Beier first cites Walls' 1942 paper saying that all species of bats, badgers and most smaller carnivores,
most rodents (besides squirrels), 20% of primates, and 80% of marsupials are nocturnal, and many more are active both night and
day. (Walls, G. L. 1942. The vertebrate eye and its adaptive radiation. Cranbrook Institute of Science Bulletin No. 19,
Bloomfield Hills, Michigan.) Thus it would be surprising if night lighting did not have significant effects on mammals.
Beier says. Various light levels affect animals in different ways. In general, light favors a predators' attempt to find food,
which is why so many prey animals seek the cover of darkness. So when it is dark, predators are at a disadvantage. Coyotes howl
more during the darkness of a new moon as it is harder to hunt mice then. Owls hunt more successfully in brighter conditions,
but more of their prey hides then. While in dark nights, it is harder for them to find prey, so they have to work harder to find
it. Other small species increase their activity, for it is easier for them to find their insect prey, such as bats, and some
Few studies exist that have focused on artificial night lighting on wild mammals; however, some papers do
describe the moonlight's effect on nocturnal behavior and biological clocks. Using these and other papers regarding light and
vision, Beier's paper covers likely effects of artificial lighting on mammals, such as foraging patterns changes, predation risk
increases, biological clock disruptions, road mortality increases, and disruption of dispersal movements through artificially lit
Disruption of Foraging Behavior and Increased Risk of Predation
A variety of nocturnal mammals avoids open areas in moonlit conditions. The avoidance restricts foraging activity and movements,
reduces total duration of activity, or concentrates foraging and longer movements during the darkest periods of night. Herbivores
just eat less food during moonlit nights to avoid predators. A bright artificially glowing skies at night may make every night
seem as if lit by a full Moon, perhaps resulting in animals being continually underfed. While artificial night lighting may in
the short term seem beneficial for the predators, it is not natural; it is not beneficial for the prey species and may not even
benefit the predator species in the long term. Bats seem to concentrate around streetlights, looking for an easy meal among the
insects captured around the lights. Such concentrations
could lead to faster disease transmission among predator species not to mention the faster depletion of their food source.
Disruption of Biological Clocks
Studies demonstrate that 10 to 15 minute exposures to moderately bright light, equivalent to twilight levels, can shift the
circadian clock by 1-2 hours for nocturnal animals. (Halle, S., and N. C. Stenseth (eds.). 2000. Activity patterns in small
mammals: an ecological approach (Ecological Studies, 141). Springer, Berlin.). This finding suggests that artificial night
lighting can disrupt circadian patterns in the wild. In one study, wild-caught nocturnal mice were illuminated for 15 minute
periods of moderate brightness by daylight, incandescent light, and fluorescent lights at different times in the mice's
circadian cycle. The phase of the mice's biological clocks shifted most strongly 2-3 hours after the experimental
twilight. The daylight stimulus produced a greater delay in activity (about 2.5 hours) than the two types of artificial
light (each about 1.5 hours).
The light regime and the circadian clock also influence production of some hormones, notably melatonin,
which mediates almost every physiological or behavioral rhythm in mammals (Bartness and Goldman. 1989. Mammalian pineal
melatonin: a clock for all seasons. Experientia 45:939-945). In all species, melatonin production is high at night and
suppressed during daytime, although reaction to melatonin often differs between diurnal and nocturnal species. Among its many
roles, melatonin suppresses tumor growth by regulating production and tumor use of linoleic acid. In a laboratory experiment,
Dauchy et al. (1997) (Light contamination during the dark phase in photo-periodically controlled animal rooms: effect on
tumor growth and metabolism in rats. Laboratory Animal Science 47:511-518) determined that minimal light contamination of
0.2 lux, simulating a light leak around a laboratory door during an otherwise normal dark phase, disrupted normal circadian
production of melatonin and promoted tumor growth in rats. Compared with rats experiencing a cycle of 12 hours light and 12 hours
total darkness per day, rats experiencing light contamination produced 87% less melatonin, similar to the 94% decline observed in
rats held in full light 24 hours per day. There were corresponding dramatic increases in tumor growth. Remarkably, low-intensity
light exposure during the subjective dark phase had virtually the same effect as constant light in blocking melatonin production
and stimulating tumor growth.
