Population density mediates the interaction between pre‐ and postmating sexual selection

Abstract

When females mate with more than one male, sexual selection acts both before and after mating. The interaction between pre- and postmating episodes of selection is expected to be context dependent, but few studies have investigated how total sexual selection changes under different ecological conditions. We examined how population density mediates the interaction between pre- and postmating sexual selection by establishing replicate populations of the horned dung beetle Onthophagus taurus at low, medium, and high densities, and then using microsatellite-based parentage analyses to measure male fitness. We found that mating success and fertilization success were positively correlated at all three densities, but the strength of the correlation decreased with increasing density. We also found a shift from negative to positive linear selection on testes mass as density increased, and opposing selection on weapons and testes at high densities. These patterns suggest that the importance of postmating processes increases with increasing population density, which reduces the selective advantage of weapons for premating contest competition, and increases the selective advantage of large ejaculates for postmating sperm competition. We expect that density-dependent selection on testes mass has contributed to the phenotypic variation observed between natural populations of O. taurus that differ in density.

Costs of weaponry: Unarmed males sire more offspring than armed males in a male‐dimorphic mite

“…Regardless of what the underlying mechanisms are, our results reveal a direct link between the presence or absence of precopulatory attributes and reproductive output in the absence of sperm competition. Theory predicts that with increasing population density, the number of mates each female copulates with will increase, so the benefits of precopulatory attributes (to increase the chance of obtaining females) decrease and the benefits of post-copulatory attributes (to increase the chance of producing offspring when mating) increase (McCullough, Buzatto, & Simmons, 2018;Parker & Birkhead, 2013;Parker & Pizzari, 2010). It follows that the reproductive benefit of investing in post-copulatory attributes rather than precopulatory attributes also increases with increasing population density (McCullough et al, 2018;Parker & Birkhead, 2013;Parker & Pizzari, 2010).…”

“…Theory predicts that with increasing population density, the number of mates each female copulates with will increase, so the benefits of precopulatory attributes (to increase the chance of obtaining females) decrease and the benefits of post-copulatory attributes (to increase the chance of producing offspring when mating) increase (McCullough, Buzatto, & Simmons, 2018;Parker & Birkhead, 2013;Parker & Pizzari, 2010). It follows that the reproductive benefit of investing in post-copulatory attributes rather than precopulatory attributes also increases with increasing population density (McCullough et al, 2018;Parker & Birkhead, 2013;Parker & Pizzari, 2010). Indeed, under low food conditions, bulb mite males are mostly fighters (Smallegange, Fernandes, & Croll, 2018), but under strong density-dependent conditions, male morph expression is biased towards scramblers (Smallegange & Deere, 2014).…”

Sexual selection and its evolutionary consequences in female animals

“…Approximately equal positive Bateman gradients occur in two species with male parental care, the frog Allobates fermoralis (Ursprung et al, ) and the sea spider Pycnogonum stearnsi (Barreto & Avise, ); in one species where sperm from multiple matings are inferred to be necessary to maximise fertilisation success, the Dalmatian wall lizard ( Podarcis melisellensis ; Huyghe et al, ); in two species with dominant and sneaker males, the bull‐headed scarab ( Onthophagus taurus ; McCullough, Buzatto & Simmons, ) and the brown trout ( Salmo trutta ; Serbezov et al, ); in two species where male gametic investment rivals that of females, the mega‐sperm fruit flies Drosophila bifurca and D. lummei (Bjork & Pitnick, ); in one species where sexual conflict over mating frequency has likely led to sophisticated cryptic female choice, the water strider Gerris gilletei (Gagnon, Duchesne & Turgeon, ); in two species with substantial male or mutual mate choice, the small‐mouth salamander ( Ambystoma texanum ; Gopurenko, Williams & DeWoody, ) and the eastern chipmunk ( Tamias striatus ; Bergeron et al, ); and in several birds for which the benefits of polyandry remain unknown but are suspected to be indirect (Woolfenden, Gibbs & Sealy, ; Fitze & Le Galliard, ; Gerlach et al, ; Collet et al, ). Additionally, equal but flat Bateman gradients have been reported twice in both sexes of the Eurasian blue tit ( Cyanistes caeruleus ; García‐Navas et al, ; Schlicht & Kempenaers, ), suggesting a relaxed role of sexual selection in both sexes.…”

Spatially clustered resources increase aggregation and mating duration inDrosophila melanogaster

“…Several empirical studies have supported this prediction, including in crickets (Gage and Barnard, 1996), beetles (McCullough et al, 2018), bugs (García-González and Gomendio, 2004), platyhelminths (Giannakara et al, 2016), fish (Candolin and Reynolds, 2002), and rodents (Firman et al, 2018;Ramm and Stockley, 2009).…”