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.

Condition-dependent ejaculate size and composition in a ladybird beetle

“…There is a growing appreciation for the potential influence of resource availability on ejaculate expenditure (Williams et al 2005;Tazzyman et al 2009). Our finding that ejaculate traits are differentially sensitive to male condition supports this emerging view.…”

“…Whenever high-condition males have lower marginal costs of ejaculate production than low-condition males, they are expected to transfer larger ejaculates at mating (even as they invest less reproductive effort per ejaculation ;Parker 1990;Tazzyman et al 2009). Empirical studies involving experimental manipulations of condition generally support this prediction, reporting that highcondition males produce larger ejaculates (Gwynne 1990;Delisle & Bouchard 1995;Watanabe & Hirota 1999;Jia et al 2000;Ferkau & Fisher 2006;Lewis & Wedell 2007;Blanco et al 2009; but see Wedell 1993), transfer more sperm (Fedina & Lewis 2006;McGraw et al 2007;Perez-Staples et al 2008), and produce more ejaculate-derived nuptial gifts (Jia et al 2000; but see Wedell 1993).…”

Size dependence in non-sperm ejaculate production is reflected in daily energy expenditure and resting metabolic rate

“…In systems where females are polyandrous, males are selected to increase sperm and seminal fluid production (Parker, 1970(Parker, , 1984(Parker, , 1990(Parker, , 1998Parker and Pizzari, 2010;Tazzyman et al, 2009). Limits on energy allocation are a fundamental assumption in sperm competition models that predict context-dependent ejaculate allocation strategies (size dependence: e.g.…”

“…Perry and Rowe, 2010;Rahman et al, 2013;Simmons et al, 1996;Simmons and Kotiaho, 2002;Simmons and Parker, 1992). In general, these models predict that male ejaculate allocation is affected by the cost of mate acquisition (Tazzyman et al, 2009). Male phenotypes that are more likely or have lower costs to obtain a mate are expected to expend less on ejaculates than male phenotypes that are less likely or have higher costs to obtain a mate (Parker and Pizzari, 2010;Tazzyman et al, 2009).…”

Evolutionary Modeling Predicts a Decrease in Postcopulatory Sperm Viability as a Response to Increasing Levels of Sperm Competition

“…As in many previous sperm competition models (e.g., Ball and Parker 1996; Parker et al 1996Parker et al , 1997Tazzyman et al 2009), this trade-off is expressed as . Here R is the R p n (c ϩ a) males' fixed total energy budget, n is the number of matings, c is the cost of achieving matings, and a (as above) is the cost of the ejaculate.…”