The only problem was that studying wild kangaroos and wallabies would have required more long-distance commuting than Hunsaker found appealing. So he started to reflect upon the marsupials closer at hand. Geological evidence indicates that the first marsupials developed in North America about 100 to 120 million years ago. For the most part, mammals out-competed them here, but one species that appeared around 50,000 years ago, Didelphis virginiana, was a tenacious little survivor. (Its correct common name is the Virginia opossum; the word "possum" is slang, if commonplace.) Hunsaker says topological and climatic barriers confined the Virginia opossum to the Southeast for most of its history. It took a human being to transport at least a couple of them across the Rockies and the great Southwestern deserts around 1890. Once here, the opossums thrived. "A basic rule of thumb is that all species expand to the limits of their environment, and the possum's a good example of that," Hunsaker says. "Today they're one of the most common mammals in the state."
Hunsaker thinks whoever imported them into California was dreaming about dinner fare -- he can't imagine anyone wanting an opossum for a pet. "They're mean. They bite," he says. In the years when he was studying the animals, he was nipped so many times he became allergic to opossum saliva. "And they've got extremely strong jaws." Backing up that chomping power are more teeth than found in the mouths of any other American land mammal (50, as opposed to 42 for coyotes, foxes, and bears), and "they can snap real quick at you." But he could imagine working with them as research subjects.
These reflections coincided with an opportunity to work in South America, home to more than 70 species of opossums (as opposed to North America, where the Virginia opossum is now the sole remaining marsupial). As part of a U.S. State Department technology-transfer project, he was assigned to the Colombian Department of the Interior, and over a two-year period he observed a number of the South American marsupials in the wild. Some of them looked like otters or squirrels, bright-eyed and inquisitive. "That's where I got the idea that these marsupials were much more sophisticated than anybody had ever thought."
Hunsaker wound up buying land in Colombia and using it as a research station. After his stint with the State Department project ended, he continued traveling with graduate students from SDSU to do more research in South America, encouraging other American scientists to follow suit. He set up a New World marsupial laboratory on the San Diego State campus, and for a number of years, he and his students conducted experiments involving both the Virginia opossum and its South American cousins. "We did a lot of behavioral work," he recalls. "But the field work was easiest to do on the [North American] opossum, because they're local."
Some were living on the campus, so Hunsaker taught his students how to set up wire traps that would detain the animals without injuring them. (The open ends snapped shut when the animal stepped on a treadle in the center of the device.) "We'd put a little notch in their ears or put a tag on them or just paint a big number on their backs," he says. After releasing the opossums, "We'd see if we could find them again." More often than not, they couldn't. The professor and his students also trapped and released opossums in the Santa Margarita River valley (south of Temecula) over a period of years, and that work confirmed that the local marsupials are almost constantly on the move. They do build nests out of leaves (which they can carry by coiling up their prehensile tails) but normally don't use any den for more than a few nights. "They're very opportunistic animals," Hunsaker says. "They eat everything, and they don't have really well-defined territories like some other animals do. They just kind of work the area." To anyone who frets about spotting an opossum in the back yard, Hunsaker offers this advice: "Wait a week, and it'll be gone."
The only time males and females interact is during the 36 or so hours when the female is in heat, a state in which she might find herself between one and three times per year. Mates find each other by following scents laid down in urine and saliva. The males "have a very characteristic reaction to a marked object," Hunsaker has written. Among the more bizarre stories published about the Virginia opossum's sex life have been reports of the species copulating through the nose by using the male's two-pronged penis and, later, babies being "expelled into the pouch of the mother by an explosive sneeze." The truth, according to Hunsaker, is more mundane. Males court the females by nuzzling and following them, and "Receptive females are very passive and allow the male to approach closely, [and] sniff the genital region...." The Virginia opossum male mounts "the female on the back, grasping her neck in his jaws and her shoulder region with his forefoot," while restraining her hind legs with his hind feet. "The complete act of copulation lasts for approximately 20 minutes, with a range of 5 to 30 minutes.... Ejaculation apparently occurs about 15 minutes after intromission, since the thrusts increase in force and then cease." The female disentangles herself from the male's embrace and then toddles off, never to lay eyes upon him again.
Hunsaker says he continued his work on opossums for a short while after the publication of The Biology of Marsupials. But violence springing from the Colombian cocaine traffic was growing so intense "it got to the point where I couldn't take a graduate student down there, and I would have been nuts to go myself." He'd also reached a point where he felt he'd learned most of what he had wanted to know about the North American opossum. He'd concluded that the animal's behavior was more reptilian than originally expected. With vocalizations limited to just four distinct sounds (a lip-clicking, a hiss, a growl, and a screech), the animal instead relies upon "a lot of chemical communication." Opossums "don't live in social hierarchies or family groups, and that's very typical of what we find with the reptiles." Furthermore, he'd come to suspect that "whatever IQ means, there are some lizards that probably have an ability to learn that's as good or better than an opossum's."