What Kind Of Photoreceptors Do Animals Have

Proc Biol Sci. 2005 Sep 7; 272(1574): 1745–1752.

Photoreceptor sectral sensitivities in terrestrial animals: adaptations for luminance and colour vision

D Osorio

¹Schoolhouse of Life SciencesUniversity of Sussex, Brighton BN1 9QG, Uk

Thou Vorobyev

²Vision, Touch and Hearing Enquiry Centre, School of Biomedical Sciences, University of Queensland, Brisbane, Queensland 4072, Australia

Received 2005 Apr 14; Accustomed 2005 May 16.

Abstruse

This review outlines how optics of terrestrial vertebrates and insects meet the competing requirements of coding both spatial and spectral information. There is no unique solution to this problem. Thus, mammals and honeybees use their long-wavelength receptors for both achromatic (luminance) and colour vision, whereas flies and birds probably use dissever sets of photoreceptors for the ii purposes. In particular, nosotros look at spectral tuning and diversification amongst 'long-wavelength' receptors (sensitivity maxima at greater than 500 nm), which play a primary office in luminance vision. Data on spectral sensitivities and phylogeny of visual photopigments can be incorporated into theoretical models to suggest how optics are adapted to coding natural stimuli. Models indicate, for case, that fauna color vision—involving 5 or fewer broadly tuned receptors—is well matched to most natural spectra. We can too predict that the particular objects of interest and signal-to-racket ratios will affect the optimal heart pattern. Nonetheless, it remains difficult to account for the adaptive significance of features such as co-expression of photopigments in unmarried receptors, variation in spectral sensitivities of mammalian 50-cone pigments and the diversification of long-wavelength receptors that has occurred in several terrestrial lineages.

Keywords: color vision, evolution, ecology, retina, photoreceptor

1. Introduction

Thomas Immature (1802) observed that jointly sampling spatial and spectral signals with a single assortment of photoreceptors presents a trouble, because at that place cannot exist an 'infinite number' of detectors for different frequencies of light at each point on the retina. He suggested that humans have three. The potential difficulties are illustrated when, in TV pictures, fine luminance patterns, such as pinstriped cloth, are aliased to appear as coloured Moiré patterns. Humans are indeed trichromatic, but it remains uncertain how much we suffer from aliasing betwixt spatial and spectral signals, and whether photoreceptor spectral sensitivities and spatial layout reflect a compromise between their alien requirements (Williams et al. 1993; Osorio et al. 1998; Williams & Hofer 2004).

The distinction betwixt spectral and spatial signals is inherent to prototype physics, but this does not specify how information should be coded. Many animals utilize achromatic (eastward.g. luminance or effulgence) and chromatic signals for separate purposes (Srinivasan 1985; Livingstone & Hubel 1988; Giurfa et al. 1996, 1997; Schaerer & Neumeyer 1996; Jones & Osorio 2004). Monochrome pictures can exist enjoyed virtually as much every bit a colour, considering achromatic intensity represents most signal power, and hence visual information (Osorio et al. 1998; Ruderman et al. 1998; van Hateren et al. 2002). Primates use luminance signals for tasks such every bit motility, form and texture perception. Spectral information is represented by chromatic signals, which are based on differences betwixt receptor quantum catches rather than absolute values. Chromaticity is probably relatively stable (abiding) in natural illumination, so that it gives information about surface reflectance, pigmentation and other material properties (Rubin & Richards 1982; Gegenfurtner & Kiper 2003). Colour vision is therefore likely to be important for object detection or classification.

