Written by Rachel Garwin
A week ago, I joined my friend Teage (a Field Naturalist alum) and a group of his UVM students on an “owl prowl,” Teage’s own euphonic term for a night hike. We gathered at the edge of Centennial Woods, where gauzy tufts of white pines and bare hardwood twigs strained the clear moonlight. A wall of darkness met our eye-level gaze, while the raspy sounds of drying beech leaves in the understory added to a sense of disquiet. The primary goal for the evening was to listen for flying squirrels and call them in. Before we entered the darkness, however, we observed a requisite pre-night-hike ritual.
Since leaving a well-lit environment too soon for the darkened woods might lead to undesirable confrontations with tree trunks and branches, Teage informed us about night vision while our eyes adjusted. Human eyes require 20-30 minutes to become accustomed to low light conditions. Coincidentally, it finally gets dark about 30 minutes after the sun first sets. The students oohed appreciatively at the revealed secret of the universe.
I considered Teage’s implication, reflecting on whether twilight length was consistent enough to provide a uniform selective pressure. The period between sunset and full dark (termed “Civil Twilight”) varies with latitude and season, as it reflects the time the sun takes to drop 6° below the horizon. On the same night, civil twilight in Burlington, VT, lasted 9 minutes longer than in Bogota, Colombia, a city near the equator. Atmospheric conditions and local weather can also affect our perception of available light from the setting sun, which increases variability. Pole-to-pole variation aside, the length of civil twilight appears consistent enough at low and middle latitudes to suggest the plausibility of the relationship (though it does not prove it). What mechanisms would select for correctly timed physiological processes? Perhaps some hairy, fanged predator was involved?
Suddenly, I heard Teage ask, “Rachel, do you have anything to add before we head into the woods?”
Bits and pieces of the night hikes I used to lead rushed to mind. Out of the torrent, what would be most relevant to this group of students? Our pupils dilate to accept more light, just like a camera aperture changes size. Our eyes comprise not only lenses (e.g., the cornea) that focus light, but also a receptor structure (the retina), which translates received light into neural signals our brains can understand. The retina, in turn, is made of two types of cells: rods and cones. Rods are far more abundant (about 17 for every cone cell); however, the cones are concentrated in the center of the retina. Responsible for receiving color and fine resolution, cone cells are better suited for working in high-light environments. Rod cells cannot understand color, but they register exceptionally more light than cone cells. The operational ranges of the two cells overlap to some degree; in true twilight, both cells help parse the dim picture before us.
Instead of a long-winded physiology lesson, I settled on a practical and safety-oriented piece of advice. “If you’re having trouble finding the trail in the dark, try using your peripheral vision. Your rod cells—the receptor cells that are really good at picking up light—are arranged on the periphery of your retina, so that part of your eye sees better in low light conditions.” With that, we were off.
As the trail contoured the side of a hill, I followed it with my feet as much as with my eyes. Hard packed dirt spotted with only a few fallen leaves firmly resisted my feet, whereas my wandering steps sunk into noisy leaf litter. After a few quiet minutes, we angled from the trail and picked our way down the gentle slope. Looking out of the bottom of my eyes—as if I wore tiny slivers of half-moon spectacles—proved the best technique for avoiding the tangle of hardy ferns and woody shrubs. Teage motioned for us to stop; this would be our first attempt to call the flying squirrels.
I listened intently. Wind gusted through the upper boughs of white pines and red maples. Still-hanging, papery beech leaves rubbed together, sending bursts of unwarranted excitement running through my mind. Sirens howled close-by. One of Teage’s students fired up the iPod, and high-pitched “chip chip chip” alarm calls radiated into the night to serve as bait. I cupped my hands around my ears to exclude the droning car engines circling the outskirts of the woods. Still nothing. Perhaps the squirrels’ huge eyes, dominated by rod cells, picked us out as we muddled through their habitat.
I relaxed my eyes and marveled at the increased input from my peripheral vision. Intermediate wood ferns stood distinct from the ground; before, they had dissolved into the dimness. It seemed we had been away from bright light long enough for rhodopsin, a photopigment in the rod cells, to build up. Rhodopsin and other photopigments in the cone cells help increase light sensitivity within the receptor cells, though at different rates and degrees. Cone cells, which never develop the light sensitivity attained by rod cells, take only 5-7 minutes. Rod cells, however, may need over 30-45 minutes to achieve full light sensitivity. In the presence of too much light, these photosensitive compounds break down; enough time in dark conditions is thus needed for photopigments to build up to a functional level.
The flying squirrels proved reticent, so we walked through the underbrush back to the trail. The undergrads hesitated less between steps, and they seemed to run into fewer obstacles. After walking a circuit across Centennial Brook and back along the lower slope on the other side, we paused to make their eyes’ night adaption more explicit. Crouched beneath a closed hemlock canopy, we covered our left eyes and stared at the flame of Teage’s lighter with our right ones. I smiled, startled by the pervasiveness of the “Pirate Patch” myth. As lore would have it, pirates did not wear eye patches to cover gaping eye sockets. Instead, they kept one eye in darkness to allow them to see below and above deck without needing time for rhodopsin to develop. While no historical evidence supports this story, the folks at MythBusters put their weight behind its plausibility and likely had a role in its propagation.
Teage flicked off the lighter, and we switched our “eye patch” to the light-blinded eye. Most students commented they could see more precisely with their night-adapted eye than with the light-blinded one. Removing my hand, I winked back and forth at the hemlock boughs above. Sure enough, my left eye discerned individual twigs against the dark sky; my right eye saw only fuzzy dimness. While not proof that Blackbeard covered one eye so he could rush up to a darkened deck from a lamp-lit cabin, our experience supported the possibility.
We emerged from beneath the dense hemlock canopy onto a grassy hillside, where moon-cast shadows danced at our sides. No longer relying on peripheral vision, the students carelessly walked down the trail towards home. I smiled. An hour ago, they had hung together timidly in similar light levels, still uncomfortable with moderate darkness. Now they practically ran. While the flying squirrels had remained elusive, the students found something more powerful: the ability to stride confidently through the night.