"What on Earth?"
By Ray Barbehenn
Raccoons are among a handful of mammals that remain common, even as houses are built in their habitats. Raccoons are common, but they are not often seen. They venture through "our" land mainly at night, as do white-tailed deer, opossums, deer mice, black bears, and skunks. The success of raccoons is also due to the wide range of food that they eat – even including our garbage.
Suzy Oliver was lucky to see the raccoon in the above photo while it was still foraging at about 9:00 in the morning. It was wading in a marshy area in Quivira National Wildlife Refuge in Kansas on Sept. 2, 2023. It could have been hunting for any number of things, including frogs, aquatic insects, fish, and crayfish. More often, raccoons are spotted at night in our headlights or with a flashlight. Their eyes appear to be glowing brightly back at us. This same phenomenon can be seen in cats, deer, and other animals that are partly or entirely nocturnal.
Why do the eyes of Raccoons appear to glow at night when light shines on them?
A. They have a mirror-like backing in their eyes.
B. They are releasing excess heat as infra-red light.
C. They have enlarged eyes for night vision, which reflect a large amount of light from their surfaces.
D. All of the above.
The answer is at the bottom of the page!
Fun Facts and Musings
Raccoons look a bit like little bears. And, they do have some strong anatomical resemblances to bears, especially in their skulls: similar skull shapes and a set of all-purpose teeth. Even the "Father of Taxonomy," Carl Linnaeus, categorized raccoons and bears together in the mid-1700s. However, taxonomists now place Raccoons and bears in separate family groups based on differences in their DNA. The only close relatives of Raccoons are in Central and South America, including Coatis, Kinkajous, and Ringtails.
Raccoons live from Central America through the entire U.S. and into southern Canada. They do well in a wide range of habitats – from mountains to marshes to urban landscapes. Therefore, what Raccoons eat varies greatly with their geographical area, habitat, and the season. If given a choice, they prefer sweet fruits, such as persimmons. In our area, the rapid spread of pawpaws is also aided by their sweet tooths. Of course, persimmons and pawpaws are only on a short, seasonal menu, and raccoons usually end up picking from a wide variety of other items. In addition to the aquatic species mentioned above, they seek out berries, insects, acorns, worms, bird eggs, snails, and even small vertebrates, such as mice, salamanders, and snakes.
Raccoons are famous for their amusing behavior of "washing" their food. Rather than cleaning their food, they are believed to be improving their abilities to feel and manipulate it with their paws. Since they do not have opposable thumbs for grasping food with one paw, they have to hold their food with pressure between both of their front feet. Perhaps wetting their feet makes their tough pads softer and more sensitive.
The eyes of raccoons, and many other animals that are partly or entirely nocturnal, are adapted to increase their sensitivity in the dark. Two key structures in their eyes that enhance their light-gathering efficiency are the retina and tapetum. The retina is near the top of my list of "Most Amazing Biological Structures." This is the thin (0.5 mm) layer of cells that sits like a tiny radar dish at the back of the eyes of vertebrate animals. What is most amazing about the retina is its layer of specialized cells ("rods" and "cones") that transform photons of light into electrical signals. This transformation is the key step for being able to see, since only electrical impulses can travel through nerves to the brain. The retinas of nocturnal animals contain a large fraction of the highly sensitive rod cells. While these cells are more sensitive to light than the cone cells, they do not allow for color vision, as the cone cells do. You will know when you are seeing mainly with your rods when it gets dark out and you are no longer able to see colors!
Good night vision depends not only on having an abundance of rods in the retina, but also a reflective tapetum. This is a thin layer just behind the retina that acts like a mirror. The tapetum reflects light that has passed through the retina back to the retina again, boosting the amount of light that is available for night vision!
How can the tapetum, a biological structure, act like a mirror? Different animals use different chemicals to make the tapetum reflective, including crystals of zinc and/or vitamin B2. There may be up to 20 sheets of cells in a tapetum, each enriched with these reflective crystals.
Why don't we have a tapetum? Wouldn't it be great to see well at night? As usual, the answer is, "It's a trade-off." The features that enhance night vision detract from daylight vision. For example, if we had more rods for night vision we would have less room for cones in our retinas, producing worse daytime acuity and worse color vision. It might also be uncomfortably bright in the daytime – like walking outside after an eye exam!
Of course, humans have created technologies that allow us to see at night: infra-red imaging and light intensification. These technologies are based on detecting heat (infra-red light) or amplifying low levels of light, respectively. Interestingly, these technologies were not inspired by the mechanisms in the eyes of nocturnal animals; animals cannot see infra-red light, and night-vision cameras do not have a structure like a tapetum behind the imaging screen.
Two diseases carried by raccoons should be of some concern to hikers (though nothing like the concerns about Lyme disease): Raccoon rabies and raccoon roundworm. Rabies is a virus that can be transmitted in the saliva of raccoons if they bite a person or another animal. Rabies has become common in raccoons, accounting for over 60% of animals that test positive for rabies in the eastern United States. This is a serious disease, and people who are bitten need prompt medical attention before symptoms develop. 60,000 people in the U.S. receive treatment each year after being bitten by an animal (often a raccoon). Because of this cautious approach, fewer than 10 people die from the disease each year.
Raccoon roundworm is a nematode that is present in Raccoon droppings and the surrounding soil. Significant contamination of the soil is likely around raccoon "latrines" (spots where Raccoons mark their territories). The ingestion of the tiny nematodes or their eggs can lead to life-altering symptoms and fatalities. Although this remains a rare disease, certain behaviors can greatly increase the likelihood of contracting it, such as the habit of some small children of putting soil in their mouths. And, if you are one of those unusual people who have accidentally eaten ripe persimmons off the ground near a raccoon latrine, it may not be such a rare disease for you! (Talking about myself here. I am also reconsidering eating pawpaws off the ground without a thorough washing.)
Answer: A!
Send your photos and ideas for topics to Ray at rvb@umich.edu. Thanks to Lisa Frehill, who sent amusing photos of Racoons searching a garbage can.