This clock adaption is not a surprising finding for the length of the night varies year round. Adapting to
the variation should benefit any animal. However, in winter, the nights are longer, it is colder out and there is less food
available. Those animals that do not hibernate probably need more time to find food for their survival. Lights at night,
therefore, should reduce the number of hours that they have to find food, leading to hungrier animals.
Effect of Street Lighting on Roadkill of Mammals
Researchers theorize that increased highway illumination is not effective at reducing deer-vehicle accidents in the United States,
though no studies have actively checked this out. It is thought that artificial night lighting causes difficultly for nocturnal
mammals to avoid collisions with vehicles. This could be called the deer-in-the-headlights effect. In such a moment, an
animal experiences a rapid change in brightness, such as when a car's headlights coming around a corner or when the animal steps
onto a lit highway. For nocturnal species which only use rod cells in their eyes to see, such a sudden change in illumination,
saturates their retinas rendering the animal instantly blind. Literally, they cannot see where to go to avoid getting hit
by a car, so they stand frozen in the way, uncertain of what to do and then unfortunately get hit. Although many nocturnal
mammals do have a rudimentary cone system and can switch over to it, the switching time can take a few seconds to do so, during
which they are temporarily blind. In that time, a car could have hit them. Once they do switch to the cone system, the
dark areas outside the light now become so black, so they will be unable to see into them, and the animal may become lost
or disoriented. They may even be unwilling to return to the new shadowy regions from where it came. Once they do venture into
the dark areas, it will take them 10 to 40 minutes before their rod cells can function as effectively as before and their
night vision fully returns.
So, if you are a driver at night, in such a situation, and you have stopped your car without hurting the animal,
I would suggest that you then turn your headlights off, while leaving your parking lights on. The animals are now effectively night
blind. Your parking lights, however, should enable them to use enough of their cone vision to see into the road sides to travel
there and no longer be completely blind.
Disruption of Dispersal Movements and Corridor Use
Moreover, he says, lighting can be very disorienting for animals that are trying to move at night. So wildlife corridors
can be compromised by even a single light and so prevent animals from moving to crucial landscapes. In 1995, Beier found that
pumas avoid brightly lit areas when traveling at night and move toward the darkest horizon. He noted instances in which a puma,
exploring new habitat for the first time, stopped during the night at a lit highway which crossed its path of travel. For the now
night blind animal, the terrain beyond the highway is dark.
In several instances, the animal would bed down until dawn, selecting a location where it could see the terrain
beyond the highway after sunrise. The next evening, the puma would attempt to cross the road if wild land lay beyond or would turn
back if industrial land lay beyond. (Beier, P. 1995. Dispersal of juvenile cougars in fragmented habitat. Journal of Wildlife
This wait it out approach just so the animal can see, obviously means that the puma has less time to hunt and
so goes hungry, which decreases its chance for survival. The co-editor of the book Ecological Consequences of Artificial Night
Lighting and co-director of the Urban Wildlands Group, Travis Longcore says that "If the corridors aren't dark, the animals they're
protected for aren't going to use them."
Artificial light puts ecosystem services of frugivorous bats at risk.
Nocturnal bats have only rods cells in their retinas. Bats are not truly blind (they do have eyes), but are
actually blinded by bright lights. Hence they are highly sensitive to lights.
Each year, large swaths of rainforest are cleared and converted to farm land. Roads with streetlights are
then built to connect these farms to cities to move their crop yields. However, rainforest is a poor soil to grow crops in and
when their soil fertility fails and it becomes uneconomical to grow crops there. So the land is often abandoned, but the roads
and the streetlights remain.
Lewanzik & Voigt found that the streetlights repel the seed dispersing, fruit-eating bats. While the bats
rank second only to birds in seed-spreading, no other animal does so out into the open lands of a destroyed forest. Bats have
less to fear from predators when flying out in the open lands at night. So these streetlights not only repel the bats but also
inhibit the regrowth of the rainforest.
Thus, the artificial light pollution can retard and inhibit the regeneration of one of our planetís largest
"sinks" for the greenhouse gas CO2 sinks, a tropical rainforests, by it disrupting the behavior of the fruit-eating bats
that help rainforest plants re-colonize the land with their "seed-rain".