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strength and direction of preferences; when the two
factors are set against each other, interest overrode
pleasure in determining the preference. When naive
monkeys were tested for preferences for coloured pho-
tographs paired with plain fields of light, they first
showed negative preferences and signs of fear. How-
ever, as they became more experienced, the signs of fear
dropped away and they showed positive preferences for
colour photographs (the same pattern of response
change was observed when films were introduced in-
stead of photographs). The photographs were divided
into six classes, and the rank order of preferences
exhibited by the monkeys was ‘other animals’, ‘mon-
keys’, ‘men’, ‘flowers’, ‘abstract paintings’ and ‘foods’.
In a further experiment, Humphrey [48] used novelty
preference for slides to investigate how rhesus monkeys
could differentiate between individual animals of the
same species; the finding was that monkeys to which
domestic animals were unfamiliar treated individual
domestic animals of the same species as being closely
similar, but treated individual monkeys as being differ-
ent from each other. However, monkeys who had been
exposed for 6 months to many pictures of animals,
treated all individuals as different from each other.
Demaria and Thierry [27] conducted a rather similar
experiment with female stumptailed macaques. These
females were submitted to slides displaying individual
primates or non-primates and the results showed that
subjects looked longer at slides of individuals of their
own species than at slides depicting other macaques
species; moreover, they looked more at adult females
carrying infants than at adult females alone. With
pictures of non-primates animals, subjects looked most
at slides of felids. However, spontaneous social re-
sponses, like facial expressions, were very rare.
Fox [36] observed the responses of young and adult
dogs (the breed was not given) presented for the first
time with a life-size dog painting. The young dogs
sniffed more at hind leg and inguinal regions, while
adults sniffed more at ear and anal areas; as all these
areas are normally investigated by conspecifics, we can
infer that the subjects had perceived the correspondence
between the painting and a real dog.
In sheep, Kendrick and Baldwin [53] recorded re-
sponses of cells in the temporal cortex of awake sub-
jects and demonstrated that some of these specifically
responded to slides (photographs and drawings) of
faces (but not to upside-down faces or profiles). More-
over, different groups of these cells were influenced by
relevant social factors, such as dominance, breed, famil-
iarity, and facial expression. Franklin and Hutson [37]
investigated the reactions of sheep to full-size colour
photographs of one sheep and to colour films of mov-
ing sheep and found that the sheep reacted to 2-D
images as if they were real animals: subjects were slow
to approach a sheep facing them, but they approached
without hesitation or followed a sheep displayed in
profile; reactions which were heightened when the im-
age was moving. The most attractive stimulus was the
film of sheep moving across the screen towards the exit.
Several studies are available concerning the ability of
birds to spontaneously display adapted responses to
pictures of biologically relevant situations. For exam-
ple, dark-eyed juncos which could choose between
slides of their winter and summer habitats spent more
time in front of the pictures that were consistent with
their season of capture and laboratory photoperiod
conditions compared to the inconsistent habitat [79].
Klopfer [57] performed imprinting tests with Peking
ducklings, using various decoys or images and demon-
strated that the ducklings followed both decoys and
images. However, the images elicited responses that
were not strictly equivalent to the three-dimensional
decoys; in effect, the ducklings reacted differently to
different decoys (according to their colours, and ac-
cording to those they were accustomed to follow) but
not to different two-dimensional representations of
ducks. One aspect of the task is particularly interesting
in this experiment: the comparison between decoys and
images, because the only difference between those two
representations is the presence or absence of the third
dimension, that is, the lack of three-dimensionality was
sufficient to cause a decrease in attention or in re-
sponses to feature differences.
4.1.2. Reactions to motion pictures
Rosenthal et al. [84] presented green swordtail fe-
males with video-recorded sequences of the same male
which was either engaged in an active courtship display,
or which performed similar levels of feeding activity, or
which remained inactive (control sequences showed
food particles in movement or an empty aquarium).
Female behaviour patterns differentiated between the
pre-stimulus, stimulus and post-stimulus periods for the
three stimuli showing a male, but not for the two
controls; courtship displays elicited more activity than
any other stimulus, and there were no significant differ-
ences between the responses to the feeding and inactive
sequences.
4.2. Acquired responses in picture recognition
We will now examine studies of acquired reactions
which fall into the category of responses that could
possibly be evidence for the ability of picture recogni-
tion in animals.
In an experiment conducted by Tomonaga et al. [98],
a sample of students and a language trained female
chimpanzee (called Ai) were trained to recognise video
still pictures of individual faces of humans or chim-
panzees, presented at various orientations. The experi-
ment yielded two main findings indicative of picture