Vision in the peafowl (Aves: Pavo cristatus)
Vision, Touch and Hearing Research Centre, School of Biomedical Sciences, The University of Queensland, Brisbane, QLD 4072, Australia
e-mail: n.hart{at}mailbox.uq.edu.au
Accepted 9 September 2002
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Summary |
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Key words: microspectrophotometry, colour vision, avian retina, visual pigment, cone oil droplet, photoreceptor, visual ecology, ganglion cell topography, ocular media, peafowl, Pavo cristatus
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Introduction |
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Vision is the primary sense for most birds and, in addition to predator and prey detection, vision is obviously extremely important for intraspecific communication among peafowl, particularly in the assessment of male quality. However, almost nothing is known about their visual capabilities. This paper reports microspectrophotometric measurements of the spectral absorption characteristics of the visual pigments and oil droplets found in their retinal photoreceptors. These data are combined with measurements of the spectral transmittance of the ocular media to predict photoreceptor spectral sensitivities. Data on the topographic organization of the retinal ganglion cell layer are also presented.
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Materials and methods |
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Microspectrophotometry
Following enucleation, retinal tissue was prepared for analysis using a
microspectrophotometer (MSP) as described elsewhere (Hart et al.,
1998,
1999
,
2000a
,
c
). Photoreceptors were
mounted in a solution of 340 mosmol kg-1 phosphate-buffered saline
(PBS; Oxoid, UK) diluted 1:3 with glycerol (BDH) and adjusted to pH 7.3 with 1
mol l-1 NaOH. Separate retinal preparations were made for the
measurement of oil droplet absorptance spectra and these samples were mounted
in pure glycerol.
Absorbance spectra (330-750 nm) of individual photoreceptor outer segments
and oil droplets were measured using a computer-controlled,
wavelength-scanning, single beam MSP (Hart
et al., 1998). Sample and baseline scans were made from cellular
and tissue-free regions of the preparation, respectively. The dimensions of
the measuring beam were adjusted according to the size of the outer segment
being measured, and varied from approximately 1 µmx1 µm for oil
droplets and small cones to 2 µmx10 µm for rods. Rod outer
segments were fairly robust, measuring approximately 15 µm long and 3-3.5
µm in diameter. Cone outer segments, on the other hand, were usually folded
over, or otherwise distorted, so it was difficult to estimate transverse
pathlength or outer segment length. For this reason, specific absorbance per
µm of cone outer segment was not calculated and the absorbance at the
max of the mean difference spectrum was given instead
(Table 1).
|
Data were recorded at each odd wavelength on the `downward' long- to
short-wavelength spectral pass and at each interleaved even wavelength on the
`upward' short- to long-wavelength spectral pass. Each scan (either sample or
baseline) consisted of two downward and two upward spectral passes in
alternate succession; spectral passes of the same direction were averaged
together. To reduce the effects of inscan bleaching, only one sample scan was
made of each outer segment, but this was combined with two separate baseline
scans. Averaging the two absorbance spectra obtained in this way improved the
signal-to-noise ratio of the spectra used to determine the wavelengths of
maximum absorbance (max) of the visual pigments
(Bowmaker et al., 1997
).
Following the `pre-bleach' scans, outer segments were bleached with full
spectrum `white' light from the monochromator for 5 min and an identical
number of sample and baseline scans made subsequently. The `post-bleach'
average spectrum thus created was deducted from the pre-bleach average to
produce a bleaching difference spectrum for each outer segment.
To establish visual pigment-oil droplet pairings, the spectral absorptance
of the oil droplet associated with the outer segment (if present) was also
measured. A single sample scan was made of each droplet and combined with a
single baseline scan. Each scan consisted of only one downward and one upward
spectral pass, which were not averaged together. Higher quality oil droplet
spectra, showing less evidence of by-passing light
(Lipetz, 1984), were obtained
from the retinal preparations mounted in pure glycerol, which reduces
wavelength-dependent scattering of the MSP measuring beam
(Hart et al., 1999
).
Analysis of visual pigment absorbance spectra
Baseline and sample data were converted to absorbance values at 1 nm
intervals. Upward and downward scans were averaged together by fitting a
weighted (delta function) three-point running average
(Hart et al., 2000c). Pre- and
postbleach absorbance spectra were then normalized to the peak and
long-wavelength offset absorbances determined by fitting a variable-point
unweighted running average to the data. Following the method of MacNichol
(1986
), a regression line was
fitted to the normalized absorbance data between 30% and 70% of the normalized
maximum absorbance. The regression equation was used to predict the wavelength
of maximum absorbance (
max) following the methods of
Govardovskii et al. (2000
).
