Iris Pigmentation and Comparative Psychology:
An Annotated Bibliography
Morgan Worthy
July 2, 2009
Introduction
Eye color is a polygenic trait that is determined, at least in part, by the type and amount of pigment found in the iris of the eye. The two main pigments in the iris are pheomelanin (yellow to red) and eumelanin (brown to black). Blue eyes result from small granules of pigment that scatter light.
Eye-darkness as a research variable, unlike race, strain, subspecies, has the advantage of retaining similar meaning across species. I began the present line of research (with Alan Markle) with archival studies that compared athletic performance of black and white American athletes. Those studies were published in the Journal of Personality and Social Psychology in 1970. Soon after that I switched my own focus to eye color rather than race and after 1975 studied only animals.
Various findings have supported my suggestion (Worthy, 1974,1999) that dark eyes are associated with speed or quick reactions. Independent laboratory studies with humans have confirmed a relationship between dark eyes and quick reactions. Dark-eyed animals are good at “moving without waiting”(e.g. immediate, direct pursuit hunting or active escape); light-eyed animals are better on more deliberate tasks or “waiting without moving” (e.g. lying-in-wait to ambush or using passive escape). These differences, in turn, may help to explain such things as differences in habitat preferences.
A number of the following references deal with the basic hypothesis relating eye-darkness to speed and quickness. Multiple studies have also found that differences in eye-darkness are associated with differences in hearing and hypertension.
No effort is made here to deal with children’s behavior, single-study findings, or to document all the behaviors that apparently are not related to eye darkness.
Unfortunately, too much of the eye color research has dealt with humans and not enough with animals. Nonetheless, taken as a whole, the following findings suggest that eye-darkness can be an important variable in comparative psychology.
Craig, A.J.F.K. and Hulley, P.E. (2004) Iris colour in passerine birds: Why be bright-eyed? South African Journal of Science, 100, 584-588.
Eye colors for 1143 species of passerine birds were classified as Pale, Red or Brown. These species eye colors were then tested for correlation with various other species traits within each of ten family-level groups. Most of the significant findings (29 of 36) were in the five families with most diverse eye colors. A flavor of the many findings can be given with two quotations that deal with feeding behavior.
Corvini (crows): “A coloured iris [Pale or Red] is present in close to 50% of the woodland and forest corvids, but is exceptional (one species) in birds of open grassland and thus the association of a coloured iris with feeding in vegetation and a dark iris with ground feeding is clearly closely linked to habitat preferences.” (p. 586)
“The broad feeding categories used in our analysis are not an adequate test of Worthy’s proposal. However, a dark iris was correlated with ground feeding in three groups (Corvini, Malaconotinae and Ploceidae), whereas the ground-feeding Sturnini were mostly pale-eyed.” (p. 587)
This study demonstrated the value of including a measure of eye darkness when comparing species within a family of birds with diverse eye colors. The authors conclude, “We hope that this overview will promote the inclusion of iris coloration as a topic for investigation in future studies.” (p. 588)
Cressswell, W. (1996) Surprise as a winter hunting strategy in Sparrowhawks, Accipiter nisus, Peregrines, Falco peregrinus and Merlins, F. columbarius. Ibis, 138, 684-692.
This study was not done with eye color in mind, but it provides support for the idea that yellow-eyed predators (in this case Sparrowhawks) use surprise as a strategy more than do darker-eyed predators (in this case Peregrines and Merlins) to catch the same prey. Surprise is also a common hunting strategy of the most yellow-eyed large family of birds, herons, and the most yellow-eyed large family of mammals, cats. For surprise hunting (ambush) to be successful the predator must wait until it is advantageous to launch a sudden attack.
[Predators that do not immediately attack, but rather wait to ambush elusive prey are very clever at using available cover to gain the element of surprise. Environmental cover is an asset for animals that hunt by means of ambush or use passive means of escape from predators. By contrast, for animals that are quick to engage in immediate direct-pursuit hunting or quick to escape from predators by immediately fleeing, an open environment is probably an asset. A study of habitat descriptions for all species of birds of India found that significantly more light-eyed species than dark-eyed species were described as living in a “dense” habitat (p.<0.02; Worthy, 1999, pp. 78-80). See, also, the quotations from Craig and Hulley (2004) above about habitat differences related to eye color].
Da Costa, E. A., Castro, J. C. and Macedo, M.E. (2008) Iris pigmentation and susceptibility to noise-induced hearing loss. International Journal of Audiology, 47(3): 115-118.
