Grey Fox

The gray fox (Urocyon cinereoargenteus) is a solitary, nocturnal canine found often in our lovely deciduous forests. They have an extensive range throughout North America (below Canada) and the northern cap of South America in Venezuela and Colombia. They used to be more common in the Eastern United States, but human influence has allowed them to dominate across a greater area until they have spread from coast to coast.

A grey fox in Red Rock Canyon, Nevada -- James Phelps 2004

A grey fox in Red Rock Canyon, Nevada — James Phelps 2004

There are 16 recognized subspecies of gray fox, 7 of which are North of Mexico. The grey fox and channel island fox (Urocyon littoralis) are the only living members of their genus. Genetically, they are similar to the tanuki (Nyctereutes procyonoides) of Asia and the bat-eared fox (Otocyon megalotis) of Africa. The tanuki is the only other kind of canine that can climb well.

The grey fox appeared during the mid-Pliocene, 3.6 million years ago (mya). They likely migrated into Northeastern US in the post Pleistocene epoch during the Medieval Climate Anomaly warming period from  AD 950 to 1250, which was followed by the Little Ice Age from roughly 1350 to 1850. The evolution of the grey fox, and other canines, is fascinating, but it is detailed and could be tedious to read, so I plan to touch on it in another post.

Adult male and female grey foxes -- Apterex 2009

Adult male and female grey foxes — Apterex 2009

Grey foxes live to be around 8-10 years old in the wild. They are around 76-113cm in length, and weigh around 3.6-7kg. Their coloration is red on their face and legs, with a grey dorsum (back) and a ventrum (belly) that can range between white and orange-red. Their tails are around 28-44cm long and exhibit a black dorsal stripe, along with a continuation of the same red and grey. They lack the black stockings of the red fox (Vulpes vulpes). They molt once a year between early Summer and Autumn. Their skull is fairly distinct, as their temporal ridges (upraised lines of bone on the top of the skull) are widely separate, forming a U-shape. They have little sexual dimorphism (visible differences between sexes), other than females being somewhat smaller than the males. They are also unique in having oval, rather than slit, pupils, unlike the genus Vulpes.

A black and white house cat (Felis catus) showing yellow eyeshine -- Una Smith 2008

A black and white house cat (Felis catus) showing yellow eyeshine — Una Smith 2008

They have a tapetum lucidum, which helps them see well during their nocturnal activities. The tapetum lucidum is a highly reflective membrane just behind the retina (the back of the eye with the cells that actually sense light). The tapetum lucidum reflects light back onto the retina, and more light means better vision at night. You might be familiar seeing this in a dog or cat at night, when their eyes seem to glow different colors, like green, yellow or red.

An illustration of The Fox And The Grapes illustrated by Milo Winter, from The AEsop for Children by Project Gutenberg, 1919

An illustration of The Fox And The Grapes illustrated by Milo Winter, from The AEsop for Children by Project Gutenberg, 1919

They are an opportunistic omnivore. They hunt small mammals, like rabbits, small rodents, lizards, and birds, but they also eat a lot of wild plants. They particularly enjoy fruits when they are available, eating acorns, berries, grapes, and much more. In some areas, they can be primarily insectivorous and herbivorous. If they manage to collect more food than it can eat in one sitting, it might store the excess and mark the spot with their urine in order to find it again later. Their ability to climb helps them reach food in the trees that might otherwise only be available to birds and smaller animals. This reminds me of the Aesop’s fable “The Fox and the Grapes”. I will paraphrase it here:

A fox traveling along notices some grapes hanging high from a tree. Quite hungry, he leaps to catch the grapes, but as high as he jumps, he can’t reach them. Huffing, he remarks, “Those grapes look sour anyway.”

The fox in the fable was probably not a grey fox, since they would be quite able to climb up to eat the grapes. I suppose the new moral for this story would go to show that we should use our unique strengths to make the best of a situation, rather than to get irritated and give up.

A pair of grey foxes at a golf course near San Francisquito Creek in Santa Clara County -- Bill Leikam 2011

A pair of grey foxes at a golf course near San Francisquito Creek in Santa Clara County — Bill Leikam 2011

Grey foxes excel at climbing trees thanks to their particular anatomy. In some places, they are known as the “Tree Fox” for this reason. Their claws are strong and curved at the ends, so they can get a good hold into bark. They have been witnessed climbing up a tree at a nearly vertical angle! Their legs are shorter than the red fox, allowing it to climb better. The trade-off is that red foxes are able to run more quickly. They can descend the tree by either walking down it backwards, or running down head-first if the tree is slanted. They are also able to jump between branches. They are so good at climbing that they will often make dens up in a hollow tree, or perhaps in a hole someone else has dug.

