World 

 urry-weight 



by Michael Archer 



Of the many kinds of extraordinary 

 mammals that have come and gone, only 

 three subclasses sui"vive today: the egg- 

 laying monotremes (platypuses and echid- 

 nas); the usually pouched marsupials (for 

 example, opossums, honey possums, 

 wombats, koalas, kangaroos, and bandi- 

 coots); and the unpouched placentals 

 (such as rats, bats, elephants, and hu- 

 mans). Although not all marsupials have a 

 pouch, this external nursery is one of the 

 most commonly recognized features of the 

 group. To anatomists, details of the repro- 

 ductive system and remarkably early 

 births (some only eleven days after fertil- 

 ization) are even more distinctive features. 

 Early births and an accessible pouch have 

 given marsupials more control over the 

 business of raising offspring. If times are 

 tough, as they frequendy are in the unpre- 

 dictable deserts of Australia, a mother can 

 decide whether or not to continue to invest 

 precious energy in a pouched young. If the 

 decision is against, she can "diapause" the 

 young developing in the uterus, or if an 

 offspring is suckling, she may reach into 

 the pouch, remove the young from the nip- 

 ple, and discard it — increasing the 

 chances that she will live to breed again 

 when conditions are better. This and other 

 reproductive differences have probably 

 distinguished marsupials from placentals 

 for more than ninety million years, dating 

 from the time when marsupials and pla- 



centals diverged from a coirmion ancestor, 

 probably somewhere in the dinosaur-rid- 

 den forests of North America. 



Because many of Australia's marsupi- 

 als, such as the koala, are cute and cuddly, 

 as well as biologically different from our 

 own group, they have attracted a lot of at- 

 tention since their discovery in the 

 1700s — unfortunately, not all of it mag- 

 nanimous. Most of us who have fallen in 

 love with the marsupials of this continent 

 have at one time or another suffered a con- 

 descending smile from a North American 

 or English colleague. Some of these 

 Northern Hemispherites think of marsupi- 

 als as evolutionary casualties that should 

 be shoe-homed into a single order — rather 

 than the eleven in which they are currently 

 placed. Placentals are dignified as Eutheria 

 (meaning "good" mammals — because we 

 are one of them), while marsupials are hu- 

 miliated taxonomically as Metatheria 

 (which means "between" mammals). 



I've often wondered if marsupials were 

 described in this way because they in- 

 spired feelings of subclass inadequacy in 

 their pouchless placental classifiers — 

 "pouch envy," to give the embarrassing 

 condition a name. Placental males, how- 

 ever, have even more to worry about. As if 

 nifty female pouches weren't threatening 

 enough, the pendulous scrota of some 

 male marsupials, such as the honey pos- 

 sum's, contain testicles that weigh in at 4 



A rhino-sized marsupial Diprotodon emerges from the undergrowth at far 



right, startling a threesome of giant "kangaroos." This painting from the 



1920s was originally intended by artist Charles Knight to highlight 



Palorchestes, an animal hiown at the time from just a few bones. The beast 



was later found to be, not a kangaroo, but a vastly different, quadrupedal 



Australian herbivore. Although the depiction arose from a misconception, 



the magnificent Pleistocene bounders featured here still convey a 

 sense of the strange kangaroos that once dominated the island continent. 



Painting by Ctnaries R. Knighl; courtesy of the Field Ivluseum of Naturai History, Neg. No. CK27T 



44 Natural History 4/94 



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