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First published online February 27, 2009
Journal of Experimental Biology 212, 835-842 (2009)
Published by The Company of Biologists 2009
doi: 10.1242/jeb.023572
Silent tidbitting in male fowl, Gallus gallus: a referential visual signal with multiple functions
Centre for the Integrative Study of Animal Behaviour, Department of Brain, Behaviour and Evolution, Macquarie University, Sydney, NSW, 2109, Australia
* Author for correspondence (e-mail: kls{at}galliform.bhs.mq.edu.au)
Accepted 6 January 2009
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Summary |
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 TOP Summary INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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Key words: multimodal communication, visual displays, food calls, referential signals
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INTRODUCTION |
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TOPSummaryINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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;
Hauser, 1997)]. Signals that
evoke such highly specific receiver responses in the absence of contextual
cues are considered functionally referential
(Evans, 1997). Previous
research has demonstrated the existence of acoustic
(Evans et al., 1993a;
Evans and Evans, 1999;
Seyfarth and Cheney, 1990) and
visual (Gould and Gould, 1988)
referential signals and these now appear to be taxonomically widespread,
occurring in species as diverse as non-human primates (e.g.
Di Bitetti, 2003;
Macedonia, 1990;
Seyfarth et al., 1980), birds
(Bugnyar et al., 2001;
Evans et al., 1993a;
Evans and Evans, 2007),
suricates (Manser, 2001) and
social insects (von Frisch,
1993).
Some of the same species that produce referential signals also have
multimodal communication. The honeybee, Apis mellifera, waggle dance
encodes the direction, distance and profitability of a food source
(von Frisch, 1974;
Gould and Gould, 1988). During
the waggle dance, the direction of the wagging run indicates the direction of
food. This visual display is accompanied by vibratory bursts, the duration of
which is correlated with the distance to food. The combined display is hence
an example of multimodal referential communication.
Multimodal communication has two putative benefits: ensuring that the
intended receiver perceives the signal, and maximizing information content
(Hebets and Papaj, 2005;
Partan and Marler, 1999;
Partan and Marler, 2005;
Rowe, 1999). Redundant or
`backup' signals (Johnstone,
1996; Zuk et al.,
1993) convey the same information through each modality, thereby
increasing the likelihood of signal detection in a noisy environment. By
contrast, nonredundant or `multiple message' displays
(Johnstone, 1996;
Møller and Pomiankowski,
1993; Zuk et al.,
1993) transmit different information via each channel,
potentially increasing the rate of information transfer
(Candolin, 2003;
Partan and Marler, 1999;
Partan and Marler, 2005). If
one component of a redundant multimodal display is functionally referential,
then the other component might be expected to elicit a similarly specific
receiver response when produced independently. However, this prediction has
never been experimentally tested.
We focus here on the visual component of a food-related audio-visual
multimodal display produced by fowl (tidbitting). The acoustic component of
this display (food calling) consists of series of pulsatile, cluck-like sounds
(Marler et al., 1986a;
Stokes and Williams, 1971),
which are known to be functionally referential. Audio playbacks are sufficient
to evoke substrate-search responses, in the absence of other cues
(Evans and Evans, 1999), and
the behavior of the hens is mediated specifically by the expectation of
finding food (Evans and Evans,
2007).
Under natural conditions, discovery of a palatable item by a male in the
presence of a hen reliably elicits the multimodal tidbitting display
(Evans and Marler, 1994;
Marler et al., 1986a;
Marler et al., 1986b;
Stokes and Williams, 1971).
This performance often entices one or more hens to approach the tidbitting
male and food-search near him, sometimes taking the food item directly from
his mandibles (Gyger and Marler,
1988; Marler et al.,
1986a; Marler et al.,
1986b). In this hierarchically structured system, hens prefer to
mate with males that provide food
(Pizzari, 2003) and, in the
presence of a hen, dominant males respond to a subordinate's food calling and
tidbitting display with overt aggression, often chasing him away from the food
and then food calling themselves (Stokes
and Williams, 1972). On occasions, subordinate males alter the
signal by producing only the visual component, suppressing the call.
Experimental playbacks demonstrate that these unimodal displays still attract
hens to the silently tidbitting male
(Smith and Evans, 2008).
