The Mauvs are a large humanoid species which developed an industrial civilisation that ended in a nuclear war. The survivors were rescued by the Hurrians who had bee nmonitoring them. They helped them rebuild their world, instituting a eugenics selection program to make the Mauvian species more docile. The Mauv were unable to pay for the Hurrians' service as their world had been rendered resourceless by the war and the whole species, forced to pay in servitude, rather than material goods, became slaves to the Hurrians.
The Mauvs have thinly-haired bodies, coarse-featured faces, broad noses and flat cheekbones. There appearance contrasts with their submissive nature, as they never question authority, and never disobey orders from the Hurrians.
Source: Isaac Asimov, The Gentle Vultures, by Isaac Asimov (1957)