This study, done in Brazil, tested 2407 noise-exposed workers for hearing loss. Light-eyed workers had more hearing loss than did dark-eyed workers. This replicated earlier studies that found the same effect in high-noise textile and aviation settings.
[Pigmentation in the inner ear serves a protective function. Amount of melanin in the inner ear may be correlated with amount of melanin in the iris].
Friedman, G.D., Selby, J.V., Quesenberry, C.P., Newman, B. and King, M.C. (1990) Eye color and hypertension. Medical Hypotheses. 33(3), 201-206.
“Compared to persons with lighter eye color, those with brown eyes were more prone to develop hypertension, with relative risk of 1.5 (95% confidence interval 1.18-1.96) compared to all persons with nonbrown eyes. The association persisted after control for race, sex, body mass index, alcohol use, educational level, parental history of hypertension, and among whites, for ethnic origin as crudely estimated by last name.” From the abstract.
Hale, B.D., Landers, D.M., Snyder-Bauer, R. and Goggin, N. (1980) Iris pigmentation and fractionated reaction and reflex time. Biological Psychology, 10, 57-67.
My hypothesis that dark eyes are associated with the ability to make immediate, quick reactions grew out of observations and archival studies of athletic performance. Landers and his colleagues at Penn State University brought that hypothesis into a laboratory setting in which simple reaction time could be carefully measured. In a series of well-controlled studies, they demonstrated that dark eyes in humans, independent of race, are associated with the ability to make quick reactions to visual and auditory stimuli. They also provided evidence that this effect is mediated by the Central Nervous System.
Because this particular experiment was the last in a series of seven studies, they then did a meta-analysis which was reported as follows: “Thus, the findings across studies have consistently shown that dark-eyed subjects have shorter premotor time and simple RT [reaction time] latencies than light-eyed subjects. Considering that Worthy’s hypothesis has been experimentally tested seven times with seven different samples (Landers et al., 1976, Experiment 2; Landers et al., 1977; Wolf and Landers, 1978; Post, 1977; Snyder-Bauer, 1979; Telford et al., 1978; and Experiment 1 of this study), a combined probability value would more accurately reflect the reliability of the eye color phenomenon. Using a z-transformation procedure suggested by Mosteller and Bush (1954), a z value was obtained that could not occur by chance any more than one time in 10 million (table 2). Worthy’s hypothesis, therefore, reliably predicts RT differences between eye color groups from one study to the next.” p. 61.
[Darker eyes may be correlated with a greater amount or distribution of neuromelanin that facilitates the activity of CNS neurotransmitters.]
Hiller, R. (1982) Race, iris pigmentation, and intraocular pressure. American Journal of Epidemiology, 115(5), 674-683.
Abstract: “The association of intraocular pressure with age, sex, race, iris pigmentation, systemic blood pressure, and family income was evaluated using data from the Health and Nutrition Examination Survey of 1971-1972. In general, mean intraocular pressure was highest for blacks with brown irides and progressively lower for whites with brown irides, whites with neither brown nor blue irides, and whites with blue irides. Multilinear regression analysis showed positive associations of intraocular pressure with systolic blood pressure (p < 0.001), age (p < 0.001) and amount of iris pigmentation (p < 0.001). The association with iris pigmentation held for both a combined race/iris color variable and for iris color among white persons. When race rather than iris pigmentation was used in the regression equation, it was a weaker (p < 0.03) but still significant risk factor for higher levels of intraocular pressure. Intraocular pressure was negatively associated with family income (p < 0.004). Despite the significant associations, the proportion of variance in intraocular pressure that was explained by these variables was small (R2 = 0.06).
Johnsgard, P.A. (1988) North American owls, Washington: Smithsonian Institution Press.
On page 55 this observation is made, ”It is of interest that nearly all the species of owls having dark brown iris coloration, virtually invisible in the dark, are highly nocturnal owls.” Dark eyes being “virtually invisible in the dark” is probably not the key to this difference in behavior related to eye color. Cech, et al. (2001) report, “Nocturnal owls have a large number of hearing receptors in their brains (more than 47,000 by one count), whereas diurnal owls, such as the Little Owl (Athene noctua) of Eurasia, have fewer (11,000).” p.34 Apparently, with hearing keen enough, dark-eyed owls can fly about in total darkness listening for prey without having to use the stealth employed by light-eyed owls that rely on both vision and hearing to locate prey and must guard against being seen by the prey. (At this point, there is only indirect evidence for a relationship between dark eyes and keen hearing in owls.)
