Boldness, behavioral inhibition and learning

Abstract

While many studies have examined both boldness and behavioral inhibi-tion from psychological, psychiatric and physiological perspectives, little in the way of controlled experimental work has examined the costs or benefits of bold-ness and behavioral inhibition. Predator inspection behavior in fish is a model system for just such an approach. Here, we examined one potential benefit to boldness by examining the relationship between boldness/behavioral inhibition (measured by predator inspection behavior) and associative learning in the guppy (Poecilia reticulata). We found a positive relationship between male bold-ness and a simple associative learning task, in that males that learned to associ-ate a cue with food were bolder than those that did not.

Full text

Boldness, behavioral inhibition and learning
L.A. DUGATKIN
1
and M.S. ALFIERI
2
Department of Biology, University of Louisville, Louisville, KY 40292, USA
Received 29 July 2002, accepted 19 December 2002
While many studies have examined both boldness and behavioral inhibi-
tion from psychological, psychiatric and physiological perspectives, little in the
way of controlled experimental work has examined the costs or benefits of bold-
ness and behavioral inhibition. Predator inspection behavior in fish is a model
system for just such an approach. Here, we examined one potential benefit to
boldness by examining the relationship between boldness/behavioral inhibition
(measured by predator inspection behavior) and associative learning in the
guppy (Poecilia reticulata). We found a positive relationship between male bold-
ness and a simple associative learning task, in that males that learned to associ-
ate a cue with food were bolder than those that did not.
KEY WORDS
: boldness, inhibition, risk-taking.
Introduction . . . . . . . . . . . . . . . . . 43
Materials and methods . . . . . . . . . . . . . . 45
Results . . . . . . . . . . . . . . . . . . 46
Discussion . . . . . . . . . . . . . . . . . 47
References . . . . . . . . . . . . . . . . . 48
INTRODUCTION
It is a promising sign that one emerging area of common interest between
behavioral ecologists and those in the mental health community is the evolution of
behaviorally-inhibited vs bold personality types (M
ARKS 1987, KAGAN et al. 1988,
K
AGAN 1994, MARKS & NESSE 1994, WILSON et al. 1994, ZUCKERMAN 1994, NESSE &
W
ILLIAMS 1995, STEVENS & PRICE 1996, HAMER & COPELAND 1998, MCGUIRE & TROISI
1998). While a great deal of work has been undertaken in understanding both inhi-
bition and boldness from psychological, psychiatric and physiological perspectives
Ethology Ecology & Evolution 15: 43-49, 2003
1
To whom all correspondence should be addressed. Phone: (502) 852-5943, Fax: (502) 852-
0725 (E-mail: Lee.Dugatkin@Louisville.edu).
2
Current address: Department of Biology, P.O. Box 5838, Winona State University, Winona,
MN 55987-5838, USA.