In contrast to these detailed analyses of food calling, the visual display
has only been described in general terms. Davis and Domm
(Davis and Domm, 1943)
characterized tidbitting as a repeated, rhythmic motion of the head and neck,
including repeated picking up and dropping of the food item. Wood-Gush's
(Wood-Gush, 1954) description
was similarly brief and he treated the two display components as a single
action, whereas Davis and Domm (Davis and
Domm, 1943) recognized that these often occur separately and
treated them as distinct. Stokes and Williams
(Stokes and Williams, 1971)
provided the most detailed overview of the tidbitting display, although they
did not quantify the specific motions performed or determine if there was any
consistency to the sequence order. Many visual displays are stereotyped
(Wiley, 1983) and include an
alerting component, which engages the attention of the intended receiver
(Fleishman, 1988;
Rowe, 1999) thereby improving
signal detectability in noisy environments. A more precise analysis of the
structure of the tidbitting display is an essential prerequisite for further
study of signal design.
We have previously shown (Smith and
Evans, 2008) that the visual display elicits similar overall
levels of food searching to the multimodal display and the acoustic component
alone. However, the multimodal and visual display evoke higher levels of
inspection – a conspecific recognition behavior
(Dawkins, 1995;
Guhl and Ortman, 1953) –
than the audio alone. Hence the acoustic and visual components of male
tidbitting can be classified as perceptually redundant (sensu
Partan and Marler, 1999) with
regard to food search duration. However, the additional social response
suggests that the visual display has an additional function.
These findings are consistent with the idea that the visual display might
be a food-related signal with response specificity comparable to that of food
calls, in which case tidbitting would have the unusual property of being a
multimodal signal in which the components in each modality were functionally
referential. However, there are several possible alternative explanations for
the hen's food search response. These include a release from vigilance in the
presence of an alert companion (Artiss and
Martin, 1995) or a general increase in foraging in response to any
male movement.
In this study, we tested the specificity of hen responses to tidbitting motion. First, we classified the most common movements and described their typical order over the course of the tidbitting display. We gave males food items in the presence of an unfamiliar hen and scored the types of motor pattern produced and the probability of transitions between them. This defined the gross structure of the visual display and tested whether the temporal sequence was constrained. Results also informed the design of the playbacks that followed.
We then conducted an experiment to test perceptual processing. Stimuli were designed to evaluate a range of motions, with varying levels of spatiotemporal similarity to normal tidbitting. Hens were shown high-definition videos of a male performing four different movement patterns and control footage of an empty cage. We then analyzed video recordings of the hens' food searching and social behavioral responses. If the visual display were functionally referential, we would predict an increase in the food searching behavior specific to normal tidbitting, such that responses to this playback would be greater than those to any other type of motion and to the empty cage control.
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MATERIALS AND METHODS |
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TOPSummaryINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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; Collias,
1987; Andersson et al.,
2001; Schütz and Jensen,
2001) from which all domesticated strains have been derived
(Fumihito et al., 1994;
Fumihito et al., 1996). In
particular, Sebrights have not been subjected to artificial selection for
rapid growth or egg production. Birds were housed in 1.0 mx1.0 mx0.6 m (length x width x height) cages in a climate-controlled room maintained at 22°C on a 12 h:12 h day:night cycle and given ad libitum access to high-protein food (Gordon Specialty Feeds, Sydney, Australia) and water in their home cages. Twenty-five males participated in experiment I, which measured the structure of tidbitting. Each male was housed with a single female. Twenty-four different hens participated in experiment II, which tested specificity of response. These hens were house in same-sex pairs.
Recordings for creation of playback stimuli and behavioral testing were conducted in a sound-attenuating chamber (Ampisilence S.p.a., Roberssomero, Italy 2.38 mx2.38 mx2.15 m), which was lined with 10 cm `Sonex' foam baffles (Illbruck, Minneapolis, USA) on the side walls and 15 cm baffles on the ceiling to prevent reverberation.
Experiment I: tidbitting performance
Test apparatus
During each trial, the male was confined to a steel-framed wire cage (0.60
mx0.45 mx0.86 m), which had an audience hen cage of similar size
abutting the left side wall and a food dispenser on the opposite wall (for
details, see Evans and Evans,
1999). We monitored tests via a Sony 1450QM color
monitor, connected to a Panasonic WV-CL320 video camera in the sound chamber.
A Panasonic WJ-810 time-date generator provided a `stopwatch' at the bottom of
the video display for timing test sessions. All tests were video recorded
using a Panasonic AG-7750 VHS-format deck.
Design and test procedure
Males were placed in the test cage for 15 min each on three consecutive
days to habituate them to the test environment. We placed each male's
cage-mate in the adjoining audience cage during these sessions to facilitate
habituation. No food was presented.
