title: The evolution of continuous variation in ejaculate expenditure strategy.
abstract: Sperm competition theory has largely focused on the evolution of ejaculate expenditure strategies across different species or populations or across discrete mating roles on which sperm competition operates differentially. Few studies have considered the extent to which male ejaculate expenditure is influenced by continuous change in male phenotype within a population. Here we model how optimal ejaculate expenditure responds to two sources of continuous variation: (1) the quantity of resources allocated by a male to mating within a breeding season and (2) the resource cost of obtaining a mate. We find that variation in the amount of resources available for mating does not alone produce selection for differing ejaculate investment strategies. However, when there is variation in the cost of obtaining a mate, males with a lower cost will be selected to invest fewer sperm per mating than males whose cost is higher. Any parameter decreasing this cost will also select for decreased ejaculate investment per mating. These results provide a novel insight into the evolution of male ejaculate expenditure strategies, revealing that individual constraints on the ability to secure matings can lead to variation in ejaculate expenditure even when the risk of sperm competition is the same for all males.
authors.affiliation: CoMPLEX (Centre for Mathematics and Physics in the Life Sciences and Experimental Biology), University College London, London WC1E 6BT, United Kingdom. s.tazzyman@ucl.ac.uk
authors.forename: Samuel J
authors.initials: SJ
authors.lastname: Tazzyman
authors.forename: Tommaso
authors.initials: T
authors.lastname: Pizzari
authors.forename: Robert M
authors.initials: RM
authors.lastname: Seymour
authors.forename: Andrew
authors.initials: A
authors.lastname: Pomiankowski
mesh_terms: D000818:Animals; D005075:Biological Evolution; D004542:Ejaculation; D005260:Female; D008297:Male; D011157:Population Dynamics; D012621:Seasons; D013094:Spermatozoa