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Body mass is considered a major determinant of home range size, but usually at a large scale of body mass variation. The exact scale where body size becomes more important than particular adaptations of each species is not clear, and uncertainty in the estimate of home range size is a possible cause of weak intraspecific scaling. We determine the scaling to body mass of two alternative movement measurements, daily home range (DHR) and its intensity of use (IU), in three species of didelphid marsupials, Didelphis aurita, Philander frenatus, and Metachirus nudicaudatus (Didelphimorphia, Didelphidae). The expected scaling exponents DHR a M 0.5 and IU a M -0.25 were derived from the scaling to body mass of home range and daily movement distance. Animals were tracked in Serra dos Orgaos, Rio de Janeiro, Brazil, using a spool-and-line device. Individuals of the three species were compared combining intra and interspecific variation in a single analysis, with species, body mass, and thread tracked as covariates. The model best supported included only body mass as the independent variable, with DHR a M 0.435 and IU a M -0.218 , close to the expected values. The second best supported model included species identity, but with a non-significant effect. It was surprising that body mass was more important than species identity in a comparison involving only three species, and considering the morphological and locomotory adaptations of the three species. Body mass may become more important than species identity when the scale of variation approaches one order of magnitude.

Species of didelphid marsupials (Didelphimorphia, Didelphidae) differ in their use of the forest strata, but it is not clear whether these differences are in fundamental or realized niches. The fundamental niche of seven species of didelphids (Caluromys philander, Didelphis aurita, Gracilinanus microtarsus, Marmosops incanus, Metachirus nudicaudatus, Micoureus demerarae, and Philander frenatus) was compared using their performance in arboreal walking. The association between performance and vertical use of the forest also was tested accounting for phylogenetic and allometric effects. Tests consisted of making the animal cross five 3 m long horizontal supports of different diameters, 1 m from the ground. The cycle of maximum speed was chosen to measure stride length, frequency and velocity. Arboreal species performed better than the terrestrial ones, but a major part of the variation in stride length (70.95%) and stride frequency (88.10%) was associated with body size. Part of the variation in stride length independent of body size (14.05%) was associated with the degree of vertical use of the forest, after phylogenetic effects were accounted for. Fundamental niches of six of the seven species were discriminated with the performance tests used. Discrepancies between the realized and fundamental niches can be inferred for two of these species, D. aurita and P. frenatus.

Didelphids use vertical strata in different ways, suggesting the existence of a wider variety of niches than simply arboreal or terrestrial. This variety could be related to the differential ability to deal with support diameters and inclines, and might be important for the coexistence of species in local assemblages. From data obtained with the spool-and-line technique, the use of the vertical space and above-ground supports is described in four species of an assemblage of didelphid marsupials in south-east Brazil. The stratification in this and other assemblages in the Atlantic forest and in the Amazon is also compared. Animals were trapped in Serra dos Orgaos, state of Rio de Janeiro, and equipped with a spool-and-line device before release. The paths were tracked by measuring variables related to support diameter, incline, distance and height moved above ground. The diameter of supports used was positively related to body size, and the incline to the dominant direction of movement (horizontal vs vertical). The more cursorial species, Metachirus nudicaudatus, only once moved above ground in > 3200 m of paths followed. Didelphis aurita moved mostly on the ground, but was the only species that occasionally reached the canopy. Philander frenata also moved mostly on the ground, occasionally used the understorey, but never reached the canopy. The more arboreal species, Marmosops incanus, moved mostly in the understorey, but never in the canopy. This pattern of stratification is similar to that observed in other sites in the Atlantic forest and in the Amazon.

Phenotypic evolutionary rates are reported for cranial characters of eight species of large-bodied Neotropical marsupials: Didelphis aurita, D. marsupialis, D. albiventris, Philander opossum, P. frenata, Lutreolina crassicaudata, Chironectes minimus, and Metachirus nudicaudatus. These rates were found to be lower than expected if cranial diversification in these opossums had occurred by mutation and genetic drift only, and it is clear that a greater diversification was prevented. As all parameters used in estimating rates were very conservative, the conclusion that stabilizing selection has predominated during the evolution of the skull of large-bodied opossums is fairly robust. We also show that directional selection sustained for 150 generations (158 years) or less is capable of producing differences of the same magnitudes as those found between various pairs of species. Therefore, we conclude that even where a particular differentiation has been caused by directional selection, neutral rate tests are unlikely to infer it. This is because following a morphological shift, stabilizing selection will progressively erase evidences of directional selection.

An attempt to determine the locomotor activities of Mayulestes ferox (Borhyaenoidea) and Pucadelphys andinus (Didelphoidea) from the early Paleocene site of Tiupampa (Bolivia) is presented. The functional anatomy of the forelimbs of these South American marsupials is compared to that of some living didelphids: Caluromys philander, Micoureus demerarae, Marmosa murina, Didelphis marsupialis, Monodelphis brevicaudata and Metachirus nudicaudatus. Deductions from bone morphology to myology and locomotor behavior in the fossils are inferred from the comparisons with living forms. Some features of the postcranial skeleton, indicative of arboreal adaptations, are found in the extinct marsupials: anteriorly projected acromion, hemispherical head of the humerus, extended humeral lateral epicondylar ridge, medially protruding humeral entepicondyle, proximal ulnar posterior convexity, and deep flexor fossa on the medial side of the ulna. But other features are related to a more terrestrial pattern: the well-developed tubercles of the humeral head, the elongated olecranon process of the ulna, and the oval shape of the radial head. Mayulestes had clear arboreal abilities, but, as a predaceous mammal, probably hunted on the ground. Pucadelphys was less specialized, close to the living Monodelphis, a terrestrial insectivorous form with some skeletal features related to arboreal locomotion that are probably plesiomorphic for marsupials.