The anthropomorphic bias of those who would relegate marsupials to an

The anthropomorphic bias of those who would relegate marsupials to an inferior evolutionary status is most apparent in their recourse to data on brain structure and behavior. Unlike humans and other placentals, marsupials lack the corpus callosum, which facilitates inter-hemisphere transfer of data acquired through the senses. Yet it cannot be inferred that marsupials are thus deprived of such function. Didelphis Virginiana, one of the opossums, makes use of the anterior commissure, an adaptation that is also found in reptiles and monotremes. Diprodontons, including kangaroos and koalas, supplement the anterior commissure with the fasciculus aberrans. While the modes of neocortical interconnection may be diverse, the work of Johnson, Heath, and Jones points to the conclusion that, functionally speaking, the cortices and neocortices of both groups of mammals exhibit parallel connections. Parker also notes "a similar range of brain size to body weight ratios and of neocortical expansion."

Another stigma borne by marsupials is the consensus that they are less intelligent than placentals. Yet Williams argues that, all else being equal, natural selection will favor instinctive over learned behavior as being more biologically efficient and that it is the accidental death of the young that is the prime selective pressure for the evolution of intelligence. Seen in this light, marsupials have a competitive edge; their gestation period is brief and the young remain in the pouch for an extended period exposed only to those dangers which also affect the mother. There they are directly exposed to the mother's food supply and can observe her behavior at leisure. Placentals, on the other hand, not only have a longer gestation period but, once their young are born, must often leave while foraging. Such absences increase the risk of mortality and decrease the opportunity to learn. Thus, among placentals, selection would favor the appearance of intelligence in the young and protective behavior in the mother.

Marsupials are not known to exhibit maternal protective behavior. In fact, Serventy has reported that frightened female kangaroos will drop their pouch-young as they flee, drawing a predator's attention to the less able offspring while the adult escapes. This behavior, whether purposeful or accidental, instantaneously relieves the female marsupial of the mechanical difficulties of pregnancy with which her placental counterpart would be burdened, while marsupials can replace any lost young quickly. Thus, in the absence of any need for close maternal supervision, sacrificing their offspring in this manner may well have been favored in selection. Pointing to the absence of the "virtue" of maternal protectiveness in marsupials is an instance of how mistaken are those theorists who see similarities with humans as marks of evolutionary sophistication.

1. According to the passage, similarities between marsupials and placentals will most likely be found in

A.brain function
B.brain anatomy
C.maternal behavior
D.the corpus callosum
E.selection for intelligence



2. According to the passage, which of the following favor(s) the development of intelligence as a trait of placental mammals?

I) The need to leave their young while foraging
II) The comparatively great risk of accidental death of the young
III) The opportunity for the young to observe the mother at leisure

A) I only
B) III only
C)I and II only
D) II and III only
E)I, II, and III



3. The author's attitude toward those who consider marsupials to occupy an inferior evolutionary position would most probably be one of

A)criticism because they ignore evidence that marsupials are more intelligent than usually supposed
B)disagreement because current studies support the opposite view
C)disagreement because they apply human standards in an inappropriate context
D)agreement, but on the basis of marsupials' lack of maternal protective behavior rather than their brain structure
E)neutrality, on the grounds that the concept of evolutionary inferiority has not been defined with precision



4. All of the following authorities are cited for their work on marsupials EXCEPT

A) Johnson
B) Parker
C) Heath
D) Williams
E) Serventy