Friday, May 26, 2006
When and where seen: On May 17 at 7:30 a.m., D'Ann Brownrigg spotted an opossum in their backyard on Acton Street. It was walking slowly along the top of a stone wall. Tom Brownrigg had time to get the camera and take pictures. While opossums are not very rare, they are nocturnal and most of us only glimpse them at night in the beam of the car headlights or, sadly, see them as road-kill. It is rare to see one in daylight hours.
Strange and wonderful: The marsupials are sometimes called living fossils because they have retained their structural characteristics for 50 million years. Marsupials are so-named because of the pouch (properly called a marsupium). It serves as an intensive care nursery for the young which are naturally born in a very early stage of development after only 12 to 13 days of gestation. They are about half-an-inch long and weigh less than one-hundredth of an ounce. Their hind limbs are just embryonic buds but their fore limbs are sufficiently developed for them to climb into the pouch and attach themselves to a nipple. Most opossums have 13 nipples although the range is from nine to 17. If there are more babies than nipples, some babies die — there is no sharing in the pouch. With separate passages for breathing and drinking, the little opossums can multi-task without a worry about choking, drowning or suffocating. After approximately two months, they let go of the nipples and either ride around on the mother's back or just hang out in the pouch having a drink of milk when necessary. They are weaned in two weeks and fully independent in another three or four weeks. Adult opossums have a prehensile tail and an opposable toe on the rear feet, both of which are unique in native animals of North America. They also have 50 teeth —almost twice the number of most other mammals.
Identifying characteristics: The opossum is about the size of a domestic cat but the legs are shorter and the body heavier. The face is white, the ears are round and black, and the body has a grizzled gray look. The tail is naked and pink except for the base which is black. Because of the opposable toe, the tracks are very easy to distinguish from other five-toed creatures.
Behavior: Opossums are more or less nomadic. They shelter in abandoned dens of other animals, tree cavities, brush piles, or under buildings and seldom use the same place for two nights in a row. They walk slowly and awkwardly but swim well and are good tree-climbers. If cornered, they may hiss and growl, or attempt to escape, or in rare cases "play possum," i.e., go suddenly into a catatonic state of feigned death which can last up to six hours and end just as quickly as it began. They are omnivorous. They don't hibernate and are not well suited to freezing temperatures. The ears and tail are often damaged by frostbite. They can live for seven years but the average life span is less than two due to road-kill and predation by dogs, foxes, bobcats, hawks and great horned owls.
References: Alfred J. Godin, Wild Mammals of New England; Donald & Lillian Stokes, Guide to Animal Tracking and Behavior (good drawings of tracks); Opossum Society of the US at www.opossumsocietyus.org; Peter Alden & Brian Cassie, Audubon Guide to New England (excellent photo of a mother with her young).
Submissions for the Biodiversity Corner are encouraged from everyone. Send a photo, a note about a sighting, or the whole column to Kay Fairweather at email@example.com
Upcoming nature walks during Massachusetts Biodiversity Days
June 3, 10:00 a.m. Foss Farm. Focus on insects. Trip leaders are Dick Walton and John Huehnergard.
June 6, 4:15 p.m. Towle Land. Focus on fungi. Trip leader is Larry Millman.
June 8, 3:30 p.m. Cranberry Bog. Focus on dragonflies. Trip leader is Susan Emmons.
June 10, 7:00 a.m. Benfield A. Focus on birds. Trip leader is Tom Brownrigg.
For more information go to www.maccweb.org/biodiversity_days.html. Select "Join a field trip."
© 2006 The Carlisle Mosquito