Scans from each photoreceptor type that satisfied established selection
criteria (Hart et al., 1999
;
Levine and MacNichol, 1985
)
were averaged and reanalysed. Criteria were relaxed for SWS single cones (see
Abbreviations), as it was impossible to obtain spectra that did not show at
least some distortion of the short wavelength limb.
Analysis of oil droplet absorptance spectra
Sample and baseline data were converted into absorptance and normalized to
the maximum and long-wavelength offset absorptances obtained by fitting an
unweighted 13-point running average to the data
(Hart et al., 1998). Oil
droplet absorptance spectra are described by their
cut,
which is the wavelength of the intercept at the value of maximum measured
absorptance by the line tangent to the oil droplet absorptance curve at half
maximum measured absorptance (Lipetz,
1984
). For comparison with other studies (e.g.
Partridge, 1989
), the
wavelength corresponding to half maximum measured absorptance
(
mid) is also given
(Lipetz, 1984
).
Spectrophotometry of ocular media
Absorbance measurements of the cornea, aqueous humour, lens and vitreous
humour from one bird were made over the range 200-800 nm using a Shimadzu
UV2101 PC UV-VIS scanning spectrophotometer fitted with a Shimadzu ISR-260
integrating sphere assembly to reduce the effects of light scattering by the
tissue samples. Because the eyes were too big to measure in their entirety,
the ocular media were measured separately
(Hart et al., 1999), and
pathlengths were determined from measurements of a radially sectioned frozen
eye (see below).
The lens was dissected away from the anterior segment of the eye and placed in a rectangular aluminium insert, designed to fit inside a standard (10 mm pathlength) quartz cuvette, in which a 7.3 mm diameter hole (the same diameter as the lens) had been drilled to coincide with the measuring beam of the spectrophotometer and in which the lens could be positioned in its normal orientation relative to the incident light. Thin plastic rings were lodged inside the insert hole in front of and behind the lens to prevent movement. The cornea was excised from the sclera and measured whilst sandwiched between two stainless steel mesh inserts inside a standard cuvette. Both cornea and lens were bathed in 340 mosmol kg-1 PBS, which was also placed in the identical inserts and cuvettes used as reference samples.
Vitreous humour was removed from the vitreal body and placed in the hole (4.5 mm diameter) of an aluminium cuvette insert identical to that used to measure lenticular absorbance. The vitreous, which is a highly viscous gel, was trimmed in the insert to give a pathlength of exactly 10 mm. Aqueous humour was removed from the anterior chamber, using a hypodermic syringe, and measured in a 200 µl, 10 mm pathlength quartz cuvette. Both humours were measured relative to distilled water.
The spectrophotometer performed a single spectral pass from 800 nm to 200 nm, recording absorbance at 1 nm intervals. The spectral full width at half maximum bandwidth of the monochromator used by the spectrophotometer was set at 5 nm to maximise light transmission and signal-to-noise ratio, which are otherwise low when using an integrating sphere.
Determination of optical pathlengths in the peafowl eye
Pathlengths of the aqueous and vitreous humours along the optic axis were
estimated from scaled photographs of a frozen eye, hemisected sagittally using
a cryostat. Eyes were frozen at -20°C and attached to the chuck of a motor
driven microtome using OCT embedding compound (BDH). The eye was orientated
such that sections made by the cryostat were parallel to the optic axis, and
10 µm sections were made at -20°C until the edge of the lens was
visible. Photographs of the eye, and a scale ruler positioned adjacent to the
cut face of the eyeball, were then taken after every 10 sections,
approximately 0.1 mm intervals. The negatives obtained were projected with a
magnification of approximately x13 using a photographic enlarger and the
pathlengths of the aqueous and vitreous calculated according to the scale
ruler.
Expansion of the eye upon freezing has a negligible effect on the calculated pathlengths. If the eye is modeled as a sphere 21 mm in diameter (mean of axial and equatorial diameters), and it is assumed that the thermal expansivity of the aqueous and vitreous humours is similar to that of water, the total pathlength along the optic axis of the frozen eye would only be approximately 35 µm longer than at body temperature. This increase in pathlength due to freezing (approximately 0.2%) was less than the likely error in estimating the pathlength by measuring an enlarged photograph (±1.4-5.3%).
