The Auk 127(2):283 292, 2010 The American Ornithologists’ Union, 2010. Printed in USA.
HUMAN VISION CAN PROVIDE A VALID PROXY FOR AVIAN PERCEPTION OF SEXUAL DICHROMATISM
NATHALIE SEDDON,1,4 JOSEPH A. TOBIAS,1 MUIR EATON,2 AND ANDERS ÖDEEN3
1Edward Grey Institute, Department of Zoology, University of Oxford, South Parks Road, Oxford OX1 3PS, United Kingdom; 2Natural History Museum and Biodiversity Research Center, University of Kansas, Lawrence, Kansas 66045, USA; and 3Department of Animal Ecology, Evolutionary Biology Centre, Uppsala University, Uppsala, Sweden
Abstract.—The study of sexual dichromatism has played an important role in the development and testing of evolutionary theory. However, previous work has often relied on human vision to assess plumage color and contrast, an approach challenged by the finding that dichromatism is often visible to birds but invisible to humans. We explicitly tested whether the use of human vision undermines previous comparative analyses in antbirds (Thamnophilidae). Focusing on a sample of species, we used () molecular sequencing of the SWS opsin gene to assess spectral sensitivity and () spectrophotometry and color discrimination models to compare human and avian perception of dichromatism. We show that antbirds, like the majority of avian families studied to date, are violet- sensitive (VS) rather than ultraviolet-sensitive (UVS). We also demonstrate that species perceived as monochromatic by humans may look dichromatic to antbirds, but that human and avian perceptions of dichromatism are nonetheless positively correlated. To assess whether this relationship validates the assumptions of published comparative analyses, we re-ran the analyses using avian-perceived dichromatism; the results remained qualitatively unchanged. Although it is clear that the use of spectrophotometry and visual models can improve measurements of plumage coloration, we conclude that scores generated from human perception provide a meaningful estimate of sexual dichromatism for the purposes of comparative analyses, at least in antbirds. Furthermore, our results suggest that discrepancies between human and avian perceptions of sexual color differences may be relatively minor in avian families with VS visual systems. Received July , accepted October .
Key words: antbirds, comparative analysis, plumage reflectance, sexual dichromatism, spectral sensitivity, Thamnophilidae, ultra- violet vision. La Visión Humana puede Proveer una Aproximación Válida a la Percepción del Dicromatismo Sexual por parte de las Aves
Resumen.—El estudio del dicromatismo sexual ha jugado un papel importante en el desarrollo y puesta a prueba de la teoría evolutiva. Sin embargo, los estudios han dependido frecuentemente de la visión humana para evaluar el color y contraste del plumaje. Este enfoque ha sido desafiado por el hallazgo de que, con frecuencia, las aves pueden ver dicromatismo en situaciones en las que éste es invisible para los humanos. Evaluamos de forma explícita si el uso de la visión humana representa un problema para los estudios comparativos realizados previamente en los hormigueros (Thamnophilidae). Enfocándonos en una muestra de especies, empleamos () secuenciación molecular del gen SWS de las opsinas para evaluar la sensibilidad espectral y () espectrofotometría y modelos de discriminación de colores para comparar la percepción del dicromatismo por parte de los humanos y de las aves. Nuestros resultados demuestran que especies de la familia de los hormigueros, como la mayoría de las familias de aves que han sido estudiadas, son sensibles al violeta y no son sensibles al ultravioleta. También demostramos que las especies que son percibidas como monocromáticas por los humanos podrían parecer dicromáticas para las aves, pero las percepciones del dicromatismo de los humanos y de las aves están correlacionadas positivamente. Para evaluar si esta relación valida las suposiciones de los análisis comparativos publicados, repetimos dichos análisis usando el dicromatismo percibido por las aves y obtuvimos resultados cualitativamente iguales. Aunque es claro que el uso de la espectrofotometría y los modelos visuales pueden mejorar las medidas de la coloración del plumaje, concluimos que los puntajes generados a partir de la percepción humana proveen un estimativo del dicromatismo sexual que resulta sensato para los propósitos de los análisis comparativos, al menos en los hormigueros. Además, nuestros resultados sugieren que las discrepancias entre los humanos y las aves en la percepción de las diferencias de colores entre sexos podrían ser pequeñas en las familias de aves que tienen sistemas visuales sensibles al violeta.