Bachelor evidence suggests that nigh species resemble humans in using chromatic and achromatic signals for dissever tasks (§2), simply in that location are substantial differences in retinal sampling. For instance, primate luminance mechanisms combine outputs of long (L; red) and medium (M; light-green) cones, while chromatic mechanisms compare responses of all three cones (Wyszecki & Stiles 1982; Livingstone & Hubel 1988). Honeybees (Apis mellifera) also use all iii of their spectral receptors for colour vision, with the long-wavelength receptor providing a 'luminance' signal that is used for motion and class perception (Backhaus 1991; Giurfa et al. 1997; Vorobyev et al. 2001). In dissimilarity to primates and bees, cyclorraphid flies (Diptera) and birds probably have carve up sets of photoreceptors for luminance and color vision (figure 1; Hardie 1986; Strausfeld & Lee 1991; Osorio et al. 1999; Anderson & Laughlin 2000; Jones & Osorio 2004). In birds, these are double and unmarried cones, and in the flies, short and long visual fibres, respectively. This dual arrangement has obvious costs, but avoids compromises between the competing demands of coding chromaticity and luminosity. For case, narrowing photopigment spectral tuning with coloured filters probably benefits color vision, but reduces accented sensitivity (Douglas & Marshall 1999; Hart 2001; Stavenga 2002; Vorobyev 2003). Flies tailor further features of receptor physiology, such equally response speed and proceeds to the specific needs of chromatic and luminance coding (Anderson & Laughlin 2000). For case, at whatever given adapting intensity, brusk visual fibres accept faster responses than long visual fibres. This is expected because chromatic signals have a lower signal-to-noise ratio (SNR) (van Hateren 1993).

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Vertebrate cone and insect photoreceptor spectral sensitivities, normalized to λ _max. (a), (b) Humans and honeybees have iii spectral types of photoreceptor, and trichromatic colour vision. Homo Yard and L cones, and the bee's long-wavelength receptor also provide luminance signals. (c), (d) In birds and flies, respectively, the double cone (D) and short visual fibre (SVF) signals are used for luminance, while the bird's four types of unmarried cone and the fly'south long visual fibres (LVFs) give chromatic signals. The fly photoreceptors are named according to their location in the rhabdom (R7 or R8), and by their color stake (p) or yellow (y). The 5 types of fly receptor illustrated are found across virtually of the eye, only in specialized regions are replaced past others (Hardie 1986). Vertebrate and bee visual photoreceptors incorporate only a single type of photopigment, but flies as well have a UV sensitive antennal pigment, which accounts for their complex spectral sensitivity (Hardie 1986; Stavenga 2004).

The diverseness of retinal designs implies that there is no universal solution to jointly sampling spatial and spectral information. To begin to sympathise this diversity, we starting time with the relatively simple question of spectral tuning of photoreceptors. In a globe where evolution had optimized performance, one could identify the part of a system by establishing what it did best. Species differences would depend upon their sensory ecology. The existent world is not optimal, only adaptive variation in photopigment and receptor spectral sensitivities, and its molecular basis is relatively easy to empathise. Models of visual coding can so propose a criterion for evaluating functioning, based on (noisy) receptor responses to natural stimuli.

(a) Photopigment spectral sensitivities

The wealth of data on photoreceptor spectral sensitivities and the phylogeny of photopigment proteins (opsins) is an fantabulous basis for comparative studies (effigy 1). In improver, the spectral sensitivity of a photopigment is adamant by its acme (λ _max; Govardovskii et al. 2000), which makes it like shooting fish in a barrel to model receptor sensitivities and to simulate the effects of evolutionary alter. Vertebrates take five genetic classes of visual photopigment (Hisatomi et al. 1994, Yokoyama 1994), of which four emerged before the departure of cyclostomes and fishes (Collin et al. 2003). Several groups, including birds and many lizards, retain all five classes. These cistron families are mostly known (Yokoyama 1994) as: RH1, which are rod pigments with λ _max from approximately 460 to 530 nm, and cone paint classes, RH2, λ _max: 460–530 nm; SWS1, λ _max: 350–440 nm; SWS2, λ _max: 430–470 nm and LWS/MWS λ _max: 495–575 nm (Bowmaker & Hunt 1999; the spectral ranges include fishes). The LWS/MWS family unit is then named because it includes primate medium- (Thou) and long- (L) wavelength sensitive cone pigments, only these latter pigments are genetically closely related, and we refer to the family simply as LWS. Arthropod photopigments evolved independently from those of vertebrates. They fall into iii families—'UV', 'bluish' and 'green'—which diverged before the radiation of insects (Briscoe 2000; Briscoe & Chittka 2001).

