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First published online June 13, 2008
Journal of Experimental Biology 211, 2052-2057 (2008)
Published by The Company of Biologists 2008
doi: 10.1242/jeb.017194
Multimodal signaling in fowl, Gallus gallus
Centre for the Integrative Study of Animal Behaviour, Macquarie University, Sydney, NSW 2109, Australia
* Author for correspondence (e-mail: kls{at}galliform.bhs.mq.edu.au)
Accepted 16 April 2008
 |
Summary |
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 TOP Summary INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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Key words: multimodal communication, audiovisual displays, redundancy, tidbitting
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INTRODUCTION |
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TOPSummaryINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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),
birds (Partan et al., 2005),
frogs (Narins et al., 2005;
Grafe and Wagner, 2007),
spiders (Hebets and Uetz,
1999) butterflies (Papke et
al., 2007) and fishes
(McLennan, 2003) all produce
signals that engage more than one of the intended receiver's sensory systems.
Such coordinated signals are thought to enhance either efficacy or information
content (Guilford and Dawkins,
1991; Candolin,
2003; Partan and Marler,
1999; Partan and Marler,
2005; Hebets and Papaj,
2005). In redundant, or `backup' signals
(Johnstone, 1996), the
additional modality increases the likelihood of detection in a noisy
environment, while in nonredundant or `multiple message' signals
(Møller and Pomiankowski,
1993; Johnstone,
1996) it increases the rate at which information can be
transmitted or the number of possible messages. An additional complexity is
that performance of the two components can either be obligatory [fixed
(Smith, 1977)] or flexibly
combined [fluid or free (Smith,
1977)] depending on intrinsic factors, such as motivational state,
and extrinsic ones, such as social context.
Among birds, the most common pairing of sensory modalities is auditory and
visual (Hebets and Papaj,
2005). For example, many songbirds produce elaborate physical
displays, such as bows, jumps and flights, in conjunction with their
vocalizations during courtship (Balsby and
Dabelsteen, 2002; Chandler and Rose, 1998;
Bostwick and Prum, 2003;
Partan et al., 2005). This
combination of vocal and visual displays can also be found in the stereotyped
courtship feeding displays, known as tidbitting, of many galliforms
(Stokes and Williams,
1972).
Food calls (Marler et al.,
1986a; Collias and Joos,
1953; Kruijt,
1964) are the distinctive, pulsatile sounds that
characteristically accompany tidbitting movements
(Davis and Domm, 1943).
Previous studies have shown that food calls are functionally referential
(Evans and Marler, 1994;
Evans and Evans, 1999). More
recent work has revealed that this is one of the few examples of a
representational signal [i.e. the behavior of hens is mediated specifically by
information about food (Evans and Evans,
2007)]. The pulsatile nature of food calls
(Stokes, 1971;
Stokes and Williams, 1972)
should also make them easy to localize, in the same way as other sounds in the
fowl vocal repertoire with similar spectral and temporal structure
(Wood et al., 2000).
The visual component of the tidbitting display involves a repeated,
rhythmic motion of the head and neck, including picking up and dropping a food
item (Davis and Domm, 1943;
Stokes and Williams, 1972;
Evans and Marler, 1994). Hens
often approach the tidbitting male and search for food near him or take the
food directly from his mandibles (Stokes
and Williams, 1972; Marler et
al., 1986a; Marler et al.,
1986b; Gyger and Marler,
1988). There is clearly the potential for the visual information
in this putative multimodal signal to act synergistically with that encoded in
the acoustic modality to affect female behavior
(Evans, 1997).
It is revealing that the design of galliform visual ornaments and the
structure of their calls have been explored separately, such that both types
of signal are now well understood (Petrie
and Halliday, 1993; Evans and
Marler, 1994; Evans and Evans,
1999), but very little is known about the way in which females
integrate information across these sensory modalities. To understand the
design of such complex signals it is necessary to determine the way in which
modalities interact.