44
L.A. Dugatkin and M.S. Alfieri
(see KOGAN & WALLACH 1964; ZUCKERMAN 1979, 1991, 1996), little in the way of con-
trolled experimental work has examined the costs or benefits of inhibition and bold-
ness (see W
ILSON et al. 1994 for an exception). From a behavioral ecology perspec-
tive this omission is striking, as a thorough understanding of the evolution of inhi-
bition and boldness can only be attained through a knowledge of the costs and ben-
efits associated with these traits.
In order to better understand whether natural selection has operated on inhi-
bition and boldness, we must focus experimentally on a single context in which
these traits emerge. Predator inspection behavior in fish may prove an ideal system
for just such a cost/benefit analysis. In many species of fish, one to a few individu-
als move away from their school and approach a putative predator to gain informa-
tion about this potential danger, in a behavior termed “predator inspection” (P
ITCH-
ER et al. 1986). Predator inspection has now been documented in more than half a
dozen species of fishes (D
UGATKIN & GODIN 1992b, PITCHER 1992). Potential benefits
of inspection behavior include: (i) determining whether the potential danger is in
fact a predator, (ii) announcing to an ambush predator that it has been spotted,
(iii) assessing the motivational state of the predator (e.g., is it hunting?), and (iv)
obtaining information on the distance between the school and the predator
(M
AGURRAN & PITCHER 1987).
In prior work on whether inspectors are capable of individually recognizing
one another, guppies (Poecilia reticulata) differed significantly in their tendency to
inspect predators, and these differences were consistent over time (D
UGATKIN &
A
LFIERI 1991, DUGATKIN & WILSON 2000). Some guppies consistently fail to inspect,
while others consistently inspect often and in very close proximity to the predator
(similar behavior has been observed in wild populations of guppies; D
UGATKIN &
G
ODIN 1992a). It could be argued that guppies that consistently do not inspect
predators could be labeled “inhibited,” while guppies that consistently inspect
appear to map onto the concept of “boldness” (K
AGAN 1994).
Given that there are consistent differences among inspectors that may map
onto “inhibited” and “bold” personality types, an examination of the costs and/or
benefits of inspection behavior is essential to better understand the evolutionary
implications of these personality traits. In terms of costs, the most obvious ques-
tion is whether the extent to which an individual inspects a predator is correlated
with mortality risk. Do bold inspectors truly put themselves at risk, compared with
their (inhibited) noninspecting shoal mates? Isn’t it possible that bold individuals
simply “warn” potential predators that they have been seen, and that any attack
would be fruitless, thus making bold fish as least as safe, if not safer, than inhibit-
ed fish?
Work on guppies and sticklebacks (Gasterosteus aculeatus) suggests that pre-
dation rates are greatest on males who inspect predators closely, and weakest on
those who rarely inspect (inhibited fish: D
UGATKIN 1992, PARKER & MILINKSI 1997;
but see G
ODIN & DAVIS 1995a, 1995b and MILINSKI & BOLTHAUSER 1995). As such, it
seems that there is a clear and evolutionarily relevant cost that is being paid by
bold individuals. The cost is even more dramatic when it is noted that any informa-
tion obtained by bold individuals is passed on (in one way or another) to all fish in
their group (M
AGURRAN & HIGHAM 1988).
From an adaptationist perspective, one would hypothesize that there must be
some compensating benefit(s) obtained by bold individuals or natural selection
would have culled this trait from the population. In an experiment designed to test
the Handicap Hypothesis (Z
AHAVI 1975), GODIN & DUGATKIN (1997) uncovered one

45Boldness and behavioural inhibition
such benefit in that bold male guppies were more attractive to females than their
inhibited counterparts.
Here we examine another potentially important cost/benefit difference related
to bold vs behaviorally-inhibited male guppies during predator inspection behavior.
We test whether there exists a relationship between boldness/behavioral inhibition
(measured by predator inspection behavior) and associative learning in the context
of foraging (see B
UDAEV 1998 for other work on “personality” and avoidance learn-
ing in the guppy).
MATERIALS AND METHODS
Guppies were purchased at a local pet store. These guppies were raised in an environ-
ment where they would have had no experience with piscine predators before the experiment
described herein. In the laboratory, 32 male guppies were housed in individual 2.5 gal “home
tanks” during all noninspection periods. All trials consisted of three stages: predator inspec-
tion, learning phase I, and learning phase II; in all cases the predator inspection trials preced-
ed the learning phases.
Predator inspection phase
Boldness and behavioral inhibition during predator inspection were recorded for each
male, once a day for 2 min for 3 consecutive days. For each inspection trial, a male was
taken from its home tank and placed in a 10 gallon aquarium that was juxtaposed to a small-
er (2.5 gal) tank containing a Crenicichla alta predator (a predator natural to the guppy’s
native streams in Trinidad and Tobago).
During a 5-min acclimation period, an opaque partition blocked the view between the
guppy and predator tanks. Once the opaque partition was raised, the inspection behavior of
the guppy was videotaped for 2 min. A grid on the bottom of the tank allowed us to measure
the position of the guppy relative to the predator (i.e., how bold or inhibited the fish was). All
trials were videotaped from a camera suspended 91.4 cm above the experimental apparatus.
When viewing the videotapes, the position of the guppy was noted every 15 sec and a mean
inspection score for the 3 days of testing was computed for each individual. This measure
was used as a boldness/behavioral inhibition score for each guppy (similar results are
obtained if “maximum inspection” values are used).
Learning phase I
The phase I learning component of the experiment began on the day following the last
inspection trial, and lasted 10 days, with two trials per day taking place. At the start of a
phase I learning trial, a clear Plexiglas ring was placed in the center of a male’s home tank.
Introduction of the ring caused little or no stress to the individual guppies, as guppies imme-
diately began swimming around the tank normally after the ring was placed in the tank. Ten
seconds after the ring was placed in the tank, a small amount of flake fish food was dropped
inside the ring and the fish was allowed to feed for 2 min. Fish that approached the ring and
performed the nipping behavior associated with foraging (at or inside the ring) before the
food was placed in the ring, on two of three consecutive trials, were labeled “trained”, and
those who failed to meet this criteria were labeled “untrained”. At the end of each day of tri-
als, any flake food remaining in a fish’s tank was removed and all rings were cleaned of any
flake food residue.

