considered that a
difference was significant when P value (P) was lower
than 0.05.
In order to complete the analysis, using the same method
with a generalized linear mixed model (Bates and Maechler
2010), with the dog as a ‘random-effect’’, and the task as a
‘fixed-effect’’, a comparison of the numbers of sessions
with the same dogs was carried out for successive periods
(A, B, C, D and E). The successive periods were defined as
periods of monotonic increase or decrease periods in the
number of sessions. This procedure establishes the best
way to explain the task effect as a factor, or as an ordinal
value, for each period. A significant task effect as ordinal
value highlighted whether response tendencies are signifi-
cantly increasing or decreasing.
Anim Cogn
123
Ethical note
The protocol (schedules and duration of the session blocks
of our experiment) was approved by the Ethical Committee
of VetAgro-Sup (Lyon, France) registered as number 1058,
complying with French law.
Results
Task 0
All nine dogs met the criterion of success for Task 0 (a
single pair: a bowl of kibble S? vs. a black screen S-;
Figs. 5, 6). Dogs needed from 6 to 29 sessions (Med-
ian = 11; Fig. 6) to complete the training Task 0.
Training tasks with ‘S?=dog’ (Task 1–3)
For the training tasks with ‘dog as S?’, the nine dogs
reached the criterion of success for all the tasks (Figs. 5, 6).
Dogs needed 2–13 sessions in training Task 1 (a single
pair: the dog D1, S?, vs. a black screen S-: Median = 5;
Fig. 6), 2–3 sessions in training Task 2 (a single pair: the
dog D1, S?, vs. a blue screen S-: Median = 2; Fig. 6) and
2–12 sessions in training Task 3 (a single pair: the dog D1,
S?, vs. the cow C1, S-: Median = 3; Fig. 6).
The generalized linear mixed model showed that the
number of sessions needed to reach criterion on Task 0 (a
single pair: a bowl of kibble vs. a black screen) was sig-
nificantly (P \ 0.001) higher than for the reference Task 3
(a single pair: the dog D1 vs. the cow C1; Table 2; Fig. 6).
In contrast, Task 2 (a single pair: the dog D1 vs. a blue
screen) seemed less difficult (P & 0.05) than Task 3
(Table 2; Fig. 6).
Generalization tasks with ‘S?=dog’’ (Task 4–6)
For the generalization tasks with dog as S?, all nine dogs
reached the criterion of success for all the tasks (Figs. 5, 6).
Dogs needed 2–13 sessions in generalization Task 4 (12
dogs S? vs. 12 cows S-: Median
= 4; Fig. 6), 2–10 ses-
sions in generalization Task 5 (12 dogs S? vs. 6 cows ? 6
other species, S-: Median = 6; Fig. 6) and 2–6 sessions
Tasks
Number of sessions
0
10
20
30
012345678910
Babel
012345678910
Bag
012345678910
Bahia
Bounty Canaille
0
10
20
30
Cusco
0
10
20
30
Cyane Sweet Vodka
Fig. 5 Individual changes in
the number of sessions to reach
the criterion, according to the
type of the task, arranged
sequentially along increasing
difficulty (11 tasks from 0 to
10), for each of the 9 subjects
Anim Cogn
123
for generalization Task 6 (12 dogs S? vs. 12 ‘non-dog’
species S-: Median = 4; Fig. 6).
The generalized linear mixed model showed that the
number of sessions needed for Tasks 4, 5 and 6 were not
significantly different than for Task 3 (Table 2; Fig. 6).
Training tasks in reversal reward contingencies
with ‘S?=non-dog species’ (Task 7–8)
For the training tasks, when the stimulus-reward contin-
gency was reversed, the nine dogs met the success criterion
Type of Task
Number of sessions to reach the criterion
012345678910
0
5
10
15
20
25
30
35
40
***
***
*
(with
Bounty)
*
(without
Bounty)
Training
S+=dog
Generalization
Reversal training
Reversal generalization
S+=« non dog » species
Fig. 6 Influence of the type of task on the overall performances of
the dog subjects (N = 9). The 11 tasks are arranged sequentially, in a
chronological order also corresponding to an increasing complexity.
