which they could interact with, without a tool
available in order to create a possible incentive
for later planning. Thereafter, the apparatus was
removed in the presence of the subject. One hour
later, the ravens were offered, at a different location, a forced-choice selection from a tray containing the functional tool and three nonfunctional
distractors. After their selection, a 15-min delay
ensued before the apparatus was installed. The
birds received 14 trials each. Only the first trial
was preceded with a baited apparatus when the
bird lacked a tool.
In the bartering condition, the birds were first
given 35 trials of training on exchanging a
specific token for an immediate food reward. To
provide a planning incentive, the ravens were
exposed to an experimenter asking for the
token when they did not posses it. One hour
later, a tray with the token and three distractors
was offered at a different location by an experimenter with no history of bartering with the
birds. The selection procedure differed from
the tool condition because the ravens were
given three trays in immediate succession
(to be comparable with some studies on
great apes) so that the subject could select,
and later exchange, three tokens in one
trial. After 15 min, the bartering experimenter showed up at a location not visible from the selection compartment. All
subjects received 12 trials each.
In the tool condition, the subjects successfully selected and used the tool to solve the
task in an average of 11 trials out of 14 (min,
8 trials; max, 12 trials), or 78.6%. In trial 9,
one female invented a way to open the apparatus without the tool (and was therefore
excluded from subsequent tool conditions
in the rest of the study). When excluding
her novel solution, the mean success rate
was 86% for all subjects (min, 78.6%; max,
100%). In the bartering condition, the birds
selected in total 143 tokens out of 144. On
average, they exchanged 77.6% of the selected tokens (min, 58.3%; max, 86.1%). At
least one token was exchanged in 91.6% of
the trials (fig. S2).
Combining the results from the tool and
bartering conditions yielded in total eight
first trials with five subjects (two ravens took
part in only one condition each). All subjects
selected functional objects in their first
trials. In six trials, the subjects used the items
in the future task (75%). Four of the five
birds succeeded in their very first task (of
the two conditions) before they had experienced the delay in any task or experienced
the consequences of their choices (table S1).
These trials were the first time any subjects
saved the items by caching them.
Experiment 2 extended the delay be-
tween item selection and use to 17 hours
(overnight). It consisted of six trials per con-
dition, and only one selection per trial was
offered in the bartering condition. In the
tool condition, the three ravens selected
and used the tool in 88.8% of the cases
(min, 5; max, 6). In the bartering condition, the
mean success rate of the four ravens was 95.8%
(min, 5; max, 6).
Experiment 3 tested planning in a self-control
context to determine whether ravens can act with
future events in mind by disregarding an immediate, valuable food reward in favor of an item
that could give access to an even more valued
reward occurring only after a 15-min delay. In
both conditions (14 trials each), subjects were presented with a tray that included the distractor
objects, the tool or token, and an immediate reward. In contrast to a control condition in which
all ravens selected the immediate reward on
100% of trials when no tool or token was available (10 trials per subject), subjects selected the
tool on average in 73.8% of the trials (min, 8;
max, 12) and the token in an average of 73.2%
of trials (min, 7; max, 12). When the subjects did
not choose the functional object, they invariably
took the immediate reward (Fig. 1).
Experiment 4 again presented the birds with
bartering and tool-use tasks with the immediate
reward available so as to determine whether they
took the delay into account. That is, whether
they would select the functional item more often when the reward was spatiotemporally closer
than in experiment 3. If the item would carry an
intrinsic value only, it should be selected equally
often regardless of the delay. In both conditions
(14 trials each), subjects walked past the reward
opportunity (either the apparatus or the experimenter) to select an item from the tray from
where they did not see the future reward. In both
conditions, all subjects declined the immediate
reward and instead selected and used the functional item in 100% of trials. This is a significant increase relative to experiment 3 on both
group and individual levels (fig. S3). An overview of the results in all experiments is provided in Table 1.
This study suggests that ravens make decisions for futures outside their current
sensory contexts, and that they are domain-general planners on par with apes. In the
tool conditions, including self-control, the
ravens were at least as proficient as tool-using apes (1, 2, 15). In the bartering conditions, the ravens outperformed orangutans,
bonobos, and particularly chimpanzees
(3, 4, 15). [Detailed comparisons are available in (14).] The first trial performances
show that the ravens’ behaviors were not
a result of habit formation, and that they
perform better than 4-year-old children
in a comparable set-up (16).
