Females commonly mate with more than one male¹, and polyandry has been shown to increase reproductive success in many species^{2, 3, 4}. Insemination by multiple males shifts the arena for sexual selection from the external environment to the female reproductive tract, where sperm competition or female choice of sperm could bias fertilization against sperm from genetically inferior⁵ or genetically incompatible males^{6, 7}. Evidence that polyandry can be a strategy for avoiding incompatibility comes from studies showing that inbreeding cost is reduced in some egg-laying species by postcopulatory mechanisms that favour fertilization by sperm from unrelated males^{8, 9, 10}. In viviparous (live-bearing) species, inbreeding not only reduces offspring genetic quality but might also disrupt feto-maternal interactions that are crucial for normal embryonic development^{11, 12, 13}. Here we show that polyandry in viviparous pseudoscorpions reduces inbreeding cost not through paternity-biasing mechanisms favouring outbred offspring, but rather because outbred embryos exert a rescuing effect on inbred half-siblings in mixed-paternity broods. The benefits of polyandry may thus be more complex for live-bearing females than for females that lay eggs.

1. Department of Biology and Program in Ecology, Evolution and Conservation Biology, University of Nevada, Reno, Nevada 89557, USA

Correspondence to: Jeanne A. Zeh¹ Correspondence and requests for materials should be addressed to J.A.Z. (Email: jaz@unr.edu).

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