Some species show a pattern of decreased investment in matings with familiar partners and increased investment in matings with novel partners (Beach and Jordan 1956 Dewsbury 1981 Pizzari et al. In many species, including humans, individuals have the opportunity to mate with either a familiar partner (one with whom they have already mated) or with a novel partner (one with whom they have not previously mated). In the current study, we tested whether ejaculate parameters change in response to the familiarity or novelty of the female stimulus used to induce sexual arousal. For example, men who viewed sexually explicit images suggesting sperm competition (i.e., two men with one woman) produced a greater proportion of motile sperm in their ejaculates than men who viewed sexually explicit images suggesting no competition (i.e., three women Kilgallon and Simmons 2005). To date, there has been very little research examining differential allocation of ejaculate components in humans however, work thus far suggests that it does occur (reviewed in Leivers and Simmons 2014 see also Baker and Bellis 1993 Kilgallon and Simmons 2005 Leivers et al. Males of some species have also been found to differentially allocate seminal fluid proteins (reviewed in Perry et al. 2003 delBarco-Trillo and Ferkin 2006 Thomas and Simmons 2007 Lüpold et al. 2002 Kelly and Jennions 2011 also see Evans et al. Differential allocation of sperm based on exposure to competitor males and on female qualities occurs in a wide range of species (reviewed in Wedell et al. In the case of ejaculate composition, selection may favor differential allocation of sperm and other ejaculate components depending on intrinsic (e.g., the male’s own condition) or extrinsic factors (e.g., presence of competitors or condition of the females Perry et al. Thus, our results suggest that human males ejaculate more quickly and invest more in ejaculates with novel females.Ĭompetition between males for fertilization of a female’s eggs can result in selection on a variety of phenotypic traits, including morphology, behavior, physiology, and ejaculate composition. Time to ejaculation also decreased significantly upon exposure to a novel female. However, ejaculate volume and total motile sperm count significantly increased when males were exposed to a novel female. When males were exposed successively to the same female six times, we saw no change in ejaculate parameters between the first and sixth exposures to the same female. We analyzed ejaculates produced through masturbation with stimulation from sexually explicit films. We tested for effects of stimulus habituation and novelty on ejaculated semen parameters in humans. In some species, males reduce their investment in sperm quantity or quality upon repeated matings with the same female and increase such investment when mated to a novel female. Males in many species differentially allocate sperm and seminal fluid depending on certain social variables, including perceived sperm competition and female reproductive status.
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