Landmark use and development of navigation behaviour in the weakly electric fish Gnathonemus petersii (Mormyridae; Teleostei)
Department of Psychology, St. Lawrence University, Canton, NY 13617, USA
* Author for correspondence (e-mail: pcain{at}stlawu.edu)
Accepted 11 September 2002
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Summary |
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Key words: electric fish, Gnathonemus petersii, electrolocation, landmark, spatial navigation, development, cognitive map
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Introduction |
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According to Bennett (1996),
successful navigation requires some memory of the position of landmarks. Local
landmarks aid navigation by providing information about the environment, such
as the distance and direction to a specific goal
(Cheng and Spetch, 1998
).
Goldfish (Carassius auratus) learned to locate a food source more
quickly if there was a visual landmark present
(Warburton, 1990
). Honeybees
(Apis mellifera; Cartwright and
Collett, 1983
) and gerbils (Meriones unguiculatus;
Collett et al., 1986
) acquired
information about the distance to a goal from the size of landmarks when the
goal was reached. When the size of a previously learned landmark was modified,
bees and gerbils adjusted their search for the goal relative to the size of
the landmark.
As G. petersii learned the aperture location in the absence of
local landmarks, they modified their approach and increased the height at
which they made contact with the divider wall more closely to the aperture
height (Cain, 1995;
Cain et al., 1994
). Both
intact and `electrically silent' fish increased the height at which they
contacted the divider wall when the hydrostatic pressure was increased by
increasing the water level (Cain,
1995
) or when it was increased in a pressurized aquarium (P. Cain
and J. Manchester, unpublished observations). Hydrostatic pressure acts as a
global landmark or reference (Braithwaite,
1998
). Similar to compass direction or distal visual information,
it provides information about the location of a goal, especially to an
organism that must orient and navigate in three dimensions.
G. petersii is a nocturnal bottom-dweller that lives in streams
and lakes in Africa (Blake,
1977; Moller et al.,
1979
). These fish possess an electric organ in their tail that
generates an electric organ discharge (EOD) used in communication and
electrolocation (Heiligenberg,
1977
; Lissmann and Machin,
1958
; reviewed by Moller,
1995
). G. petersii use electrolocation to navigate in
novel environments. Once familiar with the environment, they rely on an
internal representation of their environment developed from electrosensory
input and hydrostatic pressure (Cain,
1995
; Cain et al.,
1994
). The present study investigated whether G. petersii
uses landmarks in order to orient and acquire direction and distance
information contributing to an internal representation of their
environment.
Von der Emde et al. (1998)
showed that G. petersii were able to determine the distance to
different objects via electrolocation. The boundaries of more-distant
objects were less sharply defined in the fish's electrosensory receptive field
than closer objects. If fish learned to associate a landmark with an aperture
and gauged the distance to the aperture based on the electrosensory image of
the landmark, we hypothesized that modifying the size of the landmark would
affect the locomotor trajectory that the fish took to the aperture. As with
honeybees and gerbils, if the sphere increased in size, fish would maintain
the electric image size by swimming upward towards the aperture sooner and, as
a result, would contact the divider wall at a higher place; conversely, if the
landmark size decreased, fish would contact the divider at a lower place.
Because of the perceptual illusions for distance determination of spheres by
weakly electric fish described by von der Emde et al.
(1998
), we believed that the
fish would be more likely to modify their approach trajectory in order to
match the learned electric image with the perceived image.
This led us to examine whether or not a change in the size of such
landmarks would affect the fish's navigation once an internal representation
was established. When sticklebacks were presented with conflicting information
from local and global cues after becoming familiar with a feeding situation,
about two-thirds oriented using global cues, while the remainder relied on
local cues (Huntingford and Wright,
1989). We were therefore interested to see whether global cues,
such as hydrostatic pressure, or local landmarks were the primary sensory
input to guide the fish's locating behaviour.