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Studies of avian sexual dichromatism have been central to reliable” (Armenta et al. b). As yet, the degree of mismatch the formulation of evolutionary theory from the outset (Darwin between dichromatism as detected by humans and by birds has , Wallace ) and remain prominent in reviews of sexual not been examined directly, and its influence on the conclusions selection (Andersson ) and speciation (Price ). Indeed, of previous analyses remains to be tested. the focus on sex differences in plumage color and pattern has in- Given the differences in visual physiology between humans tensified, partly because strong positive relationships have been and birds, it is inevitable that information is lost when plumage reported between avian dichromatism and measures of sexual se- coloration is interpreted by human observers (Endler , Ben- lection, such as the degree of polygyny, frequency of extrapair pa- nett et al. , Cuthill , Håstad and Ödeen ). Evidence ternity, and relative testes size (Møller and Birkhead , Owens that the lost information is biologically significant stems from () and Bennett , Dunn et al. ). These findings have led to research that documents the ubiquity of UV-reflecting plumage in widespread use of dichromatism as an index of sexual selection in birds (e.g., Eaton and Lanyon ); () the discovery that % of comparative analyses. In recent decades, quantification of avian passerine species classified as monochromatic by human observ- dichromatism for this purpose has been pervasive, involving nu- ers are probably perceived as sexually dichromatic by an avian vi- merous studies of speciation (Barraclough et al. , Møller and sual system (Eaton ); and () numerous studies that show that Cuervo , Owens et al. , Morrow et al. , Phillimore et spectral reflectance invisible to humans is used in a range of inter- al. , Seddon et al. ) and extinction (McLain et al. , specific and intraspecific interactions, including species recogni- ; Sorci et al. ; Doherty et al. ; Morrow and Pitcher tion and mate choice (e.g., Bennett et al. , ; Andersson et ), as well as a wide range of other topics, including individ- al. ; Arnold et al. ; Bleiweiss ). The broad implica- ual mortality (Promislow et al. ), immune defense (Møller tion is that all studies that use human estimates of dichromatism ), signal evolution (Garamszegi et al. ), molecular evolu- are potentially invalid (Bennett et al. , Vorobyev et al. , tion (Nadeau et al. ), and response to climate change (Spot- Eaton ). However, this is not necessarily the case for compar- tiswoode et al. ). ative analyses, because most previous work has scored dichroma- Since the pioneering studies of Owens and Bennett () tism on a scale, usually from to , and the extent to which this and Owens and Hartley (), a common method for measuring approach is confounded depends on how well human-perceived dichromatism has been to score sex differences from painted illus- dichromatism predicts avian-perceived dichromatism. trations or specimens. This technique assumes that human vision The overall aim of the present study is to assess the validity of allows a reasonable approximation of dichromatism, or of bird col- human vision as a proxy for avian perception, specifically in color oration generally. Thus, a major objection to this approach stems discrimination and perception of dichromatism. To achieve this, from the fundamental differences between human and avian vi- we provide the first direct assessment of sex-specific plumage re- sual physiology (Cuthill et al. a). In particular, humans are flectance in the antbirds (Thamnophilidae) and the first estimate blind to the near ultraviolet (UV; – nm), to which many of spectral sensitivity of the antbird eye. We use color spectro- birds are sensitive, and humans have three retinal cone types, photometry to measure plumage reflectance from museum spec- compared with at least four in birds (Bowmaker et al. ). On imens, molecular sequencing to determine spectral sensitivity the basis of these differences, visual models predict that humans (Ödeen and Håstad , Ödeen et al. ), and color discrim- and birds see color very differently, particularly when reflectance ination models to estimate avian perception of dichromatism spectra are more complex or UV-rich (Vorobyev et al. ). They (Vorobyev and Osorio ). These models generated values of also predict significant mismatches between human and avian perceived sex differences in color across the avian visual spectrum perception of plumage color contrasts (Håstad and Ödeen ) (– nm) for each species (hereafter “dichromatismavian”), and—crucially—that a high proportion of bird species classified allowing comparison with scores generated by human observers as monochromatic by human observers are perceived as dichro- (hereafter “dichromatismhuman”). Finally, to determine the extent matic by birds (Eaton , ). to which dichromatismhuman is a valid surrogate for dichroma- These findings call into question the central assumption tismavian in comparative analyses, we used both measures to re- of many previous studies that human and avian perceptions of analyze data from a recent study of antbirds (Tobias and Seddon dichromatism are correlated. Nonetheless, the human-vision a). Our results reveal close concordance between human and method is so convenient that such analyses continue to be pub- avian perception, which suggests that, for the purposes of compar- lished, with their assumptions defended by logic rather than data ative analyses, human-generated scores can provide a meaningful (e.g., Seddon et al. ). The main supporting argument runs proxy for avian-perceived dichromatism, at least when spectral thus (Badyaev and Hill ): complexity and UV reflectance are low. The fact that so many interesting patterns related to sexual METHODS dichromatism have been revealed in comparative studies that ignore the UV portion of the spectrum suggests that the visi- Study system.—The antbird family contains species of small ble portion of the spectrum must, in many cases, be a reason- to medium-sized (.– g) insectivorous passerines found in able approximation of the overall coloration and dimorphism of forest, woodland, and scrub throughout Central America and a species. South America at elevations –, m above sea level (Zimmer This hypothesis received indirect support from a recent com- and Isler ). It provides an excellent system for this study be- parison of techniques, which again concluded that studies using cause it is speciose, with much interspecific variation in plum- human vision to assess avian sexual dichromatism “should be age dichromatism, and it therefore gives us power to explore the APRIL 2010 — HUMAN AND AVIAN PERCEPTION OF PLUMAGE DICHROMATISM — 285
relationship between human and avian perceptions of this trait. A in the dominant color and two measures in the alternate color. large collection of specimens is available for study, and males and When regions contained more than one color (rare in antbirds), females of all species have been illustrated in detail by the same we took all measures from the dominant color. When regions con- artist (Zimmer and Isler ), facilitating estimation of dichro- tained a distinct color patch in at least one sex (i.e., black throats in matism by human observers. Antbirds have also been subject to males), we took measures from the center of the patch. recent comparative analyses based on sexual dichromatism (Sed- For the purpose of analysis, we averaged raw spectral data don et al. , Tobias and Seddon a), which allows us to test into -nm bins from across the entire avian-visible wavelength the validity of previous assumptions. range (– nm), thus producing a series of reflectance Plumage reflectance.—Spectral reflectance was measured values for each measure. Averaging reflectance for each plumage from antbird specimens housed at the Natural History Museum, patch produced reflectance curves per sex, reflectance curves Tring, United Kingdom. We sampled species, using three speci- per species, and a total of reflectance curves for analysis. Fol- mens of each sex where possible (male specimens per species [mean lowing Eaton and Lanyon (), each reflectance curve was clas- o SE] . o .; female specimens per species . o .; n sified as having a mean UV reflectance (– nm) of %, specimens). For each specimen, we recorded species, subspecies, –%, –%, or % of incident light, with any UV reflectance sex, catalogue number, locality, collector, and date (see online Ap- % treated as biologically meaningful. We then compared mean pendix). We selected the nominate subspecies where possible and reflectance between the UV spectrum (– nm) and the ensured that all specimens were of the same subspecies. Specimens human-visible spectrum (– nm) for each of the five body that showed signs of molt or juvenile plumage were not sampled. regions (sexes pooled) and by averaging across body regions to Fresh specimens were unavailable and therefore we, like au- quantify reflectance for the plumage as a whole. Finally, we com- thors of other reflectance studies (e.g., Gomez and Théry ), pared UV reflectance between males and females separately for used older specimens. Collection dates spanned more than a cen- all five body regions, and then by averaging across body regions to tury (–), with the bulk of specimens (.%) collected in quantify UV dichromatism for the plumage overall. a -year period (–). Specimen age has been shown to in- The antbird visual system.—The spectral sensitivity of the fluence plumage reflectance, particularly when specimens are avian retina varies widely between species. The wavelength of years old (Armenta et al. a), and it is therefore likely that our maximum absorbance (δmax) of the short-wavelength-sensitive sin- measurements do not fully capture dichromatism in wild birds. gle cone (SWS) falls into one of two classes (Cuthill et al. b):
