Body Weight: its Relationship to Tissue Composition, Segmental Distribution of Mass, and Motor Function III.* The Didelphidae of French Guyana

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Abstract
Two sets of anatomical data are presented for six marsupial genera from French Guyana: tissue (skin, muscle, bone) and organ (alimentary tract, brain, eyes) proportions, and segmental distribution of mass (head, tail, thighs, etc.). As a percentage of total weight, skin is about equal in all genera, but the percentage of muscle is low in Caluromys and Didelphis (32%) and exceptionally high in Metachirus (45%); the percentage of viscera is low in Marmosa (6%) and high in Caluromys (10%). Philander, Caluromys, and Marmosa are similar in the segmental distribution of weight: large head, moderately sized thighs, and heavy, prehensile tail. Caluromys and Philander differ in the percentage of total muscle, size of the eyes, and weight of the alimentary tract. Only Monodelphis and Metachirus strongly diverge from the group, and Metachirus possesses many characteristics of high-speed terrestrial cursors such as Dolichotis and Lepus: high percentage of muscle in the body; muscular arms and thighs; elongated and lightened feet; and heavy lumbar extensor muscles. Basically, however, the Didelphidae remain small, prehensile-tailed, scampering and climbing omnivores with only moderate hindlimb dominance. They have not differentiated structurally as much as the prosimians in comparable niches in West Africa or Madagascar. Certain forces have acted conservatively upon body size and locomotor tissues.

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