One approach for classifying a multimodal signal is to present the
components both separately and in combination, and to compare the type and
intensity of the responses evoked (Partan
and Marler, 1999; Partan and
Marler, 2005). A range of methods has been employed for such
tests, including audiovisual playbacks
(Evans and Marler, 1991;
Uetz and Roberts, 2002;
Partan et al., 2005), and 3D
models (Narins et al., 2005;
Balsby and Dabelsteen,
2002).
Previous work with fowl has shown that they can recognize the feeding
movements of a conspecific on video and discriminate these from other types of
motor activity (McQuoid and Galef,
1993). Furthermore, preferences acquired from video sequences
transfer to their real equivalents
(McQuoid and Galef, 1993). The
consistent effectiveness of video playback for evoking natural responses in
fowl (Evans and Marler, 1991;
Evans et al., 1993a;
Evans et al., 1993b), together
with recent advances that have substantially improved image realism
(Ikebuchi and Okanoya, 1999;
Ophir and Galef, 2003),
encourage the use of this technique for exploring the perception of multimodal
signals.
In the present study, we used high-definition audiovisual playbacks to
determine whether tidbitting by male fowl is a redundant signal, with each
modality acting as a backup to enhance signal transmission. This is the first
study to test systematically the perceptual integration of a display that is
ubiquitous in galliforms (Stokes and
Williams, 1972).
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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.
Video and audio recording
We used a high-definition 3-CCD video camcorder (Sony HDR-FX1) and a
Sennheiser MKH-40 microphone to make video and audio recordings of 12 male
fowl food calling and tidbitting. The new HDV video standard captures
substantially more detail (1920 pixelsx1080 lines) than the VHS
camcorders available for pioneering video playback studies [240 lines maximum
resolution (Clark and Uetz,
1990; Evans and Marler,
1991)], and achieves an approximately fourfold resolution
increment over the DV format adopted more recently [576 lines
(Ord et al., 2001;
Ord and Evans, 2003;
Carlile et al., 2006)]. The
frequency response of the soundtrack was flat (±1 dB) over the full
range audible to birds.
Males were confined in a 0.60 mx0.45 mx0.86 m wire cage, 0.8 m from the camera, within a sound-attenuating chamber (Ampisilence S.p.a, Rovassomero, Italy; 2.38 mx2.38 mx2.15 m). We used the same plasma display later employed for playbacks to monitor the video signal and adjusted the camera zoom to ensure that the image of the male was precisely life-sized. An unfamiliar audience hen was present in a separate cage approximately 30 cm from that of the male. After a 10 min acclimatization 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.
Design of playback stimuli
We wished to compare the response of hens to playback of the full
multimodal signal to that evoked by each of the two modalities in isolation.
Planned comparisons were hence designed to test whether removing either
modality caused a significant decrement in signal efficacy. In addition, we
wished to assess responses in the three signaling treatments relative to
spontaneous behavior, thereby establishing whether each of the modalities
evoked a significant response in absolute terms.
Treatments consisted of Multimodal, Audio only, Visual only, and a silent Empty cage control. The Multimodal tidbitting stimulus was the coordinated vocal and visual display of an adult male (see Movie 1 in supplementary material). The Audio only treatment consisted of food calls at the same amplitude, accompanied by a video of an empty cage. The Visual only treatment consisted of a male performing the tidbitting display, accompanied by ambient sound chamber noise to control for the background sound present in food call playbacks. The Empty cage video was identical to this in every respect, except that the male was absent. All four stimulus types had mealworms on screen from the moment at which these had been delivered to the male, thus controlling for the sudden appearance of a preferred food item.
We reviewed raw videos from each of the 12 males and arbitrarily chose four of the eight that had tidbitted continuously for >1 min without eating any of the mealworms, and spent most of this time in a lateral orientation to the camera. The first criterion controlled for the amount of food present in all playbacks. The second was developed from observations of natural interactions in aviaries, which showed that approaching hens most often have a side view of a displaying male (C.L.S and C.S.E., unpublished data). We used Final Cut Pro 5.1 (Apple Computers) to isolate a 60 s sequence of multimodal tidbitting from each of the four males and then manipulated this to create the corresponding audio only and visual only stimuli. Completed playback sequences were 15 min in duration. The first 10 min consisted of silent empty cage, to allow the hen to settle down. This was immediately followed by one of the four types of 60 s test stimulus and then by a further 4 min of empty cage sequence. The test stimuli began and ended with a 0.5 s fade transition to avoid a startle response. The first 10 min and the last 4 min of every trial were hence identical in every playback; only the 60 s stimulus differed according to experimental treatment. We selected this stimulus duration based upon observations of males tidbitting in the presence of a hen that was unable to retrieve the food item (N=12, mean=1 min 49 s). This allowed sufficient time for the hen to respond, while avoiding habituation to the playback stimuli, or extinction of unreinforced responses.
