How do helpers slow senescence in a Tibetan cooperatively breeding bird?

Sociality is known to be capable of slowing individual senescence, but it is unclear whether the effect differs for reproduction versus survival in a sex-specific manner. Here we predict that social benefits are directed toward (1) somatic maintenance in harsh environments where high survival prospects of adults over young intensify the trade-off between current and future reproduction, and (2) females that invest more in reproduction and have a greater marginal effect if their survival is improved by reducing the cost of reproduction. These two predictions are tested with cooperatively breeding Tibetan ground tits (Pseudopodoces humilis). Across the lifetime, both mothers and fathers with helpers did not differ in brood size at fledging from their counterparts without helpers. The presence of helpers reduced survival senescence of both parents, but mothers benefited more than fathers from receiving help. Consequently, the inherent sex difference in life span and along with lifetime reproductive success, as expressed in breeders of never-receiving help, became no longer obvious. The model of social modulation for senescence should facilitate the persistence of cooperative breeding in challenging environments.