However, we believe that specimen age is unlikely to affect our re- ultraviolet sensitive (UVS; δmax: – nm) and violet sensi- sults, for three main reasons. First, the effects of age are gener- tive (VS; δmax: – nm); for reviews, see Hart (, ) ally minor, even in older specimens (Bleiweiss , Armenta et and Beason and Loew (). The sensitivity of avian single cones al. a, Doucet and Hill ). Second, we found no statistically is tightly correlated with sequence variation in the SWS gene significant effects of specimen age on mean levels of reflectance (Wilkie et al. , Yokoyama et al. , Carvalho et al. ), for any plumage patch in either the UV or human-visible parts of which makes it possible to estimate δmax for the visual pigment by the spectrum (see below). Third, measurements of fresh plumage molecular sequencing of genomic DNA (Ödeen and Håstad , samples collected from mist-netted individuals suggest that the Ödeen et al. ). We used this approach to identify the likely spectra of live antbirds are similar to those of old specimens, with spectral sensitivity in antbirds. similarly low levels of UV reflectance (Tobias and Seddon b). Genomic DNA was extracted from blood samples taken from Nonetheless, to minimize any potentially confounding effects of one individual each of three distantly related species of antbird: age on our calculations of dichromatism, we ensured that male Peruvian Warbling Antbird (Hypocnemis peruviana), Chestnut- and female specimens were collected at a similar time within each tailed Antbird (Myrmeciza hemimelaena), and Black-spotted species (mean age difference [o SE] . o . years; correlation Bare-eye (Phlegopsis nigromaculatus). Samples were taken from between mean age of males and females within species: Pearson’s birds caught in r m mist nets at the Centro de Investigación r ., P ., n ). y Conservación de Río Los Amigos (CICRA; n`S, n`W), We measured spectral reflectance using an Ocean Optics Madre de Dios, Peru; . μL of blood was extracted from the bra- USB spectrometer (range: –, nm), a DH-FHS chial vein and stored in % ethanol. We amplified a key frag- pulsed xenon lamp (–, nm), and a bifurcated fiber-optic re- ment of the SWS opsin gene containing the residues of amino flection probe (QR--UV/BX) mounted in a matte black holder acid positions – (following bovine rhodopsin numbering) (RPH-). The latter excluded external light and kept the probe and located in the second α-helical transmembrane region. Ex- perpendicular to the measurement surface at a fixed distance of tractions, polymerase chain reaction (PCR; without explicit ex- mm. Data were collected in SPECTRASUITE and expressed as tension time), and cycle sequencing were carried out following the proportion of reflectance relative to an Ocean Optics WS- Ödeen et al. (). We used primer SUGa in sequencing (`- white standard, which reflects –% of incident light. Following AYTACATCYTGGTGAACATCTCS-`, modified for the present Doucet et al. (), we took five measurements from the center of study from SUCa). each of five body regions: head (forecrown, crown, and nape), up- We identified the spectral tuning amino acid sites , , and perside (mantle and rump), underside (throat, breast, and belly), (Wilkie et al. , Yokoyama et al. ), to estimate δmax of wings, and tail. In all cases, measurements were taken from the the SWS cone pigment. Positions and , in particular, are re- upperside of feathers and from parts of feathers visible without sponsible for shifts in δmax; CS has a nm effect in Budgerigar spreading or ruffling the plumage. When plumage regions were (Melopsittacus undulatus) wild-type pigment (Wilkie et al. ); patterned with bands, spots, or stripes, we took three measures nm in Zebra Finch (Taeniopygia guttata; Yokoyama et al. 286 — SEDDON ET AL. — AUK, VOL. 127
); and SF can short-wave-shift pigeon and chicken SWS Using the same specimens from which spectral reflectance pigments by and nm, respectively (Carvalho et al. ). Mi- data were collected, two observers scored dichromatism on a scale nor shifts in δmax of the budgerigar pigment have been achieved by of (monomorphic) to (maximum dichromatism). Following AS with nm and TV with nm (Wilkie et al. ). Two standard methodology (e.g., Owens and Bennett , Owens and other key vertebrate tuning sites, namely (Shi and Yokoyama Hartley ), we allocated a score of ( no difference between ) and (Hunt et al. ), fell outside the range of our DNA the sexes), ( difference in shade or intensity), or ( difference amplifications. However, neither appears to be significantly in- in color or pattern) to each of the five body regions described volved in tuning the avian SWS opsin (Carvalho et al. ). We above. These were totaled to produce a composite dichromatism assumed that the effects of the key tuning sites were additive. This score for each species (dichromatismhuman), which we then directly assumption disregards potential interactions between the tuning compared with our estimates of dichromatismavian, using both VS sites (see Shi et al. ) but still provides approximations of δmax and UVS retinal models. sufficient to distinguish between VS and UVS in the avian retina Using the same scoring system, we also quantified dichroma- (Ödeen et al. ). tism from color illustrations (Zimmer and Isler ). This allowed Measures of dichromatism.