The variable is the number of sessions to reach the criterion. The box
plots present the median (the bold line within the box), the bottom of
the box represents the first quartile, the top of the box the third
quartile, the dotted lines with horizontal segments figure the overall
range of the variable (Minimum and Maximum). The different types
of tasks are mentioned below the box plots. The horizontal dotted line
indicates the minimum number of sessions to reach the criterion, that
is, 2. Out values are represented by a small circle. The P value for the
test related to Table 1 is represented with ***(P B 0.001),
**(0.001 \ P B 0.01), *(0.01 \P B 0.05),
.
(0.05 \ P B 0.1) and
NS (0.1 \ P); in task 10, results of P value are represented with and
without the dog Bounty, cf. Table 2. For more information about the
type of tasks, please see the text and Table 1
Table 2 Estimate of parameters
Task
i
Parameter Estimate Standard error Expected value P Comments
a (Intercept) 1.490 0.173 \2e-16 ***
0 b
0
1.150 0.179 14 1.21e-10 ***
1 b
1
0.158 0.213 5.2 0.459 NS
2 b
2
-0.495 0.254 2.7 0.051
3 b
3
Reference task
4 b
4
0.158 0.213 5.2 0.459 NS
5 b
5
0.178 0.212 10.8 0.400 NS
6 b
6
0.048 0.226 4.6 0.827 NS
7 b
7
-0.578 0.261 2.5 0.027 *
8 b
8
1.169 0.124 14.3 6.33e-11 ***
9 b
9
0.257 0.208 5.7 0.214 NS
10 b
10
0.413 0.201 6.7 0.040 *With bounty at task 10
-0.454 0.261 2.82 0.082 Without bounty at task 10
The P value for the test for comparison of the parameters estimate to 0 is represented with *** (P B 0.001), ** (P B 0.01), * (P B 0.05),
(P B 0.1) and NS (0.1 \ P)
Anim Cogn
123
for all the tasks (Figs. 5, 6). Dogs needed 2–4 sessions in
reversal training Task 7 (a single pair: the cow C2, S? vs. a
blue screen S-: Median = 2; Fig. 6) and 3–27 sessions in
reversal training Task 8 (a single pair: the cow C2, S? vs.
the dog D2, S-: Median = 17; Fig. 6).
The generalized linear mixed model showed that the
number of sessions needed for Task 8 (a single pair: the
cow C2 vs. the dog D2) was significantly (P \ 0.001)
greater than that for Task 3 (a single pair: the cow C1 vs.
the dog D1; Table 2). In contrast, the number of sessions
needed for Task 7 (a single pair: the cow C2 vs. a blue
screen) was significantly (P \ 0.05) lower than for Task 3
(Table 2; Fig. 6).
Generalization tasks in reversal reward contingencies
with ‘S?=non-dog species’ (Task 9–10)
For the generalization tasks in reversal reward contingen-
cies, the nine dogs met the success criterion for all the tasks
(Figs. 5, 6). Dogs needed 2–14 sessions in the reversal
generalization Task 9 (12 cows S? vs. 12 dogs S-:
Median = 4; Fig. 6), and 2–3 sessions in the reversal
generalization Task 10 (12 ‘non-dog’ species S? vs. 12
dogs S-: Median = 3; Fig. 6).
The generalized linear mixed model showed that the
number of sessions needed for Task 9 was not significantly
different from Task 3 (Table 2). With respect to Task 10,
the results were different depending on whether the dog
Bounty was taken into account or not. Bounty needed a
considerable number of sessions to reach criterion on Task
10 (39 sessions). With Bounty included, median perfor-
mance in Task 10 was significantly more difficult than in
Task 3 (P \ 0.05; Table 2; Fig. 6). Without Bounty’s data,
Task 10 was marginally less difficult than the reference
Task 3 (P \ 0.1; Table 2; Fig. 6).