Mormyrids of different sizes responded differently in an earlier
investigation of navigation behaviour (P. Cain and W. Nolin, unpublished
observations). During the breeding season, mormyrids migrate to newly flooded
swamps to spawn (Corbet, 1960;
Okedi, 1969
). It is possible
to distinguish juvenile, sub-adult and reproductively mature adult
developmental stages in G. petersii based on anal-fin ray-bone
expansion (Pezzanite and Moller,
1998
). Ontogenetic shifts in habitat preference by saltwater
fishes provide evidence of changes in behaviour as fish mature
(Danilowicz, 1997
;
Macpherson, 1998
). In
addition, significant age-related differences in exploratory and investigatory
behaviour have been documented in rats (Rattus norvegicus;
Renner et al., 1992
;
Renner and Pierre, 1998
;
Renner and Rosenzweig, 1986
).
These findings led us to investigate the effect of development stage on
landmark use by fish in navigation.
The current investigation examines the following hypotheses: (1) G. petersii learn to find a goal faster with a landmark present than without one; (2) once fish learn the location of the goal in the presence of a constant-size landmark, they will modify their locomotor behaviour to maintain the original, learned distance/size relationship between the goal and the landmark if the landmark size is changed; (3) when the fish have a choice between hydrostatic-pressure cues and landmarks, they will choose hydrostatic-pressure cues because of their universal (and therefore primary) presence; and (4) navigation behaviour changes during development from juvenile to adult.
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Materials and methods |
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Experimental apparatus
Experiments were conducted in a dark room with an apparatus similar to that
described and illustrated in Cain et al.
(1994). An aquarium (92
cmx46.5 cmx45.5 cm) was divided into two equally sized
compartments with a clear plastic wall. The divider was split in the middle
and slotted to receive four clear plastic squares measuring 10 cmx10 cm.
A circular aperture (diameter 6.85 cm) in one of the squares provided the only
access from one compartment to the other. The aperture was positioned with its
centre 25.5 cm from the floor (Fig.
1). Based on computations by Heiligenberg
(1977
) and our previous work
(Cain, 1995
;
Cain et al., 1994
), there was
no evidence that fish at or beyond 15 cm from the divider wall, the
electrolocation boundary, could detect the aperture using electrolocation or
any other sense. Water depth was marked in 5 cm increments at the divider wall
and the electrolocation boundaries on the front of the aquarium. Each
compartment was illuminated independently by two red 5 W lamps (C7
,
Osram Sylvania, Yonkers, NY, USA) placed behind translucent white paper
covering the back wall of the aquarium. G. petersii can detect
long-wavelength light (575-725 nm), although the fish are most sensitive to
525 nm light (Ciali et al.,
1997
). Under these conditions, we were able to observe and
videotape the behaviour of the fish while providing them with a nearly dark
environment.
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We constructed three sets of landmarks, one of each diameter (3.9 cm, 6.8 cm or 8.3 cm), by attaching one end of a plastic tube to two plastic spheres of equal diameter. The opposite end of each tube was attached to a Plexiglas square. Each plastic tube was cut such that the maximum distance each plastic sphere extended was 12.5 cm from the divider wall (Fig. 1). Each square could be placed in the slotted opening so that the center of the landmarks was 20 cm directly below the aperture and 6 cm off the floor.
Training
The fish were divided into two groups: small fish (N=8) ranging
from 113 mm to 135 mm SL (124±6.6 mm, mean ± S.D.) and large
fish (N=9) ranging from 145 mm to 178 mm SL (159±14.4 mm).
These assignments reflect two developmental stages: sub-adults and early
adults, respectively (Pezzanite and
Moller, 1998). Each group was trained with the medium landmark
(diameter 6.8 cm). A control group of large fish (N=4) ranging from
145 mm to 181 mm SL (157±16.5 mm) and a control group of small fish
(N=6) ranging from 110 mm to 132 mm SL (121±6.9 mm) were
trained without landmarks.