—Dichromatism was calculated a direct test of the validity of previous comparative studies in ant- from spectral reflectance data using two complementary meth- birds that have scored dichromatism in this way (Seddon et al. , ods. In the first, we simply calculated the maximum Euclidean dif- Tobias and Seddon a). Note that dichromatismhuman scores gen- ference across all bins in reflectance between males and females erated by the two observers were strongly correlated (specimens: r within bins and then compared the mean maximum sex differ- ., P ., n species; illustrations: r ., P ., n ence between the UV and human-visible spectra for each body re- species) and that the mean of their scores was used. gion. In the second, we calculated dichromatism for each plumage Testing the assumptions of comparative studies.—In a recent region within each species using the Vorobyev-Osorio color dis- analysis, we found that dichromatismhuman (generated from color crimination model (Vorobyev and Osorio ). The model calcu- illustrations) was positively associated with ecological generalism lates a distance in avian perceptual color space (ΔS) between two in the antbird family (Tobias and Seddon a). Specifically, we colors, defined by the quantum catches and estimates of receptor found that mean level of dichromatism was positively correlated noise of each photoreceptor type in the avian retina. Thus, ΔS for with mean niche width, as measured by habitat range (r ., n a given plumage region quantifies sexual dichromatism explicitly genera). To investigate whether this finding was influenced by with respect to the avian visual system. the use of human rather than avian perception of dichromatism, To calculate ΔS for each plumage-region comparison, we fol- we re-ran the analysis and compared mean habitat range against lowed the calculations detailed in Eaton (). However, because () mean dichromatismhuman and () mean dichromatismavian. We amino acid sequencing suggests that antbirds may, in general, used the same subset of species from antbird genera (i.e., possess a violet-sensitive (VS) visual system (see below), ΔS val- those included in the present study) to calculate both. For meth- ues were calculated for each plumage region with spectral sensi- ods used to calculate habitat range for each species and genus, see tivity data from Common Peafowl (Pavo cristatus; data provided Tobias and Seddon (a). by Hart ). These data likely provide a good estimate for other Statistics.—Because common ancestry leads to non-inde- species with VS visual systems, because visual pigment charac- pendence of samples, classical statistics tend not to be applied in teristics (and, thus, cone-cell sensitivities) are highly conserved multispecies comparative analyses. However, previous work has over much of the avian visual range (Hart ). However, be- revealed that dichromatism has a low phylogenetic signal within cause actual antbird spectral sensitivity is unknown, we also ran and between avian families (Phillimore et al. ), including ant- the models based on a UV-sensitive visual system, with cone-cell birds (Seddon et al. , Tobias and Seddon a). Moreover, spectral-sensitivity data from Blue Tit (Parus caeruleus; Hart et the key aim of our study was not to look for evidence of corre- al. ). Values of ΔS generated with a UVS system were strongly lated evolution between traits (Price ) but to ascertain how positively correlated with those generated with a VS system (r well human scores of dichromatism reflect avian perception of di- .; see online Appendix); thus, we present only the latter here. chromatism. We therefore treat species values as independent and Sexual dichromatism was calculated on a patch-by-patch ba- compare traits using simple linear regression. sis, so we averaged ΔS values within each species to produce an Reflectance and dichromatism were compared within plum- overall estimate of the level of dichromatism perceived by birds age patches using paired t-tests, and dichromatismhuman and di- (dichromatismavian). The units of ΔS are jnd (just noticeable dif- chromatismavian were compared using Pearson correlation. All ferences), where . jnd is the threshold value for discrimina- data were log transformed prior to analyses to meet the assump- tion of two colors. Thus, ΔS values . jnd indicate differences tions of parametric statistics. Results are presented as means o SE. in color that are most likely not visually discernible by birds, The complete data set is provided in the online Appendix. whereas values . jnd indicate a magnitude of color differentia- tion above the visual discriminatory abilities of birds (Vorobyev RESULTS et al. , Vorobyev , Siddiqi et al. , Eaton ). Visual performance can vary between species and viewing conditions Plumage reflectance.