The minimal AIC method was used to analyze the
increasing and decreasing trends in the number of sessions
needed by the subjects to succeed in the subsequent tasks
(Fig. 6; Table 3). The models demonstrated that there were
significant increases or decreases in the number of sessions
according to the rank of the task (P \ 0.001 except for
period B (P \ 0.01); Table 2). For decreasing periods,
tasks 0–2 (A), tasks 5–7 (C) and tasks 8–10 (E), the trend
was a decrease in the number of sessions for each suc-
cessive task. For increasing periods, tasks 2–5 (B) and tasks
7–8 (D), the trend was an increasing number of sessions for
each successive task (Table 2; Fig. 6).
Progression across the tasks differed between individual
dogs (Fig. 5). One dog (Vodka) presented an extreme
pattern. This dog took more time for the learning Task 0
(with 29 sessions), but then, it succeeded rapidly with all of
the following tasks, needing only 5 sessions for Tasks 1
and 2, and after that, no more than 3 sessions for each task.
This dog needed fewer and fewer sessions to meet the
criterion for the subsequent tasks. Ultimately, this dog was
among those which needed the lowest number of general-
ization sessions (i.e., 12 sessions = 144 trials). On the
contrary, Bounty needed increasing numbers of sessions to
reach the criterion for the later tasks, with a peak at 39
sessions for the final generalization Task 10. Bounty was
among those needing the greatest number of generalization
sessions (i.e., 56 sessions = 672 trials).
Discussion
Our results explore the dog’s ability to visually discrimi-
nate 2D pictures of the faces of various species depending
on whether they represent dogs or not. Behavioral studies
investigating the capacities of dogs to use visual cues for
face identification are still relatively sparse compared with
humans and other animals such as non-human primates,
sheep and heifers (Leopold and Rhodes 2010). Compared
to previous studies investigating such abilities in domestic
dogs (Range et al. 2007b; Racca et al. 2010; Somppi et al.
2012), our study is the only one using as stimuli species
other than dogs and humans, that is, domestic species (cats,
cows, sheep, horses, etc.) and wild species (tigers, birds,
rodents, etc.).
Moreover, in our study, the dogs were confronted by a
large diversity of stimuli: for the images of dog faces, the
four morphological types of dogs were used in balanced
proportions, from the smallest such as Chihuahua (1 kg)
to the largest such as mastiff (100 kg). Also images of
Table 3 Estimate of parameter with the standard error in brackets
Parameter Period A Period B Period C Period D Period E
a
a 2.60 (0.133)*** 0.775 (0.289)** 3.48 (0.686)*** -11.4 (1.79)*** 8.77 (0.916)***
b -0.870 (0.102)*** 0.196 (0.0715)** -0.346 (0.118)*** -1.75 (0.227)*** -0.784 (0.105)***
Task 0 1 2 23455 6 7 7 8 8 9 10
Expected value 13.5 5.6 2.4 3.2 3.9 4.7 5.8 5.7 4.1 2.9 2.3 13.5 12.1 5.52 2.53
The P value for the test for comparison of the parameter estimate to 0 is represented with *** for P \ 0.001 and ** for 0.001 \ P \0.01
a
Poisson regression done without ‘Bounty’ dog
Anim Cogn
123
‘non-dog’’ species included about forty different species in
roughly equal proportions. As a result, our subjects were
confronted by more than 144 pictures of morphologically
different dogs versus others species (144 being the number
of trials performed by the dog which was the fastest to
complete the successive tasks), whereas the number of
stimuli in ‘non-dog’’ category was less than 30 in previous
studies (Racca et al. 2010, Somppi et al. 2012).
Thus, our study may suggest that dogs can form a visual
category of ‘dog pattern’’, as assumed in rhesus macaques
(Yoshikubo 1985). We may then hypothesize that there
may exist some invariants in dog morphotypes that allow
the nine subjects to group pictures of very different dogs
into a single category despite the great diversity in canid
species. The rapid generalization from a single training
instance (a single pair: a dog picture versus a cow picture in
Task 3) to multiple new instances in Task 4 goes against
the ‘category size effect’’, which has been thoroughly
explored in pigeons (cf. Soto and Wasserman 2010): in
general, when animals are trained on category discrimi-
nations, generalization is quite poor when only a few
instances are used in training, let alone only one. There is
one well-known exception, where training pigeons to
respond to a single oak leaf silhouette image led to instant
generalization to all oak leaf silhouettes (Cerella 1979).