This apparent difference in owls, related to eye darkness, of what senses are used to locate prey and when they hunt may be part of a larger pattern involving other animals as well. At least as a hypothesis, I would suggest that lighter-eyed predators rely more on vision to locate prey than do their close relatives of comparatively darker eye colors. The type differences I have in mind include those seen in (a) typical herons vs. the Boat-billed Heron, and (b) plovers vs. sandpipers. Warnock and Warnock (2001) say of the last pair, “Many scolopacids [sandpipers] have bills with tactile and chemo-sensitive nerve receptors at the tips that allow them to locate prey through touch, smell, and pressure gradients. The use of these senses opens up feeding options unavailable to the more exclusively visual plovers. Species with these sensory adaptations are as active at night as they are during the day.” p.279 [References: Cech, et al., and Warnock and Warnock quotations are from Elphick, D., Dunning, J.B., Jr. and Sibley, D.A. (Eds.)(2001) The Sibley guide to bird life and behavior. New York: Alfred A. Knopf.]
Kleinstein, R.N., Seitz, M.R., Barton, T.E. and Smith, C.R. (1984) Iris color and hearing loss. Am. J. Optom. Physiol. Opt., 61(3), 145-149.
Textile workers who work in low noise areas have no differences in hearing loss related to eye color. However, among those workers who work in high-noise areas, light-eyed workers experience more hearing loss than do dark-eyed workers.
Landers, D.M., Obermeir, G.E., and Patterson, A.H. (1976) Iris pigmentation and reactive motor performance. Journal of Motor Behavior, 8, 171-179.
Simple reaction time was found to be related to eye color of Caucasian college students. Of three eye color groups, the quickest were the dark brown-eyed students, and next quickest were the light brown-eyed students. Slowest were the blue-eyed students. The effect was found for both visual and auditory stimuli.
Petterson, R. (1964) Silently by night. New York: McGraw-Hill.
On page 111, the author makes this observation--“The eyes [of fruit bats as compared to insectivorous bats] are usually quite large and often light brown rather than dark umber or black: upon occasion even they are a rich amber. ” This is an interesting observation because it is similar to what is seen in birds. Birds that eat fruit have a wide range of eye colors that, on average, are close to the midrange between yellow and black (e.g. red or light brown). By contrast, birds that feed in-the-open and on-the-wing in a manner similar to insectivorous bats (swifts, swallows, and nightjars) have eye colors that are mostly dark brown or black. A systematic comparison [Worthy (2000) pp 2-3] found that families of birds identified as “aerial feeders” have significantly darker eyes than do families of birds identified as fruit eaters.
Thomas, G.B., Williams, C.E. and Hoger, N.G. (1981) Some non-auditory correlates of the hearing threshold levels of an aviation noise-exposed population. Aviation, Space, & Environmental Medicine, 52(9), 531-536.
Abstract: “Performed retrospective analyses on data gathered from the 1963 follow-up of the US Navy’s Thousand Aviator Study. Ninety-seven Ss were identified as having normal hearing and 104 as having impaired hearing. All were male with a mean age of 47 years. The only successful predictors of hearing impairment were eye color and amount of smoking. Thirty-one other physical, psychological, and sociological measures failed to appear differentially in the two groups.”
In this study, as in similar studies, the finding was that light-eyed people suffered more noise-induced hearing loss than did dark-eyed people.
Wolf, M.D. and Landers, D.M. (1978) Eye color and reactivity in motor behavior. In: Landers, D.M. and Cristina, R.W. (Eds.) Psychology of motor behavior and sport. Champaign, IL: Human Kinetics Publishers, 255-263.
Abstract: “There were 20 subjects within each of the light-eyed white, dark-eyed white and dark-eyed black groups. Each subject performed on a knee-reflex time and simple response-time apparatus. Skin color was assessed to determine if it was a factor affecting the behavioral measures. Dark-eyed superiority was found across the races on the reaction time component (p < .00l) and on total response time (p < .02). Movement time, reflex time, and skin color effects were all nonsignificant. The eye-color phenomenon was therefore limited to the reaction-time component and probably has a central nervous system locus.”
Worthy, M. (1978) Eye color, size and quick-versus-deliberate behavior of birds. Perceptual and Motor Skills, 47, 60-62.