Adaptive variation of visual pigments is well known, especially amongst fishes whose receptor sensitivities tend to match the ambience illumination spectrum equally it varies with water depth and quality (Lythgoe 1979; Bowmaker 1995). Shifts in spectral sensitivity are caused by amino acid substitutions at a few specific sites in the opsin molecule, so that point mutations can essentially affect visual phenotype (Yokoyama & Radlwimmer 1999; Briscoe 2001; Hunt et al. 2001; Yokoyama & Radlwimmer 2001). Best known are primate LWS pigments, where substitutions at iii sites shift λ _max from 535 to 560 nm (Neitz et al. 1991; Asenjo et al. 1994; Nathans 1999). Almost certainly, these disquisitional sites have been subject to stabilizing choice (and convergent option) in separate primate lineages (Deeb et al. 1994; Surridge et al. 2003). At a broader phylogenetic scale, there are parallels between groups. For example, the amino acid substitutions that differentiate pigments in the long-wavelength range (λ _max>510 nm) are akin in mammals, fishes and butterflies (Yokoyama & Yokoyama 1996; Briscoe 2001; Yokoyama & Radlwimmer 2001).

In the face of the simple relationship between photopigment genotype and spectral phenotype, and for adaptive variation in fishes, several workers have commented on the uniformity of paint spectral sensitivities within sure taxonomic groups of land animals. Examples include bees and wasps, anole lizards, birds and One-time Globe (catarrhine) primates (Peitsch et al. 1992; Jacobs & Deegan 1999; Briscoe & Chittka 2001; Hart 2001; Loew et al. 2002). Within each of these groups, receptor sensitivities vary piddling (figure 1), and seem to be unaffected by habitat, feeding behaviour or the species' ain colours. This uniformity is consistent with the view that signalling colours, such as those used in sexual displays, exploit a fixed sensory organisation (Allen 1879; Ryan 1990). Even so, as Wallace (1879) pointed out, there is little support for the prediction (Allen 1879) that species feeding on fruit and flowers should have superior color vision, and more colourful displays than others. Primates may be an exception, in that past comparison with other mammals, they accept superior colour vision, a preference for feeding on fruit and are relatively colourful.

The evolutionary conservatism of photoreceptor spectral sensitivities should non be exaggerated. For instance, the shortest wavelength (SWS1) pigments of birds accept repeatedly switched from an ancestral class, λ _max ca 410 nm, to a 365 nm grade (Odeen & Håstad 2003). λ _max of mammalian LWS pigments varies from 495 to 565 nm (Jacobs & Deegan 1994; Yokoyama & Radlwimmer 1999). Meanwhile, long-wavelength pigments have diversified by factor duplication in groups such equally Primates and Lepidoptera, a discipline to which we return below.

2. Luminance spectral sensitivity

Although not directly concerned with spectral information, the spectral sensitivity of achromatic signals is likely to be affected past two main considerations: (i) the SNRs in signals from captivated photons and (2) the rate of thermal isomerization of photopigment. Thermal isomerization is indistinguishable from photon absorption and hence is a source of dissonance.

In principle, achromatic signals might be derived by summing the outputs of any number of different spectral types of photoreceptor, but normally, outputs of a unmarried type of photoreceptor are used for tasks such as motion perception and form vision. By analogy with homo vision, this can be chosen a 'luminance signal' (Wyszecki & Stiles 1982; Livingstone & Hubel 1988).¹ Mammals utilize long-wavelength sensitive (50) cones (Jacobs 1993), while birds and fishes probably use double cones (which contain the LWS paint; Schaerer & Neumeyer 1996; Jones & Osorio 2004). Amid insects, bees use their long-wavelength receptor (Srinivasan & Lehrer 1988; Giurfa et al. 1996, 1997), and flies, the curt visual fibres (Hardie 1986; Heisenberg & Buchner 1977; Anderson & Laughlin 2000). These luminance receptors are generally the about arable type (excluding vertebrate rods), which allows high sensitivity and spatial resolution. The relatively contempo evolutionary divergence of primate M and L cones may explain why their outputs are combined (Mollon 1989).