Retinal whole-mount preparation and analysis
The retina and vitreous of one eye was removed intact by dissection in 340
mosmol kg-1 PBS and the pigment epithelium adhering to the
photoreceptor layer removed gently with a fine paintbrush. The free-floating
retina was fixed for 30 min in 4% paraformaldehyde in 0.1 mol kg-1
phosphate buffer (pH 7.2) and then washed in PBS. The retina was cleared of
vitreous and floated onto a gelatinised slide (Fol's mounting medium;
Stone, 1981) with the ganglion
cell layer uppermost. Relieving cuts were made at the periphery to enable the
retina to lie flat, and the preparation flooded with fresh Fol's medium. The
retina was covered with Whatman #50 filter paper soaked in 16%
paraformaldehyde in 0.1 mol l-1 phosphate buffer. A large coverslip
was placed on top of the filter paper and a small weight (85 g) applied to the
coverslip to ensure that the retina fixed flat to the slide (modified from
Moroney and Pettigrew, 1987
).
The preparation was stored in a moist chamber for 24 h, after which the weight
was removed and the slide washed in distilled water. The retina was then
allowed to dry on the slide slowly in a moist chamber over several days to
avoid cracking.
The retina was defatted in xylene (two changes, each 30 min), rehydrated through a descending alcohol series (100%, 95%, 70%, 50% ethanol and then distilled water for 10 min each) and stained for Nissl substance in an aqueous solution of 0.05% Cresyl Violet titrated to pH 4.3 with glacial acetic acid for 25 min. After rinsing in distilled water, the stained retina was passed through 70% and 95% ethanol (30 s each) before immersion in a differentiation solution (95% ethanol titrated to pH 3.6 with glacial acetic acid) for a further 30 s. The retina was finally dehydrated (95% ethanol, followed by two changes of 100% ethanol, each 30 s) and cleared in xylene before mounting in DPX (Aldrich).
Counts of Cresyl Violet-stained neurons in the ganglion cell layer were made at 1 mm intervals (0.5 mm intervals in the regions of highest density) across the retina using a Zeiss Axioplan microscope at a total magnification of x1000. Initially, all cell bodies that stained for Nissl substance were counted, with the exception of the very darkly stained spindle-shaped glial cells associated with the optic nerve fibre layer.
The retina was then recounted in an attempt to distinguish ganglion cells
from putative displaced amacrine cells in the ganglion cell layer. Ganglion
cells were identified by their relatively larger size, polygonal cell bodies,
abundant darkly staining cytoplasm containing clumped Nissl substance and
paler staining nucleus; putative displaced amacrines were identified by their
smaller oval or teardrop-shaped perikarya with little cytoplasm and relatively
darker, homogeneous staining (Chen and
Naito, 1999; Ehrlich,
1981
; Hayes, 1984
;
Stone, 1981
), see
Fig. 1. Like ganglion cells,
the soma size of displaced amacrine cells decreases towards the centre of the
retina (Hayes, 1984
).
Consequently, in the central retina of the peafowl where Nissl-stained
cells were present at high densities, ganglion cell somata were increasingly,
circular or oval in shape, and somata overlapped considerably a
population of relatively smaller neurons morphologically similar to the
putative displace amacrines in the more peripheral regions could still be
distinguished. Nevertheless, it is always possible that some displaced
amacrines were counted as ganglion cells and small ganglion cells in the
central retina were wrongly classified as displaced amacrine cells. Therefore,
both sets of count data are presented.
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Cell densities were plotted on a x10 scale drawing of the retina
traced onto graph paper using the stage micrometer reading. Points of
isodensity were calculated by linear interpolation and the line segments
connecting these were smoothed by eye
(Wathey and Pettigrew, 1989)
to give the contour lines shown in Fig.
6A,B. Shrinkage of the retina during preparation was estimated by
measuring the change in area of a chicken Gallus gallus domesticus
retina prepared in an identical fashion to that of the peafowl. Retinal
outlines were traced from enlarged (x16) scaled photographs of a
whole-mounted chicken retina taken before and after fixation and after
Nissl-staining. The outlines and scale bars were digitised using a flat-bed
scanner connected to a microcomputer and retinal areas calculated using Image
Tool V3.00 for Microsoft Windows (University of Texas Health Service Centre,
San Antonio, USA). Shrinkage of the free-floating retina during fixation was
2.3%. The retina shrunk a further 8.7% throughout the Nissl-staining
proceedure, although shrinkage occurred predominantly at the retinal
periphery. However, cell counts were not corrected for shrinkage because, in
the case of the peafowl, a total retinal shrinkage of 11% would decrease the
estimate of maximum spatial resolution by less than 1 cycle
degree-1.