—Our analyses revealed that antbird plum- (Vorobyev et al. ), but in general, at jnd . for threshold, two age has a relatively low level of UV reflectance. Of the plumage colors are barely distinguishable under ideal conditions; as jnd be- patches analyzed, .% reflected % of incident UV light, .% re- comes larger, two colors are more easily discernible under wors- flected %, and .% reflected % (for examples of reflectance ening viewing conditions (Siddiqi et al. ). curves, see Fig. ). Accordingly, we found that the mean percentage APRIL 2010 — HUMAN AND AVIAN PERCEPTION OF PLUMAGE DICHROMATISM — 287
reflectance per -nm bin of UV wavelengths averaged across all body regions was low (. o .%, range: .–.%), with very little variation from one region to the next (Fig. A). Only the front (i.e., chest and belly) of antbirds reflected % UV light (. o .%). Most reflectance detected in antbird plumage involved wave- lengths in the human-visible part of the spectrum. Specifically, mean percentage reflectance per -nm bin of visible wavelengths was significantly greater in the human-visible than in the UV parts of the spectrum, both across the plumage as a whole (. o .% vs. . o .%; t –., P ., n species) as well as in each of the five body regions (Fig. A). Analyzing the reflectance data by sex, we found that male plumage reflected significantly more UV than female plumage in three of the five body regions (Fig. B) and across the plumage as a whole (. o .% vs. . o .%; t –., P .). Note that, controlling for the effects of sex and body region, there was no overall effect of specimen age on plumage re- flectance in either the UV or the human-visible part of the spec- trum (GLMM: . F ., . P .). Antbird visual system.—Using the PCR forward primer SUa, we amplified base pairs (bp) of the SWS DNA se- quence in Hypocnemis peruviana, and using SUa we ampli- fied bp in Myrmeciza hemimelaena and bp in Phlegopsis nigromaculatus. The gene sequences were virtually base-pair identical. The sole difference was in the phenylalanine present in amino acid position (outside the second α-helical transmem- brane region but still amplified), which showed a synonymous substitution: cytosine in the third position in P. nigromacula- tus but thymine in the other two species. This indicates that all three species bear identical amino acid sequences for the section of the second α-helical transmembrane region in the SWS opsin (amino acid positions –). The sequence FSGFLCCIFSVFTV (with the key tuning sites , , and marked in bold) predicts
that δmax nm and, hence, that there is a violet-sensitive (VS) type of photoreceptor in the retinas of these three species. The sequences have been deposited in GenBank (accession numbers GQ–GQ). Extent of dichromatism.—We found that the maximum Eu- clidean distance in spectral reflectance between males and fe- males was significantly greater in the human-visible spectrum than in the UV spectrum for all body regions (Fig. C). In other words, a high proportion of measured dichromatism in antbirds is visible to humans. However, using retinal models, we found some evidence of cryptic dichromatism. Values of ΔS ranged from . to . jnd (just noticeable difference) among all plumage FIG. 1. Mean reflectance curves across avian-visible wavelengths, in- patches sampled, and .% ( of ) had ΔS . jnd and, cluding the ultraviolet (UV; shaded, 300–400 nm), for selected plumage thereby, met the criteria of being dichromatic to birds (Fig. D). patches in three typical antbird species: (A) Amazonian Antshrike (Tham- With the threshold discrimination set at . jnd, we found that all nophilus amazonicus), (B) Stripe-backed Antbird (Myrmorchilus strigi- species sampled are likely to be perceived as dichromatic by avian latus), and (C) Checker-throated Antwren (Epinecrophylla fulviventris). vision, including the four species in our sample that are classified Data are means derived from 5 measures per patch taken from 2–3 males as monochromatic by humans: Bicolored Antbird (Gymnopithys (gray line) and 3 females (black line) of each species; arrows mark the leucaspis), Russet Antshrike (Thamnistes anabatinus), Ocellated plumage patch from which measures were taken. The vertical distance Antbird (Phaenostictus mcleannani), and White-plumed Ant- between these lines is a measure of dichromatism. Error bars show the bird (Pithys albifrons). Only the Ocellated Antbird was classified 95% confidence intervals for reflectance values every 100 nm. Mantle as monochromatic using a more conservative threshold of . jnd feathers in A and throat feathers in B look strongly dichromatic to hu- (following Eaton ). Further, we found strong positive rela- mans; mantle feathers in C are brighter in the male but essentially mono- tionships between dichromatismavian and dichromatismhuman us- chromatic. Illustrations are reproduced and adapted from Handbook of ing both the VS (Pearson’s r ., P ., n species; Fig. ) the Birds of the World with the permission of Lynx Edicions. and UVS retinal model (r ., P .). Re-running these 288 — SEDDON ET AL. — AUK, VOL. 127