Among mammals, the spectral sensitivity of L cones varies; primates accept 560 nm LWS pigments, while sheep and cows take a 555 nm pigment (Jacobs et al. 1998), owl monkeys (Aotus spp.) have a 545 nm paint (Jacobs et al. 1993), ground squirrels (Spermophilous spp.) have a 520 nm pigment (Kraft 1988), rats, mice and rabbits accept a 510 nm pigment (Yokoyama & Radlwimmer 2001) and the Mongolian gerbil (Meriones unguiculatus) has a 495 nm pigment (Jacobs & Deegan 1994). Amniotes accept rod pigments with λ _max close to 500 nm. By comparison, in birds and anole lizards, the LWS pigment λ _max is nearly ever 565 nm (Hart 2001; Loew et al. 2002; although some lizards, including Anolis carolinensis, have a 620 nm (A2) pigment, Provencio et al. 1992).

For rhodopsins with a retinal (A1) chromophore, 565 nm is close to the longest known value of λ _max, and this may be an upper limit imposed by photopigment chemistry. Below this (hypothetical) limit, the λ _max for mammalian LWS pigments is unlikely to exist selectively neutral between 500 and 560 nm (Yokoyama & Radlwimmer 1999, 2001). New World monkeys (Platyrrhini) provide direct evidence for selection considering they have a single LWS opsin gene that is polymorphic for the Yard/L pigments in the 535–560 nm range. Longer wavelength (L type) pigments appear to exist selectively favoured in homozygous monkeys, which are dichromats. Genetic equilibrium is maintained because 535 nm pigments confer an advantage to heterozygous individuals, which are trichromats (Surridge et al. 2003; Osorio et al. 2004).

(a) Terrestrial spectra, thermal isomerization and luminance spectral sensitivities

Where SNR is adamant past photon absorption (as opposed to thermal isomerization), the optimal value of λ _max will tend to favour maximizing photon catch. Presumably, this is why fish photopigment sensitivities match illumination spectra. Celestial illumination has a comparatively broad spectrum (effigy 2 a), but in light reflected from or transmitted through leaves, chlorophyll produces a relatively narrow spectrum (figure 2 b; Endler 1993). For standard daylight, quantum catch of visual photopigments will increase with λ _max (effigy ii c), but for greenish forest light, quantum grab is practically contained of λ _max in the range of 500–560 nm. Thus, maximizing sensitivity may explicate why luminance mechanisms tend to utilise receptors with λ _max>500 nm, but variation in illumination spectra is unlikely to account for the differences among land animals.

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(a) Illumination spectrum of standard daylight in quantum units. Illumination spectra contain differing proportions of direct sunlight, blue skylight, and light filtered through leaves, which resembles the leaf reflectance spectrum. (b) Hither, illustrated by the average from a large sample of rainforest species. Leaf spectra peak at 555 nm, and increment sharply across 680 nm. (c) Dependence of quantum grab on the wavelength position of the photopigment containing A1 pigments. A2 pigments, which tin can tiptop at up to 620 nm, requite very like curves. Illumination is assumed to be either standard D65 daylight (ca figure two a) or wood light. Calculations assume the receptor views a surface whose reflectance is uniform across the spectrum, and that the absorption at the acme does non depend on λ _max. Wood light is assumed to be the product of D65 illumination with the reflectance of leaves, which is an extreme green light. Breakthrough catch is normalized to that for a receptor peaking at 645 nm. The quantum take hold of in forest lite increases sharply at the tiptop wavelengths greater than 600 nm. Ordinarily visual pigments contain a vitamin A1 chromophore (or in some insects, a chromophore based on xanthophylls; Hardie 1986), which allow λ _max to reach about 570 nm. Nonetheless, the lizards employ vitamin A2 as a chromophore (Provencio et al. 1992), which gives an LWS pigment with λ_max 620 nm. A2 pigments are common in fishes and information technology unclear why they are rare in amniotes.

