Sperm competition and diversity in rodent copulatory behaviour

Abstract

The copulatory behaviour of male mammals is characterized by striking diversity in patterns of copulatory stimulation and ejaculation frequency. We conducted comparative analyses of rodents to investigate the potential influence of sperm competition in the evolution of copulatory behaviour. We found that increasing sperm competition is associated with faster rates of stimulation and earlier ejaculation among species in which males perform multiple intromissions prior to ejaculation, but with no overall change in the number of intromissions per ejaculation. Increasing sperm competition is also associated with a higher frequency of repeated ejaculations with the same female, and with shorter refractory periods between repeated copulations. Increasing sperm competition level thus appears to select for male ability to ejaculate more rapidly and more frequently with each female mated. As prolonged copulations are known to reduce female remating rates, these findings indicate that males may experience opposing selection pressures on copulatory behaviour with respect to offensive and defensive aspects of sperm competition. We conclude that sperm competition is likely to be an important selection pressure explaining diversity in mammalian copulatory behaviour.

Social cues of sperm competition influence accessory reproductive gland size in a promiscuous mammal

“…[12,18], but see [50]). Even without an overall increase in the number of sperm transferred, repeated ejaculation may be beneficial to spread the delivery of sperm transferred to the same female under competitive conditions [12,51], and/or to spread delivery of fewer sperm per ejaculate to more females under conditions of high sperm competition intensity [5].…”

“…[12,18], but see [50]). Even without an overall increase in the number of sperm transferred, repeated ejaculation may be beneficial to spread the delivery of sperm transferred to the same female under competitive conditions [12,51], and/or to spread delivery of fewer sperm per ejaculate to more females under conditions of high sperm competition intensity [5]. Hence it is possible that the increased investment in seminal vesicles shown by male bank voles in our 'high' competition group may function to promote more frequent ejaculation, but with a reduced sperm investment per ejaculate when compared with males in the 'low' competition group, as predicted by the 'intensity' model of sperm competition [5,6].…”

“…For example, it is well established that testes are relatively large in those species for which levels of sperm competition are high [4,8], and that selection under sperm competition leads to increased testis size [9]. In mammals, high levels of sperm competition are also associated with relatively high rates of sperm production [10], high-quality ejaculates [11] and more frequent ejaculations [12].…”

Evolution of genitalia: theories, evidence, and new directions

“…(Eberhard and Gelhaus 2009), and inferred from male morphology in moths (Forbes 1941;Gwynne and Edwards 1986) and wasps (Richards 1978). Copulation in various mammals also involves behavior apparently designed to stimulate the female with the genitalia (summary Eberhard 1996; see also Dixson 1998), and some aspects of copulation behavior in rodents correlate with indicators of increased probability of competition with sperm from other males (Stockley and Preston 2004). In addition, the male genitalia of several insects and spiders perform long, highly rhythmic series of taps, or squeezes on membranous portions of the female, that also suggest that stimulation of the female is important; these include a dryomyzid fly (Otronen 1990), a buprestid beetle (Eberhard 1990), a sciarid fly (Eberhard 2001c), several sepsid flies in different genera (Eberhard and Pereira 1996;Eberhard 2001bEberhard , 2003Eberhard , 2005, a pholcid spider (Huber and Eberhard 1997;Peretti et al 2006), some scathophagid flies (Hosken et al 2005), several species of tsetse flies (Briceño et al 2007;Briceño and Eberhard 2009), and the hesperiid butterfly Urbanus dorantes and the katydid Idiathron sp.…”

Sexual Conflict and Sperm Competition

“…Another MASC is to prolong mating, either by extending copula duration, pre-or postejaculation (e.g., Cordoba-Aguilar et al 2009), or copulating repeatedly with the same female (e.g., Laird et al 2004;Stockley and Preston 2004;Preston and Stockley 2006). There are many examples of males strategically altering copulation to benefit either from the guarding effect of this behavior (see above) or from transferring larger ejaculates to potentially achieve greater fertilization success (e.g., Wedell 1998;Martin and Hosken 2002).…”