Each fish was released into an illuminated compartment at the wall opposite the divider. The task was for the fish to locate and swim through the aperture to the non-illuminated compartment during a 300 s interval. As fish became familiar with the compartment and learned the aperture location, they would spend less time near the floor of the compartment, reduce the amount of time in the illuminated compartment and swim more directly to the aperture. These changes in locomotor behaviour resulted in faster times to and through the aperture, increased height of contact with the divider wall as the fish's trajectory more closely approached the height of the aperture, and increased number of crossings. The side of the aquarium in which the fish were released was alternated by trial-day. The fish's movements were monitored with a low-light video camera and recorded on videotape for later analysis.
If the fish swam through the aperture within 300 s, lights were turned off, and, after 15 s of darkness, the lights in the compartment where the fish was located were turned on and the fish was again allowed 300 s to swim to the non-illuminated compartment. In this way, it was possible for the fish to make numerous crosses from an illuminated to a non-illuminated compartment. However, if the fish failed to locate the aperture after 300 s, it was returned to its home tank after 60 s in darkness and given a time score of 301 s. This procedure was followed for a maximum of 10 min per day (one trial) for 10 days. The water level remained constant at 35 cm, the aperture was 25.5 cm from the floor, and the landmark was always 6.8 cm in diameter and 15 cm directly below the aperture. Testing followed immediately upon completion of 10 training trial-days. Fish were netted and either the water level was raised or a larger or smaller landmark replaced the medium-sized landmark. Presentation of novel stimuli was counterbalanced across fish over subsequent trial-days after 5 min refresher trials under training conditions.
Landmark tests
Each fish was netted and confined at the water surface while the medium
landmark was replaced with the large (8.3 cm diameter) or small landmark (3.9
cm diameter). The test began when the fish was released from the net into the
illuminated compartment. Because we were not interested in how long it took
the fish to learn the new conditions, we scored dependent variables for each
fish for 180 s. If the fish swam through the aperture, the lights were turned
off for 15 s, then turned on in the compartment where the fish was located. If
the fish did not swim through the aperture, the lights were turned off, the
fish was given a time score of 181 s and was returned to its home tank after
60 s.
Water-level test
A fish was netted and held in the net at the water surface while the water
level was raised by 10 cm (from 35 cm to 45 cm) within 5 min with all other
conditions remaining the same. The test began when the fish was released into
the illuminated compartment. Each fish was allowed 180 s to locate and swim
through the aperture. When the fish swam through the aperture, lights were
turned off for 15 s and turned on in the compartment where the fish was now
located. If the fish did not swim through the aperture, the lights were turned
off and the fish was returned to its home tank after 60 s. We scored dependent
variables for 180 s.
Analysis
We recorded three dependent variables: (1) the time (s) that the fish
needed to locate the aperture after it crossed a 15 cm boundary corresponding
to the electrolocation distance (see
Moller, 1995), (2) the height
(cm) at which the fish first made physical contact with the divider wall and
(3) the number of times that the fish crossed through the aperture from an
illuminated compartment to a non-illuminated compartment. Data are expressed
as means ± S.E.M. Learning the location of the aperture was defined by
a decrease in the time to locate the aperture, an increase in the height at
which the fish contacted the divider and an increase in the number of aperture
crossings. In earlier investigations, fish learned the aperture location
within four trial-days (Cain,
1995
; Cain et al.,
1994
). We compared the mean time to locate the aperture for the
first three trial-days with the mean time over trial-days 4-10. We also
determined the responses of fish to changes in landmark size and water depth.
We compared these dependent measures for the first 3 min of trial-day 10 with
those of the first 3 min of each of the test situations.
Data were analyzed using mixed factors or univariate analysis of variance (ANOVA; SPSS V.10; SPSS Inc., Chicago, IL, USA). For the within-subject analyses, violations of the sphericity assumption, as shown by Mauchly's test, were corrected using the HuynhFeldt epsilon correction for degrees of freedom. Pair-wise comparisons were conducted with independent and paired-sample t-tests (SPSS V.10). A confidence level of 95% (P<0.05) determined significance.