An culling and long-standing proposal is that the spectral sensitivity reflects a compromise between the need to maximize photon catch and the effects of night noise, which favours short-wavelength receptors (Barlow 1957). Illumination spectra at night are much like those of daylight (Henderson 1977), and given that a 560 nm pigment maximizes photon catch (figure 2 c) information technology is a puzzle why rod sensitivity peaks at 500 nm. The difference between photopic (light adjusted) and scotopic (dark adapted) spectral sensitivities is known as the Purkinje shift. Barlow (1957) suggested that the rate of spontaneous activation of photopigment molecules by thermal isomerization, or 'dark lite' increases with λ _max, considering the energy barrier for thermal isomerization falls with λ _max (see as well Ala-Laurila et al. 2004). Dark noise raises absolute threshold (i.due east. the weakest light detectable over a dark groundwork), and so is most important in dim light. In fact, thermal isomerization rates are affected by λ _max, but other causes underlie the 10^iv fold divergence in rates of spontaneous isomerization betwixt rod and cone pigments (Ala-Laurila et al. 2004). Thus, the Purkinje shift may exist attributable to the wavelength-dependence of thermal isomerization, merely there is no clear evidence that this effect accounts for the diversity of mammalian LWS pigments. The simplest prediction being that λ _max should exist lower in species that apply cones in dim light.

(b) Broadening spectral sensitivity: mixtures of visual pigments and antennal pigments

Some groups including rodents and butterflies co-express short- (S) and long- (50) wavelength sensitive opsins within photoreceptors (Szel et al. 2000; Arikawa et al. 2003), which broadens spectral sensitivity. Often, the Fifty : S ratio varies systematically beyond the retina, to class gradients. Typically, the proportion of short-wavelength pigment is greatest in the receptors that view the sky (Szel et al. 2000; Briscoe et al. 2003). This arrangement seems logical (considering the heaven is blueish), and might maximize the contrast of objects seen in silhouette. Nonetheless, using the best single type of pigment (figure iii) nearly always maximizes photon take hold of, and the reward of mixing pigments is unclear.

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Effects of mixing long- (L) and short- (S) wavelength sensitive pigments on quantum catch. Dependence of the breakthrough grab on the ratio L : S in a cone containing a mixture of an L pigment peaking at 565 nm and an S pigment peaking at 435 nm. Calculations are for two optical densities. The solid line corresponds to a receptor with a peak absorptance of 0.4, and the dashed line a elevation absorptance of 0.99. This shows that even for very long receptors (with high optical density), the breakthrough catch for a cone containing just L pigment exceeds that for a receptor with a mixture of visual pigments.

In dipteran flies, UV sensitivity of the main type of photoreceptor (R1-6) is often enhanced past an antennal pigment, which transfers lite energy to the rhodopsin (figure i; Hardie 1986). This paint is idea not to reduce the rhodopsin concentration, and may increase quantum catch past upwards to xx% depending on the stimulus spectrum (Stavenga 2004).

iii. Color vision

Colour vision can be defined as the ability to discriminate variation in the spectrum of light from changes in overall intensity (Kelber et al. 2003). This requires comparing of responses in two or more spectral types of photoreceptor, only how many types are needed to code natural spectra? The broad spectral tuning of visual photopigments limits their spectral resolution (Barlow 1982), which means that eyes cannot discriminate two closely spaced wavelengths from a single intermediate spectral line. Whether wide tuning is a limitation depends on the spectral signals, and ordinary reflectance spectra tend to vary relatively smoothly. Equally with a scene viewed through fog, the absence of fine spectral detail means in that location may be no benefit from high resolution.