On the fourth day, the male was again placed in the test cage, but with an unfamiliar hen in the audience cage. Unfamiliar hens were used as audience because males are most likely to display for an unfamiliar female, although the performance does not differ based on familiarity (C.S.E., unpublished). After a 5 min delay, which allowed the male to settle after handling, the food dispenser was activated, delivering five corn kernels onto the floor of his cage.
Analysis of head movements
We recorded the type of movement and position of the male's head in space
in every video frame (PAL standard; 40 ms time interval). Movements fell
naturally into three discrete classes, which we coded as follows: `twitch'
(Fig. 1A): a rapid horizontal
side-to-side motion of the head with the neck held fully upright; `short bob'
(Fig. 1B): abrupt vertical
movement of the head from a fully upright position to a point halfway above
the floor, returning to the upright position; and `long bob'
(Fig. 1C): vertical movement of
the head, plunging through the full arc toward the floor, often picking up the
food item with the mandibles, and ending with the head in the upright
position. We recorded all transitions between these motor patterns and
calculated mean probabilities, which were then summarized in a kinematic plot
(Lehner, 1979). Results of
this analysis informed the design of the experiment II stimuli.
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Experiment II: female response to visual tidbitting display
Test apparatus
The test apparatus consisted of a 106 cm Sony high-definition flat panel
plasma display (resolution 1080 by 1960 pixels), mounted at floor level next
to a 1.2 mx0.30 mx0.5 m cage with a remote-controlled wire door
0.4 m from one end. Previous research has demonstrated the hens can recognize
the movements of conspecifics on video screens
(McQuoid and Galef, 1993) and
that video playback evokes natural anti-predator and social responses
(Evans and Marler, 1991;
Evans et al., 1993a;
Evans et al., 1993b). The HDV
format used in this study provides an approximately four-fold increase in
resolution over standard-definition digital video and has successfully been
used in multimodal playbacks with hens
(Smith and Evans, 2008).
Decisions about the overall layout of the test setup were informed by the
well-described properties of the fowl visual system. Hens recognize
conspecifics using close binocular inspection of the other bird's head and
neck region (Guhl and Ortman,
1953), but they are myopic in the frontal field and so unable to
determine individual identity from distances greater than 30 cm
(Dawkins, 1995;
Dawkins, 1996;
Dawkins and Woodington, 1997).
We hence positioned the end of the test cage 30 cm from the plasma display, a
distance at which a hen should attempt social recognition by fixating on the
screen. Note that this spatial separation was also sufficient to prevent hens
from resolving individual pixels, with a concomitant loss of verisimilitude.
The long axis of the cage was perpendicular to the plasma display and the
remote-controlled door was at the end farthest from it.
Stimulus design
We used a 3-CCD high-definition video camera (Sony HDR-FX1) to make
high-definition (1920x1080 lines) video recordings of 12 male fowl,
Gallus gallus, performing the tidbitting display. During recording,
the video signal was monitored on the plasma display later employed for
playbacks, allowing us to adjust the camera zoom so that the image of the male
was precisely life-sized. Males were confined in a 0.60 mx0.45
mx0.86 m wire cage, 0.8 m from the camera. The audience hen was held in
a separate cage, approximately 30 cm from that of the male. After a 10 min
acclimation period, four mealworms were delivered from a remote-controlled
food hopper mounted above the male's cage. This usually evoked tidbitting and
food calling from the male.
We selected four males using the same criteria as in our previous study
(Smith and Evans, 2008). Raw
footage was then edited using Final Cut Pro 5.1 (Apple Computer) to create
five stimuli from each male, each of 15 min duration. These consisted of an
empty cage for the first 5 min, to allow the hen to settle, followed by 5 min
of a male standing alone, which provided a baseline measurement for food
searching in the company of a vigilant companion, then 60 s of one of the five
test sequences, followed by 4 min of empty cage. Each successive phase of the
test stimuli began and ended with a 0.5 s fade transition to avoid evoking a
startle response. The first 10 min and the last 4 min of every trial were
hence identical across every condition. Only the 60 s test sequence differed
across conditions.
Test sequences were designed systematically to assess the specificity of female responses to male display movements. The five stimuli were Silent tidbitting (normal signal), Matched-frequency motion in the opposite direction, Silent crows, Inactive male and Empty cage. Decisions about design of the stimuli were informed by the results of experiment I, as detailed below.
