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Locomotion in land and water requires different adaptations, hence semiaquatic mammals must deal with conflicting demands of the 2 environments. Semiaquatic rodents of the tribe Oryzomyini are considered semiaquatic mostly based on habitat use and morphology, but locomotory specializations were not yet determined for most species of the group. We compared water absorption of the fur and swimming behavior between 2 species of oryzomyine water rats (Nectomys) and 2 terrestrial species of Cerradomys. We used adult rodents captured in the wild but acclimated to laboratory conditions. Water absorption rate was determined by the relative increase in body mass after a 5-min swimming session. Animals were videotaped swimming in an aquarium to determine gaits, body posture, maneuverability, and speed. Water absorption rate was significantly lower in semiaquatic species, with no significant difference between sexes. Bipedal paddling was the more frequently used gait by all 4 species, but semiaquatic species were faster and maintained a more hydrodynamic body posture, with a short gliding phase during the gait cycle. Only semiaquatic species were capable of floating effortlessly, and used the swimming bound, a gait similar to the half bound of terrestrial locomotion. Submerged swimming was the fastest swimming gait, used by 1 terrestrial and 1 semiaquatic species. The better performance during bipedal swimming of semiaquatic water rats was related to the improved buoyancy provided by reduced water absorption of the fur, which seemed to represent an important adaptation to move in the water without compromising locomotion on land. Key words: Cerradomys, locomotion, mammalian behavior, Nectomys, swimming

The reasons Lutreolina crassicaudata is always captured in close proximity to water are not clear. We investigated locomotory behavior and performance in swimming, running, climbing, and jumping of L. crassicaudata. One adult male was videotaped in the laboratory while swimming, walking on the ground and on a horizontal tube 1.2 m from the ground, climbing a tree trunk angled 45[degrees], and jumping gaps between supports. The locomotor cycles in these different activities were described by speed, stride length, stroke or stride frequency, time of power and recovery phases or stance and swing phases, and by displacement of points on the animal. L. crassicaudata employed a quadruped paddling gait in swimming. Swimming speed was similar to that of terrestrial didelphids, but stroke frequency and buoyancy ability were more similar to those of the water opossum. Different gaits were used for locomotion in each habitat type and we conclude that L. crassicaudata cannot be considered a specialized species for aquatic locomotion. Key words: animal behavior, climbing, Didelphidae, locomotion, Lutreolina, swimming