(Reuters) - Frustrated by high fees and uninspiring returns, pension funds, sovereign wealth funds and other big institutional investors are demanding U.S. activist hedge funds be more transparent about their investment ideas and charge less, investors and fund managers say. Activist hedge funds push for changes at companies, ranging from ousting management and board directors to buying back stock and divesting non-core assets. Traditionally they have used capital from their total pool of investor money to build stakes in companies, without telling clients in advance what their targets and strategy will be.