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)

Physiology

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 5.1.3.4/4.1.3.4. 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)

Reproduction

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)

Defenses

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!