A fur bag made of the grey fox -- Kuerschner 2011

A fur bag made of the grey fox — Kuerschner 2011

The grey fox has been hunted historically, and is still harvested today, for their beautiful fur, like many other creatures. Fortunately, this has not affected their populations greatly. They are listed as a species of Least Concern. Apparently, early European settlers (and fox hunters) who came to North America didn’t like that the grey fox preferred to climb up a tree or into a hole instead of leading a long chase like the red fox. The settlers introduced their red foxes into some of their colonies for a better hunting experience.

A red fox snarling at a grey fox on the San Joaquin National Wildlife Refuge, CA -- USFWS Pacific Soutwest Region 2013

A red fox snarling at a grey fox on the San Joaquin National Wildlife Refuge, CA — USFWS Pacific Soutwest Region 2013

This introduction has led to red foxes being common in the eastern United States. At first, the two species were found in different areas, as the red fox preferred boreal forests (colder, more conifers like pine trees) and the grey fox was found in deciduous forests (warmer, trees shed their leaves in the Fall). The red fox has spread out throughout the US, but the grey fox apparently still dominates in areas where they overlap. Notably, the grey fox can be found in a remarkable range of habitats from the deciduous forests, the open desert or rocky canyons of Arizona, even to grasslands and, of course, urban areas.

A grey fox in North Carolina. She has four kits under the wooden paneling behind her. -- Dcrjsr 2010

A grey fox in North Carolina. She has four kits under the wooden paneling behind her. — Dcrjsr 2010

Grey foxes have an annual breeding season that varies depending on where they are found. Roughly, this can be between late Winter and early Spring. They form a monogamous pair. The female is pregnant for about seven and a half weeks. Their litter can be between one to seven kits, who are able to hunt along with their parents at three months old. At four months old, the kits get their adult teeth and are able to find their own food. However, the family stays together until Autumn. The kits learn from their parents and play with each other to practice the skills they will need as an adult. In Autumn, the kits become sexually mature adults and leave to find their own territories and families. The offspring might still meet their parents, if they live close enough, but they no longer have extensive interactions.

Grey fox kit at the Baylands -- Bill Leikam 2011

Grey fox kit at the Baylands — Bill Leikam 2011

As mentioned earlier, the grey fox has spread through a large area, partially due to their adaptability to humans in their habitat. From what I gather, it seems that both the grey and red fox (Vulpes vulpes) are able to adapt to living around people. They have a fairly flexible diet, one that can include small pets like chickens or rabbits, but is more likely to be rodents in your yard, or any fruit available, like grapes or strawberries. They are said to avoid garden vegetables and adult cats (who could be a fierce rival in a fight). They rarely carry rabies, and are overall rather benign as far as wild guests go. If you have a fox living near you, please leave it alone, and be glad for the view you get of a natural animal. If you truly do not wish to see them again, this link has many suggestions for scaring them away humanely.

A grey fox in captivity at the Lindsay Wildlife Museum in Walnut Creek, CA. -- Milonica 2012

A grey fox in captivity at the Lindsay Wildlife Museum in Walnut Creek, CA. — Milonica 2012

The grey fox seems to be a beautiful and resourceful creature. Their general nocturnal, shy nature speaks of caution, though sometimes they can be more bold when forced to tolerate humans in close proximity. Often, I hear of foxes being associated with mischief, but I do not really see that here. They seem to represent adaptability and a strength that doesn’t need to be boasted of. They are flexible in their lifestyle, and also stay their course of survival despite the challenges of human encroachment into once-wild lands. It is rather inspiring, and they carry a regal image in my mind. I definitely respect the grey fox.

A grey fox resting in the rocks -- Gary Stolz, USFWS 2013

A grey fox resting in the rocks — Gary Stolz, USFWS 2013


The First Step — Virginia Opossum

One the day that I began my research, I decided to find a list of native Californian animals and go in order. First on the list was the Virginia opossum. I thought, I know about opossums already. They’re very common, the only North American marsupial, they can play dead, and that’s the sum of it. Well, just for completion, I thought I should go ahead and read. I’m very glad I did. It turns out they are much more impressive than I gave them credit for.

Virginia Opossum in winter coat; Cody Pope 2007

Virginia Opossum in winter coat; Cody Pope 2007

General Description

The Virginia opossum (Didelphis virginiana) is found extensively across Central and North America. They are mainly found in deciduous woodlands, but have also been sighted in prairies, marshes, and farmlands. In more arid places like Texas, they often stick to riparian woodland (riparian means along the river). They aren’t great burrowers, so they tend to use dens from other animals, or natural shelters like hollow logs.

They first evolved in South America, but were one of many species to head North during the Great American Interchange 3mya (million years ago). This was the time when the isthmus of Panama was formed (which we later had to dig through to create the Panama Canal). Originally, the opossum was found mainly in the South-eastern parts of the United States, but it was introduced to the west coast during the Great Depression. Considering the great variety of recipes, and the needs of the time, it is generally thought that it was brought as a food source.

