Female remating decisions and a shorter inter-mating interval diminish last-male sperm precedence
Highly variable within and across species, patterns of sperm use not only are often driven by post-copulatory sexual selection but can also be impacted by experimental design. In investigations of paternity bias using competitive double matings, the inter-mating interval is a temporal factor that can affect sperm use patterns if the first male’s sperm is used or lost at an appreciable rate between matings or if its viability or relative competitiveness is influenced by the time since ejaculation. Rapid loss of first-male sperm within the female after mating has been established in the seed beetle (Callosobruchus maculatus), a model system in sperm competition studies. However, our understanding of sperm precedence in this species, which disproportionately favors the last (second) male to mate, is based on long inter-mating intervals. Here, I determine the effect of a shortened inter-mating interval on second-male paternity (P 2) and, importantly, the extent to which females in this species are willing to remate immediately. I find that P 2 is significantly reduced when females remate immediately than when they remate 24 or 48 h after the first mating and that immediate remating is common, indicating that there is a substantial potential for female remating decisions to influence the intensity of sperm competition within species. To understand the variation in inter-mating intervals from a female perspective, I further identify key differences between females that did and did not remate at three inter-mating intervals (0, 24, and 48 h after the initial mating) and discuss potential mechanisms for the observed variation in female refractoriness.
Females often mate with multiple males, and in insects it is common for the last male to mate to obtain the greatest share of paternity. One possible explanation for this bias towards last-male sperm use is the passive loss of sperm from the first male with time since mating. Here, I examine the effect of different intervals between matings in a seed beetle species to determine if first-male paternity is increased when females remate sooner. My results show that first males do achieve higher paternity when females remate sooner, which is more common than expected in this species. I also show the differences between females that do and do not remate, which can help explain potential causes for variation in female remating behavior. Together, these results show the significant impact that female remating decisions can have on male competition and, ultimately, their reproductive success.
KeywordsPolyandry Sexual selection Paternity Female remating behavior Sperm precedence Sperm competition
I thank my committee members—Drs. Janis Dickinson, Hudson Kern Reeve, Linda Rayor, and Mike Webster—and fellow departmental colleagues for critical discussion and comments; Frank Messina for providing the stock population of beetles, extra oviposition substrate, and advice on culture maintenance; Ken Kemphues for help with access to and use of the irradiator; Pat Sullivan and Andy Clark for their input and help with experimental design; Abigail Giancola and Hannah Yoo for beetle culturing assistance; Lynn Johnson for statistical advice; Lars Washburn for use of a microbalance; family and friends for their support; the city of Austin for their tacos and sunshine; and two anonymous referees for their comments that greatly improved the manuscript.
Compliance with ethical standards
The datasets generated and/or analyzed during the current study are available in the Dryad Digital Repository, [http://dx.doi.org/10.5061/dryad.7h7h3].
This research was supported by grants from the Department of Neurobiology and Behavior at Cornell, ATHENA Fund and the Linda and Samuel Kramer Graduate Student Fellowship at the Cornell Lab of Ornithology, the Cornell Sigma Xi Student Research Grant, the American Museum of Natural History, and the National Science Foundation Graduate Research Fellowship (Cornell NSF Grant DGE-1144153).
Conflict of interest
The author declares that she has no conflict of interest.
All procedures performed in studies involving animals were in accordance with the ethical standards of the institution or practice at which the studies were conducted.
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