Largerbodied Folivores

The phalangeroids that weigh from about 1 kg in body mass and higher (up to a maximum of about 5 kg) make their living eating leaves, particularly leaves of Eucalyptus. Microbial fermentation of cellulose is accomplished in an enlarged cecum (Smith, 1984a). Several of these larger phalangeroids present interesting comparisons to primates based on obvious external physical characteristics, as is evident from the name of Hemibelideus lemuroides. Compelling comparisons have been drawn between the slow-climbing cuscuses (Phalanger and relatives) and the Asian slow lorises (Nycticebus), which differ from each other in the presence of a long prehensile tail in the former and only a remnant stub in the latter (Figure 7). The cuscuses have orbits more convergent than those of other phalangerids (Cartmill, 1972) but less so than in Nycticebus, snouts that are relatively truncated, and short lorisine ears. They utilize their hands and feet—which have reduced claws compared to other phalangerids and pseudocheirids—to travel like lorises via cautious slow climbing. They prefer to maintain a grip on branches with three extremities at a time, and only attempt leaps when moving along open ground. Interestingly, the cus-cuses are the least folivorous of these larger phalangeroids, relying on a diet of leaves, fruit, seeds, and animal prey (Hume et al., 1997). They are dietary generalists but locomotor specialists.

In contrast, the closely related brushtails (Trichosurus) are both locomotor generalists and dietary generalists, and have proven to be successful colonizers following their introduction to New Zealand. They usually eat a high proportion of leaves in addition to smaller amounts of fruit, shoots, animal prey, and other items (Kerle, 1984; Proctor-Gray, 1984; Statham, 1984).

Figure 7. The possum Phalanger (A) and the primate Nycticebus (B), both slow-climbing arboreal mammals. Photograph (A) Reproduced with permission from Nature Focus © Australian Museum; photograph (B) by D.T.Rasmussen.

Figure 7. The possum Phalanger (A) and the primate Nycticebus (B), both slow-climbing arboreal mammals. Photograph (A) Reproduced with permission from Nature Focus © Australian Museum; photograph (B) by D.T.Rasmussen.

Brushtails have more of a galago cast than do the cuscuses, with sharp, projecting snouts, large erect ears, usually bushy tails, and sternal scent glands, but postcranially, they are more closely comparable to Didelphis (the familiar New World opossum). Trichosurus moves in the trees and on the ground, and occurs in a wide variety of habitats. A third member of this clade, Wyulda, is restricted in distribution to southern Australia where it lives in terrestrial rock piles but forages up into trees (Humphreys et al., 1984; Muncie, 1999). Wyulda shows considerably fewer primate-like attributes to casual inspection than do its relatives—if anything, it might make an interesting analogy to the tree shrew Anathana, which also lives in rock piles and forages in trees.

Pseudocheirids, or ringtail possums, eat a highly folivorous diet, mainly consisting of eucalypt leaves (Proctor-Gray, 1984; Thompson and Owen, 1964). They are distinguished from phalangerids by having higher, more elaborate shearing crests on their molars for processing leaves. While most ringtails are arboreal, one species, the rock ringtail possum (Petropseudes dahli) lives on rock outcrops. Apparently in correlation with its terrestrial habits, it shows a "longer snout, shorter tail, shorter legs, and shorter claws" than do its arboreal relatives (Nelson and Kerle, 1983). Typically, members of Pseudocheirus and Pseudochirops are slow, deliberate climbers, resembling the cuscuses of the family Phalangeridae. Indeed, the similarities between certain members of the paraphyletic ringtails and the paraphyletic cuscuses suggest that their common ancestor (the ancestor of the combined clade of Phalangeridae and Pseudocheiridae) was a deliberate quadruped with a generalist to folivorous diet; the species Phalanger orientalis and Pseudocheirus herbertensis are noted to be particularly similar to each other despite their family level separation. In contrast, the lemuroid ringtail possum (H. lemuroides) is a highly arboreal rainforest possum with a flatter face and longer limbs than those of Pseudocheirus dahli; this species is expert at leaping through the canopy (Figure 8). Remarkably, Hemibelideus has slight folds of skin less than 25 mm in width along the sides of the trunk (JohnsonMurray, 1987), and it spreads the limbs wide during a leap like a gliding possum—whether these flaps represent the origin of a gliding membrane from an ancestor like Pseudocheirus or the remnants of one from an ancestor like Petauroides (see below) is a fascinating question, with profound implications either way. The prehensile tail of Hemibelideus shows an interesting parallel with ateline platyrrhines in being heavily furred over most of its length except the ventral surface of the grasping tip (also characteristic of some other pha-langeroid folivores). Hemibelideus is an animal with a lemur-like countenance, the tail of a spider monkey, and incipient or vestigial gliding membranes. Although nocturnal, the lemuroid ringtail is reported to be gregarious, with

Figure 8. The possum Hemibelideus lemuroides (A) compared to the primate Eulemur fulvus (B). Photograph (A) reproduced with the permission of the Environmental Protection Agency, Queensland, © State of Queensland; Photograph (B) by D.T.Rasmussen.

Figure 8. The possum Hemibelideus lemuroides (A) compared to the primate Eulemur fulvus (B). Photograph (A) reproduced with the permission of the Environmental Protection Agency, Queensland, © State of Queensland; Photograph (B) by D.T.Rasmussen.

groups of two or three being common (Winter and Atherton, 1984)—one group of eight animals was reported in a single tree (Winter, 1983).

Another large, heavily clawed pseudocheirid is the greater glider (Petauroides volans), which resembles the greater galago (Galago crassicau-datus) in body size, shape, and general facial proportions, including the large ears (Figure 9). The similarity even extends to having gray and black color phases (Ride, 1970). In terms of its diet and social system, the greater glider has been specifically compared to the prosimian Lepilemur (Henry, 1984). The greater glider produces one young at a time which takes two years to mature, a remarkably slow reproductive rate for a mammal of its size. Ecologically, the glider differs from the galago in its folivorous habits and, of course, in its exceptional ability to glide (several dozens of meters per flight). The parallel development of gliding in several lineages of phalangeroids and in primate-like dermopterans raises the question of why there are no gliding primates.

Figure 9. The possum Petauroides volans (A) compared to the primate Galago cras-sicaudatus (B). Photograph (A) by A. Smith; photograph (B) by D.T.Rasmussen.

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