Now then, I already knew what they look like and how big they are, but I suppose for the sake of the exercise, I should read about that, too. Turns out, this little marsupial was going to be a lesson on hubris for me. They range from around 13-37 inches (35-94cm) long, and run from 11oz – 14lb (including both male and female measurements). That is quite a difference! I learned that they have one of the greatest ranges in size in the world for a mammal. They are much larger in the Northern areas of North America, and are much smaller in the tropics.

Virginia opossum in Western Canada --(Drcyrus 2007)

Virginia opossum in Western Canada — (Drcyrus 2007)

I can only assume this has to do with the temperature, as a larger body allows an animal to stay warmer more easily. However, don’t take my word on that as a fact (just as a hypothesis). This relates to Bergmann’s rule which essentially states that animals are larger in colder environments, and smaller in warmer areas. This can apply between different species in a genus, but also between different populations of the same species, like with this opossum. The reasoning relies on what is called the surface area to volume ratio.

When an endotherm (“warm-blooded” animal) produces its body heat, it is simultaneously losing heat to the surrounding air when it is cold outside. When a given mass of the animal produces heat, it is losing it through the area of its skin (surface area) that is touching the air. So when it’s cold, an animal wants more mass and less surface area.

This adorable young opossum will have a harder time keeping warm than an adult! -- (Liam Wolff 2011)

This adorable young opossum will have a harder time keeping warm than an adult! — (Liam Wolff 2011)

For those of you that would like to see some numbers, let’s talk about a theoretical endothermic cube creature. If you have a 1m (meter) cube, their surface area is 6m2, whereas a 2m cube has a surface area of 24m2. The SA:V ratio of the 1m cube is 6m-1, while the 2m cube is 3m-1. Basically, this means that when an animal is producing heat, a smaller body will have to work harder to keep warm, because they are losing heat faster.

Regardless of why, it is pretty impressive that the opossum adapted so well to such a broad range of climates. It is continuing to expand northward into Canada. This makes it rather surprising to learn about its small brain for its body size. While not terribly smart, they do have more tricks up their fuzzy sleeves.

Virginia opossum in a tree in northeastern Ohio --(2008)

Virginia opossum in a tree in northeastern Ohio –(2008)


They have long, hairless prehensile tails. This is mostly useful for grabbing onto branches or move a small object. They can hang from their tails, but their tails aren’t strong enough to make it the primary purpose. Mainly, it seems their tails and opposable thumbs help them climb around better. Most of the opossums I have seen do not seem very graceful, but they are very prevalent despite that. I have often spotted them scurrying along the top of a wall or fence around a suburban area.

Virginia Opossum skull -- (Dawson 2006)

Virginia Opossum skull — (Dawson 2006)

They also have 50 teeth, as you can see, which they use to eat just about anything. Their dental formula is A dental formula lists each kind of teeth on the upper and lower jaws in the order of incisors, canines, pre-molars and molars. So this opossum has 5 incisors, 1 canine, 3 pre-molars and 4 molars on the upper jaw. This is a ton of teeth compared to most other mammals. (Imagine the dental bills!)

They will eat a variety of plants, smaller animals, fruits, insects, and even carrion. In urban areas, they can also be found eating garbage or pet food. This flexibility in their diet probably helps a great deal in allowing them to survive not only in a wide range of climates, but also in and around human settlements.

Their teeth reflect this omnivorous diet. They have a mixture of good flesh-piercing canines and premolars, incisors suitable for cutting vegetation, and tricuspid molars great for grinding up everything. Their skull also features a large sagittal crest (rising vertically from the top of the skull, from behind the eyes to the back). This is an area where jaw muscles attach. Because the crest is so prominent, you know that the opossum does a lot of chewing.

Opossum tracks (larger) and vole tracks (smaller) -- (Michael Lensi 2003)

Opossum tracks (larger) and vole tracks (smaller) — (Michael Lensi 2003)

Virginia opossums have opposable thumbs on their hind legs. I had no idea that any other animal had opposable thumbs, so I think this is pretty cool. Some cursory searching shows that there are many animals that have opposable thumbs, but it is still fairly rare relative to the number of animals that exist.

When I saw this picture, I was struck by how much the left print resembles a human handprint. It definitely makes me feel more connected to them in a visceral way. Hands are a very important feature that we use all the time, not just for tools, but also for socializing. We shake hands, high five, flip the bird, thumbs up/down, salute, and so on. It is a good reminder that humans are not alien from the rest of the animal kingdom. However, it is also good to remember that we are related to things that don’t have features similar to ours, like jellyfish and bacteria.