FIG. 2. Variation among body regions in (A) mean reflectance across the full spectrum (sexes pooled, averaged by 10-nm bin), (B) reflectance in the ultraviolet (UV) in each sex (averaged by 10-nm bin), (C) maximum Euclidean sex differences in reflectance (across all 10-nm bins), and (D) dichroma- tismavian ($S). In A and C, reflectance and dichromatism are compared between the UV spectrum (300–400 nm) and the human-visible spectrum (VIS; 400–700 nm), respectively, and show how both are greatest in the visible part of the spectrum. Panel B reveals that male antbirds reflect more UV light from all plumage patches than females. Panel D shows that peak dichromatismavian occurs in the head region (i.e., the area of plumage with the lowest levels of UV reflectance). Bars show means o SE; n 71 species; P 0.001 for all paired comparisons (t-tests) denoted with an asterisk (A–C). analyses using scores generated from color illustrations produced models; further work could usefully investigate this possibility. near-identical results (VS: r ., P .; UVS: r ., P However, including brightness would not change the overall con- .). Hence, dichromatism scores estimated using human vision clusion of the present study. captured a high proportion of interspecific variation in dichroma- Comparative study.—We found a strong positive relationship tism perceived by the birds themselves, regardless of whether the between dichromatism and habitat range across antbird genera us- scores were generated from specimens or color illustrations. It is ing both dichromatismhuman (slope o SE, specimens: . o ., possible that a better fit between human and avian assessments of F ., df and , P .; illustrations: . o ., F plumage dichromatism in antbirds could be achieved by includ- ., df and , P .) and dichromatismavian (. o ., ing measures of brightness or achromatic contrast in our visual F ., df and , P .). Although dichromatismhuman APRIL 2010 — HUMAN AND AVIAN PERCEPTION OF PLUMAGE DICHROMATISM — 289
FIG. 3. Relationship between human estimates of dichromatism in ant- birds scored from museum specimens (dichromatismhuman) and levels of dichromatism perceived by antbirds (dichromatismavian). The units on the y-axis are values of $S calculated from color discrimination models, as- suming a violet-sensitive (VS) visual system (r2 0.34, n 71 species). explained slightly more intergeneric variation in habitat range (r .) than dichromatismavian (r .), the slopes were very similar (Fig. ). Hence, the use of dichromatismavian did not quali- tatively change the results of this comparative analysis.
DISCUSSION
Visual physiology and plumage reflectance.—Molecular analyses of the SWS opsin gene revealed that, for all three antbird species sampled, the wavelength of maximum absorption was ~ nm. This compares with nm for the equivalent cone in humans (Dartnall et al. ). It is also similar to the values reported for FIG. 4. Relationship between dichromatism and habitat range across 71 the only other suboscine passerines previously analyzed: White- antbird species in 41 genera, using (A) human estimates of dichromatism bearded Manakin (Manacus manacus; nm) and Brown-crested (dichromatismhuman, scored from museum specimens) and (B) levels of di- Flycatcher (Myiarchus tyrannulus; nm) (Ödeen and Håstad chromatism (dichromatismavian; LOG10$S; perceived by antbirds), as cal- ). These estimates should be considered approximate because culated with color discrimination models that assume a violet-sensitive they were based on three key amino acid positions, and such cal- (VS) visual system. Slopes were similar in both cases; r2 0.29 in A and culations can misjudge extremes in spectral tuning by ≤ nm r2 0.24 in B. Data on habitat range are from Tobias and Seddon (2009a). (Ödeen et al. ). However, even if our estimates are out by o nm, our findings strongly indicate that antbirds have VS rather than UVS photoreceptors. orders; Wright and Bowmaker , Ödeen and Håstad , It is often assumed that most birds possess a UVS short-wave Håstad et al. b, Ödeen et al. ). One recent analysis of re- photoreceptor, but there is increasing evidence that the VS visual flectance data from ~, species concluded that UVS might be system is widespread. Using microspectrophotometry (e.g., Hart more prevalent in birds than molecular data suggest (Mullen and et al. ) and molecular analyses (e.g., Ödeen and Håstad ), Pohland ), but this was mainly