This study was based on 3264 species of birds. A simple 2-point scale of eye darkness was used. Species with only “brown,” “dark brown” or “black” eyes were rated as dark-eyed; all other species (including those with “light brown” eyes) were rated as light-eyed. The eye darkness measure for families was simply the percentage of dark-eyed species. The families were rank ordered from herons with 0% dark-eyed to hummingbirds, 100% dark-eyed. Content analyses of brief family descriptions in an ornithology textbook [Van Tyne, J. and Breger, A.J. (1965) Fundamentals of ornithology. New York: Wiley] were used to identify behaviors related to eye darkness at a statistically significant level. Results: Families mentioned as having “fast” flight were darker-eyed than families mentioned as having “slow” flight. Families identified as feeding on-the-wing were darker-eyed than other families. The families mentioned as using passive defense were lighter-eyed than other families.
A second study, reported in the same article, controlled for experimenter bias and for differences in physical size. (Light-eyed birds tend to be larger than are dark-eyed birds.) Ornithologists were asked, by mail, to make (blind) ratings on a 7-point scale of quick vs. deliberate behavior for flight, feeding and escape of taxonomic families of birds with 30 or more species in the population. Included in the study were all families for which at least 15 ornithologists provided ratings--a total of 34 families. Eye color information was available for 12 or more species in each of these families. Percentages of dark-eyed species in each family were converted to arcsine values and a measure of physical length was converted to logarithmic values. The corrected split-half reliability coefficients were .96 for the eye-darkness measure and above .90 for ornithologists’ pooled quick-versus-deliberate ratings for each of the three areas of behavior. Eye-darkness correlated -.58 with size (p < 0.001). With size controlled, eye darkness correlated with rated quickness in flight at .55 (p < 0.001), in feeding habits at .44 (p < 0.01), and in escape habits at .52 (p < 0.01).
[Using a simple 2-point scale served to establish the fact that eye color is, indeed, related to differences in behavior between families of birds. However, 2-point and 3-point scales of eye darkness, while having the advantage of good face validity, fail to discriminate well at either end of the scale. For some behavioral differences to come into clear focus, “yellow” needs to be scored differently from “red” and likewise at the other end of the scale, “dark brown/black” needs to be scored differently from “brown.”]
Worthy, M. (1999) Eye color: A key to human and animal behavior. Lincoln, Nebraska: Iuniverse.
This is a re-issue of a book which had been in print for only a few months in 1974 when the publisher went out of business. The book was then out-of-print and unavailable for 25 years. Most of the book consists of archival studies presented in support of the idea that dark eyes are associated with quick reactions and light eyes are associated with hesitation, inhibition, and tasks that are self-paced and do not require immediate, quick reactions.
For example, one archival study involved published eye colors for 204 breeds of domestic dog. Whereas only 23% of other breeds were light-eyed, 73% of the 33 breeds listed in the category, “Pointers and Setters”, were light-eyed (p < 0.001). Because pointers and setters were bred to freeze and wait when prey is first sensed, these breeds of dog, similar to cats or herons, illustrate an association between light eyes and deliberate behavior as seen in animals that ambush or stalk prey. By contrast, terriers, similar to weasels, are almost all dark-eyed and hunt by means of immediate and fearless direct-pursuit.
Differences in deliberate vs. quick motor responses of animals are viewed as relevant to other differences such as habitat. In addition to motor responses, various more tentative observations and findings related to humans and animals in other areas are reviewed. Suggestions are offered of how eye darkness may relate to areas such as physiology, perception, sociability and disease.
A short Epilogue was added in 1999 that brought two things up-to-date. The first was that predators that ambush or stalk prey are not just light-eyed, but to be more specific, their eyes tend to be yellowish in color. (It has to be remembered, though, that many yellow-eyed animals are not predators of any kind.)
The other update was to go beyond thinking of just two types of tasks relevant to deliberate vs. quick behavior. Tasks can be thought of as having different time demands such that they fall into three categories. Those are (1) Must Wait tasks, (2) Can Wait (self-paced) tasks, and (3) Must Not Wait tasks. With Must Wait and Must Not Wait tasks, time is of the essence and it is on those tasks that eye color is most predictive. Animals that specialize in Must Wait tasks tend to be light-eyed. Animals that specialize in Must Not Wait tasks tend to be dark-eyed. Animals of any eye color may specialize in Can Wait tasks and thus averaged eye colors for such specialists (e.g., fruit-eaters or other herbivores) will often fall near the midrange of eye darkness.
The five point scale of eye darkness that was used in 1974 was later revised (Worthy, 2000).