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Results |
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The amount of time to locate and swim through the aperture decreased significantly for each group: controls (F5.5,49.8=5.97, P<0.001); large fish (F9,72=11.9, P<0.001); small fish (F9,63=12.4, P<0.001). Over the 10-day period, large fish in the presence of the medium landmark did not differ from controls (with no landmark present) in the amount of time taken to find the aperture. Small fish took significantly longer to locate and use the aperture than controls (F1,16=8.4, P=0.01). Large fish found the aperture faster than small fish with the same landmark present (Fig. 2; F1,15=9.15, P=0.009).
We divided the trial-days into two blocks and compared trial-days 1-3 with
trial-days 4-10 (Cain, 1995;
Cain et al., 1994
). There was
a significant difference in the amount of time to the aperture between the
blocks (F1,23=28.31, P<0.001) across groups.
There was also a significant difference between groups
(F1,23=4.59, P<0.012). The amount of time
(228.5±21.5 s) that small fish in the presence of the medium landmark
took to find the aperture over the first three trial-days was significantly
greater (F1,16=8.14, P=0.012) than that observed
in controls (112.9±25 s) and significantly greater
(F1,15=12.68, P=0.003) than the time taken by
large fish (116±21.1 s). There was no difference in time taken between
large and control fish over the first three trial-days.
Height
All groups increased the height at which they contacted the divider wall
(F7.4,177.7=22.52, P<0.001) over the 10
trial-days (Fig. 3). There was
no significant difference in the height at which large and small control
groups contacted the divider over 10 trial-days. Each group controls
(F9,81=9.95, P<0.001), large fish
(F9,72=9.03, P<0.001) and small fish
(F9,63=6.21, P<0.001) significantly
increased the height at which they contacted the divider wall. While the mean
height at which controls (16.7±1.43 cm) and large fish (14.8±6.8
cm) contacted the divider did not differ over 10 trial-days, the mean height
that small fish (12.2±7.58 cm) contacted the divider was significantly
lower than that for controls (F1,16=4.64,
P=0.047).
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When we compared the heights of contact between trial-day blocks 1-3 and blocks 4-10, we found no significant difference across groups or between groups. Also, there was no significant difference in the height at which controls, large and small fish contacted the divider over the first three trial-days. Control fish continued to increase the height of contact, approaching the bottom edge of the aperture (22.1 cm from the floor), until trial-day 7, while the large and small fish levelled out. On trial-day 7, there was a significant difference (t22.8=2.82, P=0.014) between controls (21.3±1.38 cm) and large and small fish (15.9±1.4 cm). In the first 3 min of trial 10, small fish contacted the divider at a significantly lower height than did the larger fish (t15=2.18, P=0.045) (Figs 3, 5B medium landmark).
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Crossing
The number of crossings, across all three groups, increased significantly
(F6.7,160.3=13.77, P<0.001), with the control
group crossing more times than both the large and small fish. Controls
(F6.6,59.2=6.12, P<0.001), large
(F9,72=3.87, P<0.001), and small fish
(F9,63=4.92, P<0.001) significantly increased
the frequency with which they crossed from one compartment to the other over
the 10 trial-days (Fig. 4). The
controls crossed more frequently than did small fish over 10 trial-days
(F1,16=4.19, P=0.057), although this was not
significant. However, there was no difference in the number of crossings
between the large and small control groups over the 10 trial-days. There were
significant differences in the number of crossings between trial-day blocks
1-3 and 4-10 (F1,24=147.48, P<0.001) across
groups. There were no significant differences between groups in the number of
crossings over trial-days 1-3, trial-days 4-10 or over the total 10
trial-days. Over trial-days 1-3, the difference in the number of crossings
between controls (22.3±5.3) and small fish (7.9±4.1) is
significant (t16=2.07, P<0.025).
