About 160 million years ago dinosaurs ruled, but small early mammals scurried about. All the continents were joined together as Pangaea. This super-continent started breaking up 30 million years later, when rifting began to open the Atlantic Ocean. An ancestral marsupial found in North America was very similar to the modern opossum. It was probably arboreal, with a prehensile tail and clawless, opposable big toes.
As rifting continued, South America became an island continent, with a fauna of primitive placental herbivores (plant eaters), sloths and anteaters, giant running killer birds, and marsupials. There were no placental carnivores (meat eaters), and the marsupials evolved into a variety of omnivores and carnivores, including a large saber-tooth. When the land bridge of Panama opened, modern camellids, tapirs, and felines entered, and out-competed the archaic herbivores and carnivores.
The marsupial uterus is split, so the space is too crowded for a placenta and as a consequence, fetal development in the uterus is rudimentary. The opossum embryos are “born” at 12 to 14 days, migrate to the pouch on the belly, attach to teats, and complete their development for another 60 days. The female has 12 teats arranged in a circle and one in the center. When mature enough, the young opossums may cling to their mother’s back when she is on the move. The average litter size is six to eight kits, and the female may have two litters a year.
Other unusual traits of the opossum are 18 incisors, a pacing gait, (both limbs on one side of the body move at the same time), and they seem to be resistant to rabies and Lyme disease, perhaps because they have a lower body temperature than placental mammals. Opossums are the about the size of a medium raccoon, but have a brain only 1/5 the size.
Opossums are famous for “playing dead.” This is really an involuntary faint when frightened. The lips are drawn back, baring the teeth and allowing foamy saliva to drool out. Their eyes are almost closed and a putrid green fluid is secreted from the anal glands. These defenses may fool some predators, but not cars and not pit vipers, which hunt by sensing heat. Opossums have consequently developed partial or total immunity to the snake’s venom. If the opossum does not faint, it will act ferocious, with hissing, snarling, and screeching. As opossums have a short (two-year) lifespan in the wild, but double that in captivity, these defenses are not too effective.
Today, opossums are increasing their range, moving from California north into British Columbia. They are omnivorous, eating just about anything: fruits, insects, small animals, garbage, and carrion. They are nomadic and solitary, do not dig burrows, but may use their tails to carry brush for making a nest.