Above image by Annie Fischinger (creative commons license)
Guest Post by Nathaniel J. Dominy, Professor of Anthropology and Biological Sciences, Dartmouth College; OpEd Project Public Voices Fellow (Full bio below this post)
Robert L. May graduated from Dartmouth College in 1926 and soon afterwards published a classic monograph of animal social behavior, Rudolph the Red-nosed Reindeer. May reported observations of reindeer disporting themselves and excluding an individual, Rudolph, for having a uniquely large, red nose. The radiance of this nose, which May described as “dazzling” in daylight and “glowing” at night, later proved advantageous on Christmas Eve, when thick Arctic fog disrupted the flight preparations of Santa Claus and his team of eight reindeer. Under these conditions, Rudolph’s nose produced sufficient light for safe aviation and the global distribution of gifts (Figure 1 below).
This account of Rudolph’s nose and its brilliance under specific atmospheric conditions is familiar to most biologists, but it is also viewed as a spurious anatomical novelty, which could explain why the sensory ecology of luminescent noses is practically unstudied.
New findings on the color vision of reindeer may elucidate the potential selective advantages of a luminescent nose. For example, it was discovered recently that the vision of Arctic reindeer (Rangifer tarandus tarandus) is sensitive to ultraviolet (UV) light, a rare trait among diurnal mammals. The ecological function of this visual sensitivity is uncertain, but it could enhance the ability of reindeer to detect objects that absorb UV light, such as predators and plant foods, against a background of snow cover, which reflects UV light to different extents. The potential fitness benefits of discriminating UV-absorbing targets could be greatest in mid-winter, when the sun is low on the horizon and high atmospheric (Rayleigh) scattering produces UV-enriched light.
Still more surprising is the unparalleled plasticity of reindeer eyes, which change color seasonally. The tapetum lucidum of the retina (the reflective tissue responsible for “eye shine”) shifts from a rich golden color during the summer months to a deep blue color during the winter months, perhaps to increase visual sensitivity to shorter (bluer) wavelengths under dim (mesopic) conditions. Taken together, these recent findings point to a specialized visual system and ecology that is strikingly suboptimal for late-December fog.
Light transmission in fog
Fog is an aggregation of suspended water droplets or ice crystals immediately above the surface of the Earth. It forms when moist air is cooled below its dew point or frost point and some of the water vapor condenses; and, by definition, human visibility is < 1000 m. Robert May described ground fog “as thick as white fizz” and near zero visibility (it was “dark and drear“), which suggests one of two possible fog types: radiation fog or ice fog. Radiation fog is produced when the ground cools the air above it by contact. Ice fog occurs when warm air interacts with extremely cold air and the water vapor sublimates, or changes directly into a solid, to form tiny ice crystals suspended in the air.
The opacity of fog is wavelength-dependent. Longer (redder) wavelengths travel farthest, but the distance varies as an inverse function of droplet size. If the droplet sizes exceed the wavelength of light, then Mie scattering extinguishes all light regardless of wavelength.
Such scatter prevails in most fog types, rendering them optically opaque, but a red luminescent nose is predicted to outshine any other kind of nose,
with greater transmission in ice fog (typical ice particles: < 10 μm) than radiation fog (typical droplet sizes: 10-20 μm). To explore the extent to which Rudolph’s nose might be optimized for vision in fog, it is necessary to know the spectral composition of the radiant light.
The color of Rudolph’s nose
A clue to the color, or chromaticity, of Rudolph’s nose emerged in Barbara Hazen’s (1958) adaptation of May’s original work. In this version, Rudolph hides behind a holly hedge so that “his bright red nose blended in with the bright red berries.” Figure 2 (below) illustrates this episode and the reflectance spectra of ripe holly fruits (Ilex aquifolium). If we assume that such spectra are representative of Arctic holly fruits, and we assume a similar radiant spectrum from Rudolph’s nose, then we can estimate radiant light with spectral peak of ca. 700 nm. Such a peak is not only supremely red, but also the maximum redness visible to most mammals.
Such conjecture on the color of Rudolph’s nose is necessarily speculative, but it does suggest a nose that is optimized for light transmission in fog, particularly in late December when reindeer eyes are sensitized to seeing blue wavelengths that attenuate rapidly in fog.
Reindeer, then, appear to have greater need for a red fog light than other mammals, which could explain why Rudolph’s nose was so effective for aviation in thick fog.
Expected increases in efficiency and safety during long-distance flight could confer fitness benefits; however, these advantages may be offset by high thermal costs. The noses of reindeer have a rich microvascular system and are therefore quite warm, a trait that prevents freezing but results in substantial heat loss. Excessive heat radiation in his glowing nose could put Rudolph at risk of hypothermia under cold weather conditions.
It is therefore imperative for children to provide high-calorie foods to replenish Rudolph’s low energetic reserves on Christmas Eve.
On balance, the selective advantages of a red luminescent nose appear to outweigh the costs, which raises questions about the frequency of the trait in the population. It may be on the rise; however, the frequency of fog is decreasing worldwide, a pattern that may offset the occasional selective advantages of a luminescent nose. Another hypothesis holds that Rudolph’s red nose is infected with nasal parasites and simply inflamed. Such contrasting hypotheses invite testing, and it is hoped that future research on the optical properties of Arctic light and fog will shed, dare I say, new light, on the extraordinary biology and sensory ecology of reindeer.
For a child-friendly version of this blog post, please see: Frontiers for Young Minds.
Filed under: satire, holiday spirit
Nathaniel Dominy is a Professor in the Departments of Anthropology and Biological Sciences at Dartmouth College. His research is focused on the functional ecology and anatomy of humans and nonhuman primates, with a specific interest in how these organisms discern, acquire, and assimilate edible tissues. The present blog post, his first, was motivated by his four-year-old daughter, Eleanor, who asked, “Why don’t all reindeer have glowing noses?”