Cerella interprets this unexpected result by saying that the
oak leaf pattern is ‘‘transparent’’ to the pigeons, that is, they
do not have to learn its extension. Could this hypothesis
apply with the ‘dog pattern’’? This might be possible in
light of the performances in the first sessions of general-
ization (Task 4), and especially in the first trials of the first
session of that task: the performances of the dogs were
generally above chance for the majority of them.
But there might be another explanation for this very
rapid generalization: the performance could be the conse-
quence of (1) progressive training with the same picture of
the dog D1 versus a black screen (Task 1), and then versus
a blue screen (Task 2) and further a single cow C1 (Task
3); (2) the fact that the instances in Task 4 included only
cows as S- (not other species) against dogs as S?. When
in Task 5 a greater diversity appeared in the trials with
other species than cows presented against dogs, the sub-
jects had more trouble in reaching the criterion, although
this difference was not significant. Dogs’ morphology
varies more than that of cows (Wayne and Ostrander 2007).
In Task 4, the dogs may have developed a strategy ‘not to
choose the cow picture’ in order to choose the picture of a
dog. But this strategy was no longer possible in Task 5 with
various species (not only cows) pitted against various dogs.
It may be that ultimately it was only in Task 5 that the
subjects understood the ‘dog categorization’ required by
the experimenter. The ‘dog category’ is an insight which
has been especially explored in various species. For human
babies, cats are treated as a kind of dog, but dogs are not
treated as a kind of cat (Eimas et al. 1994; Quinn and
Eimas 1996). Experiments conducted on humans and
pigeons confronted by pictures of dogs and cats showed
that pigeons and humans do not form categories using the
same features (Ghosh et al. 2004; Goto et al. 2011). We
assume that such differences may exist between dogs and
humans, and further investigations are needed to support
the idea and the nature of ‘a dog species pattern’ in dogs
as Cerella (1979) suggested with the oak leaf pattern for
pigeons. According to the five levels of categorization of
Herrnstein (1990), from simple discrimination based on
perceptual cues to categorization based on complex con-
cepts, we cannot conclude more than that dogs based on
their categorization of dog faces on perceptual cues.
Another unusual feature of our study was the use of a
reversal of the stimulus-reward contingency. This proce-
dure has not often been used in category discrimination
studies, particularly in dog studies. However, reversal
learning in categorization studies can be used to strengthen
the demonstration of categorization abilities. Spence’s
theory (1960) predicts that the more the initial discrimi-
nation is learned, the more difficult it will be to learn the
reversed stimulus-reward contingency. This theory has
been supported by numerous studies (e.g., in pigeons:
Vaughan 1988; heifers: Coulon et al. 2007). In contrast, our
dogs succeeded easily in the first reversal task (Task 7).
This could be explained by the ‘overtraining reversal
effect ORE’ (Sperling 1965; Sweller 1973), where under
some conditions, and contrary to Spence’s theory, over-
trained subjects (as our dogs could be considered by
numerous successive training) acquire the reversal dis-
crimination more easily than those trained only to criterion.
On the other hand, the ORE effect may not be the only
explanation of the small number of sessions required for
the first reversal task. We may suppose, as observed in the
individual performances in the first session of Task 7, dogs
rapidly learned to choose the screen containing an image,
not just a dog, versus a blue uniform screen. This expla-
nation seems consistent with the peak in the following task
8 (a same single pair: the cow C2 as S? versus the dog D2
as S-), showing the persistence of the initial habit as
described by Spence (1960).