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Tests
Water level
At 35 cm water depth, the distance from the aperture centre to the water
surface was 9.5 cm; at a depth of 45 cm, this distance increased to 19.5 cm.
The floor-to-aperture distance was 25.5 cm, with the landmark surface 15 cm
below the aperture and 12.5 cm from the divider wall. These did not change
with the increase in water depth.
Large fish took longer (t8=-2.96, P=0.018) to find the aperture, from 26.8±4.8 s in the first 3 min of trial-day 10 to 115.0±33.0 s in the first 3 min after the water level was raised (Fig. 2). By contrast, the amount of time to locate and swim through the aperture by controls and small fish did not change significantly after the water level was increased.
The height at which the controls (no landmark) contacted the divider wall increased significantly from 18.2±1.95 cm in the first 3 min of trial-day 10 to 27.7±3.9 cm (t9=-2.47, P=0.036) (Fig. 3). Fish in the presence of a landmark contacted the divider at a lower height than did the controls after the water depth was increased: large fish at 13.6±3.4 cm (t17=2.7, P=0.015), small fish at 15.8±2.8 cm (t16=2.39, P=0.03). There was no difference between large and small fish on this measure.
When we compared the number of crossings by all three groups in the first 3 min of trial-day 10 with the number of crossings in the first 3 min after the water level had been increased (Fig. 4), we found a significant difference (F1,24=6.24, P=0.02). A paired-samples t-test revealed no difference in the frequency of crossing in the first 3 min before and after the water-level increase by controls and small fish. However, large fish decreased their number of crossings from 3.11±0.39 in the first 3 min of trial-day 10 to 1.78±0.62 after the water level had been raised (t8=2.31, P=0.05).
Large landmark (8.3 cm diameter)
Both large and small fish took significantly longer to swim through the
aperture after the landmark increased in size
(Fig. 5A). The large fish took
significantly longer (85.8±23.1 s; t8=-2.48,
P=0.038) to swim through the aperture when the landmark size was
increased than on trial-day 10 (26.8±4.8 s). The small fish also
increased the time taken to swim through the aperture (84.6±22 s;
t7=-3.63, P=0.008) after the size increased as
compared with trial-day 10 (28.3±8.0 s).
In the first 3 min of trial-day 10, small fish contacted the divider at a significantly lower height than did the larger fish (t15=2.18, P=0.045) (Fig. 5B), medium landmark). However, contrary to what we expected, the heights at which both groups of fish made contact with the divider wall did not change after the landmark size increased from 6.8 cm to 8.3 cm diameter.
Both large and small fish made fewer crossings after the landmark increased in size (Fig. 5C). In the first 3 min after the increase in landmark size, the large fish reduced (t8=2.6, P=0.032) the number of crossings from 3.1±0.39 to 1.7±0.5, while the small fish (t7=2.86, P=0.024) reduced their crossings from 3.8±0.93 for the first 3 min of trial-day 10 to 1.75±0.41.
Small landmark (3.9 cm diameter)
The small fish took longer (124.97±28.4 s) to find the aperture
after the landmark size decreased from 6.8 cm to 3.9 cm diameter
(t7=-3.78, P=0.007), while the large fish showed
no difference (Fig. 5A). As
with the large landmark, there were no differences in the heights at which the
large and small fish made contact with the divider wall after the small
landmark replaced the medium landmark (Fig.
5B). However, the large fish decreased
(t8=2.58, P=0.03) the number of crossings
(1.56±0.47) from that recorded at trial-day 10 (3.11±0.39). The
decrease by small fish from 3.8±0.93 crossings in the first 3 min of
trial-day 10 to 1.62±0.99 was not significant
(Fig. 5C).
