Playing opossum. (Photo by Mary Holland)

Although there are few outward signs of such activity, the woods and fields are alive with mammals giving birth this month—fishers, eastern gray squirrels, eastern cottontail rabbits, and Virginia opossums, to name but a few. The young of the first three species have been developing within their mothers' uteruses for the past four to eight weeks, depending on the animal. After this gestation period, the mothers give birth to fully formed young weighing anywhere from 15 (eastern gray squirrel) to 40 (eastern cottontail rabbit and fisher) grams. Immediately after their birth, the young fishers', squirrels' and rabbits' lives are relatively easy, in that they don't have to make much of an effort in order to receive essential sustenance—their mother's milk. Life is a bit more challenging for newborn opossums.

To begin with, an opossum embryo develops within its mother's uterus for the amazingly brief period of 12 days and 8 hours. Most of the growth and development that takes place prior to birth in the uterus of most mammals occurs within the pouch of marsupials. The young, underdeveloped newborns complete their gestation outside, rather than inside, their mother's body, hence their newborn nickname, "living embryos." Within the span of 12 minutes, between five and 25 (usually six to nine) young are born (opossums living this far north tend to have larger litters than those living in the south). Because of the brief gestation, the bodies of newborn opossums are not fully formed—only the head and front limbs are developed to any extent. Proportionately, the young opossums are also much smaller than the newborns of other species of mammals. Weighing just .13 grams at birth and measuring less than half an inch in length, a baby opossum is approximately the size of a honeybee. In spite of its early developmental stage and its small size, the young opossum must work hard for its first meal, harder than many other newborns. Under its own steam it must travel to the source of nutrition that is essential for its survival.

The young opossum emerges from the birth canal swinging its forelimbs in a swimming motion, and soon grasps hold of its mother's belly hairs. Although the mother assumes a sitting position while giving birth, which shortens the distance the young have to travel in order to reach her milk, the newborns must expend considerable effort in crawling blindly up into her marsupium, or pouch. Inside this fur-lined nursery are 12 teats arranged in a U-shape, with a 13th located in the center. Usually there are ample spigots to go around, but should more than 13 opossums be born and succeed in reaching the pouch, it is first-come, first-served, with those not securing a teat perishing.

Once it has located and attached itself to a source of milk, the young opossum is more or less locked into place by the swelling of the tip of the teat inside its mouth. Small projections on the lips, tongue, and palate of the newborn opossum also help anchor it to the teat. For 60 days the baby opossums nurse within this warm environment. During this time, as the young marsupials grow, the mother's teats actually lengthen, serving as tethers that allow the maturing opossums to have some freedom of movement within the pouch while still attached to their milk source. At the end of two months the opossums clamber out from their mother's pouch and crawl up on her back, where they cling to her long fur for much of the next two to four weeks.

Even though humans have been aware of the mechanics of marsupial birth and development for centuries, it remains unusual enough to be noteworthy. Captain John Smith, in 1612, described the opossum and its marsupial properties in words that this author could not improve upon: "An Opassom hath a head like a Swine, & a taile like a Rat, and is of the Bignes of a Cat. Under her belly shee hath a bagge, wherein she lodgeth, carrieth, and sucketh her young."

Naturalist and writer Mary Holland lived in Harvard for many years before moving to Vermont. She can be contacted at mholland@vermontel.net. Visit her natural history blog at www.naturallycuriouswithmaryholland.wordpress.com.