Maloney (1986) used principal components assay to show that within the visible spectrum three components are sufficient to represent about of the variation in spectra of 'natural formations' from the Soviet Wedlock. The smallest signals that can usefully be coded depend upon the level of racket, and when this effect is taken into account trichromacy and rhodopsin tuning curves (with a half-width of about 100 nm) may be well matched to natural spectra (van Hateren 1993). The model predicts a smaller number of more broadly tuned receptors as the SNR falls (due east.g. in dim light).

Given that the relatively small number of receptor types found in animal eyes are more or less sufficient to encode natural spectra nosotros can get on to ask what factors may underlie differences between species; for example, in the spectral locations of receptors, or the narrowing of spectral tuning with coloured filters (figure i). Piece of work on the number of receptors has used sampling and information theory and considered a wide range of spectra (Barlow 1982; van Hateren 1993), but clearly, the particular objects of interest and the behavioural uses of color vision may be pregnant. One can approximate receptor responses to object spectra, and simulate performance of colour vision in a plausible task, such as finding fruit, identifying the maturity of leaves or selecting mates (Vorobyev & Osorio 1998; Kelber 1999; Sumner & Mollon 2000a,b; Regan et al. 2001). To test a hypothesis well-nigh evolutionary adaptation, a hypothetical fix of receptors that is predicted to be optimal for a given purpose can be compared with reality. In that location are studies on spectra from woodlands viewed by dichromatic eyes (Lythgoe & Partridge 1989; Chiao et al. 2000); fruit and leaves consumed by primates (Osorio & Vorobyev 1996; Sumner & Mollon 2000a,b; Osorio et al. 2004); flowers visited by bees (Chittka & Menzel 1992; Vorobyev & Menzel 1999); and bird plumage colours (Vorobyev et al. 1998).

Although these studies differ in their details, the main observations are consistent (Lythgoe & Partridge 1989; Vorobyev 1997; Vorobyev et al. 1998; Vorobyev & Menzel 1999; Chiao et al. 2000; Vorobyev 2003). Notably, they confirm that spectral information can be coded with at most five types of receptor. Increasing the spectral separation of receptors, or narrowing their sensitivities with oil droplets will more often than not increase the discriminability of natural spectra.

More interestingly, models go far clear that the optimal centre design is dependent upon iii main factors; namely, the spectra of interest, the behavioural job and photoreceptor noise. For instance, the uniformly spaced receptors of honeybees are optimal for discriminating flower colours (Chittka & Menzel 1992; Vorobyev & Menzel 1999), whereas, detection of fruit against leaves, may favour the closely spaced Fifty and M pigments of trichromatic primates (figure 1; Osorio & Vorobyev 1996; Sumner & Mollon 2000a,b). The large spectral overlap of the L and Yard receptors is probably advantageous because fruit and leaves reflect more strongly, and/or exhibit greatest variation in reflectance at long wavelengths. Finally, the demands of color continuance may favour relatively closely spaced and narrowly tuned receptors (Osorio et al. 1997).

(a) Diversification of long-wavelength receptors and leafage colouration

Simplification is a common grade of evolutionary change, and information technology is no surprise that nocturnal and fossorial groups, such mammals and snakes, have lost 1 or more of the bequeathed classes of cone pigment (Yokoyama & Yokoyama 1996; Sillman et al. 2001; Arrese et al. 2002). Increment in complexity is peradventure more than interesting, and amidst land animals is prominent among long-wavelength receptors (λ _max>510 nm). Thus, birds and reptiles retain the five ancestral sets of visual pigments, but coloured oil droplets on the LWS single cone substantially red-shift and narrow its peak compared with the double cones, which also contain the LWS photopigment (figure i; Hart 2001).

Mammals lack coloured oil droplets in their cones, but trichromacy has emerged in Old Globe monkeys (Catarrhine) and howler monkeys (Alouatta) post-obit independent duplications of the ancestral LWS factor (Nathans 1999; Surridge et al. 2003). (A fruit bat Haplonycteris fischeri, has duplicate LWS genes, although the spectral sensitivities of the 2 pigments probably do not differ; Wang et al. 2004.)