46
L.A. Dugatkin and M.S. Alfieri
Learning phase II
After the 10-day phase I learning stage, trained fish were provided with a new task. In
the phase II learning stage of a trial, twice a day for 10 consecutive days, two plastic rings —
one colored orange and one colored green — were simultaneously placed in each male’s home
tank. Ten seconds later, food was placed into one of the rings and fish were allowed to feed
for 2 min. The same color ring had food placed in it for all phase II learning trials for a given
fish and the color was chosen randomly at the start of phase II. The position of the colored
rings — left or right of the tank — was alternated between trials to avoid side biases. Learn-
ing in this context was defined as a fish rising up to the correct colored ring and nipping (at
the ring or inside it) before food was placed in it for two of three consecutive trials.
RESULTS
A t-test on phase I learning was used to compare the inspection scores of
trained vs untrained guppies. Significant differences were found across these
groups, with trained fish being bolder than untrained fish (Fig. 1: unpaired t-test df
= 30, t = 3.07, P < 0.005).
A t-test was also employed to examine differences in phase II learning (two
guppies died between phase I and II, reducing our sample size to n = 30). Although
not statistically significant at 0.05 level, a strong trend toward differences were
found across trained and untrained groups, again with fish meeting the learning
criteria for phase II being bolder than those that failed to meet this criteria (Fig. 2:
df = 28, t = 1.94, P = 0.06).
0
2
4
6
8
Mean inspection score
Trained Not Trained
Fig. 1. — The mean inspection score (± SE) of those who were trained and those who were
not during phase I. Lower scores indicate closer inspection, on a scale of 1-10, where 1 is
closest to predator. Significance levels shown are the result of comparisons between trained
and untrained fish.
P < 0.005

47Boldness and behavioural inhibition
DISCUSSION
Bold and inhibited phenotypes are both seen in many contexts in humans as
well as nonhumans. To better understand how natural selection might favor such a
behavioral polymorphism, a thorough understanding of the costs and benefits of
boldness and inhibition is essential. Ethical problems often forbid experimental
work that might shed light on the costs and benefits of boldness and inhibition in
humans, and as such, the animal literature in this area may provide insight not
only with respect to questions relevant to behavioral ecologists and evolutionary
biologists, but to scientists studying human behavior as well.
One of the best-studied systems of risk taking in animals is predator inspec-
tion behavior in fishes. For example, the costs and benefits of inspection behavior
are slowly being deciphered in both guppies and sticklebacks (D
UGATKIN 1997).
These studies have focused primarily on mortality rates (costs) and mating benefits
associated with being either a bold or inhibited inspector. Given that guppies are
very consistent in their predator inspection scores (D
UGATKIN & ALFIERI 1991,
D
UGATKIN & WILSON 2000), the results presented here suggest a potentially impor-
tant selective advantage with respect to learning in individuals with bold tempera-
ments.
Differences in what we are calling associative learning in phase I and II
learning trials could be due to at least two factors. These results might indicate a
true difference in associative learning skills across bold and inhibited individuals
that have experienced the same learning regime. An alternative explanation is that
bold individuals were less fearful of the ring being placed in their home tank, and
0
1
2
3
4
5
6
Mean inspection score
Learned Did not Learn
P = 0.06
Fig. 2. — The mean inspection score (± SE) for those who learned in phase II and those who
did not. Lower scores indicate closer inspection, on a scale of 1-10, where 1 is closest to pred-
ator. Significance levels shown are the result of comparisons between fish who learned and
those who did not.

48
L.A. Dugatkin and M.S. Alfieri
hence they formed a stronger association between this ring and food than did
inhibited fish. If this were the case, differences in associative learning may be due
to the fact that bold and inhibited fish did not really experience the same learning
regime after all. Rather, these differences may have been due to fear of the feed-
ing ring leading to differential opportunities for learning in bold and inhibited
fish. However, as mentioned earlier, the placement of the ring into the home tank
of fish did not appear to frighten the fish for more than an instant. It is worth
noting that even if the inhibited fish were indeed more frightened of the ring than
were their bolder counterparts, this is a mechanistic, not a functional explanation
for the differences in associative learning between those fish that learned and
those that did not.
With respect to the possible benefits associated with bold behavior in guppies,
our work suggests a positive relationship between male boldness and the likelihood
of learning a simple associative learning task. In conjunction with prior work on
the mortality costs associated with boldness, and the putative benefits of male
boldness in terms of attracting mates, a clearer picture of the evolutionary forces
driving boldness and behavioral inhibition in guppies is coming into focus.
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