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Estimating visual resolution
Peak cell density data from counts of the whole-mounted retina were used to
estimate the theoretical resolution limits for the peafowl eye. The posterior
nodal distance (PND) of the eye was estimated by multiplying its axial length
(measured from an enlarged scaled photograph) by 0.60. This ratio is identical
to that measured empirically for the chicken
(Schaeffel and Howland, 1988),
starling Sturnus vulgaris
(Martin, 1986
) and blackbird
Turdus merula (Donner,
1951
), and similar to the mean for diurnal eyes (0.67) from a
variety of vertebrate species calculated by Pettigrew et al.
(1988
).
The distance d subtended by one degree on the retina was
determined from the calculated PND and the formula:
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Assuming that ganglion cells are the limiting factor for spatial resolution
and that they are packed in a hexagonal array, the mean cell-to-cell spacing
S was calculated using the formula:
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Results |
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Good quality absorbance spectra were obtained for almost all of the
photoreceptors, but it can be seen in Fig.
2 that the mean absorbance spectrum of the SWS visual pigment is
distorted on the short wavelength limb, having a slightly higher absorbance at
each wavelength than would be expected on the basis of a visual pigment
template (Govardovskii et al.,
2000). This is most probably due to the build up of stable
photoproducts in the outer segment as a result of in-scan bleaching, similar
to those that can be seen in the post-bleach absorbance spectra of all the
cone types (Fig. 2).
Consequently, the
max values calculated for the SWS visual
pigment may be less reliable than the other cone types. However, the
max is predicted from the absorbance values on the long
wavelength limb, and the
max values calculated from the
difference spectra shown (Fig.
3, Table 1) are not
very different from those calculated from the pre-bleach spectra.
Both the principal and accessory members of the double cone pair contain a
LWS visual pigment that is spectrally identical to the one found in the LWS
single cones (Figs 2,
3). In the dorsal retina, the
P-type oil droplets in the principal member had a mean cut
at 479 nm and appeared pale green under bright field illumination. In the
ventral retina, however, P-type oil droplets had
cut at
longer wavelengths (mean
cut 497 nm) and appeared pale
yellow. The pale greenish-yellow A-type oil droplet in the accessory member
had a
cut at 488 nm. The oil droplets of both members of
the double cone pair are located nearer the sclera than the single cone oil
droplets. Of the single cones, the C- and T-type oil droplets of the SWS and
VS cones, respectively, are located closer to the vitreous than the R- and
Y-type oil droplets.
Spectrophotometry of ocular media and calculated pathlengths
Calculated pathlengths of the aqueous and vitreous humours along the optic
axis of the peafowl eye were 3.2 mm and 10.6 mm, respectively, for an eye
measuring 19.8 mm in axial length (lens axial thickness 4.3 mm). Measured
absorbances for the aqueous and vitreous (10 mm pathlength; see above) were
scaled appropriately, summed with the absorbance data for the cornea and lens,
and the combined absorbance converted to transmittance for display
(Fig. 5). The wavelength of 0.5
transmittance (T0.5) was at 365 nm and the ocular
media ceased to transmit light below approximately 330 nm.
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Retinal ganglion cell distribution
The isodensity contour maps of both total cell counts
(Fig. 6A) and presumptive
ganglion cells only (Fig. 6B)
are similar and reveal a prominent area centralis in the central retina,
approximately 2 mm nasal and dorsal to the top of the pecten. The density of
presumptive ganglion cells (Fig.
6B) in the ganglion cell layer decreased concentrically from a
peak of approximately 35,609 cells mm-2 in the area centralis to a
minimum of 816 cells mm-2 at the dorsal periphery. Closer
examination reveals a poorly defined horizontal visual streak of high cell
density extending nasally from the area centralis.
Resolution limit
The unfixed eye from which the counted retina was taken had a measured
axial length of 19.4 mm and an estimated PND of 11.6 mm (see Materials and
methods). Thus, one degree of visual angle subtended 0.20 mm on the retina.
The Nyquist frequencies calculated from the highest cell density counts for
all cells in the ganglion cell layer (37 649 cells mm-2) and
presumptive ganglion cells only (35 609 cell mm-2) were 21.2 cycles
degrees-1 and 20.6 cycles degrees-1, respectively (not
corrected for retinal shrinkage).