based on the idea that plum- UVS has been directly demonstrated in bird species from age reflectance spectra can predict photoreceptor physiology, an families within only four orders (Ödeen and Håstad ; Håstad assumption that has yet to be tested. et al. a, ). By contrast, violet-sensitivity (VS) has been The UV contribution to plumage color is thought to be the demonstrated throughout the avian phylogenetic tree and ap- main source of disparity in human and avian perceptions of bird pears to be ancestral: it occurs in one family of oscine passerines coloration, and of sexual differences in particular (e.g., Hunt et al. (Corvidae) and in all suboscine passerines and nonpasserines ana- ). This may hold true even in birds with VS vision, because lyzed so far, with the exception of gulls (Laridae), parrots (Psittaci- their sensitivity to UV wavelengths remains greater than that of dae), and rheas (Rheidae) (i.e., species from families within humans. Nonetheless, we found that UV dichromatism in antbirds 290 — SEDDON ET AL. — AUK, VOL. 127
was relatively minor. A comparison between the sexes revealed that be sufficiently strong that the former serves as a valid proxy for the male antbirds reflected more incident UV wavelengths than females latter in interspecific comparative analyses. This is clearly the case (Fig. B), but levels of UV reflectance were generally low across all in antbirds, and potentially in other groups with VS visual physiol- plumage patches in all species. Overall, dichromatism was heav- ogy and variable levels of dichromatism. We are less able to extend ily biased toward the human-visible part of the spectrum: in each our conclusions to avian families in which plumage reflectance body region, mean reflectance in the UV was greatly exceeded by spectra are more complex or UV-rich, because in these cases hu- mean reflectance in human-visible wavelengths (Figs. and A). man vision performs less well in relation to bird vision (Vorobyev Low UV reflectance makes sense because antbird plumage is et al. ). Nonetheless, given that human-visible dichromatism dominated by black, brown, and rufous (Zimmer and Isler , and VS photoreceptors are widespread, our findings provide broad Tobias and Seddon a), colors that in other taxa have been support for the conclusions of previous comparative studies of the shown to reflect only modest amounts of UV (Eaton and Lanyon causes and consequences of sexual dichromatism. ). Reflectance spectra may, in turn, be linked to ecology. For example, many antbirds occur in dense vegetation, including the ACKNOWLEDGMENTS understory of tropical forest (Zimmer and Isler ), a habitat penetrated by low levels of UV light (Endler ) and generally An online Appendix to this article is available at http://caliber.uc- associated with low or moderate UV reflectance in avian plum- press.net/doi/suppl/./auk... We thank the Insti- age (Gomez and Théry , ). It is tempting to conclude that tuto Nacional de Recursos Naturales (INRENA) and the Asociación UV wavelengths are unlikely to play an important functional role para la Conservación de la Cuenca Amazónica (ACCA) for permis- in antbird plumage signals. However, absolute levels of UV reflec- sion to conduct research and collect blood samples at Centro de In- tance convey little about biological meaning, given that several vestigación y Conservación de Río Los Amigos. We are also grateful studies have shown that intraspecific interactions can be medi- to staff at the Natural History Museum, Tring, United Kingdom, ated by very minor ( %) differences in UV reflectance (e.g., Ben- for access to the specimen collection; to S. Evans for assistance with nett et al. , Andersson et al. , Siitari et al. ). Further spectrophotometry; to J. del Hoyo for permission to use the antbird behavioral experiments would be required to ascertain whether illustrations in Figure ; to D. Lamping for programming; and to O. UV reflectance has any important signaling function in antbirds. Håstad and two anonymous reviewers for comments that improved Use of human vision as a proxy for avian perception of an earlier draft. This research was supported by the Royal Society, dichromatism.—Most evidence of discordance between human the John Fell fund (Oxford University), the Swedish Research Coun- and bird perception of coloration stems from species in families cil Formas, and Stiftelsen för Zoologisk Forskning. well known to possess UVS (Andersson and Amundsen , Bennett et al. , Hunt et al. , Arnold et al. , Eaton LITERATURE CITED ). Consequently, the incidence of hidden sexual dichroma- tism in birds with VS visual physiology remains largely unknown. Andersson, M. . Sexual Selection. Princeton University Press, It is of interest, therefore, that all four species in our sample that Princeton. look monochromatic to humans had jnd scores ., the standard Andersson, M., and T. 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