As far as behavior goes, the virginia opossum is very nocturnal and mostly solitary. They maintain a home range (the area that they regularly live in), but can allow them to overlap when conditions are good. That is, they have enough food and other resources that can support them in close proximity.

Virginia Opossum carrying her young -- (Specialjake 2012)

Virginia Opossum carrying her young — (Specialjake 2012)


They have two breeding seasons each year: one from January-February and another from June-July. After courtship and mating, they return to being solitary. The female will bear 5-21 young, each only weighing about 3 grams. They are pink, hairless and can only manage to crawl into their mother’s pouch and nurse. They will stay there for 7 weeks, until they are finally ready to detach. The reported mortality rate for young opossums is very high, especially since there are only 13 teats available to nurse from, so any more that are born cannot be fed. At least, that is what the literature says. This video shows a mother carrying 15 young, so perhaps when conditions are good, she might be able to feed more.

It has been surprisingly difficult to find information on the interaction between parent and young after they are done nursing. Apparently this is not an interesting topic. However, I can report with a reasonable guess that the young spend some time riding on their mother’s back, and perhaps also following her on foot when they are larger. They likely observe what she eats and other behaviors that they need to learn to be successful adults, until they are old enough to leave.

This video, by what might be a nature group from Missouri, states that the young leave the mother at around 3.5 months old. It also shows an opossum carrying some leaves in her tail in order to make a nest.

Even though opossums have a lot of young, and only stay with them for a few months, it is apparent that they do care about their offspring. They carry them in a pouch, then on their back. It makes me think that a human parent trying to emulate the opossum, might care a great deal for their children while they are young, but allow them to transition into acting like an independent adult when the time is right. They might encourage their child’s individual pursuits, supporting them, then letting them blossom into adulthood without trying to hold on too much, to the point where their now-adult child feels smothered.

Irritated opossum -- (PiccoloNamek 2006)

Irritated opossum — (PiccoloNamek 2006)


Opossums are not very exciting when it comes to interacting with other species, at least not in a traditional way. Their mouths full of pointy teeth look rather intimidating, but they don’t tend to use them in fights where they are out-matched. Instead, they usually play dead.

This seems to be an involuntary reaction in response to extreme fear. Incredibly, when under enough stress, they can enter a full coma for up to four hours! This is stunning, not just in the extent at which they can hold the ruse of being dead, but that after falling into a coma they can emerge and return to normal life. In humans, a coma is a pretty extreme event that needs a lot of recovery time.

Watching the opossum might be a good way to learn how to avoid confrontation at times when it isn’t productive. For example, if you find yourself having trouble reigning in your anger, particularly against friends or family, it might be good to try to be more like the opossum and avoid the confrontation. That way, your relationships can survive, rather than potentially burning bridges with those who care about you.

Obviously, though, there are many ways to deal with different situations, and I don’t want to advise anyone to do anything. I am merely thinking about ways that a hypothetical situation might arise.

The opossum also has other defenses that aren’t so melodramatic. They happen to be highly resistant to snake venom, a fact which I found both surprising and impressive! They also resist rabies, likely because they have a lower body temperature than other placental mammals (a lower body temperature is common among marsupials).

They also successfully kill most of the ticks that parasitize them, which helps them avoid the Lyme disease that ticks can carry. In general, they don’t transmit many diseases to humans, unlike other animals common in urban environments, like rats.

It seems like the North American opossum is much tougher than I first gave them credit for. Despite this, they don’t have very long lifespans. In the wild, they generally live for about 2 years, and at most for 4 in captivity.

They rapidly decline in old age. Their life span is short, even compared to other marsupials–who don’t tend to live as long as placental mammals–and compared to animals of a similar size.

The proposed explanation for this is based on their general lack of defenses to predation. If playing dead doesn’t work, then they’re stuck. This leads to the idea that since they likely won’t live very long anyway, they had no push for evolving the biochemical means to live longer. Old age generally involves organs starting to fail, without the body keeping up the process of repairing them as they do when the body is younger. It costs a lot of energy to resist the effects of aging, after all.

This idea is supported by a population of opossums that has been isolated for a few thousand years on the small Sapelo Island off the coast of Georgia. There are no natural predators there. The opossums live up to 1-2 years longer there. It seems that since they are less likely to be eaten, those opossums that evolved the ability to resist aging for longer are actually relevant to the forces of natural selection.

Final Perspectives

Learning about the opossum this week has been very illuminating. Personally, it taught me not to underestimate any creature, no matter how plain it sounds or how much I think I know about them.

Opossums seem to embody a basic perspective on life: live well while you can. They show that it’s okay to be tough, but it can also be okay to be pacifistic. They are fine with solitude, but invest in those close to them.

What is your perspective? I would love to hear from you!