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Discussion |
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After becoming familiar with their environment, G. petersii may
continue to swim from one compartment to another as an operant response to
changes in their environment such as (1) the mormyromast electroreceptor
response pattern across the fish's body as it swims through the aperture, (2)
changes in the external electric field as a result of the lights turning on
and off and (3) a change in light intensity when swimming from an illuminated
compartment to a non-illuminated one (Cain
et al., 1994). Our data show that G. petersii of
different developmental stages (1) responded differently to similar navigation
and orientation tasks, (2) incorporated landmarks into their navigation and
orientation behaviour, (3) were able to detect and respond to changes in
landmark size and (4) relied on landmarks as the primary source of information
in the presence of conflicting information between hydrostatic-pressure cues
and landmarks.
Size/development differences in performance
G. petersii display a sexually dimorphic ventral body wall
indentation resulting from anal-fin ray bone expansion
(Pezzanite and Moller, 1998).
Pezzanite and Moller (1998
)
defined developmental stages in G. petersii based upon this
characteristic and correlated them with standard body length. According to
their categories, our large fish are large sub-adults and adults (145-181 mm)
and our small fish are large juveniles and sub-adults (110-132 mm). In the
current study, we expected that intact G. petersii would locate and
swim through an aperture in a clear plastic wall more rapidly in the presence
of a landmark. Our expectations were accurate for large fish (large sub-adults
and adults) but not small fish (large juveniles and sub-adults). G.
petersii consistently demonstrated size-related orientation and
navigation behavioural differences. Small fish initially took longer to swim
through the aperture after crossing the electrolocation boundary, made contact
with the divider wall at a lower height and made fewer crossings in the first
three days.
Armstrong et al. (1997)
examined the effects of trials or experience, starting region, and size on the
propensity of wild Atlantic salmon (Salmo salar) parr (69-114 mm fork
length) in a group situation to explore novel surroundings. They found no
effect of trial or starting region, but larger parr were more likely to
explore than smaller fish. No size breakdown of larger and smaller fish was
provided. Similar differences in behaviour have been reported for juvenile and
adult rats. Renner et al.
(1992
) reported that
30-day-old rats (R. norvegicus) took significantly longer to cross
through an aperture and enter an open field arena than 60- and 90-day-old
rats. They also spent significantly less time in the arena than 60- and
90-day-old rats. The authors suggested that, as rats mature, they develop
behaviours capable of investigating novel environments.
Previous studies showed that fish rely on landmarks to orient and navigate
(Braithwaite, 1998;
Braithwaite et al., 1996
;
Girvan and Braithwaite, 1997; Reese,
1989
; Warburton,
1990
). We examined the effects of adding a landmark and found that
it affected the behaviour of both large and small fish differently. Once large
fish learned the aperture location in the presence of the landmark, they did
not increase their contact height to match the aperture but instead oriented
to the landmark and then swam up to and through the aperture
(Fig. 2).
Both controls and large fish located the aperture faster than small fish over the first four trial-days and 10 trial-days. There was no difference between the large and small controls. Why were small fish with a landmark slower than small fish without a landmark? The landmark was directly below the aperture and within the electrolocation boundary, and moving from one compartment to another was a two-stage event for small fish. Small fish crossed the electrolocation boundary and remained stationary under the landmark before crossing through the aperture. By contrast, controls of both sizes and large fish swam through the aperture after crossing the electrolocation boundary.
Cue conflicts
Von der Emde and Bleckmann
(1998) found that G.
petersii foraging for food rely on multiple sources of sensory input and
that each individual may rely on a specific combination of sensory input. Cain
(1995
) found that G.
petersii use electrolocation in conjunction with hydrostatic-pressure
cues to form a spatial representation of their environment.
Hydrostatic-pressure cues provided the fish with information of relative
`height' in the water column. These hydrostatic-pressure cues became the
primary sensory cue after the aperture location was familiar. Without a local
landmark, the internal representation and hydrostatic pressure controlled
orientation and navigation to the aperture
(Cain, 1995
;
Cain et al., 1994
).
