Friend or Foe? The perplexing opossum

Before the bitter cold descended, it seemed there was an uptick in the number of opossum encounters, at least in Thetford Center. On Jackson Brook Road and Poor Farm Road  the action centered around chicken coops. In both instances an opossum had breached the electric fence and made itself at home inside the chickens' house. And in both cases the opossum met a quick demise once discovered. A couple of weeks later, a third opossum was live-trapped in the same place. This individual fared better, It was released in the barn of a generous and humane neighbor who did not keep chickens.

Indeed, the sight of an animal reminiscent of an oversized, grizzled rat in a chicken house that was thought to be secure would set off alarm bells in any chicken-owner's head. But is a death-dealing response justified? What kind of threat do opossums pose to chickens, or anyone else for that matter? 

Possums, or more correctly opossums, are not predators. They are opportunistic feeders and scavengers that consume insects, slugs, earthworms, fruits and berries, pet food, chicken food, and whatever they fancy in the compost pile. They certainly eat eggs if given a chance. They are sometimes described as "non-confrontational hunters" because they also eat any critter that can't get away and doesn't fight back, including snakes, frogs, baby birds, baby chicks, and adult chickens if they are sick or injured. 

The analysis of the stomach contents of 187 road-killed opossums revealed the following food groups: fruit 18%; amphibians 17.3%; mammals 14.2%; insects 13.4%; grasses 6.6%; worms 5.4%; reptiles 5.3%; birds 5%; carrion 4.8%;  miscellaneous 6.7%.

While opossums get a bad rap for attacking chickens, they are welcomed by some gardeners and livestock keepers because of their reputation for controlling rats and mice according to the National Wildlife Foundation. Other sources describe rats avoiding areas inhabited by opossums   While a search for scientific observations verifying this behavior came up empty-handed, it is very likely that they sniff out and eat baby rodents that can't escape.

Another opossum attribute that has captured the public's attention is their purported ability to eliminate ticks. This stems from, a scientific study that compared six animal species, mice, chipmunks, squirrels, opossums, veeries and catbirds that are common hosts for the black-legged tick that spreads Lyme disease. All animals were housed in cages until ticks naturally present on their bodies had dropped off. Each animal was then infected with 100 unfed ticks and restrained for 4 hours to prevent the animals from immediately grooming away the ticks. The subjects were returned to their cages and observed for numbers of ticks that fed and dropped off successfully. Only 3.5% of ticks on opossums fed and dropped off, a far lower number than with the other animals. The rest of the ticks on the opossums were presumably eaten in the course of grooming. The researchers then counted the numbers of ticks dropping off newly-captured opossums, finding an average of 199. They assumed 199 represented 3.5% of the original number of ticks on the animals, worked backwards and calculated that a possum would have to host over 5,500 ticks for 199 to survive. In other words the opossums ate on average at least 5,301 of the ticks. 

This demonstrates that opossums are very good at eliminating ticks on their bodies compared with other common tick hosts. It does not indicate that opossums actively search out ticks in the environment to eat. Rather, they inadvertently pick up ticks in their wanderings and are then very good at destroying those ticks. But the very fact that they act as ecological traps for ticks is important. Without opossums, more ticks would instead attach to mice, the most commonly occurring animal host, and about half of those ticks (see graph) would feed successfully and drop off to continue the tick life cycle.

There's more to opossums than chicks and ticks, of course. If you think there’s something strange about them  - you’re right. These  animals are known as living fossils. Recognizable opossum ancestors walked the earth in the era that saw the extinction of the dinosaurs. The fossil record shows they have changed remarkably little since then. Opossums are marsupials, a group that split off from other mammals early in their evolution. They are only distantly related to all the other furry, warm-blooded animals we’re familiar with. 

And speaking of warm-blooded, the body temperature of an opossum is lower than that of regular mammals. This makes them a very poor host for the rabies virus, and opossums generally don’t get rabies.

The opossum is the only marsupial that made it to North America. They are distinguished from other mammals by the fact that they incubate their babies in a pouch rather than having a conventional pregnancy. Opossum babies are born so immature that they resemble embryos the size of a honeybee. They crawl to the mother's pouch, and each baby latches onto a teat and remains there for about two and a half months. Although litters can be large, over 20 on occasion, there are only 13 teats. Any baby that doesn't get a teat soon dies. While the babies are developing, the teat stretches to 35 times its original length, allowing the baby to essentially swallow it and anchor securely to the mother. When the babies grow too large to fit in the pouch they ride around on the mother's back till they become independent at about four and a half months.

Opossums are excellent climbers and have opposable thumbs on their hind feet, the only North American mammal with this anatomical feature. (Raccoons are dexterous but don't have thumbs.) The naked tail that many find repulsive is prehensile and used to assist in climbing. It can carry things, too, like nesting material which they pick up with their mouth, then pass under their belly for the tail to grasp.  

Another notable attribute is the opossum's ability to survive the venom of rattlesnakes, copperheads, and vipers thanks to unique proteins that neutralize these toxins. These opossum amino acid sequences are being investigated as new treatments for snakebites that globally account for up to 138,000 deaths per year.

Opossums don't move quickly and can easily be cornered by dogs or humans. They don't attack but open their mouths to reveal an impressive array of very sharp teeth. They have 50 teeth, more than any other North American mammal, and their canine teeth are very large for a non-carnivore. In spite of this, they are not aggressive and rarely bite except as a last resort. Their normal defense is to fall to the ground, often curled up, and play dead. In this mimicry of death they become stiff and draw back the lips to expose the teeth, thus resembling a carcass. In addition, the opossum drools copiously and its anal glands emit a foul odor. Behavior evolves for a reason. Perhaps even predators can be "grossed out" as videos show them losing interest and walking away.

Opossums are solitary and nomadic. If one moves into your yard, it will likely not stay for long. As nocturnal animals, their sensory world is dominated by smell through their long noses and touch via many whiskers. Their brains appear primitive in that they are tiny and lack the folds characteristic of “higher” mammals, yet they have excellent memories and are not stupid. In one study they beat rats at running mazes..

Despite their remarkable attributes, these highly adaptable, oddball animals don't live very long, one to two years on average in the wild.