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2 Post-copulatory sexual selection (PCSS) arises via traits that are expressed during and after 3 mating that increase the likelihood of an individual gaining fertilisations, relative to other 4 members of the same sex (Birkhead & Pizzari, 2002; Pitnick & Hosken, 2010). It can be seen 5 as the combination of selection pressures arising from sperm competition and cryptic 6 female choice. Sperm competition is defined as the competition between the sperm of 7 different males to fertilise the ova of a given female (Parker, 1970; Simmons, 2001). Cryptic 8 female choice is the biasing of paternity by females towards some males over others 9 (Eberhard, 1996; Thornhill, 1983). 10

11 In order for inter-or intra-sexual competition to continue after mating there must be a risk 12 that a female will mate more than once before any eggs are fertilised. Therefore, PCSS is 13 commonly said to be a consequence of multiple mating by females (eg Birkhead & Pizzari, 14 2002; Pitnick & Hosken, 2010). While this is true, a distinction needs to be made here 15 between multiple mating at the population level and at the individual level. It is the average 16 risk that a female will remate (or the average number of matings she may be expected to 17 have) that leads to PCSS, and this is dependent on the population-level female mating rate 18 (Parker, 1970; Simmons, 2001). At the individual level, some females may remate more than 19 others and at different intervals, so that there will be variation in the mated status of 20 females in the population, and in the number of male ejaculates present in the reproductive 21 tract of a given female at a given time. We note that sperm …