title: Reproductive Conflict and the Evolution of Menopause in Killer Whales.
abstract: Why females of some species cease ovulation prior to the end of their natural lifespan is a long-standing evolutionary puzzle [1-4]. The fitness benefits of post-reproductive helping could in principle select for menopause [1, 2, 5], but the magnitude of these benefits appears insufficient to explain the timing of menopause [6-8]. Recent theory suggests that the cost of inter-generational reproductive conflict between younger and older females of the same social unit is a critical missing term in classical inclusive fitness calculations (the "reproductive conflict hypothesis" [6, 9]). Using a unique long-term dataset on wild resident killer whales, where females can live decades after their final parturition, we provide the first test of this hypothesis in a non-human animal. First, we confirm previous theoretical predictions that local relatedness increases with female age up to the end of reproduction. Second, we construct a new evolutionary model and show that given these kinship dynamics, selection will favor younger females that invest more in competition, and thus have greater reproductive success, than older females (their mothers) when breeding at the same time. Third, we test this prediction using 43 years of individual-based demographic data in resident killer whales and show that when mothers and daughters co-breed, the mortality hazard of calves from older-generation females is 1.7 times that of calves from younger-generation females. Intergenerational conflict combined with the known benefits conveyed to kin by post-reproductive females can explain why killer whales have evolved the longest post-reproductive lifespan of all non-human animals.
authors.affiliation: Centre for Research in Animal Behaviour, University of Exeter, Exeter EX4 4QG, UK. Electronic address: d.p.croft@exeter.ac.uk.
authors.forename: Darren P
authors.initials: DP
authors.lastname: Croft
authors.affiliation: Behaviour and Evolution Group, Department of Zoology, University of Cambridge, Cambridge CB2 3EJ, UK.
authors.forename: Rufus A
authors.initials: RA
authors.lastname: Johnstone
authors.affiliation: Centre for Research in Animal Behaviour, University of Exeter, Exeter EX4 4QG, UK.
authors.forename: Samuel
authors.initials: S
authors.lastname: Ellis
authors.affiliation: York Centre for Complex Systems Analysis, University of York, York YO10 5GE, UK.
authors.forename: Stuart
authors.initials: S
authors.lastname: Nattrass
authors.affiliation: York Centre for Complex Systems Analysis, University of York, York YO10 5GE, UK.
authors.forename: Daniel W
authors.initials: DW
authors.lastname: Franks
authors.affiliation: Centre for Research in Animal Behaviour, University of Exeter, Exeter EX4 4QG, UK.
authors.forename: Lauren J N
authors.initials: LJ
authors.lastname: Brent
authors.affiliation: Department of Mathematics, University of York, York YO10 5DD, UK.
authors.forename: Sonia
authors.initials: S
authors.lastname: Mazzi
authors.affiliation: Center for Whale Research, 355 Smugglers Cove Road, Friday Harbor, WA 98250, USA.
authors.forename: Kenneth C
authors.initials: KC
authors.lastname: Balcomb
authors.affiliation: Pacific Biological Station, Fisheries and Oceans Canada, Nanaimo, BC V9T 6N7, Canada.
authors.forename: John K B
authors.initials: JK
authors.lastname: Ford
authors.affiliation: Centre for Ecology and Conservation, University of Exeter, Penryn Campus, Penryn, Cornwall TR10 9FE, UK.
authors.forename: Michael A
authors.initials: MA
authors.lastname: Cant
mesh_terms: D000818:Animals; D005075:Biological Evolution; D005260:Female; D008593:Menopause; D012098:Reproduction; D050685:Whale, Killer / Q000502:physiology*
keywords: cetacean; cooperation; fertility; grandmother hypothesis; human evolution; life history; senescence