Worthy, M. (2000) Animal eye colors: yellow-eyed stalkers, red-eyed skulkers & black-eyed speedsters. Lincoln, Nebraska: Authors Choice Press.
This is intended primarily as a source book for people doing animal research. The bulk of the text is a database of published eye colors for 4918 species of birds, 342 species of reptiles, 181 species of amphibians and 179 species of mammals. [My wife, Linda H. (Polly) Worthy, agreed to repeated yearly vacations near Washington, D.C. so we could spend part of our time in the many libraries there recording iris colors.]
Various systems can be used to convert these species eye colors to quantitative measures of eye-darkness. The simplest are to use a two-point scale (Worthy, 1978) or three categories (Craig and Hulley, 2004). However, there are some advantages to using a four- or five-point scale that makes finer discriminations. A five-point, revised, scale is described and used to give a score to each of the 5620 species in the database. The colors, Yellow, Orange, Red, Brown and Black were treated as progressively darker and given equally-spaced scores ranging from 0.0 for Yellow to 1.0 for Black. Color definitions and subjective judgment were used to place all other colors at one of the five levels, 0.0, .25, .50, .75, or 1.0. One rule I used was to always score in terms of the lightest color recorded for each species. Another was to move down one level if the color was preceded by “light” or “pale” and move up one step if the color was preceded by “dark.” For example, “brown” is scored as .75, “light brown” as .50 and “dark brown” as 1.0. The species scores were also averaged to give a mean eye-darkness score for each taxonomic family, order, and class. These quantitative scores make it easy to add eye darkness as a variable to multivariate analyses.
Anyone who wants to use this database can (a) choose to use the quantitative scores provided; (b) just use the color information as such; (c) modify the scoring system described; or (d) devise some other system suited to the research being done.
The strength of this database is the number of species of birds for which information is provided; the sample is much smaller in the other three classes and probably less representative as well--certainly so for mammals because large diurnal mammals are over-represented. Another weakness of the database is that it gives no information about regions, subspecies, age or sex differences. Only adult iris colors were recorded. If male and female differed, both were recorded. However, identification of iris color by sex was not retained. Neither the published source nor region were recorded. Once a particular color was recorded for a species, additional mentions of that same color by other sources were not recorded.
One new study was included in this publication that illustrates the use of these eye-darkness scores. The 1993 book, Birds of the World by Colin Harrison and Alan Goldsmith, lists families by feeding habits. Nine families were listed as “Aerial Feeders” and 20 families were listed as “Fruit-Eaters.” One family was on both lists and was excluded from the comparison of the two groups. The median eye-darkness was .84 for the eight remaining families of Aerial Feeders and .51 for the 19 remaining families of Fruit Eaters. The nonparametric Mann-Whitney U Test indicated that this was a significant difference (p < 0.002). The study compared aggregate family scores based on a total of 1463 species. For this purpose, the quantitative scores proved to have more than adequate reliability.
If the two group medians in the above study are converted back to eye colors, the typical Aerial Feeders have dark-brown eyes and the typical Fruit Eaters have red or light-brown eyes.
[A statistically significant relationship between dark eyes and on-the-wing feeding in birds was first found in a 1974 study of different species of raptors that eat insects (see Worthy [1999] p. 39). The present study found the same relationship by comparing families of birds. This relationship, then, is found at more than one level of analysis and across taxonomic orders of birds. Including bats, the relationship appears to hold across taxonomic classes as well.]
Worthy, M. and Markle, A. (1970) Racial differences in reactive versus self-paced sports activities. Journal of Personality and Social Psychology, 16, 439-443.
“The hypothesis is advanced that black athletes perform better, relative to white athletes, in sports activities that are reactive in nature than in sports activities that are self-paced in nature. Evidence in support of this hypothesis is presented from the fields of professional baseball, professional basketball, and college basketball.” [From the abstract]
Pitching in baseball and free throws in basketball are self-paced and do not rely heavily on speed or quickness. White athletes, compared to black athletes, were shown to do relatively better on such tasks than on tasks that benefit from speed and quick reactions.
[Those comparisons were made 40 years ago. More recently, I looked at Who’s Who in Baseball, 2005, (McLean, 2005) which listed all current major league baseball players. Whereas only 5 % of the American-born pitchers were black, 40% of the American-born outfielders were black (Chi-square p. < .001). Sports positions with a high percentage of black athletes (e.g. outfielder in baseball and cornerback in American football) seem to require, among other things, natural speed and quickness.]
July 12, 2006
An Annotated Bibliography
