Inbreeding produces trade-offs between maternal fecundity and offspring survival in a monandrous spider

Offspring born to related parents often have lower fitness than those born to unrelated parents, a phenomenon termed inbreeding depression. While many species have been shown to rely on pre- and/or postcopulatory mate choice to avoid inbreeding, such research has focused largely on polyandrous rather than monandrous species. The absence of postcopulatory mate choice in monandrous species suggests that precopulatory mate choice should play a more important role in inbreeding avoidance. We used a monandrous wolf spider, Pardosa astrigera, as a model system to investigate whether (1) male spiders respond differently to sibling and nonsibling females; (2) female spiders respond differently to sibling versus nonsibling males; and (3) inbreeding affects females and their offspring. Male courtship behaviour was similar for sibling and nonsibling females; although females were less likely to mate with siblings, over half did mate successfully with them. Sibling-mated females produced fewer offspring from the first egg sac and fewer total offspring, but inbred offspring survived longer in a range of environments than their outbred counterparts. This suggests that the fitness costs of reduced fecundity in sibling-mated females may be offset by higher offspring survivorship. Our results highlight the importance of considering both parent and offspring fitness when addressing the costs of inbreeding, and are the first to document the impact of inbreeding on sexual behaviour and reproductive fitness in a monandrous spider.