Each procedure that can be used to investigate catego-
rization abilities has methodological advantages and dis-
advantages. With preferential looking procedures, some
factors, such as attractiveness and interest in certain cate-
gories of stimuli which carry more informative value, can
affect the preferences for a category (Buswell 1935; Farago
´
et al. 2010). This interest could be the result of the diversity
within a category: for instance, as between photographs of
dogs and those of humans, dogs could be attracted by the
diversity of dog stimuli while human faces belonging to a
Anim Cogn
123
single human race (Caucasian) presented less diversity to
them (Somppi et al. 2012). Category-dependent gazing
behavior could indeed be a consequence of differences in
the physical complexity of the stimuli. As a matter of fact,
even though they used sophisticated eye tracking proce-
dures, the authors conclude that they ‘cannot draw any
conclusions as to whether the attention of dogs was
directed mainly by stimulus features or semantic informa-
tion, or both’ (Somppi et al. 2012). This bias could also
explain contrasting results in different studies using the
same paradigm: Somppi et al.’s dogs fixated on a familiar
image longer than on novel stimuli regardless of the cate-
gory (dogs or humans), whereas it was not the case in the
study of Racca et al. (2010). The use of an operant positive
conditioning method has the advantage of allowing the
subjects to make an unambiguous choice. To the best of
our knowledge, our study is the first one using such a
procedure to demonstrate species discrimination in dogs.
It remains true, however, that the conditioning proce-
dure also has some disadvantages, because reinforced (or
not) choices may affect the motivation of the dog, and
ultimately the outcome of the trials. However, this moti-
vation effect may influence the number of sessions needed
to reach the criterion more than the final outcome of the
trials. The repetition of tasks may lead to ‘learning set’
formation (Harlow 1949). Learning set formation refers ‘to
the learning of visual and other types of discrimination
problems more quickly as a function of training on repe-
ated series of such problems’ (Schrier 1984). Due to the
succession of the tasks, dogs had greater discrimination
experience when it came to reversal Task 9 than in the
equivalent Task 4.
Our results do not allow us to determine which dog
morphotypes or which species were easier to discriminate
by our dogs because 1) we used crossbreed dogs as stimuli,
whose morphological types were not always well defined
2) the number and diversity of stimuli presented to the dogs
was high and was not similar because they depended on the
dog’s facility to reach the criterion. In our study, the
‘Category size effect’ (Soto and Wasserman 2010),
instead of reducing the learning speed of our dog subjects,
may in fact have facilitated their extraction of the ‘Com-
mon elements’ (Soto and Wasserman 2010) in dog faces,
despite the large phenotypic diversity. ‘Error-driven
learning’ (Soto and Wasserman 2010) may have played a
role in the rapid generalization shown in the difficult
reversal task. This ‘error-driven’ learning is probably
based on the analysis of the stimuli themselves rather than
on a strategy based on choosing a side. In the latter case,
this would have not led to the rapid improvement observed.
Thus by considering the stimuli, the dogs may have learned
to reverse their responses based on ‘‘error-driven’’ learning.
This kind of learning, in relation to the ‘Common
elements’ theory (Soto and Wasserman 2010), leads us to
assume that our dog subjects only formed a ‘‘dog category’
rather than a ‘dog category’ and a ‘non-dog category’’. It
is likely that having already formed a ‘dog category’
based on a reward contingency in the generalization tasks,
the subjects may have shifted their strategy in relation to
the two stimuli, and in recognizing the ‘dog category’’,
chose the ‘non-dog’ stimuli based on the reward associ-
ated to it. While dog faces were likely categorized, the
‘non-dog’ stimuli were only ‘identified’ (Soto and
Wasserman 2010). It is then likely that the reversal learn-
ing task has helped the dogs to strengthen their categori-
zation of the dog species.
Whereas paradigms using spontaneous responses have
to deal with subjects’ motivation and attention, requiring
a reduction in the duration of the session and likely the
number of stimuli used, paradigms that use operant con-
ditioning also have to deal with frustration. Failing to
respond correctly leads to no reward. The effect of this
frustration may have varied consequences depending on
the subject’s temperament (Svartberg and Forkman 2002).
Differences in temperament and emotional stability may
explain some differences in the dogs’ performance.