Test apparatus and procedure
Tests were conducted in the same sound-attenuating chamber as recordings.
The playback apparatus consisted of a 106 cm Sony high-definition flat-panel
plasma display, together with a Nagra Kudelski DSM speaker located at the base
of the screen. Hens were confined within a 1.2 mx0.30 mx0.5 m
(length x width x height) cage, which had a remote-controlled wire
door 0.4 m from one end. Hens were initially held in the compartment behind
this door to standardize distance from the display at the beginning of each
playback.
Decisions about the overall layout of the test setup were informed by
well-described properties of the fowl visual system. Hens are myopic in the
frontal field and thus unable to determine the identity of a conspecific from
distances greater than 30 cm (Dawkins,
1995; Dawkins,
1996). Recognition depends upon close binocular inspection and is
principally dependent upon attributes of the other bird's head and neck region
(Guhl and Ortman, 1953). We
positioned the hen's cage with the long axis perpendicular to the plasma
display and the remote-controlled door at the more distant end. The closer end
of the cage was 30 cm from the plasma display, a distance chosen to be at the
outer limit of the hens' ability to recognize the male by sight, but close
enough to entice her to attempt to fixate on the screen. Note that this
spatial separation was also sufficient to prevent hens from resolving
individual pixels, with a concomitant loss of verisimilitude.
To ensure that hens would move freely about the test cage and not be startled by operation of the remote-controlled door, we acclimated them to the test environment for 15 min at the same time of day for four consecutive days. The empty cage background video was displayed, accompanied by playback of ambient chamber noise. In addition, the remote-controlled door was released once during each acclimatization period. By the fourth acclimatization 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 experienced all four experimental treatments, from one of the male exemplars, in a unique random sequence. To further control potential order effects, we also counter-balanced by re-testing hens with the same stimuli in reversed order. Hens had no social contact with the real male depicted in their video sequences for at least six months prior to the experiment and experienced all of their playbacks at the same time of day to minimize diel variation in behavior. The inter-trial interval was 48 h.
We began each test by placing the hen behind the closed wire door in the section of the cage farthest from the plasma display. The empty cage background video then played for 10 min. After this, the wire door opened, allowing the hen to approach the plasma display, and one of the four stimulus sequences began.
We used a CCD camera (Panasonic WV-CL320) connected to a color monitor
(Sony PVM-1450QM) to observe tests. The analogue video signal was converted
into MPEG-2 format using a Miglia-EvolutionTV and saved for later analysis.
Behavior during the 60 s stimulus period was scored using JWatcher Video 1.0
(Blumstein et al., 2006), which
reads the time-code of the video file to permit single-frame resolution (40 ms
in the PAL standard). We measured the duration of food search, which is
characterized by distinctive close fixation of the substrate
(Evans and Evans, 1999;
Evans and Evans, 2007) and
latency to begin food searching. In addition, we measured visual attention
directed toward the plasma display; this sometimes included intense inspection
in which the hen stretched her neck towards the screen at the height of the
male's head (see Movie 2 in supplementary material), exactly as hens
scrutinize other flock members (Guhl and
Ortman, 1953).