Examining performance in different domains—unrelated to ecological
predispositions—and on first trials is
regarded as key for revealing planning,
and previous studies on both great apes
and corvids have been criticized for not
meeting either or both of these criteria
(11, 13). This study also tested whether the
birds made decisions for future events by
inferring temporal distances to these events,
which shows that the item is not selected
because of an intrinsic value but because
of its relation to a future. Previous studies
have been questioned for not testing temporal sensitivity (12, 17).
Ravens are avian dinosaurs that shared
anancestorwithmammalsaround320million
years ago. The conspicuous similarities in
performance to great apes in tasks such as
these opens up avenues for investigation
into the evolutionary principles of cognition
and shows what the brains of some birds
are capable of.
REFERENCES AND NOTES
1. N. J. Mulcahy, J. Call, Science 312, 1038–1040
(2006).
2. M. Osvath, H. Osvath, Anim. Cogn. 11, 661–674
(2008).
3. M. Osvath, T. Persson, Front. Psychol. 4, 698 (2013).
4. M. Bourjade, J. Call, M. Pelé, M. Maumy, V. Dufour,
Anim. Cogn. 17, 1329–1340 (2014).
5. M. Dekleva, L. van den Berg, B. M. Spruijt,
E. H. M. Sterck, Behav. Processes 90, 392–401 (2012).
Table 1. Overview of the results. Values indicate the
number of trials in which the functional item was selected or
used. The numbers in parentheses indicate the maximum
possible number of successes. Each subject’s individual
performance in selecting the functional item was significant
in all experiments (exact binomial test). N.A., not tested.
Experiment 1: 15-min delay
Tool condition Bartering condition ............................................................................................................................
Subject Selected Used Selected Used ............................................................................................................................
Rickard (male) 14 (14) 12 (14) 36 (36) 29 (36) ............................................................................................................................
Siden (male) N.A. N.A. 36 (36) 21 (36) ............................................................................................................................
Juno (female) 14 (14) 12 (14) 36 (36) 31 (36) ............................................................................................................................
None (female) 11 (14) 8 (11) 35 (36) 30 (35) ............................................................................................................................
Embla (female) 11 (14) 11 (11) N.A. N.A. ............................................................................................................................
Experiment 2: 17-hour delay ............................................................................................................................
Tool condition Bartering condition ............................................................................................................................
Subject Selected Used Selected Used ............................................................................................................................
Rickard (male) 6 (6) 6 (6) 6 (6) 6 (6) ............................................................................................................................
Siden (male) N.A. N.A. 6 (6) 6 (6) ............................................................................................................................
Juno (female) 5 (6) 5 (5) 6 (6) 5 (6) ............................................................................................................................
None (female) N.A. N.A. 6 (6) 6 (6) ............................................................................................................................
Embla (female) 6 (6) 5 (6) N.A. N.A. ............................................................................................................................
Experiment 3: Self-control long delay ............................................................................................................................
Tool condition Bartering condition ............................................................................................................................
Subject Selected Used Subject Selected ............................................................................................................................
Rickard (male) 12 (14) 12 (12) 12 (14) 10 (12) ............................................................................................................................
Siden (male) N.A. N.A. 11 (14) 11 (11) ............................................................................................................................
Juno (female) 8 (14) 7 (8) 11 (14) 10 (11) ............................................................................................................................
None (female) N.A. N.A. 7 (14) 6 (7) ............................................................................................................................
Embla (female) 11 (14) 11 (11) N.A. N.A. ............................................................................................................................
Experiment 4: Self-control short delay ............................................................................................................................
Tool condition Bartering condition ............................................................................................................................
Subject Selected Used Subject Selected ............................................................................................................................
Rickard (male) 14 (14) 14 (14) 14 (14) 14 (14) ............................................................................................................................
Siden (male) N.A. N.A. 14 (14) 14 (14) ............................................................................................................................
Juno (female) 14 (14) 14 (14) 14 (14) 14 (14) ............................................................................................................................
None (female) N.A. N.A. 14 (14) 14 (14) ............................................................................................................................
Embla (female) 14 (14) 14 (14) N.A. N.A.
Experiment 2: 17-hour delay
Experiment 4: Self-control short delay
x e iment3 S lf-con o ongdelay
