title: Inbreeding parents should invest more resources in fewer offspring.
abstract: Inbreeding increases parent-offspring relatedness and commonly reduces offspring viability, shaping selection on reproductive interactions involving relatives and associated parental investment (PI). Nevertheless, theories predicting selection for inbreeding versus inbreeding avoidance and selection for optimal PI have only been considered separately, precluding prediction of optimal PI and associated reproductive strategy given inbreeding. We unify inbreeding and PI theory, demonstrating that optimal PI increases when a female's inbreeding decreases the viability of her offspring. Inbreeding females should therefore produce fewer offspring due to the fundamental trade-off between offspring number and PI. Accordingly, selection for inbreeding versus inbreeding avoidance changes when females can adjust PI with the degree that they inbreed. By contrast, optimal PI does not depend on whether a focal female is herself inbred. However, inbreeding causes optimal PI to increase given strict monogamy and associated biparental investment compared with female-only investment. Our model implies that understanding evolutionary dynamics of inbreeding strategy, inbreeding depression, and PI requires joint consideration of the expression of each in relation to the other. Overall, we demonstrate that existing PI and inbreeding theories represent special cases of a more general theory, implying that intrinsic links between inbreeding and PI affect evolution of behaviour and intrafamilial conflict.
authors.affiliation: Institute of Biological and Environmental Sciences, School of Biological Sciences, University of Aberdeen, Zoology Building, Tillydrone Avenue, Aberdeen AB24 2TZ, UK alexander.duthie@stir.ac.uk.
authors.forename: A Bradley
authors.identifier: 0000-0001-8343-4995
authors.initials: AB
authors.lastname: Duthie
authors.affiliation: Institute of Biological and Environmental Sciences, School of Biological Sciences, University of Aberdeen, Zoology Building, Tillydrone Avenue, Aberdeen AB24 2TZ, UK.
authors.forename: Aline M
authors.identifier: 0000-0001-9272-4249
authors.initials: AM
authors.lastname: Lee
authors.affiliation: Institute of Biological and Environmental Sciences, School of Biological Sciences, University of Aberdeen, Zoology Building, Tillydrone Avenue, Aberdeen AB24 2TZ, UK.
authors.forename: Jane M
authors.initials: JM
authors.lastname: Reid
mesh_terms: D000818:Animals; D005260:Female; D056084:Genetic Fitness; D007178:Inbreeding; D008297:Male; D052586:Mating Preference, Animal; D008957:Models, Genetic; D012098:Reproduction
keywords: inbreeding; inclusive fitness; mate choice; parental investment; relatedness; reproductive strategy