Tests for an overall treatment effect 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 before further analysis. When significant differences occurred in the pooled data, Tukey's honestly significant difference (HSD) test was used to conduct multiple pair wise comparisons, while maintaining the overall alpha level at the nominated value of 0.05.
|
RESULTS |
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TOPSummaryINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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Further analyses considered responses during successive 15 s time intervals throughout the playbacks. These revealed that Multimodal and Audio only elicited significantly higher levels of food searching than Visual only or Empty cage during the first 15 s of the trial. In the 15 s to 30 s trial interval, food searching during Visual only increased to a level similar to those evoked by the other two signaling playbacks and all three were significantly higher than the Empty cage control. This pattern continued for the remainder of the trial. (F3,69=21.459, P<0.0001, adjusted using Tukey's HSD, P<0.05; Fig. 1).
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0.83). The overall treatment effect was
highly significant (F3,57=14.488, P<0.001).
Post-hoc tests revealed a stepwise pattern of increasing latency:
Audio only playbacks evoked the most rapid response, followed by Multimodal,
then Video only. Empty cage had the longest latency. Hens took significantly
longer to begin food searching in Visual only than in Audio only (Tukey's HSD,
P<0.05); however, neither of these treatments was significantly
different from Multimodal (Table
1). To compare asymptotic response magnitude, we corrected food search duration for latency by expressing it as a rate, beginning with the first response. The overall treatment effect was significant (F3,45=8.066, P<0.05). Post-hoc pair-wise comparisons, adjusted using Tukey's HSD (P<0.05) revealed no differences among the three types of signal playback (Multimodal, Visual only, and Audio only), all of which evoked significantly higher responses than the Empty cage control (Fig. 2).
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Analyses at the level of 15 s time intervals reveal that hens spent significantly longer inspecting the screen in the Multimodal and Visual only treatments than in Audio only or Empty cage treatments from the beginning of playback, and that this pattern was maintained throughout the three periods that followed (F3,69=24.545, P<0.0001, Tukey's HSD, P<0.05; Fig. 3). This difference between playbacks with a visual component and those without was not attributable to variation in latency to orient toward the plasma display, which this did not differ across the three signaling treatments (Multimodal, Audio only, and Visual only; F2,46=1.266, P=2.92).
|
DISCUSSION |
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TOPSummaryINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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;
Partan and Marler, 1999;
Partan and Marler, 2005;
Hebets and Papaj, 2005). Our
approach for classifying the multimodal tidbitting signal of male fowl,
Gallus gallus, was to present the audio and visual components both
separately and in combination, and to compare the type and intensity of
responses evoked. We used high-definition audiovisual playbacks to determine
whether tidbitting should be classified as a redundant signal, with each
modality acting as a backup to enhance signal transmission. Considering the average response over the course of the 60 s playback, the multimodal display generated food search duration similar to the individual modalities presented in isolation (Table 1). This suggests that the two modalities are redundant. However, food calls elicited significantly higher total food search than tidbitting movements (Table 1), a difference not anticipated by any of the current classification heuristics. Further analysis reveals that this effect was caused by the hens responding more quickly to the Audio only playbacks than to the Visual only ones; the silent tidbitting display hence had lower initial signal efficacy. This initial lower response does not appear to be caused by a deficiency in the playback video (see Movie 2 in supplementary material) since food search during Visual only playbacks increased rapidly to the same level as that evoked by Multimodal and Audio only playbacks. After the first 15 s, all three treatments were equivalent (Fig. 1). In addition, when we compensated for differences in latency by calculating food search rates, we found that the two unimodal playbacks and the multimodal playback all evoked statistically indistinguishable responses (Fig. 2). Taken as a whole, these data suggest that the acoustic and visual components of the tidbitting display are redundant.
Our results also reveal that Multimodal and Visual only tidbitting playbacks elicited significantly higher levels of binocular fixation on the plasma screen, indicative of social inspection, than Audio only or Empty cage playbacks (Fig. 3). This is a previously unreported response to the visual display and one that suggests that tidbitting may have multiple functions.
Lastly, differences in the latency to food search and in inspection behavior were not attributable to differences in conspicuousness of the signal, at least at the relatively close test distance employed. We found no difference in the latency to orient towards the multimodal, visual, or audio playback stimuli.
Classification of multimodal signals
Food search rate adjusted for latency
(Fig. 2) presents a pattern
traditionally classified as redundancy with equivalent effects (sensu
Partan and Marler, 1999;
Partan and Marler, 2005).
