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The Giant Otter (Pteronura brasiliensis) is an endangered otter of Amazonian lakes and rivers. It is the only otter of 13 extant species to breed cooperatively in family groups, with young of several years helping to raise younger siblings. I studied giant otters' behavior and ecology in 4 oxbow lakes (or "cochas") in the Manú Biosphere Reserve, Perú during 2003-2006. The objectives of this research were: 1) to investigate whether oxbow lakes return to random or predictable faunal communities after annual flooding; 2) to document seasonal and annual patterns in the diets of giant otters on two phytoplankton-dominated oxbow lakes; and 3) to document and characterize helping behavior in giant otters. I initially classified the four study lakes into 2 lake types, phytoplankton-dominated lakes and macrophyte-dominated lakes. I obtained data on their faunal communities, limnology, and otter diet through four seasons of 2003, and during the dry-seasons of 2004-2006. I show that lakes' bird communities and caiman populations, and to a lesser degree fish communities, respond predictably by lake identity and type. Lake communities also responded to seasonal changes in 2003 data, but generalization to other years is complicated by the destructive flood in January of that year. I studied giant otters' diet using visual observations, and demonstrated seasonal and annual changes not previously reported for the species. Giant otters shift to more intensive use of small cichlid prey found in edge habitats when with young cubs in the dry season. In studying the giant otters' behavior, I showed that hunting skills and helping activities generally increase with age. This observation is consistent with a pattern of "slow learning" suggested by the Skills Hypothesis of Heinsohn (1991). I observed considerable variation in dispersal age and helping contribution within families, particularly with respect to defensive behaviors against potential threats. Finally, the elderly matriarch in one family switched from being a provider of large prey to a beggar from other family members in 2007, apparently from the effects of old age. During 8 days of observation, her offspring assisted her through sharing prey and other types of assistance.