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Discussion |
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The possession of a VS visual pigment is correlated with a shift in SWS
visual pigment max to longer wavelengths relative to SWS
visual pigments found in birds with UVS visual pigments
(Hart et al., 2000a
) and
increased spectral filtering (higher transverse absorptance and
cut at longer wavelengths) in the C-type oil droplet with
which the SWS visual pigment is associated
(Bowmaker et al., 1997
). Both
of these factors will serve to reduce the overlap between the spectral
sensitivities of the VS and SWS cone classes
(Fig. 7), potentially improving
colour discrimination and colour constancy under a variety of illumination
conditions (Barlow, 1982
;
Govardovskii, 1983
; Vorobyev
et al., 2001
,
1998
).
|
As well as reducing the overlap between adjacent spectral classes, spectral
filtering by the oil droplets in the SWS, MWS and LWS single cones shifts the
peak sensitivity of each cone to a longer wavelength than the
max of the visual pigment it contains; in the case of the
peafowl, to 477, 537 and 605 nm, respectively (Figs
7,
8). The peak sensitivity of the
VS cone is shifted to about 432 nm because of increasing absorption at short
wavelengths by the ocular media. Excluding double cones, the potentially
tetrachromatic colour vision system of the peafowl has, therefore, three
spectral loci of maximal wavelength discrimination, where the cone spectral
sensitivities overlap (Delius and
Emmerton, 1979
; Jacobs,
1981
), at approximately 462, 517 and 576 nm (Figs
7 and
8).
|
The visual systems of birds have presumably evolved primarily for finding
food and avoiding enemies (Lythgoe,
1979); both the spectral characteristics of their photoreceptors
(Hart et al., 2000a
) and
variations in the relative abundance of the different cone types across their
retinae (Hart, 2001a
;
Partridge, 1989
) appear to
reflect diet, feeding behaviour, habitat and even phylogeny rather than intra-
or interspecific variations in body coloration. It seems likely, therefore,
that rather than driving the evolution of avian colour vision, conspicuous and
sexually attractive coloration would take advantage of pre-existing visual
mechanisms (Lythgoe, 1979
). In
the case of the peafowl, we might predict that the plumage colours designed to
predict male quality would have peaks in reflectance spectra overlapping those
regions of maximal wavelength discrimination. The plumage of the male peafowl
is characterised by blue and green feathers that may well have reflectance
peaks in these spectral regions, but such speculation must be tested using
objective measurements of plumage coloration
(Bennett et al., 1994
). It may
also be productive to compare plumage reflectance spectra between males that
differ in their mating success to determine any chromatic component to the
assessment of male quality. Although only males were used in this study there
is no precedent to expect the visual systems of the males and females to
differ in the spectral characteristics or the relative abundance of their
photoreceptors (e.g. Hart et al.,
2000a
,b
,
1998
).
Microspectrophotometric data, together with estimates of the relative
abundance of the different cone types in the retina, can be used to predict
the relative threshold spectral sensitivity of a visual system
(Vorobyev and Osorio, 1998).
Such estimates have been shown to match behaviourally measured threshold
spectral sensitivities well for di-, tri- and tetrachromatic colour vision
systems operating under photopic light levels. The model of Vorobyev and
Osorio (1998
) assumes that
photoreceptor noise limits discrimination and that noise in a given receptor
colour channel is proportional to the reciprocal of the square root of the
relative proportion of a given receptor type in the retina. I have applied
this model to the potentially tetrachromatic visual system of the peafowl
(Fig. 8), using photoreceptor
proportion data described elsewhere (Hart,
2001a
) and the predicted quantum catches displayed in
Fig. 7. Unfortunately, there
are as yet no behavioural data to compare the model against.
Spectrophotometry of ocular media
As with other species that possess a VS visual pigment (wavelength of 0.5
transmittance, T0.5, 358-380 nm), the ocular media
of the peafowl (
T0.5=365 nm;
Fig. 4) transmit fewer short
wavelengths than species with UVS visual pigments
(
T0.5=316-343 nm; for a review, see
Hart, 2001b
). Consequently,
while the VS visual pigment confers considerable sensitivity to near
ultraviolet wavelengths (at least in the mallard duck;
Parrish et al., 1981
), it is
clear that the visual systems of species with a VS visual pigment are
functioning over a narrower range of short wavelengths than those with a UVS
visual pigment. However, it is not yet known if the spectral characteristics
of the ocular media determine the
max of the visual pigment
in the single cone containing the T-type oil droplet or vice versa
(Hart, 2001b
).
