Abstract
Sperm competition theory predicts semen characteristics to be affected by the social environment. We used the polygamous horse (Equus caballus) to experimentally study within-subject plasticity in response to different social environments. Stallions were sequentially exposed, over a period of 8 weeks each, to other stallions and then singly to mares, or vice versa (in adjacent boxes separated by grills). Ejaculates were collected to determine semen characteristics. Highest sperm numbers were found in stallions that were first exposed to other stallions and then to mares, while lowest sperm numbers were observed in stallions that had been exposed to mares but not yet to other stallions. One of three sperm velocity measures (curvilinear velocity) was consistently elevated in stallions that were first exposed to stallions and then to mares. Sperm number after exposure to mares and curvilinear sperm velocity after exposure to stallions were both positively correlated to average blood testosterone levels during the corresponding period of exposure. We conclude that ejaculate characteristics are plastic traits affected by the social environment in horses.
Highlights
Selection is predicted to affect the evolution of ejaculate characteristics in species where females have the possibility to mate multiply and sperm of different males may have to compete for fertilization of eggs [1,2]
The order of exposure to stallions and mares significantly affected ejaculate characteristics: Sperm numbers were highest after exposure to mares for stallions that had previously been exposed to other stallions, and lowest after exposure to mares for stallions that had not previously been exposed to other stallions (Fig 1A; see interaction term in Table 1), while sperm numbers were about in between after exposure to stallions, regardless of the order of exposure (Fig 1B)
Sperm numbers were never significantly correlated to the corresponding sperm velocity measures (|tau| always < 0.30, p always > 0.17)
Summary
Selection is predicted to affect the evolution of ejaculate characteristics in species where females have the possibility to mate multiply and sperm of different males may have to compete for fertilization of eggs [1,2]. Key ejaculate characteristics that are predicted to be shaped by this evolution are (i) sperm number per ejaculate [3] and (ii) sperm velocity [4,5] The latter is affected by sperm morphology and ATP content [5,6]. Because sperm production is costly [8], optimal ejaculate expenditure may evolve in response to the degree of sperm competition but may display plasticity in response to changes in mating roles or temporal changes in sperm competition [9]. Experiments in fish demonstrate that sperm velocity can PLOS ONE | DOI:10.1371/journal.pone.0143185 November 24, 2015
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