However, none of the current models for multimodal signaling includes an
explicit temporal dimension. This is a necessary simplification for any
classification scheme that seeks to encompass the full diversity of animal
signaling behavior; it would plainly be impractical to include time courses
for all possible response types in a general heuristic. Nevertheless, our
fine-grained analysis of tidbitting reveals that classification is sensitive
to the period over which the signal response is integrated and the time at
which the measurement is taken. No current category accurately reflects the
initial pattern of responses observed (Fig.
1), unlike asymptotic food search responses
(Fig. 2), which are
straightforward to accommodate within the Partan and Marler
(Partan and Marler, 1999)
scheme.
Nature of signal efficacy
For a signal to be effective, it must engage the attention of intended
receivers (Dawkins and Guilford,
1991; Guilford and Dawkins,
1991). This often involves a trade-off because increased
conspicuousness can also attract predators
(Ryan et al., 1982;
Roberts et al., 2007),
parasites (Bernal et al., 2006)
or interference from conspecific competitors
(Stokes, 1971). It is well
established that animals can flexibility adjust signal structure to optimize
the balance of such costs and benefits
(Ryan and Rand, 1990;
Endler, 1992). In fowl, the
cost of a subordinate male tidbitting can be loss of the food item to the
dominant male or more overt aggression
(Stokes, 1971). It is hence
intriguing that subordinate males sometimes tidbit without perceptible food
calling, and hens often respond by approaching and food searching during these
apparently unimodal displays (C.L.S., unpublished). We speculate that
subordinate male fowl may have a facultative strategy that sacrifices initial
signal efficacy for a lower likelihood of social cost. This possibility is
currently being investigated with long-term observational studies under
naturalistic conditions.
Multiple functions
Thus far, we have focused on tidbitting as a food-associated signal,
comparing responses with those evoked by food calls, which have been the
subject of several previous studies
(Marler et al., 1986a;
Marler et al., 1986b;
Evans and Marler, 1994;
Evans and Evans, 1999;
Evans and Evans, 2007).
However, our results suggest that tidbitting probably has multiple functions,
one of which is to attract hens in a mate assessment context. Hens responded
to visual only and multimodal playbacks with obvious inspection, consisting of
binocular fixation on the male's head. They spent approximately one-fourth of
the playback engaged in this behavior (Fig.
3), an amount comparable to that spent in food search. Close
visual inspection is used for individual recognition
(Dawkins, 1995;
Dawkins, 1996); it seems
likely that it also facilitates assessment of sexually selected ornaments,
such as the comb and wattles (Zuk et al.,
1992).
Stokes (Stokes, 1971)
suggested that repeated tidbitting helps to maintain a strong bond between the
male and female. Results of the current study are consistent with this idea.
Episodes of inspection evoked by the visual component of the display may
facilitate development of a link between male quality, in terms of physical
ornaments and ability to provide food, and individual identity.
Many species of galliform tidbit while making only rudimentary motions or
by simply freezing over the food item while calling
(Stokes and Williams, 1972).
By contrast, male fowl perform elaborate movements of the head and neck and
frequently manipulate the food item. In our experimental playbacks,
vocalizations alone were sufficient to elicit a high level of food searching,
with a similar latency to that evoked by the multimodal display. The principal
benefit associated with production of a complex multimodal signal may hence be
increased inspection behavior by the hen. Additional studies of tidbitting
under more naturalistic conditions will be required to test for additional
putative benefits, such as increased efficacy of the multimodal signal over
longer ranges [i.e. greater active space
(Peters and Evans, 2007)],
enhanced robustness in the presence of background noise (e.g. wind, or visual
obstructions), and greater ability to attract hens when there is competition
from other displaying males. These data will help to identify the full gamut
of factors that have influenced signal design. It will also be important to
determine whether the characteristic movements that make up the visual
component of tidbitting have effects specific enough to be categorized as a
dynamic visual signal, as current observations suggest, rather than as a
contextual cue (Leger, 1993)
that acts synergistically with food calls
(Evans and Marler, 1994).
|
Acknowledgments |
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Footnotes |
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