Silent tidbitting
Silent tidbitting consisted of the male performing the full natural
display, including twitches, short bobs and long bobs
(Fig. 1).
Matched-frequency motion in the opposite direction
Analysis of the tibitting movements (experiment I) revealed that the
majority (77%) of the movements during a typical tidbitting display include a
rapid downward sweep and return upward of the head and neck. To test whether
this characteristic, rapid, vertical movement was a crucial signal component,
independent of temporal pattern, we created stimuli with Matched-frequency
motion in the opposite direction. Each of the Silent tidbitting exemplars was
used as a template; we edited spontaneous crowing of the same male so that
each upward extension of the neck during the crowing motion corresponded to
one of the long, downward sweeping motions in the Silent tidbitting sequence
(Fig. 2A,B). Matched pairs of
completed stimuli contained movement sequences with similar timing and
amplitude of displacement in the vertical plane (range 19–22 movements)
but in opposite directions.
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), there is no
evidence that the accompanying movement has specific social salience in the
absence of vocalizations.
Inactive male
To test the effect of an alert companion, we created the Inactive male
playbacks, in which the male maintained a stationary alert posture, with
occasional spontaneous side-to-side head movements, appearing to scan
horizontally around the cage, but without any vertical displacement of the
head.
Empty cage
The Empty cage video was identical to the other stimuli in every respect,
except that the male was absent. We standardized the five test sequences for
several parameters that seemed likely to influence hen responses. To prevent
potential interactions between food calls and the motor patterns, which were
the focus of the study, all playbacks were silent. Mealworms appeared and
remained on the video screen during all five 60 s test sequences to control
for the presence of a preferred food item. Finally, to reduce the possibility
of social facilitation of food search caused by observation of a feeding
companion (Zajonc, 1965;
McQuoid and Galef, 1993), the
males were not shown consuming the mealworms. Results from experiment I
revealed no obligatory transitions between the three motor patterns
(Fig. 3), suggesting that
temporal sequence is unlikely to be an important aspect of display structure;
hence we did not manipulate transition patterns in experiment II.
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Test procedure
Hens were individually placed in the test cage for four 15 min periods, at
intervals of 48 h, to acclimate them to the apparatus and sound chamber. At
the start of each session, the wire door was closed and the hen was confined
to the end section of the cage, preventing her from approaching the plasma
display. During each session the wire door was remotely released once and the
empty cage video, without mealworms, played on the plasma screen. By the
fourth acclimation session, all hens readily emerged and walked the length of
the cage after the door opened and none exhibited signs of disturbance such as
wing-flapping or crouching.
We used a within-subjects design in which each hen was first assigned one of the four male exemplars and then a unique random sequence of the five treatments (test sequences). To ensure that the video male was unfamiliar to the hen, we applied the constraint that she should have had no social contact with the real male for at least six months prior to the experiment. Hens were tested at the same time of day to minimize diel variation in behavior. The inter-trial interval was 48 h. Each trial began with the hen behind the closed wire door in the section of the cage farthest from the plasma screen. This standardized the hen's minimum distance from the video male at the beginning of the 60 s stimulus. For the first 10 min, the door remained closed while the empty cage and then the male video played. The test sequence (i.e. one of the four movement types or Empty cage control) then began and the remote control door was opened, allowing the hen to approach the video male.
We monitored the tests using a CCD camera (Panasonic WV-CL320) connected to
a Sony color monitor (Sony PVM-1450QM). The video output was converted into
MPEG-2 format using a Miglia-EvolutionTV and saved for later analysis.
Behaviors of interest were scored using JWatcher Video 1.0
(Blumstein et al., 2006), which
reads the time-code of the video file to permit single-frame resolution. We
measured the duration of food searching, which is characterized by distinctive
close binocular fixation of the substrate
(Evans and Evans, 1999;
Evans and Evans, 2007) and two
social responses: time spent in close proximity to video male, indicated by
approach to within 0.1 m of the end of the cage closest to the screen, and
inspection behavior, which was characterized by the hen stretching her neck
towards the flat panel monitor at the height of the male's head, exactly as
hens scrutinize other flock members (Guhl
and Ortman, 1953).