Another temperament trait, perseverance, might be useful
in interpreting individual differences in performance. We
have observed that often errors came in bouts, thus
making the session unsuccessful (only 2 errors were
accepted if the session was to reach the criterion for
success). This trait could explain both the rapid success of
some individuals (Babel, Bag, Cyane, Vodka) when they
applied the right rule and stuck to it, and difficulties in
reaching the criterion when perseveration was applied to
errors, especially when dogs persistently stuck to their
preferred side (‘‘Laterality effect’’: Bahia, Bounty). Per-
severation is likely an important temperament trait in
dogs, as it has ‘doggedness’ as a synonym. Another
important aspect for interpreting dogs’ differences in
performances, and one which is quite specific to dogs, is
their degree of obedience training. On the one hand, a
very obedient dog may have initial difficulties in adapting
to the experimenter and the new commands used in the
procedures. This ‘smart obedient’ dog will then become
a ‘slow learner’ (e.g., Vodka). On the other hand, a less
obedient dog might have difficulties in adapting to the
constraints of the experiment and fail to be attentive to
the screens, and then be a ‘slow learner’ throughout the
experiments (e.g., Bahia). Another aspect that was men-
tioned by Morgan (1898) is the fact that an initially ‘slow
learner’ might become the ‘fastest learner when the
stimulus-reward contingency is reversed as he might be
more flexible once he has understood a rule (such as
Vodka). A perseverant dog might have difficulties in
reversing the rule (such as Bounty).
Anim Cogn
123
Moreover, our study also shows that dogs are able to
discriminate unfamiliar dog faces (and likely ‘non-dog’
faces) in pictures from different viewpoints (front, profile,
etc.). This capacity has already been shown in domestic
species, in sheep (Kendrick et al. 2001a; Ferreira et al.
2004) and heifers (Coulon et al. 2007, 2009, 2010). As we
were using a large variety of ‘non-dog’ species, we had to
reduce or increase the size of natural stimuli to adjust it to a
standard surface. We did that in order to prevent the dogs
making their discrimination on the size of the surface of
colored pixels different from the uniform blue background.
Although some authors suggested that still images as they
may change the natural size of the stimuli reduce their
informational content (Bovet 1999; Van der Velden et al.
2008), our dogs succeeded in spite of this drawback. This
recalls the performances of pigeons which were not
impaired when the size of stimuli was modified (Lombardi
and Delius 1990).
In our study, we presented stimuli against a standardized
background even though dogs seem able to discriminate a
dog picture with heterogeneous background such as land-
scape (cf Range et al. 2007b). This ensured that our dog
subjects’ performances were based on elements found
within the pictures, or the contour of them, rather than on
features present in the background with no relation to the
categories tested.
In conclusion, we have demonstrated that dogs are able
to discriminate their own species in 2D pictures of faces
alone, from different viewpoints, as shown previously in
several other species (for a review Leopold and Rhodes
2010). The species discrimination demonstrated in our
study might be considered as an ‘open class’ categoriza-
tion (Herrnstein 1990), as the dog faces presented covered
the great variability in dog breed. This phenotypical
diversity includes both contour and intrafigural features of
dog faces. A natural further step would be to determine the
salient features of dog faces, both common and stimulus
specific elements (Soto and Wasserman 2010) on which the
subjects relied to make their discrimination and their cat-
egorization. It might also be possible that the species ‘‘open
class’’ categorization that we proposed might be of a higher
level such as a conceptual one (Herrnstein 1990). This
would be the case if dogs were also able to group in the
same category familiar conspecifics and in other category
non-familiar ones.
The fact that dogs are able to recognize their own spe-
cies visually and that they have great olfactory discrimi-
native capacities insures that social behavior and mating
between highly morphologically different breeds is still
potentially possible and therefore that, although humans
have stretched Canis familiaris to its morphological limits,
its biological entity has been preserved.
Acknowledgments We thank Professor Charles T. Snowdon for his
useful comments and careful editing on the manuscript. Thanks are
also due to VetAgro-Sup which enabled our project to be carried out,
to vet students Cindy Ribolzi and Florent Roques for their assistance
in experimental procedure, to owners of our subjects who entrusted
their dogs to us and to Royal Canin
Ò
for providing food rewards for
dogs.
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