Topography of the retinal ganglion cell layer
It is generally acknowledged that the topographic organisation of the
retina represents an evolutionary adaptation to the habitat and life style of
a given species, both in terms of ganglion cell (e.g.
Collin and Pettigrew, 1989;
Hughes, 1977
) and
photoreceptor (e.g. Ahnelt and Kolb,
2000
; Hart, 2001a
;
Partridge, 1989
) distribution.
The macroscopic topography of the avian retina has been studied extensively
using ophthalmoscopy (e.g. Moroney and
Pettigrew, 1987
; Wood,
1917
) and, to a lesser extent, using anatomical techniques (e.g.
Binggeli and Paule, 1969
;
Budnik et al., 1984
;
Chen and Naito, 1999
;
Hayes et al., 1991
;
Hayes and Brooke, 1990
;
Inzunza et al., 1991
;
Wathey and Pettigrew, 1989
),
and a variety of organizations are evident (for reviews, see
Martin, 1985
;
Meyer, 1977
).
The Indian peafowl has a single, large, centrally located area of increased
ganglion cell density, the area centralis. In this respect it differs from
both the domestic chicken and the Japanese quail Coturnix coturnix
japonica, to which it is closely related phylogenetically
(Sibley and Monroe, 1990),
both of which have an area of increased ganglion cell density in the
dorsotemporal retina (area dorsalis) in addition to the area centralis
(Budnik et al., 1984
;
Chen and Naito, 1999
). The
presence of only one area in the peafowl retina may reflect differences in
feeding behaviour of this species compared to the chicken and quail. In the
pigeon Columba livia, and presumably other species with similar
retinal specialisations, the area dorsalis projects into a region of visual
space in front of and just below the beak, which corresponds to the region of
binocular overlap of the left and right visual fields, and is thought to
facilitate pecking at and grasping nearby objects, especially food
(Nalbach et al., 1993
). The
red jungle fowl Gallus gallus, which is the ancestor of the domestic
chicken, and the Japanese quail, both feed predominantly on the seeds of
grasses and weeds and occasionally small invertebrates (ants, beetles,
termites). The peafowl, however, prefers slightly more substantial items,
including many green crops, insects, small reptiles, mammals and even small
snakes, berries, drupes and figs (del Hoyo
et al., 1994
). These larger visual targets may obviate the need
for a specialized region of high spatial resolution in the
dorsotemporal retina. It is also possible that the peafowl has a
reduced binocular overlap in the anterior sagittal plane compared to the
chicken and quail.
The peafowl retina also displays a weakly defined visual streak extending
horizontally from the area centralis. Visual streaks are common in mammals
(Hughes, 1977) and birds
(Hayes and Brooke, 1990
;
Meyer, 1977
) that inhabit open
environments where the visual horizon is largely unrestricted. In this respect
it is interesting to note that the peafowl spends much of its time foraging on
open plains and scrubland, whereas the Japanese quail (which is only 17-19 cm
tall) prefers dense herbage less than 1 m tall and the red jungle fowl favours
the forest understorey (del Hoyo et al.,
1994
).
Visual resolution
The area centralis is used to view distant objects during monocular
fixation, and high spatial resolution is important for both predator and prey
detection (Pumphrey, 1948).
The calculated visual acuity for the lateral visual field of the peafowl (20.6
cycles degrees-1) is better than that measured behaviourally for
the lateral visual field of the pigeon (12.6 cycles degrees-1;
Hahmann and Güntürkün,
1993
) and the chicken (7.1 cycles degrees-1; see
Donner, 1951
), largely due to
its relatively long PND. Of course, the visual acuity of the peafowl might be
considerably poorer if not all of the ganglion cells in the area centralis
contribute to spatial tasks. However, estimates of visual acuity made on the
basis of anatomy can be remarkably similar to values measured behaviourally:
assuming a peak cell density of 31 500 cells mm-2 (42 000 cells
mm-2 corrected for 25% shrinkage) in the area centralis of the
pigeon (Binggeli and Paule,
1969
) and that about 85% of these are ganglion cells
(Hayes, 1984
), and a PND of
7.9 mm (Marshall et al.,
1973
), the calculated maximum visual acuity in the lateral visual
field of the pigeon would be 12.1 cycles degrees-1 (see also
Wathey and Pettigrew,
1989
).
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Acknowledgments |
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