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;
Evans and Evans, 2007). This
was calculated by measuring each hen's total duration of food searching during
the 60 s test sequence and then subtracting a baseline estimate, which was
obtained by averaging two samples: her time spent food searching during the
first 60 s after the video male appeared (min 6 of 15 min trial period) and
that during the last 60 s before the test playback began (min 9). This
approach also further controlled for day-to-day variation. Tests for overall treatment effects were conducted with repeated measures ANOVAs (SPSS 15.0.6 for Windows), using male exemplar as a blocking factor. Exemplar was never significant, so all data were pooled for further analysis. Significant differences were further explored using Tukey's honestly significant difference (HSD) to conduct multiple pair-wise comparisons; this maintained the overall alpha level at the nominated value of 0.05.
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RESULTS |
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TOPSummaryINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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Experiment II
Food search duration
Analysis of food searching duration, adjusted for baseline rate, revealed
that Silent tidbitting playbacks evoked a significantly higher response than
any of the other test sequences (Fig.
4). Responses to Inactive male, Silent crows and Matched motion in
the opposite direction did not differ significantly from one another, despite
considerable variation in the frequency of movement depicted in these three
stimulus types. In addition, Silent crows and Matched motion were not
significantly different from Empty cage (F4,88=9.61,
P<0.0001; Tukey's HSD, P<0.05).
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=0.85). The overall treatment (stimulus type)
effect was highly significant (F3,75=7.52,
P<0.001). Post-hoc tests revealed that hens spent
significantly more time close to the simulated male in the Silent tidbitting
playbacks than in any other treatment. There were no significant differences
among the other four stimuli (Tukey's HSD, P<0.05;
Fig. 5).
Inspection
As a complementary measure of social response, we measured the amount of
time hens spent in inspection behavior
(Fig. 6). Mauchly's test of
sphericity was significant (P<0.05), so we applied a Huynh-Feldt
correction (=0.78). The overall treatment effect was highly significant
(F3,68=40.04, P<0.001). Post-hoc
tests revealed that females spent significantly more time inspecting the male
on the plasma screen during the Silent tidbitting sequences than in any of the
other playbacks. None of the other treatments were significantly different
from one another (Tukey's HSD, P<0.05).
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DISCUSSION |
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TOPSummaryINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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). One possible explanation for this may be that the displays
contain far more motion in the vertical plane than in the horizontal. Such
signal design should generate a large sweep area in the visual field of
potential receivers that is robust to variation in angle of view and therefore
should be consistently conspicuous (Marr,
1982; Peters et al.,
2002; Peters and Evans,
2007). A second possibility is that the visual display is
typically accompanied by food calls, which are easy to localize and audible
over a considerable distance (Stokes and
Williams, 1971), thereby relaxing selection for a specialized
initial movement. A third possibility is that the wattles, which move
considerably, swinging from side to side, during each type of motor pattern
and produce a large proportion of the apparent motion during the display
(C.L.S., unpublished), may so enhance detectability that no constraint on
movement sequence is necessary. Planned experiments will test these
predictions.
Referential signaling
We tested whether the food search response evoked by visual tidbitting is
specifically dependent on the spatiotemporal characteristics of the display,
including frequency and direction. In its most restrictive form, this
hypothesis requires that hens respond significantly more to a silently
tidbitting male than to any other motion type. Comparisons of food-searching
duration reveal precisely the predicted pattern of responses
(Fig. 4). It is particularly
striking that Matched control movements at the same frequency as tidbitting,
but in the opposite direction, were much less effective
(Fig. 4). This implies that the
downward direction of motion is an important component of the signal.
This experiment also demonstrates that the hens' increased food searching
is not simply a consequence of a decrease in vigilance caused by the presence
of an alert companion (Artiss and Martin,
1995). Although the difference between the Inactive male and Empty
cage is consistent with such an effect, the fourfold increase in food
searching to the Silent tidbitting display over that recorded with the
Inactive male reveals an additional response specific to the tidbitting signal
(Fig. 4). We conclude the food
searching response to a silently tidbitting male is not caused by a change in
the foraging/vigilance tradeoff (Artiss and
Martin, 1995). In addition, the increase in food searching
response to Silent tidbitting cannot be attributed to social facilitation
(Zajonc, 1965;
McQuoid and Galef, 1993)
because the playback male was shown signaling, rather than searching the
substrate. We conclude that the tidbitting display is sufficient to evoke
foraging behavior and that responses to these movements have specificity
similar to that previously demonstrated for food calls
(Evans and Evans, 1999).
Tidbitting movements. therefore have all the characteristics of a functionally referential visual signal. When combined with food calling, as in the majority of displays, this constitutes the first experimental demonstration of multimodal referential signaling in a non-human vertebrate.