Duplications of long-wavelength paint genes occurred early in insect development (Spaethe & Briscoe 2004), and repeatedly more recently in Lepidoptera (Briscoe 2000, 2001, 2002; Wakakuwa et al. 2004). Butterflies take from i to three types of long-wavelength photopigment, and employ coloured filters to produce photoreceptors with λ _max from 540 to 620 nm (Stavenga 2002; Arikawa 2003). Specialized 'red receptors' are known from other insect families, including Odonata, Diptera and Hymenoptera (Hardie 1986; Peitsch et al. 1992; Yang & Osorio 1996; Briscoe & Chittka 2001).

As we take mentioned, there is petty testify that the evolution of long-wavelength sensitive receptors occurred in response to biological signals, such as fruit, flowers or mating displays (§1; Surridge et al. 2003). An culling is that receptors evolved mainly to bargain with the 'common' colours of visual backgrounds. Terrestrial spectra fall into ii principal classes (figure ii). Leaves absorb strongly below 500 nm, and have a peak due to chlorophyll almost 555 nm. Reflectance of almost all other materials (e.k. bark, expressionless vegetation, soil and animal melanin pigments) increases roughly linearly with wavelength (Osorio & Bossomaier 1992).

Lythgoe (1979) recognized the probable importance of foliage colouration. He suggested that leaf spectra are more variable above the 555 nm superlative than below, and speculated that this is why pigeons (Columba livia) have a long-wavelength single cone with its sensitivity maximum nearly 600 nm (achieved past an LWS paint with a cherry-red oil droplet). Yet, it should be noted that all birds, with the exception of penguins, probably have this type of photoreceptor (Hart 2001, 2004).

Others have drawn attending to the importance of foliage reflectance spectra in evolution of long-wavelength spectra. Kelber (1999) suggested that having multiple long-wavelength receptors allows butterflies to discriminate between dissimilar leaf greens every bit an aid to oviposition. Likewise, sawflies and their allies (Symphyta), with vegetarian (and often folivorous) larvae, take a crimson (600 nm) receptor, in addition to the usual iii that they share with other bees and wasps (Peitsch et al. 1992; i bee in this written report of 42 species also had a red receptor).

Mollon and his collaborators (Mollon & Regan 1999; Sumner & Mollon 2000a,b; Regan et al. 2001) propose a rather dissimilar scenario to business relationship for the tuning of primate L and Thousand pigments. They annotation that mature leaves of dissimilar constitute species give a small range of red–greenish (i.eastward. L–M) signals, and propose that the spectral tuning of the receptors may be adapted to minimize these signals. The small range of foliage ruddy–green signals might simplify the task of locating ripe fruit, which tend to exist reddish. Regardless of its validity, this proposal nicely illustrates how sense organs may exist adapted for specific behavioural tasks, in this case classifying fruit and leaf reflectance spectra.

4. Determination

In that location is clear evidence for adaptive variation in visual pigments amidst fishes, and likewise New Earth primates (Surridge et al. 2003; Osorio et al. 2004). Models of visual coding are useful for showing how spectral signals, receptor noise and behavioural tasks can substantially influence photoreceptor spectral tuning. They practice non clearly identify what optics are adapted to see. Photoreceptor spectral sensitivities, either as adaptations for luminance or for chromatic coding, should be a straightforward trouble in sensory ecology. It is perchance salutary that in that location are so few answers, at least regarding diurnal country animals. Open up questions include the range of sensitivity maxima among LWS pigments of rodents and other mammals, the reasons for co-expressing pigments and the gradient of co-expression that occur in rodents and collywobbles and the occurrence of multiple long-wavelength sensitive receptors in primates, butterflies and other insects.

Acknowledgments

D.O. was funded by a visiting fellowship to the Centre for Visual Science at the Australian National Academy. We thank A. Briscoe and the referees for communication.

5. Endnotes

¹In insects, achromatic signals used for behaviours such as phototaxis and celestial navigation are derived from UV-sensitive receptors.

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