In Bond, Kamil and Balda
we tested pinyon jays and scrub jays on an abstract version of a dominance
hierarchy, an implicit ordinal sequence of seven colored stimuli.
The sequence was implicit, in that birds were only shown two stimuli
at a time. During training, only adjacent pairs were presented.
The sequence was ordered, in that birds were only rewarded if they
chose to peck the higher member of the pair (Fersen et al. 1991;
Treichler & Van Tilburg 1996). Thus, on one trial, the bird
could be shown a blue and a red circle, and he would be rewarded
for pecking the blue one. On another trial, he would see a red and
a green circle, and in this case he would be rewarded for pecking
the red one. Different color sequences were used for each bird,
to control for possible differences in how easily particular colors
could be distinguished.
birds were given 36 pair-comparison trials per day. They were trained
initially on the A/B pair until they reached sufficient levels of
accuracy. Subsequently, they were given the B/C pair, and so on
through the six successive dyads in the hierarchy. When they were
fully trained on all adjacent pairs, we trained them in sessions
with mixtures of two adjacent pairs (e.g. A/B and B/C). Although
it is difficult to compare the results of these training trials
to other studies, scrub jays appeared to learn the color relationships
somewhat faster than pigeons (Fersen et al. 1991), while pinyon
jays were only slightly slower than rhesus monkeys (Treichler &
Van Tilburg 1996). At the start of the third phase of the experiment,
the two species were equally accurate and well above chance levels.
the third phase, the birds were given 100 sessions in which all
six color pairs were presented equally often. Pinyon jays improved
more rapidly than scrub jays and showed a higher over-all level
of performance. In the graph below, the filled symbols show the
average accuracy across birds for each species over this phase of
the experiment. The open figures show the results of an additional
100 sessions, indicating that the species difference was still present
even after over a year of training on the seven stimuli. This result
is consistent with the first prediction of the social complexity
hypothesis: that highly social animals should be able to keep accurate
track of a larger number of dyadic relationships.
turn to the second prediction, which concerns transitive inference.
The criterion of transitive reasoning here is fairly simple: After
training only on adjacent dyads, animals should be able to respond
correctly to novel stimulus pairings, pairs that they had never previously
seen, thereby displaying a knowledge of the underlying ordinal sequence.
Previous work on other species suggests that transitive inferences
are generally stronger and more accurate for stimuli that are further
apart on the implicit sequence, what has been called the Symbolic
Distance effect (Moyer & Bayer 1976), These data are
shown on the graph below. In the left panel, the X axis shows pairs
of stimuli at progressively greater distances from one another along
the implicit sequence, all beginning with the second-ranked stimulus.
The right panel shows the same effect beginning with the third-ranked
stimulus. Notice that both species show a clear increase in accuracy
with increasing distance along the implicit sequence. Which is to
say that both jays show some ability to make transitive inferences
(Fersen et al. 1991; Bryant & Trabasso 1971; Steirn, Weaver &
is not the end of the story, however, because there is evidence in
the literature for two different cognitive mechanisms for transitive
inference. An associative mechanism (Terrace &
McGonigle 1994) simply works on the basis of frequency of reward,
how often responses to the given stimulus have been rewarded. By any
of several models of this cognitive mechanism, the net result should
be a gradient of associative strength that is higher at the beginning
of the sequence and drops off toward the end (Fersen et al. 1991;
Zentall & Sherburne 1998; Delius & Siemann 1998). More importantly,
however, the variance in associative strength is higher toward the
end of the sequence. And this means that an associative representation
should become less accurate the further down the sequence you go,
what has been called a First Item accuracy effect
(Fersen et al. 1991). This has been shown several times in pigeons
(Fersen et al. 1991; Steirn, Weaver & Zentall 1995; Wynne 1997;
Delius & Siemann 1998).
In contrast, monkeys commonly seem to insert the results of paired
contrasts into an underlying linear array, so that the relationships
between the stimuli are directly encoded and the stimuli are associated
with their ordinal positions. This has been termed a linear (D’Amato
& Columbo 1990) or relational (D’Amato
1991) mechanism. Monkeys show no obvious first item accuracy effects,
but apparently because they find the relative positions of two stimuli
by counting down from the beginning of the sequence, they often show
a First Item effect on response time (D’Amato
& Columbo 1988, 1990; D’Amato 1991). So we have a straightforward
method of distinguishing the two cognitive mechanisms, based on the
effect of the position of the highest member of the stimulus pair.
Item results for accuracy are plotted below. Scrub jays show a significant
first item accuracy effect, both for adjacent pairs (on the left)
and non-adjacent pairs (on the right). Pinyon jays show no significant
effects of first item ranking on accuracy. The apparent trend for
pinyons in the right panel is due to a single bird and is not statistically
significant. Scrub jays thus appear to use the pigeon-like mechanism
of associative representation.
are the First Item results for correct response time, again with adjacent
pairs on the left and non-adjacent pairs on the right. The two species
do not differ in response time at the top of the sequence (B/C or
B/D), but they progressively diverge, and toward the end of the sequence
(D/E, E/F, or D/F), pinyon jays are taking significantly longer to
make correct choices. Pinyon jays thus appear to be using a monkey-like
relational or linear mechanism.
from this study are that pinyon jays learn to track multiple dyadic
relationships more rapidly and more accurately than scrub jays. And
they appear to use a more robust and sophisticated mode of transitive
inference, one that is less susceptible to error across the entire
stimulus sequence. These results provide the first quantitative demonstration
of a clear association between social complexity and cognitive representation
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