In these experiments, the relationship between hydrostatic pressure (a global cue), the landmark (a local cue) and the aperture remained stable during training for both large and small fish. The changes in the dependent variables indicate that large and small fish with the landmark present apparently learned the relationship between water depth, landmark and aperture.
When we increased the water level after training, this presented
conflicting cues and we expected that all fish would orient according to the
hydrostatic pressure, the distal cue. For control fish, the conflicting cues
were hydrostatic pressure and internal representation versus the
actual aperture location. After we raised the water level, controls increased
the height at which they contacted the divider wall and did not significantly
change the amount of time to the aperture. In the presence of conflicting
information between hydrostatic pressure, landmark position and aperture
location, large and small fish oriented to the landmark, as evidenced by the
decreased height of contact, and increased the amount of time before crossing
through the aperture. All fish reduced the number of crossings through the
aperture in response to the increased water level. These changes suggest that
fish detected novel sensory information, oriented to the landmark and took
time to adjust to the new information. The data show that the landmark was a
critical and preferred reference point for orientation. Biegler and Morris
(1996) found similar results
when they changed the location of the landmark and held the distal cues
constant.
We were interested in determining if G. petersii would modify
their locomotor trajectory if the size of the landmark changed. Biegler and
Morris (1993,
1996
) showed that a single,
reliable landmark, a proximal or local cue, exerts greater control over search
location in a stable environment than does a variable environment in which
local cues vary with respect to global cues. Their evidence supports a
hierarchical organization of spatial representations. We changed the size of
the landmark, not its location, and obtained similar results.
We predicted that because fish could determine the distance to an object
via electrosensory means and that, because of the potential for error
in determining the distance to a sphere
(von der Emde et al., 1998),
fish would rely on their internal representation and modify their approach
path to the aperture in a predictable manner. We expected that the internal
representation would provide an expectation of the electrosensory image size
of the landmark as the fish approached the real landmark. The fish would
attempt to match this image with the perceived image and modify its
trajectory. However, fish of both sizes did not significantly change their
trajectory. Instead, they appeared to perceive the changes in landmark size,
approached it and investigated the novel stimulus.
Fish respond to detected changes in their environment with an increase in
exploratory behaviour (Kleerekoper et al.,
1974; Russell,
1967
; Welker and Welker,
1958
). After the landmark size increased, both large and small
fish increased the amount of time to cross and decreased the number of
crossings through the aperture. When the landmark size decreased, the small
fish took longer to use the aperture but did not significantly decrease the
number of crossings. Large fish did not increase the amount of time to the
aperture but decreased the number of crossings through the aperture. Neither
large nor small fish significantly modified the height at which they contacted
the wall in response to changes in landmark size. Fish oriented to the stable
cue. When the landmark changed size, the location of the landmark did not
change, but the relationship between it and the substrate and aperture
shifted. These discrepancies triggered responses similar to those displayed
during the first three days of training when the fish were unfamiliar with the
environment.
Biegler and Morris (1996)
proposed that an organism will resolve a conflict between global and local
cues in a previously stable environment based on `a priori
reliability' and the `extent of the discrepancy' between current and previous
information. This could explain the reliance of the fish on the local landmark
when the hydrostatic pressure changed. When the hydrostatic pressure increased
at both the landmark and the aperture, the relationship between the landmark,
the substrate and the aperture did not change.
Significant, rapid increases in the water depth of rivers and streams
during the rainy season are normal phenomena in subtropical Africa and act as
a cue for reproduction in mormyrids (reviewed by
Moller, 1995). The
relationship between proximal and distal cues, which is relatively stable
during the dry season, would change significantly. Spatial learning of
proximal landmarks and their relation to hydrostatic pressure could help fish
solve navigational problems encountered during migrations to and from
nocturnal feeding grounds in their natural environment. In addition, the
ability to detect and respond to changes in these relationships adaptively
would facilitate predator avoidance and feeding in young fish with limited
experience and would facilitate reproduction in mature fish as they moved from
familiar areas into newly flooded swamps to breed.
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References |
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