Social responses
For males, achieving close proximity to a hen is an important factor in
mating success (Graves et al.,
1985). The playback experiment reveals that tidbitting can play an
important role: hens spent more time standing close to the silent tidbitting
male than they did during any other treatment
(Fig. 5). This visual display
was uniquely effective, both relative to other movements and in absolute
terms; none of the other video male stimulus evoked significantly more close
approach than the Empty cage control sequence. Hens, therefore, responded very
specifically to the visual display, and not simply to rapid motion or to the
presence of a simulated companion. It has been suggested that one of the
functions of the food calling might be to lure the intended receiver close to
the signaler (Stokes and Williams,
1971) and many descriptions of multimodal tidbitting have noted
that the hen typically approaches the calling male
(Collias and Collias, 1996;
Stokes and Williams, 1971;
Wood-Gush, 1954). Our
experimental results confirm that the visual display alone is sufficient to
evoke this social response.
Tidbitting may also improve memorability of both the signal and signaler,
an important aspect of visual signal design
(Guilford and Dawkins, 1991)
that has primarily been explored in the context of aposematic coloration
(Halpin et al., 2008). Hens
spent approximately four times longer inspecting the silent tidbitting male
than any of the other male stimuli, none of which differed from Empty cage
(Fig. 6). Close inspection, as
indicated by the hen binocularly fixating on the video male's head, is hence
uniquely triggered by tidbitting. Previous research
(Dawkins, 1995;
Dawkins, 1996;
Hodos, 1993) has implicated
binocular fixation with the frontal field as a critical process in conspecific
recognition and has established that hens use the area around the eye to
identify flock mates (Dawkins,
1996). Hens prefer males that tidbit more
(Pizzari, 2003;
Zuk et al., 1993), so it seems
likely that the inspection of the male's head evoked by tidbitting may
facilitate formation of an association between the appearance of a particular
male and provisioning with food. Mating does not usually occur immediately
after tidbitting (Stokes and Williams,
1971), so females must retain some memory of individual male
tidbitting performance for preference subsequently to be expressed.
Although the visual and acoustic components of this multimodal display are
redundant (sensu Partan and
Marler, 1999), with regard to predicting food availability
(Smith and Evans, 2008), the
visual display has a synergistic effect on social responses that the sounds do
not (Evans and Evans, 1999),
increasing the time spent in close proximity and inspecting the male. This
contrast presents a challenge for theoretical models of multimodal signaling,
since categorization will be sensitive to the function(s) of interest.
Unimodal production of the tidbitting signal: functional implications
Short-term changes in signal structure can reduce potential costs
(Ryan et al., 1982).
Conspicuous signals attract predators
(Bayly and Evans, 2003;
Roberts et al., 2007),
parasites (Bernal et al., 2006)
and competitors (Stokes and Williams,
1971). In fowl, dominant males obtain the majority of matings
(Pizzari, 2003) and are
aggressive towards subordinate males that tidbit, often displacing them and
taking the food item (Stokes and Williams,
1971). However, Johnsen et al.
(Johnsen et al., 2001)
observed that subordinate males were able to tidbit as frequently as alphas
when the subordinate male could display out of sight of the alpha. In flocks
living under naturalistic conditions, we have observed that subordinate males
tidbit without perceptible food calling. This behavior created additional
mating opportunities by attracting nearby hens, which approached and food
searched near him (C.L.S., unpublished). If the silent tidbitting signal is
less conspicuous than the multimodal display, then this behavior may reflect a
tradeoff by subordinates between the benefit of attracting females and the
social cost of increased conspicuousness to a dominant male. Planned studies
will test the conspicuousness of unimodal and multimodal signals and the
frequency of their occurrence as a function of social context.
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Footnotes |
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References |
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TOPSummaryINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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Bayly, K. L. and Evans, C. S. (2003). Dynamic changes in alarm call structure: a strategy for reducing conspicuousness to avian predators? Behaviour 140,353 -369.[CrossRef]
Bernal, X., Rand, A. S. and Ryan, M. J. (2006).
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(Corethrella Coquillett) to túngara frog calls
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Blumstein, D., Evans, C. S. and Daniel, J. C. (2006). JWatcher 1.0. http://www.jwatcher.ucla.edu/.
Bugnyar, T., Kijne, M. and Kotrschal, K. (2001). Food calling in ravens: are yells referential signals?. Anim. Behav. 61,949 -958.[CrossRef]
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Collias, N. E. (1987). The vocal repertoire of the red junglefowl: a spectrographic classification and the code of communication. Condor 89,510 -524.
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